Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

 

PART THREE – Clinical Management of Special Surgical Problems

Chapter 21 – Anesthesia for Pediatric Orthopedic Surgery

Aaron L. Zuckerberg,Myron Yaster

  

 

Scoliosis, 737

  

 

Epidemiology and Etiology, 738

  

 

Natural History, 739

  

 

Respiratory Sequelae of Scoliosis, 740

  

 

Cardiovascular Sequelae of Scoliosis, 740

  

 

Preoperative Evaluation, 741

  

 

Surgical Techniques, 741

  

 

Anesthetic Management of Scoliosis Surgery,743

  

 

Postoperative Management, 751

  

 

Joint Disorders, 752

  

 

Arthrogryposis Multiplex Congenita,752

  

 

Juvenile Rheumatoid Arthritis, 752

  

 

Marfan Syndrome, 753

  

 

Syndromes of Disproportionate Short Stature: Dwarfism, 753

  

 

Airway Abnormalities, 754

  

 

Pulmonary Dysfunction, 755

  

 

Cardiac Dysfunction, 755

  

 

Neurologic Dysfunction, 755

  

 

Osteogenesis Imperfecta,756

  

 

Osteopetrosis, 756

  

 

Cerebral Palsy, 756

  

 

Ilizarov Method, 758

  

 

History, 758

  

 

Anesthetic Considerations,758

  

 

Tourniquets, 759

  

 

Clubfoot, 760

  

 

Developmental Dysplasia of the Hip, 760

  

 

Slipped Capital Femoral Epiphysis, 760

  

 

Fractures, 761

  

 

Upper Extremity Blocks, 761

  

 

Lower Extremity Blocks,761

  

 

Fat Embolism Syndrome, 761

  

 

Pathophysiology, 762

  

 

Presentation, 763

  

 

Summary, 764

Anesthesia for pediatric orthopedic surgery encompasses the entire age and medical spectrum of pediatrics. It includes the newborn and the adolescent, the otherwise normal, the chronically ill, the patient with multiple complex congenital anomalies, the emergent trauma patient, and the elective inpatient and outpatient. Orthopedic surgeons operate on virtually every area of the body from the cervical spine to the pelvis to the toes. In many instances, the perioperative anesthetic plan for pediatric orthopedic patients depends more on the child's age and on the site and emergent nature of surgery than on the underlying disease or the specifics of the surgical procedure. In other cases, the underlying medical condition, associated anomalies, pathophysiology, and surgical procedure dictate the anesthetic plan. Frequently, the anesthesiologist must be aware of unusual associated syndromes that have obvious orthopedic implications and syndromes with underlying clinical significance unrelated to the orthopedic condition. Table 21-1 lists conditions that are commonly encountered in pediatric orthopedic surgery and describes their anesthetic implications.

A common feature of children with orthopedic diseases, particularly patients with congenital anomalies, generalized constitutional diseases of bone and cartilage, or connective tissue disorders, is the significant disability that affects their everyday lives. Some of these children must undergo repeated hospitalization and may require multiple anesthetics and surgical procedures. These children may have overwhelming fear and apprehension, and they may be completely terrorized by the hospital experience (“hospitalitis”). Simply approaching these children in hospital clothing may elicit screams of terror. Others, with diseases such as cerebral palsy, may be of normal intelligence but are often treated as if they were mentally incapacitated because of their inability to communicate clearly. The approach to the orthopedic patient must be individualized.

Orthopedic surgery is among the most common types of surgery performed in the United States. Technological advances permit more sophisticated orthopedic diagnoses, and they have vastly expanded the range of treatment options and operations available to the orthopedic surgeon. The technological, physiologic, and pharmacologic advances in anesthesiology have allowed the orthopedic surgeon to contemplate longer, more extensive, and more innovative operations on younger and sicker patients than was ever before possible. Regardless of the underlying condition, almost all orthopedic surgical procedures have recurring anesthetic concerns, including positioning, airway management, blood loss and fluid replacement, conservation of body temperature, and postoperative pain management. The anesthesiologist must have knowledge of the particular surgeon, the operation, the positioning of the patient, and the duration of the procedure.


TABLE 21-1   -- Anesthetic implications of commonly encountered orthopedic disorders

Disease

Surgical Interventions

Anesthetic Implications

Congenital Malformations

Amniotic band constriction

Soft tissue release

May have facial clefts

Clubfoot

Tendon lengthening, release

Dictated by associated malformations

Klippel-Feil syndrome

Release, scoliosis

Limited C-spine mobility, heart defects

Radial dysgenesis

Tendon lengthening, pollicization release

Dictated by associated malformations

Sprengel's deformity

 

Only associated Klippel-Feil syndrome

Acquired Conditions

Charcot-Marie-Tooth disease

Tendon transfer

None known

Legg-Calvé-Perthes disease

Osteotomies, pinning

As per associated diseases (sickle cell)

Osteomyelitis

Culture, aspiration

Systemic bacterial infection

Septic arthritis

Culture, irrigation

Systemic bacterial infection

Slipped femoral capital epiphysis

Pinning

Obesity

Tumors

Benign

Excision, curettage

Blood loss may be significant, pathologic fracture

Malignant

Radical excision, amputation

Blood loss; metastasis: CNS, lung; chemotherapy cardiotoxicity

Syndromes, Inherited Conditions

Apert's syndrome

Syndactyly repair

Airway usually normal, occasional cardiac defect

Ellis-van Creveld syndrome

Polydactyly

Cardiac defects, bronchial collapse

Holt-Oram syndrome

As in radial dysgenesis

Cardiac defects (ASD, VSD)

Marfan syndrome

Kyphosis

Cardiac defects (AI, MR), aortic aneurysm

Moebius sequence

Syndactyly

Micrognathia, cleft palate, cranial nerve palsy

Osteogenesis imperfecta

Pathologic fractures, scoliosis

Fractures on positioning or intubation; hypermetabolic fever, platelet dysfunction

VATER association

As in radial dysgenesis

Cardiac defects, tracheoesophageal fistula

Dwarfism

Achondroplasia

Spinal fusion, decompression

Poor cervical mobility, restrictive lung disease

Morquio-Ullrich disease

C-spine fusion

Unstable C-spine, restrictive lung disease

Systemic Disease

Juvenile rheumatoid arthritis

Varies

TMJ ankylosis, C-spine immobility or instability, carditis, occasional pulmonary involvement

Neurofibromatosis

Scoliosis

CNS tumors, occasional pheochromocytoma

Sickle cell anemia

Osteomyelitis, Legg-Calvé-Perthes disease, pathologic fracture

Anemia, sickle crisis: hypothermia, hypoxia, hypovolemia, avoid tourniquet when possible

CNS Diseases

Arthrogryposis multiplex

Releases, scoliosis

TMJ ankylosis, C-spine immobility, GE reflux, postoperative upper airway obstruction

Cerebral palsy

Releases

GE reflux, postoperative upper airway obstruction

Myelomeningocele

Lower extremity tendon releases

Hydrocephalus

Werdnig-Hoffmann disease

Scoliosis

Respiratory insufficiency, bulbar involvement—poor secretion handling, succinylcholine-induced hyperkalemia

Myopathies

Duchenne's muscular dystrophy

Releases, scoliosis

Respiratory insufficiency, cardiomyopathy, succinylcholine-induced hyperkalemia, malignant hyperthermia

Myotonia dystrophica

Releases

Succinylcholine-induced myotonic spasm, cardiac conduction system involvement, avoid direct muscle stimulation

AI, aortic insufficiency; ASD, atrial septal defect; CNS, central nervous system; C-spine, cervical spine; GE, gastroesophageal; MR, mitral regurgitation; TMJ, temporomandibular joint; VSD, ventricular septal defect; VATER association (vertebral defects, imperforate anus, tracheoesophageal fistula, radial and renal dysplasia).

 

 

 

SCOLIOSIS

Scoliosis, derived from the Greek root meaning “crooked,” is a lateral and rotational deformity of the thoracolumbar spine. With progression of the lateral spinal curvature, the spinous processes rotate toward the concave side of the curve. The ribs on the convex side are pushed posteriorly by the rotating spine, forming the characteristic gibbous deformity. The ribs on the concave side become prominent anteriorly and are crowded together. Occasionally, scoliosis is associated with kyphosis ( Fig. 21-1 ).

 
 

FIGURE 21-1  Structural changes in idiopathic scoliosis. A, As curvature increases, alterations in body configuration develop in the primary and compensatory curve regions. B, Asymmetry of shoulder height, waistline, and elbow-to-flank distance are common findings. C, Vertebral rotation and associated posterior displacement of the ribs on the convex side of the curve are responsible for the characteristic deformity of the chest wall in scoliosis patients. D, In the school screening examination for scoliosis, the patient bends forward at the waist. Rib asymmetry of even a small degree is obvious.  (From Scoles PV: Spinal deformity in childhood and adolescence. In Behrman RE, Vaughn VC III, editors: Nelson textbook of pediatrics, Update 5. Philadelphia, 1989, WB Saunders.)

 



The progression of scoliosis and the severity of its systemic manifestations correlate with the angle of curvature measured by the Cobb method ( Table 21-2 ). This is the angle between the upper surface of the “top-end” vertebra and the lower surface of the “bottom-end” vertebra. The end vertebrae are those that are maximally tilted. Perpendicular lines are extended from these end vertebrae to the center of the curve. The angle formed by the intersecting perpendiculars determines the angle of curvature ( Fig. 21-2 ). The curve is defined as facing to the right or to the left, depending on the convexity of the curve. A lateral curve of greater than 10 degrees is abnormal. Respiratory impairment rarely occurs with a curvature of less than 60 degrees.


TABLE 21-2   -- Correlation of angle of curve and symptoms in patients with scoliosis

Angle of Curvature

Significance

<10

Normal curvature

>25

Echocardiographic evidence of increased pulmonary artery pressures

>40

Surgical intervention

>65

Restrictive lung disease

>100

Symptomatic lung disease, dyspnea on exertion

>120

Alveolar hypoventilation

 

 

 
 

FIGURE 21-2  Standing posteroanterior radiograph of a 13-year-old girl with a severe right thoracic section. Notice the Cobb measurement technique. The Cobb angle is derived by drawing lines parallel to the superior surface of the proximal-end vertebra and the inferior surface of the distal-end vertebra. Perpendiculars to these lines are erected, and the angle of intersection of these lines is measured. The numbers in parentheses indicate the degree of correction of the deformity on side-bending radiographs.  (From Thompson GH: The spine. In Behrman RE, editor: Nelson textbook of pediatrics, 16th ed. Philadelphia, 2004, Elsevier.)

 



▪ EPIDEMIOLOGY AND ETIOLOGY

The overall prevalence of spinal deformities in the North American population is between 1% and 2% ( Weinstein et al., 2003 ). In the past, polio or tuberculosis infection was the most common cause of this disease. Today, most cases of scoliosis are classified as idiopathic because the basic pathophysiology remains unknown. Pedigree analysis suggests that scoliosis is a sex-linked trait with variable expression and incomplete penetrance ( Xiong and Sevastik, 1998 ; Lowe et al., 2000 ). The most common types of scoliosis are listed in Box 21-1 .

BOX 21-1 

Classification of Scoliosis

Congenital scoliosis

  

 

Vertebral anomalies

  

 

Rib anomalies

  

 

Spinal dysraphism

Idiopathic scoliosis

  

 

Infantile (<3 years of age)

  

 

Juvenile (3 to 10 years of age)

  

 

Adolescent (>10 years of age)

Scoliosis associated with neuromuscular disease

  

 

Cerebral palsy

  

 

Poliomyelitis

  

 

Myopathies

  

 

Muscular dystrophies

  

 

Syringomyelia

  

 

Friedreich's ataxia

Traumatic scoliosis

  

 

Fractures

  

 

Irradiation

  

 

Burns

  

 

Surgery

Syndromes associated with scoliosis

  

 

Neurofibromatosis (von Recklinghausen's disease)

  

 

Marfan syndrome

  

 

Osteogenesis imperfecta

  

 

Mucopolysaccharidosis

  

 

Rheumatoid arthritis

Neoplastic disease

Congenital scoliosis is a curvature of the spine that is the result of a rib or vertebral anomaly. Idiopathic scoliosis is the most common of the spinal deformities and has three periods of onset, all coincident with periods of rapid growth spurts: infantile (<3 years old), juvenile (3 to 10 years old), and adolescent (>10 years old). Progression of the deformity depends on the age of onset. Infantile idiopathic scoliosis has been associated with an increased incidence of mental retardation, inguinal hernias, congenital dislocation of the hip, and congenital heart disease. Juvenile idiopathic scoliosis can usually be managed conservatively ( Lowe et al., 2000 ). Adolescent idiopathic scoliosis is the most common form of scoliosis and occurs most commonly in girls ( Weinstein et al., 2003 ). The curve may resolve, remain stable, or progress in severity. The most significant prognosticators of curve progression in girls are age at onset, premenarchal status, and bone age ( Table 21-3 ) ( Ahn et al., 2002 ; Lowe et al., 2000 ).


TABLE 21-3   -- Incidence of scoliotic curve progression at the time of diagnosis of a 10-degree curve in girls

Age

Menarchal Status

Bone Maturity

<11 years (88%)

Premenarche (53%)

Immature (68%)

>15 years (29%)

Postmenarche (11%)

Mature (18%)

 

 

▪ NATURAL HISTORY

The natural history of scoliosis varies according to the cause and the pattern of vertebral involvement. If uncorrected, scoliosis is marked by curve progression, cosmetic deformity, back pain, and compromise of physiologic function ( Weinstein et al., 2003 ). In most cases of idiopathic scoliosis, the spinal curvature remains small, and conservative nonoperative management is appropriate ( Ascani et al., 1986 ). In 0.2% to 0.5% of cases, the curve is progressive and requires surgical intervention ( Ahn et al., 2002 ). In patients with idiopathic scoliosis, only those with thoracic apices and curves of more than 100 degrees are at increased risk of death from cor pulmonale and right ventricular failure ( Weinstein et al., 1981 ). In most patients with idiopathic scoliosis, the grim prognosis of early death and respiratory failure is untrue ( Weinstein et al., 1981 , 2003 ). When to perform this surgery is controversial. The worse the curve and the more compromised the cardiorespiratory function, the greater is the risk of perioperative morbidity and mortality.

RESPIRATORY SEQUELAE OF SCOLIOSIS

As the degree of curvature progresses, vertebral rotation results in narrowing of the thoracic cage. Lung volumes and pulmonary compliance are inversely related to the degree of this curve. Nevertheless, even asymptomatic patients have demonstrable abnormalities in pulmonary function. When the scoliotic curve is greater than 65 degrees, respiratory function is compromised. Pulmonary function tests demonstrate the characteristic pattern of restrictive lung disease. The vital capacity (normal, 60 mL/kg) is severely reduced, often to less than 60% of predicted. Of the subdivisions of vital capacity, inspiratory capacity is affected to a greater extent than expiratory reserve volume. Functional residual capacity and residual volume are not as severely affected. These alterations in lung volumes are caused by changes in chest wall compliance and the resting position of the thoracic cage, rather than parenchymal changes.

The impairment in pulmonary function occurring in scoliosis from neuromuscular disease is exacerbated by coexisting abnormalities in central respiratory drive, coordination of swallowing, and innervation of the upper airway and respiratory musculature. Pulmonary dysfunction in these patients is exacerbated by the increased frequency of respiratory infections, predilection to aspiration, and impaired ability to clear pulmonary secretions. Patients with abnormal pulmonary function test results, particularly a forced vital capacity (FVC) of less than 50%, or who have hypercapnia preoperatively will probably require postoperative (or chronic) ventilation. Maximum inspiratory and expiratory mouth pressures (PImax, PEmax) that the patient can generate against airway occlusion are the important indices for his or her ability to reexpand the lungs (sighs, PImax < -40 cm H2O) and to expel secretions (coughs, PEmax > +40 cm H2O). Unless the patient can generate more than these threshold pressures preoperatively, postoperative admission to the intensive care unit for ventilatory support should be planned ahead of time.

▪ CARDIOVASCULAR SEQUELAE OF SCOLIOSIS

Mitral valve prolapse is found in 25% of patients with scoliosis but in less than 10% of age-matched controls. Echocardiographic evidence for increased pulmonary artery pressures has been demonstrated in individuals with only modest degrees of scoliosis in the absence of abnormal pulmonary function ( Primiano et al., 1983 ). Patients with angles of curvature greater than 70 degrees develop pulmonary hypertension on exercise; patients with curves greater than 110 degrees have mean pulmonary artery hypertension at rest. Kafer has proposed that this increase in pulmonary vascular resistance is not just the result of lung compression from thoracic cage abnormalities but also an increased incidence of hypoxic pulmonary vasoconstriction ( Kafer, 1980 ; Schur et al., 1984 ). Rather, development of the pulmonary vascular bed may be impaired, resulting in a fundamental reduction in the number of functional vascular units per lung ( Kafer, 1980 ; Schur et al., 1984 ).

Any child with a myopathy or borderline respiratory status should have an electrocardiogram and an echocardiogram performed to assess the presence of cor pulmonale, ventricular wall motion, ejection fraction, and ventricular wall thickness. Many myopathies, particularly Duchenne's muscular dystrophy, involve cardiac muscle and skeletal muscle ( Milne and Rosales, 1982 ; Miller et al., 1998 ). Duchenne's muscular dystrophy is the most common muscular dystrophy occurring in children presenting for surgery. An X-linked recessive disorder, this progressive, debilitating disease affects skeletal, cardiac, and smooth muscle. Typically, afflicted boys become wheel chair dependent by the age of 10 years, and death from respiratory or cardiac failure occurs before the age of 20. Scoliosis is common, and surgery is often performed to improve the quality of life.

Numerous anesthetic challenges occur in patients with Duchenne's muscular dystrophy. Clinically significant cardiomyopathies and rhythm disturbances manifest by 10 years of age (see Chapter 32 , Systemic Disorders). Many of these children are obese because of muscle weakness, fatty degeneration of muscle fibers, and lack of exercise. Succinylcholine can cause a fatal hyperkalemia in these patients, who may present for surgery before the diagnosis has been definitively made, and the routine use of this muscle relaxant is no longer recommended in all children ( Solares et al., 1986 ; Sullivan et al., 1994 ).

▪ PREOPERATIVE EVALUATION

The most important aspects of the preoperative evaluation include determination of the location and degree of the spinal curvature, the cause of the scoliosis, the patient's history of exercise tolerance, respiratory symptoms, and the presence of coexisting diseases. A directed physical examination of the cardiorespiratory system should evaluate the presence of tachypnea, crackles, wheezing, and signs of right heart failure, such as hepatomegaly, jugular venous distention, and peripheral edema. Any preoperative neurologic deficits should be recorded. Based on the severity of the curve and the degree of respiratory impairment, the preoperative laboratory studies listed in Box 21-2 should be requested.

BOX 21-2 

Preoperative Tests for Scoliosis Surgery

Chest radiograph

Electrocardiogram

Echocardiogram

Pulmonary function tests

  

 

Arterial blood gas

  

 

Spirometry

  

 

Forced vital capacity (FVC)

  

 

Forced expiratory volume at 1 second (FEV1), FEV1/FVC

  

 

Peak expiratory flow rate (PEFR)

  

 

Peak inspiratory pressure (Pimax)

  

 

Peak expiratory pressure (Pemax)

Coagulation studies

  

 

Platelet count

  

 

Prothrombin time, partial thromboplastin time

Electrolyte panel

Liver function tests

Right heart involvement is reflected in the findings of right ventricular hypertrophy and right axis deviation on the electrocardiogram. Estimates of the degree of pulmonary hypertension may be made by evaluating the right systolic time interval and the velocity of tricuspid regurgitation on the echocardiogram. Pulmonary function tests are useful in establishing the risk of pulmonary complications in the immediate postoperative period. An FVC of less than 30 mL/kg (or less than 50% of predicted) or a forced expiratory volume at 1 second (FEV1) less than 50% of predicted usually indicates postoperative respiratory insufficiency and the need for prolonged postoperative mechanical ventilation. Peak inspiratory and expiratory forces with the airway occluded of at least -30 cm H2O and +40 cm H2O, respectively, are needed for effective sighs and postoperative coughing and expulsion of secretions.

Children with myelodysplastic syndromes are likely to develop an allergy to latex products ( Kelly et al., 1994 ; Brock-Utne, 2003 ). All children with a myelodysplastic syndrome should be considered allergic to latex, and nonlatex products (e.g., tourniquets, sterile and nonsterile gloves) should be substituted for the latex equivalents. Corticosteroids and antihistamines are not administered prophylactically.

▪ SURGICAL TECHNIQUES

The treatment of spinal curvature is dictated by the type of scoliosis and by the surgeon's expertise and preferences. Very few cases of congenital scoliosis can be managed conservatively. The mainstay of therapy for congenital scoliosis is posterior spinal fusion without instrumentation, followed by prolonged immobilization. Instrumentation in these patients has been associated with a prohibitively high rate of paraplegia, which is presumed to be the result of coexisting cord and vertebral anomalies. Although conservative therapy is the most frequently employed treatment for idiopathic scoliosis, when rapid curve progression is anticipated, surgical intervention is employed for severe truncal deformities and for pain unrelieved by medical therapy ( Weinstein et al., 2003 ).

Posterior Spinal Fusion

The goal of scoliosis surgery is to achieve a spinal fusion and stabilization of the curve. After incision through the supraspinal ligament, the paraspinous musculature is reflected. The vertebral laminae are decorticated, the facet joints are destroyed, and the spinous processes are removed so that raw cancellous bone is exposed. Bone graft obtained from the iliac crest, ribs on the convex side, or the bone bank is cut into matchstick-sized strips and packed over the decorticated surfaces, mainly on the concave side. The fusion extends from one vertebra above the curve to the second vertebrae below. Instrumentation is usually inserted to hold the spine in the best possible position while fusion is accomplished. Without a properly performed fusion, the instrumentation will ultimately fatigue.

Several instrumentation techniques are available for treatment of the scoliotic spine. The Harrington rod is a stainless steel rod that is connected to the inferior facets and pedicles of the spine by multiple ratchet hooks that are placed at the terminal aspects of the curve. Distraction is adjusted using the ratchet principle ( Harrington, 1988 ; Harrington and Dickson, 1976 ). The incidence of neurologic complications after this technique is 0.23%. The disadvantages of the Harrington rod include two-dimensional correction, curvature distraction by the end hooks, and the need for prolonged postoperative immobilization. Because of these problems, this technique is rarely used.

Segmental spinal instrumentation was introduced to improve three-dimensional correction and the ability to achieve differential distraction at multiple levels. The Luque instrumentation system consists of sublaminar wires on each side of the spinous process and a long, L-shaped rod that can be contoured three-dimensionally. The curve is corrected as the wires are tightened ( Luque, 1986 ; Luque and Rapp, 1988 ). The internal fixation achieved is more rigid than that obtained with the Harrington system, and it can be extended to the pelvis. The most common deficit after Luque rod instrumentation is a sensory dysesthesia, which is usually observed late (2 to 6 days) in the postoperative period. The proposed mechanism for these findings is expansion of an epidural hematoma in the area of the sublaminar wires (Johnston, 1986 ).

The Cotrel-Dubousset segmental spinal instrumentation system uses multiple laminar and pedicular hooks attached to a double-rod frame ( Richards and Johnston, 1987 ). This enables three-dimensional correction of complex curves and obviates the need for postoperative immobilization. It is more time consuming than the Harrington system, increases intraoperative blood loss, and has a lower incidence (0.6%) of neurologic complications than Luque rods. Double-curve patterns are more complex and require multiple hooks at multiple fixation sites, necessitating more extensive decortication and contributing to additional blood loss. The limitation of posterior spinal fusion with or without instrumentation is that the anterior growth plates, which play a major role in the development of the deformity, are not affected. Late torsional deformities can result.

Anterior Spinal Surgery

The anterior approach to spinal deformities has been advocated for several specific deformities, including severe kyphosis and lordotic paralytic curves in patients with cerebral palsy. Surgery consists of discectomies with or without instrumentation, performed alone or in combination with a posterior spinal fusion. Video-assisted thoracoscopic surgery can be used for this procedure if instrumentation is not being used ( Newton et al., 1997 ; Sucato, 2003 ). The surgical approach used to expose the anterior portion of the spine depends on the exact spinal deformity. Thoracic curves are usually approached through a left thoracotomy, and the procedure is facilitated by insertion of a double-lumen endotracheal tube and one-lung ventilation. Alternatively, single-lung ventilation in young children is performed by advancing a tracheal tube into the main stem bronchus opposite the side of surgery or by positioning a bronchial blocker into the main stem bronchus on the operative side. Multiple techniques for placing a variety of bronchial blockers outside the tracheal tube have been described for use in children ( Hammer et al., 1999 , 2002) (see Chapter 19 , Anesthesia for General, Thoracic, and Urologic Surgery). The combined curve of the thoracolumbar spine is exposed transdiaphragmatically by means of a high subcostal incision that necessitates taking the diaphragm down from its bony insertion. Lumbar curves can be approached extraperitoneally or transabdominally. In general, complications of the anterior approach include great vessel disruption, hemothorax, pneumothorax, paralytic interruption of spinal cord perfusion, and excessive angulation or compression of the spinal cord by rapid distraction of the curvature. Spinal cord injury can result from mechanical damage by a screw or disruption of segmental spinal arteries.

One-Stage versus Two-Stage Anterior-Posterior Spinal Fusion

The combined anterior and posterior spinal fusion maximizes curve correction and minimizes recurrence and pseudoarthrosis formation by obtaining a circumferential fusion. Whether to combine these procedures into a single operation or to perform them in a staged manner is controversial. The staged approach frequently requires prolonged hospitalization and allows recovery from extensive anterior procedures before proceeding with the posterior fusion ( Brown et al., 1982 ). If complications preclude continuation of the staged procedure, premature anterior fusion may compromise the ultimate correction of the curve. A single operation provides recovery that is more rapid, shorter hospitalization, longer operative times, greater blood loss, and possibly more immediate postoperative respiratory complications. O—Brien and others (1992) compared the results of a one-stage versus two-stage anterior-posterior fusion in 26 patients with progressive neuromuscular scoliosis. The mean operative time for the one-stage procedure was 6.6 hours and 7.9 hours for the two-stage procedure. There were no differences in the percentage of curve correction, blood requirements, or postoperative complications between the two groups. However, patients undergoing the one-stage correction experienced a shorter anesthetic time, were extubated more quickly, and required fewer days of hyperalimentation than the two-stage group ( Shufflebarger et al., 1991 ; O'Brien et al., 1992 ). Overall, the number of intensive care and hospital days was 60% of those for patients undergoing the two-stage repair.

Staged Segmental Scoliosis Surgery

Occasionally, the surgeon and anesthesiologist are confronted by a patient whose physical status precludes a general anesthetic. Dalens and others (1993) reported the successful performance of a staged segmental spinal correction under regional anesthesia. Based on the findings of Rao and others (1990) , who demonstrated that the spinal canal can be safely exposed under local anesthesia, epidural or subarachnoid anesthesia was provided over a 3-month period for segmental correction of three to five vertebrae at a time in six patients with American Society of Anesthesiologists (ASA) physical status 4 and with angles of curvature of 75 to 130 degrees. The duration of each procedure was 90 minutes. Neurologic function was evaluated by performing neurologic examination in awake patients and assessing painful sensations and limb mobility. Of 44 regional anesthetics, 13 were epidural, and 31 were subarachnoid. In 14 instances, subarachnoid blockade was achieved after surgical exposure under local anesthesia. The average duration of the procedure was 140 minutes. Bupivacaine and morphine were used in most instances. The administration of 0.125% bupivacaine intrathecally resulted in complete pain relief at the surgical site without any evidence of motor blockade. There were no episodes of respiratory depression in the perioperative period, and patients were discharged from the hospital within 4 days ( Dalens et al., 1993 ; Tobias, 2004 ).

▪ ANESTHETIC MANAGEMENT OF SCOLIOSIS SURGERY

Monitoring

Dramatic hemodynamic instability and substantial blood and heat loss are the hallmarks of scoliosis surgery. In addition to the monitors routinely used in conducting a pediatric general anesthetic, an indwelling arterial catheter and central venous cannula are recommended. These invasive catheters allow monitoring of beat-beat changes in blood pressure, adequacy of oxygenation, ventilation, and intravascular volume, and they provide a direct route for administering cardiotonic medications. A pulmonary artery catheter may be substituted for a central venous catheter if significant myocardial dysfunction is found during the preoperative cardiac evaluation. Body temperature may decrease during the course of a spinal procedure, and continuous monitoring and meticulous thermoprotective strategies are required to prevent intraoperative hypothermia.

Monitoring Intraoperative Complications

Scoliosis surgery is high-risk surgery. Complications are related to the surgery and prone position and include cardiovascular collapse resulting from extensive blood loss, inadequate venous return, air embolism, or latex anaphylaxis ( Weinstein et al., 2003 ); coagulopathies, acid-base imbalance, and electrolyte disturbances (e.g., hyperkalemia, hypocalcemia) from massive blood transfusions ( Lowe et al., 2000 ); inability to ventilate or oxygenate because of endotracheal tube malposition or obstruction ( Xiong and Sevastik, 1998 ), chest or tracheal compression in the prone position, and pneumothorax or hemothorax resulting from surgery ( Ahn et al., 2002 ); myoglobinuria and renal insufficiency caused by rhabdomyolysis; and visual loss and pressure-point injuries from the prone position ( Ascani et al., 1986 ). Common intraoperative problems and the monitoring used for these problems are described in Table 21-4 .


TABLE 21-4   -- Potential intraoperative complications during scoliosis repair and precautionary monitoring regimen

Problem

Solution

I. Endotracheal tube malposition in the prone position

  

A.   

Securely tape the tube before turning

  

1.   

Benzoin

  

2.   

Waterproof tape

  

B.   

After turning prone,

  

1.   

Listen to both lung fields; do not allow the stretcher to leave the operating room until satisfied that the tube has not migrated.

  

C.   

Arterial blood gas determination every hour

  

D.   

Esophageal stethoscope

II. Alteration in pulmonary compliance in the prone position

  

A.   

Arterial blood gas determination every hour

  

B.   

Proper position on frame, to ensure that the chest can expand unimpeded

III. Alteration in cardiac function in the prone position

  

A.   

Proper position on frame, to ensure that venous return is not compromised

  

B.   

Indwelling arterial catheter

  

C.   

Central venous catheter

IV. Acute hypovolemia

  

A.   

Indwelling arterial catheter

  

B.   

Central venous catheter

  

C.   

Bladder catheter

  

D.   

Two large-bore peripheral intravenous catheters in addition to the central venous catheter

V. Extensive blood loss, often occult

  

A.   

Beat-to-beat blood pressure monitoring

  

B.   

Hemoglobin measurement every hour

  

C.   

Weigh sponges

VI. Development of coagulopathy

  

A.   

Platelet count every 1 to 2 hours

  

B.   

Prothrombin time, aPTT, and fibrin split products every 2 hours

VII. Electrolyte abnormalities from transfusions

  

A.   

Frequent measurement of Na+, K+, and ionized Ca2+

  

B.   

Avoid using “old” packed red blood cells

VIII. Excessive heat loss

  

A.   

Core temperature measurements

  

B.   

Heat conservation (plastic bags; heated, humidified gas)

  

C.   

Active heating (warm air)

IX. Neurologic injuries

  

A.   

Proper positioning, particularly the eyes and elbows (brachial plexus injury)

  

B.   

Intraoperative neurologic assessment of cord function

 

 

Hypotension and cardiovascular collapse are common during this surgery, making it among the highest-risk procedures performed in pediatric surgery and anesthesia. Physicians always should presume that hypotension is caused by hypovolemia until proved otherwise. Other causes are far less common and include latex (or rarely drug) anaphylaxis, anesthetic overdose, pneumothorax or hemothorax (particularly in a single-staged anterior posterior procedure), impaired venous return resulting from the prone position, surgical manipulation, and venous air embolism. Air embolism can occur because the epidural veins are exposed during surgery and are above the level of the heart. The outcome from a massive air embolus is almost uniformly fatal.

Because cardiopulmonary resuscitation is virtually impossible to perform in the prone position, a battle plan to turn the patient supine must be well established and rehearsed. As in any emergency, it is the anesthesiologist's responsibility to declare the emergency and to call for help. Because the surgeon needs time to pack and cover the open wound with sterile towels and adhesive plastic, it is always better to begin the process early, rather than waiting until the last possible moment.

Neurologic Monitoring

Postoperative paralysis or sensory loss is the most devastating and often unpredictable complication of scoliosis surgery ( Owen, 1999 ). Neurologic injury may result from direct injury to the spinal cord or nerves during instrumentation, from excessive traction during distraction, or from compromised perfusion of the spinal cord. Because the ramifications associated with motor deficit are significantly greater than those of sensory deficit, surgically induced paraplegia has always been the major concern of scoliosis surgery.

Spinal Cord Blood Flow

The organization of the spinal cord blood supply is segmental in a cross-sectional and rostral-caudal fashion ( Fig. 21-3 ). The intrinsic spinal cord vasculature consists of the anterior median and the paired posterior spinal arteries. The vasculature supplying these vessels arises from the segmental arteries of the aorta and branches of the subclavian—the vertebral arteries—and the internal iliac arteries. The solitary anterior median spinal artery runs along the entire length of the cord in the anterior sulcus, giving off penetrating branches that supply the ventral two thirds of the spinal cord. Blood flow in the anterior spinal artery is not continuous throughout its span; instead, the anterior spinal artery functions as an anastomotic channel between the terminal branches of successive radicular arteries. Blood leaving the terminal aspects of these radicular arteries courses upward and downward in the anterior spinal artery. Between adjacent radicular arteries, there are points where blood flows in either direction. The paired posterior spinal arteries, which supply the dorsal third of the cord, also have discontinuous segments and appear more like a plexus of pial vessels than paired arteries.

 

 

FIGURE 21-3  The anatomy of blood flow to the spinal cord is distinctive in the vertical and horizontal distributions. A, Segmental blood flow along the cord axis. B, The thoracic cord depends on flow from a number of thoracic radicular arteries, principally the artery of Adamkiewicz. C, The cross-sectional distribution of blood flow is distinctive. The outer zone of the cord (white matter) is supplied by the radial arteries; the inner zone (gray and white matter) is supplied by the central arteries. Tissue in the shaded region is supplied by both sources.  (A and B, from Cucchiara RF, Michenfelder JD, editors: Clinical neuroanesthesia. New York, 1990, Churchill Livingstone; C, from Vinken PJ, Bruyn GW, editors: The handbook of clinical neurology, vol 12. New York, 1972, North Holland.)

 

 

 

These three perimedullary vessels give rise to the intramedullary arterial system: the central arteries that supply the gray matter and the deep portions of the white matter and the radial arteries that supply most of the white matter. Nonfunctional anastomotic links exist between the central arterial supply and the radial arterial supply at a given spinal segment. This border zone and the radial circulation appear at highest risk for ischemic insult.

The regional circulation of the spinal cord is divided into four segments. The cervical and lumbosacral regions each receive double the blood flow of the thoracic region (see Fig. 21-3 ). Although each vertebral level has paired segmental arteries, only six to eight important medullary arteries are formed. These medullary arteries join the spinal arteries. The segmental arteries at all other levels are functionally nonsuppliers of blood to the spinal cord itself. The vertebral arteries form the rostral origins of the anterior and posterior spinal arteries and represent the principal supply to the cervical cord. Branches of the thyrocervical and costovertebral arteries supply the lower cervical and upper thoracic cord. A radicular artery arising from T7 provides perfusion for the middle thoracic cord. The most consistent and important of the anterior medullary arteries is the artery of Adamkiewicz, the arteria radicularis magna, which usually joins the anterior spinal artery between T8 and L3. This artery is the predominate source of blood supply to the lower two thirds of the spinal cord. The implications of this design dictate the clinical manifestations of impaired cord perfusion. Watershed areas, subject to ischemia during low-flow states, exist between the anterior and posterior circulations and between the four different spinal segments. The segments of T4-7 appear to be highly susceptible to injury during periods of hypoperfusion. The dependence of the lower two thirds of the cord on the artery of Adamkiewicz puts this region at particular risk during surgical manipulation of the thoracolumbar aorta and spinal column, the lumbar artery enlargement syndrome. Although the clinical picture of this syndrome is not constant, it is marked by the development of flaccid paraplegia or quadriplegia (depending on the level of the lesion) and dissociated sensory impairment in which heat and pain sensations are affected, while deep sensation is spared.

The same principles that regulate the cerebral blood flow are operative in the control of spinal cord blood flow. As such, cord blood flow depends on the perfusion pressure (i.e., mean arterial pressure or cerebrospinal fluid pressure), integrity of the circulation, microcirculatory autoregulation, and intrinsic regulation. If the perfusion pressure falls below 50 mm Hg, spinal cord blood flow is reduced. Spinal cord blood flow autoregulates within the range of a mean arterial pressure of 60 to 150 mm Hg. Spinal cord blood flow is also regulated on an intrinsic basis in response to arterial oxygen and carbon dioxide tensions, pH, and cord temperature in a fashion identical to that of the cerebral circulation. Hypercapnia increases flow, whereas a PaO2 below 60 mm Hg results in a vasodilatation that overrides the effects of hypocarbia and autoregulation (see Chapter 18 , Anesthesia for Neurosurgery).

Minimizing Postoperative Neurologic Complications

The estimated risk of postoperative neurologic injury in patients undergoing spinal instrumentation is 0.72% to 1.6% ( MacEwen et al., 1975 ; Dawson et al., 1991 ; Nuwer et al., 1995 ; Cervellati et al., 1996 ). In a study of 7885 patients who underwent instrumentation or fusion without instrumentation, 87 patients developed acute neurologic changes, and 36% of these patients recovered without sequelae. Individuals with nonidiopathic scoliosis are at higher risk for neurologic injury. Children with congenital scoliosis suffer neurologic complications disproportionately ( MacEwen et al., 1975 ; Cervellati et al., 1996 ).

To minimize the risk of these devastating neurologic injuries, a variety of methods of intraoperative neurologic monitoring have been used. The goal of this monitoring is to identify and herald the onset of neurologic impairment and to provide the surgeon and anesthesiologist with the opportunity to implement appropriate interventions that may minimize permanent damage. These approaches include wake-up tests and the use of neurophysiologic monitoring.

Wake-up Test

Vauzelle and others (1973) first described the use of the wake-up test to assess the integrity of the spinal cord. In this technique, patients are awakened intraoperatively to assess spinal cord motor function. The wake-up test requires an anesthetic that allows rapid recovery of consciousness and motor function. Ideally, the wake-up test should be rehearsed preoperatively. During rehearsal, the patient is informed that he or she will be momentarily awakened at the time of rod insertion to test the function of the spinal cord. Patients must be reassured that they will neither remember the event nor experience pain while they are “awake.” Preoperative preparation increases the speed and success of the test.

When a wake-up test is performed, the operating room must be quiet, the surgeon must stop operating, and an observer is positioned (usually under the drapes) to look for foot movement. After discontinuation of the anesthetic, the patient is first asked to move his or her hands (“squeeze my fingers”) to evaluate the level of consciousness and then asked to move his or her feet (“wiggle your toes”). If the patient is unable to move his or her feet but can move his or her hands, spinal cord compromise is presumed, and the spinal rod instrumentation is removed immediately. Spinal cord perfusion is maximized by raising the mean arterial blood pressure, increasing the hemoglobin concentration, and normalizing arterial carbon dioxide and oxygen tensions ( Vauzelle et al., 1973 ). In one series of 166 patients in whom the wake-up test was used, 3 patients had demonstrable neurologic deficits when awakened. These deficits disappeared immediately on release of the distracting force (i.e., rods) ( Hall et al., 1978 ; Nuwer et al., 1995 ).

Hazards associated with the wake-up test include accidental extubation, air embolization produced by deep inspirations, falling off the operating room table, and dislodgment of spinal instrumentation rods and vascular catheters ( Ben David, 1988 ). There are many other limitations to this test. It tests only the anterior spinal cord (motor function) and not the dorsal column (sensory). It requires patient cooperation and has limited use in patients with baseline cognitive dysfunction. The wake-up test provides a snapshot of a single moment of spinal cord function and can realistically be performed only once or twice during a procedure. A spinal injury may be missed because it occurred after the wake-up test was performed. Depending on the anesthesiologist's skill and the anesthetic technique employed, it may take 5 to 45 minutes after a wake-up test is requested by the surgeon before wake-up status can be achieved.

Neurometric Monitoring

Sensory Evoked Potentials.

Electrophysiologic (neurometric) monitoring provides a real-time, continuous assessment of spinal cord function and does not require patient movement, arousal, or cooperation (see Chapter 9 , Equipment and Monitoring). The most common technique uses somatosensory evoked potentials (SEPs), in which the cortical and subcortical responses to peripheral nerve stimulation are monitored ( Nash and Brown, 1989 ). Typically, a peripheral mixed nerve (i.e., posterior tibial nerve, peroneal nerve, and median nerve) is stimulated at fixed intervals during a procedure. SEPs are recorded repeatedly during surgery, and their amplitude (height) and latency (time of occurrence) are compared with baseline values. Based on changes in these characteristics, it is possible to determine the functional status of the spinal cord sensory tracts. SEP monitoring requires specialized technology and expertise. To resolve the very-low-amplitude evoked potentials from background random or spontaneous cortical activity, computer signal averaging of repetitive sensory responses is required. The processed evoked potential waveform is plotted as voltage against time and is characterized by the post-stimulus latency and amplitude. The post-stimulus latency reflects the time required for impulse transmission from the site of sensory stimulation. A reduction in amplitude of more than 50% or an increase in latency of less than 10% relative to baseline values is generally considered significant.

SEPs monitor only the dorsal columns of the spinal cord and provide no direct evidence of loss of motor function or anterior spinal cord injury. Motor deficits may occur in the absence of alterations in SEPs, and numerous case reports have recorded the postoperative finding of paralysis despite unchanged intraoperative SEPs (i.e., false-negative results) ( Lesser et al., 1986 ). The most comprehensive information regarding the false-negative rate of SEPs comes from a survey of spine surgeons by the Scoliosis Research Society and the European Spinal Deformity Society, in which 342 postoperative neurologic deficits were reviewed from a collection of 33,000 cases. Of these, 28% were not detected by SEP monitoring ( Dawson et al., 1991 ). When SEP monitoring is equivocal, many recommend an intraoperative wake-up test to assess motor function ( Grundy, 1983 ).

Many pharmacologic and physiologic variables affect the latency and amplitude of SEPS and have been estimated to account for up to 44% of intraoperative SEP changes. The most important of these are the anesthetic agents, blood pressure, and body temperature, and these variables are summarized in Table 21-5 ( Grundy et al., 1981 ; Grundy, 1983 ). Nitrous oxide has no effect on SEP latency but does decrease its amplitude by 50% ( Sloan and Koht, 1985 ; Lam et al., 1994 ). All of the potent inhaled anesthetic agents produce dose-dependent increases in latency and decreases in amplitude ( Sloan, 1998). Substantial recovery of latency and amplitude are achievable with discontinuance of nitrous oxide and the inhaled vapors ( Peterson et al., 1986 ; Lam et al., 1994 ; Schindler et al., 1998 ; da Costa et al., 2001 ). In a similar fashion, the intravenous anesthetic agents increase SEP latency and decrease amplitude, with the exceptions of midazolam, ketamine, etomidate, propofol, and opioids. Midazolam has no effect on latency ( Sloan et al., 1990 ); ketamine ( Schubert et al., 1990 ; Langeron et al., 1997 ) and etomidate ( Thakor et al., 1991 ) augment SEP amplitude. Propofol has no effect on amplitude or latency, and it is highly recommended as a component of total intravenous anesthesia for scoliosis surgery ( Maurette et al., 1988 ; Scheepstra et al., 1989 ; Sloan, 1996, 1998 [234] [237]; Rundshagen et al., 2000 ).


TABLE 21-5   -- Effects of anesthetic agents on somatosensory evoked potentials

Agent

Amplitude

Latency

Halothane

Desflurane

Isoflurane

Sevoflurane

Nitric oxide (N2O)

Barbiturates

Etomidate

Ketamine

Midazolam

Opioids

Propofol

↓, Decreases; ↑, increases; ↔, remains the same.

 

 

 

Fentanyl appears to have minimal effect on SEP waveform ( Pathak et al., 1983 ; Kimovec et al., 1990 ). The amplitude and latency of the waveform are also affected by age, preexisting neurologic deficits, body temperature, PaCO2, hypoxia, and blood pressure ( Fig. 21-4 ) ( Grundy, 1983 ; Lubicky et al., 1989 ; Sloan, 1996, 1998 [234] [237]). The reliability of spinal cord monitoring may be dramatically affected by the variability of the evoked responses. Spontaneous variability in the amplitude and latency of SEP is increased and the amplitude of the waveform diminished during anterior fusions compared with posterior fusions ( Grundy, 1983 ; Lubicky et al., 1989 ). Muscle relaxants have no direct deleterious effects on the SEP but may produce a more reliable recording by providing “quieter” conditions.

 
 

FIGURE 21-4  Somatosensory evoked potentials (SEPs) change with hypotension and hypoxia. A, During the combination of distraction and hypotension, distal SEPs were unchanged. Resumption of normotension restored the SEP to baseline. B, SEP responses are exquisitely sensitive to hypoxia (PO2 = 41 mm Hg). Resumption of normoxia restored the SEP to baseline.

 

 

An anesthetic milieu that is compatible with adequate neurometric monitoring and that allows rapid awakening can be created using a variety of approaches. McPherson and others (1985) demonstrated that fentanyl-isoflurane (0.25% to 1.0%) plus oxygen and fentanyl-enflurane (0.25% to 1.0%) plus oxygen preserves SEPs better than fentanyl-nitrous oxide (50%) plus oxygen. Eliminating nitrous oxide appears to be the key (Kalkman et al., 1991a, 1991b [118] [237]). Substituting desflurane or sevoflurane for isoflurane (or enflurane) and a remifentanil infusion produces ideal SEPs and still allows for rapid wake-up if a wake-up test is required. Alternatively, the physician can substitute a continuous propofol infusion for nitrous oxide or the potent inhaled anesthetics (e.g., desflurane, isoflurane) in combination with an opioid (Kalkman et al., 1991a, 1991b [118] [237]). When using a continuous propofol infusion, some method of titration (i.e., BIS monitor or target-controlled infusion pump) is invaluable to prevent excessive dosing and accumulation of propofol ( Gale et al., 2001 ; Varveris and Morton, 2002 ). Because etomidate augments SEP amplitude, it is particularly useful in patients with abnormal preoperative SEPs. These individuals are at greatest risk for the development of postoperative neurologic catastrophes ( Sloan et al., 1988 ; Samra and Sorkin, 1991 ). Pentobarbital at doses sufficient to result in electroencephalographic burst suppression or isoelectricity preserves SEP ( Drummond et al., 1987 ).

Baseline SEP recordings are made after turning the patient to the prone position. After the patient is prone, the anesthetic depth, end-tidal carbon dioxide (CO2) levels (35 to 45 mm Hg), temperature, and blood pressure (mean arterial pressure > 60 mm Hg) should be maintained to minimize these effects on the SEPs during surgery. Throughout the surgical procedure, a physiologic and pharmacologic steady state must be maintained to effectively use SEPs as monitors of spinal cord function. Deepening the anesthetic depth during critical operative moments when the risk of neurologic compromise is highest must be avoided to minimize the potential for pharmacologically induced false-positive changes. The intraoperative changes of increased latency, decreased amplitude, or complete loss of waveform must be attributed to spinal cord injury rather than an anesthetic-induced effect.

When the baseline is being established, knowledge of the various effects of anesthetic drugs on SEPs can be advantageously used to produce optimal signal acquisition. In the setting of less than optimal baseline SEP acquisition, a strategic change in the anesthetic regimen may result in improvement of the quality of the SEP signal. For example, discontinuing nitrous oxide or desflurane, or both, and substituting an etomidate or propofol infusion can significantly improve SEP acquisition.

Using the criterion of more than 40% amplitude decrease as a significant change, excellent specificity and sensitivity are achievable. In patients with idiopathic scoliosis (i.e., neurologically intact), SEPs are reliable and can be obtained in more than 98% of patients ( Dawson et al., 1991 ; Padberg et al., 1998 ). However, in patients with preexisting diseases such as neuromuscular scoliosis, the reliability of the SEP is less than 75% but can be improved with the addition of motor evoked potential (MEP) monitoring ( Ashkenaze et al., 1993 ). Precise communication and coordination of efforts among the surgeon, the anesthesiologist, and the neurometric specialist are imperative when a change in SEP is observed. Normalization of the SEPs may occur spontaneously, with relaxation of the distraction instrumentation, or by improving spinal cord perfusion (e.g., increasing blood pressure, arterial carbon dioxide blood levels).

Motor Evoked Potentials.

SEPs are not the method of choice for monitoring motor tract function or for detecting the presence of a surgically induced motor deficit. To avoid any type of postoperative neurologic deficit, SEPs should be monitored in conjunction with some other measurement of motor tract function. The most obvious is the use of MEPs. In this technique, direct monitoring of motor function uses myogenic or neurogenic responses (Owen et al., 1988, 1989, 1991 [186] [185] [184]; Edmonds et al., 1989 ; Aglio et al., 2002 ). The myogenic motor evoked potential (MMEP) relies on direct stimulation of the spinal cord, resulting in an electromyographic response (“twitch”) (Owen et al., 1988, 1989, 1991 [186] [185] [184]; Owen, 1999 ). Because a twitch must be elicited, MMEPs necessitate an anesthetic involving no or incomplete neuromuscular blockade (<30% return of twitch height in a train of four). Incomplete paralysis is potentially hazardous during delicate surgical procedures performed on prone patients suspended on positioning frames, particularly if patients are lightly anesthetized.

Alternatively, neurogenic motor evoked potentials (NMEPs) can be used to assess the integrity of the spinal cord motor pathways ( Edmonds et al., 1989 ; Aglio et al., 2002 ). In this technique, stimulating electrodes are placed percutaneously above the site of operation at the base of adjacent spinous processes, or a magnetic coil is placed over the scalp, stimulating the motor cortex with a magnetic impulse. This results in electrical transmission down the motor nerve tracts. This neural conduction can be recorded from the lateral column of the spinal cord, from the spinal epidural space using percutaneous needles, from the peripheral spinal nerves (e.g., sciatic nerve), and from muscle. The recording of MEPs from muscle has the serious disadvantage of being highly sensitive to all volatile and other anesthetics, muscle relaxants, and several other drugs commonly used in the operating room ( Schmid et al., 1992 ). Anesthetic agents have less of an effect when NMEPs are recorded from the spinal epidural space or peripheral nerves ( Owen et al., 1991 ). Monitoring of NMEPs is difficult because of the efficiency of the central nervous system (CNS) in conducting impulses through the spinal cord and peripheral nerves to muscle groups. These extremely small electrical impulses are difficult to record over the spinal cord or peripheral nerves. Improvement in computer amplification of these signals has overcome many of these technical shortcomings. The motor response elicited from MEPs may be so significant that it may hinder the surgical procedure or move the patient's position on the frame. MEP monitoring is usually used intermittently or at specific points during the surgical procedure when pathways amenable to MEP monitoring are at risk.

Both forms of motor evoked potentials are sensitive to the effects of anesthetics ( Herdmann et al., 1996 ; Sihle-Wissel et al., 2000 ). Compared with SEPs, the use of MMEPs and NMEPs in the operating room remains in its infancy, and the effects of anesthetic agents on these monitors is unclear and based on studies involving small numbers of patients. All of the potent inhaled vapors produce dose-dependent increases in latency and decreases in amplitude ( Yamada et al., 1994 ). Like the SEPs, amplitudes and latencies of MEPs are significantly affected by nitrous oxide ( Sihle-Wissel et al., 2000 ). Of the four most commonly used intravenous anesthetic agents, continuous infusions of etomidate and methohexital preserve MEP amplitude to a greater extent than thiopental and propofol ( Taniguchi et al., 1993 ).

Choosing the Best Method.

The ability to monitor the descending motor pathways is intuitively appealing. Animal studies suggest that MEPs are more sensitive than SEPs in identifying spinal cord compromise. Using an experimental model of overdistraction in hogs, Owen and others (1988, 1991) [186] [184] demonstrated that abolition of NMEP waveforms occurred 4 to 5 minutes earlier than SEP changes and that the wake-up test was positive only 5 to 6 minutes after NMEP changes. In other words, the spinal cord injury can be identified earlier with NMEP than with SEP monitoring, and this early warning occurs before gross evidence for neural dysfunction (i.e., wake-up test result). In the series of hog studies, the SEP false-negative rate was 13.6%, and the NMEP false-negative rate was 0% when the animals were evaluated by the wake-up test 5 minutes later ( Owen et al., 1991 ; Owen, 1999 ).

Glassman described a patient with a severe spinal deformity undergoing spinal instrumentation in whom the MEPs were abolished at the moment of maximal distraction, but the SEPs remained unaltered. A wake-up test confirmed the absence of motor function ( Glassman et al., 1993 ). However, there is no consensus about which stimulatory modality (electrical or magnetic) and which evoked potential (neurogenic or electromyographic) is superior in detecting early changes in spinal cord function. Clinical comparisons of the various MEP methods with SEP and wake-up tests during spinal distraction are needed.

Blood Loss

The vast area of decorticated, raw cancellous bone that is exposed during spinal surgery results in extensive blood loss that can exceed 25 mL/kg even in uncomplicated surgery. Children with neuromuscular scoliosis often require more extensive procedures, including pelvic stabilization, than children with idiopathic scoliosis and can easily have blood losses that exceed a blood volume ( Meert et al., 2002 ). However, even if the extent of surgery is controlled, patients with neuromuscular diseases have an almost seven times higher risk of losing more than 50% of their estimated total blood volume during scoliosis surgery than normal controls ( Edler et al., 2003 ).

An estimation of circulating blood volume should be made before the induction of anesthesia. The estimated blood volume (EBV) is calculated by multiplying the patient's weight by the approximate blood volume based on age ( Table 21-6 ). From the EBV, the initial hematocrit (Hct), and the minimum acceptable hematocrit, an estimation can be made of the maximum allowable blood loss (MABL) before packed red blood cell transfusions are indicated ( Kallos and Smith, 1974 ):


TABLE 21-6   -- Approximate blood volume

Age

Total Blood Volume (mL/kg)

Premature infant

90 to 105

Term newborn

80 to 85

1 to 12 months

70 to 80

Older child

70 to 80

Adult

60 to 70

 

 

The decision to transfuse red blood cells should be based on the balance between oxygen supply and demand, which depends on the oxygen content of the blood, cardiac output, regional distribution, and metabolic needs. The trigger to transfuse is then based on the patient's risk for developing inadequate oxygenation.

Because blood loss may be difficult or impossible to accurately assess, we estimate blood loss by measuring the hemoglobin concentration hourly and by assuming that changes in central venous pressure reflect ongoing blood loss. As blood loss progresses and exceeds a blood volume, medical causes of bleeding, such as dilutional thrombocytopenia or loss of clotting factors V and VIII, become increasingly important in accounting for the inability of the surgeon to achieve hemostasis. Platelet counts and coagulation profiles (i.e., prothrombin time, partial thromboplastin time, fibrinogen, and fibrin split products) should be obtained at regular intervals when extensive blood loss occurs.

Multiple strategies have been described to minimize blood loss and the need for blood transfusions in the perioperative period. These approaches include attempts at reduction of intraoperative blood loss and reduction of transfusion of homologous blood products. Techniques that may decrease intraoperative blood loss include induced hypotension, alteration of the operative position, changes in surgical technique, and administration of desmopressin (1-desamino-8-D-arginine vasopressin [DDAVP]) and aprotinin. Techniques aimed at decreasing the use of homologous blood products include intraoperative blood salvage, preoperative autologous blood donation, and perioperative hemodilution and apheresis ( Laupacis and Fergusson, 1997 ; Bryson et al., 1998 ; Huet et al., 1999 ). Each of these techniques has demonstrated efficacy in some clinical situations.

Predonation of autologous blood is increasingly accepted by surgeons and families and has been used extensively in children ( Helfaer et al., 1998 ). The primary limitation of this method is the number of units that a child or adolescent can donate ( Goodnough et al., 1989 ). Recombinant erythropoietin, which increases red cell mass, has been used in the weeks immediately preceding surgery to maximize a patient's ability to donate autologous blood and halves the risks of needing a homologous blood transfusion (Vitale et al., 1998, 2002 [273] [272]; Goodnough, 2001 ).

Alternatively, perioperative hemodilution and apheresis, sometimes referred to as isovolumic hemodilution, can substitute for or augment autologous blood donation ( Hur et al., 1992 ; Bryson et al., 1998 ;Copley et al., 1999 ). Before incision, the patient's blood is withdrawn from a large-bore peripheral intravenous canula or from a central venous catheter and replaced with an isotonic balanced salt solution or colloid in a ratio of 3:1. When using this technique, oxygen extraction increases, and oxygen delivery decreases; careful attention to acid-base balance (i.e., lactate levels) is essential when using this technique ( Copley et al., 1999 ). The removed blood is stored in anticoagulated bags, and it is administered as needed to maintain the hematocrit at a predetermined level, usually above 20%. Complications of this technique include postoperative pulmonary edema, anasarca, and prolonged postoperative mechanical ventilation. Perioperative salvage of blood (i.e., use of Cell Saver system) allows blood lost in the operative field to be returned to the patient in the perioperative period ( Huet et al., 1999 ). How efficient this technique is, how long the salvaged red blood cells survive, and its place in limiting blood transfusion are unclear ( Ray et al., 1986 ; Blais et al., 1996 ; Abildgaard et al., 2001 ; Vitale et al., 2002 ). When using salvaged blood, it is imperative that the blood be processed and washed adequately to avoid life-threatening bleeding and pulmonary complications caused by transfusion of cellular and tissue debris. Salvaged blood can also result in profound hypotension (e.g., citrate, air embolism) and hemolytic and bleeding complications from centrifugation, cellular debris, or anticoagulant overdosage.

DDAVP increases plasma factor VIII and von Willebrand factor concentration when given intravenously, thereby decreasing the activated partial thromboplastin time and enhancing fibrinolysis by increasing levels of plasminogen activator. DDAVP has been used successfully to prevent bleeding during surgical procedures in patients with mild to moderate hemophilia or von Willebrand disease. Infusions of DDAVP have shortened or normalized bleeding times in patients with chronic renal failure. This apparent improvement in qualitative platelet function lasts between 4 and 8 hours. Despite early reports by Kobrinsky and others (1987) that demonstrated a 25% reduction in operative blood loss when DDAVP was administered preoperatively to patients undergoing scoliosis repair, others have not been able to reproduce these findings. There is no evidence that DDAVP reduces the blood loss incurred by patients undergoing scoliosis repair ( Guay et al., 1992 ; Theroux et al., 1997 ; Alanay et al., 1999; Henry et al., 2001 ).

Deliberate Hypotension

The most common blood-preservation technique is deliberate hypotension ( Yaster et al., 1986 ; Petrozza, 1990 ; Tobias, 2002 ) (see Chapter 12 , Blood Conservation). Numerous techniques to reduce blood pressure have been used, typically with drugs that are direct venous and arterial vasodilators, such as nitroprusside and nitroglycerin. Ganglionic-blocking drugs (e.g., trimethaphan), α2-adrenergic agonists (e.g., clonidine), β- and α1-antagonists (e.g., propranolol, esmolol, prazosin, hydralazine), dopamine agonists (e.g., fenoldopam), inhaled anesthetics (e.g., halothane, isoflurane, sevoflurane, desflurane), major conduction blockade (i.e., epidural), and calcium channel blockers (e.g., nicardipine) are used alone or in combination with the direct-acting venous and arterial vasodilators to lower blood pressure (Yaster et al., 1986 ; Hersey et al., 1997 ; Post and Frishman, 1998 ; Tobias, 1998, 2000, 2002 [258] [257] [256]; Degoute et al., 2003 ; Hackmann et al., 2003 ).

Deliberate hypotension can be used in any patient who is otherwise healthy. It is contraindicated if there is any evidence of end-organ injury or ischemia. Blood pressure is usually lowered to mean arterial pressures of 50 to 60 mm Hg. In the past, the most commonly used technique was a continuous infusion of nitroprusside (1 to 8 mcg/kg per minute) combined with β-blockade, using propranolol (1 to 2 mg), labetalol, or an esmolol infusion ( Tinker and Michenfelder, 1976 ). β-Blockade is necessary when hypotension is induced with nitroprusside because nitroprusside produces a reflex tachycardia and increases cardiac index. These combine to limit the ability of nitroprusside to lower blood pressure ( Yaster et al., 1986 ). Nitroglycerin, which is often used for deliberate hypotension in adults, fails to reduce mean arterial blood pressures below 60 mm Hg in children when administered in doses as high as 40 mcg/kg per minute ( Yaster et al., 1986 ). Potent inhalation agents, such as halothane or sevoflurane, also can be used to induce hypotension in children and adolescents but may interfere with evoked potential monitoring ( Tobias, 1998 ) and delay the ability to rapidly perform a wake-up test. When combined with low-dose sevoflurane or desflurane, remifentanil is extremely effective. Regardless of the technique used to induce deliberate hypotension, the appropriate use of adjuvants in the anesthetic technique can aid achieving the desired effect of hypotension. Children can be premedicated with oral clonidine (4 to 8 mcg/kg) 2 hours before surgery to reduce anxiety and to facilitate the induction and maintenance of hypotension during surgery ( Hackmann et al., 2003 ).

Although deliberate hypotension clearly reduces blood loss, provides a relatively bloodless surgical field, and facilitates surgical dissection, its use in scoliosis surgery is controversial. Animal studies have suggested an additive effect of hypotension and surgical pressure on the spinal cord in producing neurologic injury ( Brodkey et al., 1972 ). In dogs, Griffiths and others (1979) have shown that cord compression can alter dorsal column conduction at perfusion pressures that did not affect blood flow. Use of deliberate hypotension mandates normocarbia and adequate oxygen carrying capacity.

Vigilant monitoring is essential when deliberate hypotension is used. An arterial catheter for beat-beat monitoring of blood pressure and for frequent arterial blood gas sampling and hematocrit determinations is essential. Because patients must remain normovolemic at all times, a central venous pressure catheter is a useful monitor. Nitroprusside has been shown to decrease arterial oxygen tension in children and to increase the PAO2 - PaO2 ( Yaster et al., 1986 ). We can assume that the lowered oxygen carrying capacity of low-hematocrit blood, when coupled with hypoxemia and hypocapnia from overzealous ventilation, can only exacerbate cord ischemia caused by surgical traction or hypotension. Other end organs that must be protected from hypotension include the heart and kidney. The heart is easily monitored by following the ST segment of the V5 lead of the electrocardiogram. The kidney is monitored by bladder catheterization and measuring urine output hourly. Renal perfusion pressure can be assumed to be inadequate if urine output falls to less than 1 ml/kg per hour.

Aprotinin

A number of investigators have examined the role of aprotinin and other antifibrinolytics as hemostatic agents in limiting perioperative blood loss and transfusion requirements ( Kannan et al., 2002 ; Cole et al., 2003 ). Aprotinin is a serine protease inhibitor that acts on the clotting cascade as an antifibrinolytic, an antiinflγtory, and a platelet membrane stabilizer. The hemostatic effect of aprotinin is ascribed to its antifibrinolytic properties and its ability to preserve platelet function. In particular, in high doses it inhibits the kallikrein-kinin system, decreasing its activation in the presence of tissue trauma. It also may involve, conserve, and restore platelet function with a protective effect on specific platelet receptors (Royston, 1992, 1995 [214] [213]). Aprotinin may play a protective role against disseminated intravascular coagulopathy through its anti-VIIa activity. Aprotinin dosages are variably expressed: 1 mL = 1.4 mg = 10,000 kallikrein inhibitor units (KIU).

In a randomized, double-blind, controlled trial, Cole and others (2003) found a 40% reduction in blood loss in children at high risk for blood loss, such as those with neuromuscular scoliosis or those undergoing reoperation. In that study, after a test dose of 1 mL (10,000 KIU) given 10 minutes before the loading dose, aprotinin was administered as a 240-mg/m2 load over 30 minutes before incision. The loading dose was followed by a continuous infusion of 56 mg/m2 per hour throughout the case and was maintained for 4 hours after surgery ( Cole et al., 2003 ). If a patient had more than 1 m2 of body surface area, the maximum dose, 280 mg (200-mL vial), was given as the loading dose, and maintenance was 70 mg (50 mL)/hour (see Chapter 32 , Systemic Disorders).

Positioning

An anesthetized patient is extremely vulnerable during positioning for scoliosis surgery. The patient is unable to feel the extreme discomfort of certain positions that alter the normal mechanics of the body. Vulnerable parts, such as peripheral (ulna) nerves, male genitalia, the nipples, and the anterior superior iliac spine (to protect the lateral femoral cutaneous nerve of the thigh), must be especially protected to avoid injury by pressure or stretching. Of the peripheral nerves, the brachial plexus is most vulnerable to stretch in the prone position ( Martin, 1997 ). Stretching of the lower trunks of the brachial plexus is most likely to occur when the head is turned to the contralateral side, the ipsilateral shoulder is abducted, and ipsilateral elbow is bent. Although efforts to prevent neuropathies are frequently debated, there is little hard evidence to support specific management recommendations.

The patient's eyes are vulnerable to corneal abrasion and to optic vein engorgement and retinal ischemia. Postoperative visual loss is a devastating and poorly understood injury. Over the past decade, there has been much speculation that the frequency of perioperative blindness has been increasing among patients undergoing major spine surgery. Roth and others (1996) found a much higher incidence of blindness (1 in 600) among patients undergoing complex spinal surgery. A report from the Mayo Clinic did not support this finding ( Nuttall et al., 2001 ). To help analyze the risk factors involved, the American Society of Anesthesiologists has established an anonymous, postoperative visual loss registry (www.asaclosedclaims.org).

Ischemic optic neuropathy, which affects the anterior or posterior portions of the optic nerve, is the most common cause of postoperative visual loss. Visual loss may also be caused by retinal arterial occlusion and cortical blindness. Awakening with visual impairment may be one of the most frightening and catastrophic postanesthetic complications that a patient could sustain. It is also an enormous medicolegal liability problem. Commonly cited risk factors in patients undergoing scoliosis surgery in the prone position include hypotension, anemia, and external compression of the eye. Unfortunately, blindness has occurred even when these risk factors did not occur ( Lee and Lam, 2001 ; Roth and Barach, 2001 ).

Decreased perfusion pressure in the retina or optic nerve may be caused by decreased mean arterial pressure or increased pressure in the venous drainage of the retina or optic nerve. Prolonged decreases in blood pressure, especially in patients with disturbed autoregulation, may be deleterious. Increased venous pressure (i.e., with internal jugular vein compression or ligation, prolonged head-down position, or large quantities of fluid infusion) also decreases perfusion. With prone positioning, increases in intraocular pressure might be potentiated by these factors. The combination of decreased blood pressure and increased venous pressure seems to pose the greatest risk.

The prone position increases intraabdominal pressure, which impairs ventilation by decreasing chest compliance and by limiting chest expansion. It also engorges epidural veins, which increases intraoperative bleeding and potentially increases the risk of postoperative epidural hematoma formation. Increased intraabdominal pressure leads to compression of the inferior vena cava, which impedes venous return to the heart. This decreases cardiac output and engorges epidural veins. To facilitate venous return and avoid increased intraabdominal pressure, specially designed frames have been developed that allow the abdomen to hang free and facilitate respiratory movement ( Relton and Hall, 1967 ; Nuttall et al., 2000 ).

Turning the anesthetized patient from the supine to the prone position requires careful orchestration by the entire surgical and anesthesia team. Cervical spine injury, dislodgement of intravascular and urinary catheters, accidental endotracheal tube extubation or dislodgement, inability to ventilate once prone, and dramatic changes in cardiac output can easily occur. During the turn, monitoring, with the exception of an esophageal stethoscope, is difficult. Once turned, auscultation of both lung fields is imperative. The adequacy of pressure-point padding (e.g., elbows, breasts, scrotum, feet) must be ensured to avoid peripheral nerve compression (i.e., ulnar nerve) and soft tissue damage.

The surgical plan and preference determine the patient's position and the device used to stabilize the patient's head. A pin-type or horseshoe-shaped holder may be employed and requires careful application and padding to avoid catastrophic oculofacial injuries. Alternatively, the head may be rested on a securely arranged stack of pillows to avoid exaggerated neck flexion, rotation, or ear trauma.

Anesthetic Techniques

Given the potential complications of neurologic injury and profound hemorrhage associated with spinal surgery, the anesthetic technique employed should have minimal effects on neurophysiologic monitoring, allow for a wake-up test, and provide for hemodynamic stability to minimize complications. The combination of remifentanil with desflurane or propofol allows for SEP and NMP monitoring and for a rapid wake-up response if necessary.

Patients with preexisting neurologic deficits, such as cerebral palsy, paralytic scoliosis, and congenital scoliosis, have variable SEP waveforms of weak amplitude. The use of an etomidate infusion in these patients can augment the amplitude of the evoked responses and increase the reliability of SEP monitoring ( Lubicky, 1989 ). Etomidate suppresses the adrenal cortical stress response for 8 to 24 hours after its use ( Wagner et al., 1984 ; Kenyon et al., 1985 ). Whether patients administered etomidate should be prophylactically treated with corticosteroid replacement continues to be debated ( Wagner et al., 1984). Vitamin C supplementation may be an attractive therapeutic alternative because it increases cortisol concentrations during etomidate infusions ( Boidin et al., 1986 ).

▪ POSTOPERATIVE MANAGEMENT

Although pulmonary failure and the need for postoperative ventilation are the primary problems of scoliosis surgery, other common postoperative concerns include ongoing blood loss, disseminated intravascular coagulation, hypovolemia, and development of syndrome of inappropriate secretion of antidiuretic hormone (SIADH), paralytic ileus, and pain. The most important initial decision at the conclusion of surgery is whether to extubate the trachea. This decision is based in large part on the patient's preoperative pulmonary function and on the intraoperative course. Scoliosis surgery produces an immediate and transient decrease in vital capacity of 40% or more in all patients ( Schur et al., 1984 ). These changes in vital capacity have been demonstrated in patients more than 24 months after repair (Upadhyay et al., 1993 ). Patients with preoperative FVC levels of less than 50% of predicted and patients with nonidiopathic scoliosis should be extubated in the pediatric intensive care unit after cognitive faculties and respiratory muscle strength have returned to baseline. Other common pulmonary complications in the postoperative period include atelectasis, pneumothorax, and pleural effusions.

Syndrome of inappropriate antidiuretic hormone (SIADH) is common and manifests as hyponatremia, hypo-osmolality, decreased urine output, and increased urine osmolality. The increased plasma volume reduces the hemoglobin concentration and can be identified by following the red blood cell's mean corpuscular volume (MCV). As the level of ADH increases, plasma water increases, resulting in free water entry into red blood cells. The MCV increases, and the hemoglobin and mean corpuscular hemoglobin content (MCHC) decrease. In contrast, the MCV should not change in the face of postoperative blood loss ( Mason et al., 1989 ).

Because the surgery is so extensive, pain is an expected complication in all scoliosis patients. Postoperative pain can be treated with intravenous opioid therapy (i.e., patient-controlled analgesia [PCA]) or regional anesthetic techniques, or both ( Tobias, 2004 ). For PCA use, patients are started on a basal infusion of morphine (0.02 mg/kg per hour) or hydromorphone (0.004 mg/kg per hour), combined with a low-dose (0.25 mcg/kg per hour) naloxone infusion ( Gan et al., 1997 ; Maxwell and Yaster, 2003 ). The patient or nurse can trigger the PCA machine's bolus button, which provides an additional dose of morphine (0.02 mg/kg) or hydromorphone (0.004 mg/kg) ( Yaster et al., 1997 ; Monitto et al., 2000 ). Many centers are using neuraxial techniques for postoperative analgesia ( Tobias, 2004 ). Typically, a single epidural catheter is inserted by the surgeon at the T8-9 level before wound closure. Alternatively, two catheters are placed at the top and bottom of the wounds by the surgeon. These catheters are intermittently or continuously infused with opioids (i.e., morphine, hydromorphone, or fentanyl) or local anesthetics (i.e., 0.625% to 1.25% bupivacaine solutions), or both.

Nonsteroidal antiinflγtory drugs (NSAIDs), such as ketorolac, have a morphine-sparing effect and result in fewer opioid-induced side effects such as somnolence, constipation, and pruritus (Reuben et al., 1997, 1998 [206] [205]). The use of ketorolac does not increase bleeding, the need for transfusions, or reoperation ( Reuben et al., 1998 ; Vitale, 2003 ). Nevertheless, for reasons that are discussed later, the use of ketorolac in scoliosis patients should be avoided, because in human and laboratory studies, NSAIDs significantly inhibit spinal fusion ( Glassman et al., 1998 ; Martin et al., 1999 ).

NSAIDs such as ketorolac are thought to have deleterious effects on bone healing and fracture repair, although the actual clinical effects appear to be controversial at this point ( Huo et al., 1991 ; Dimar et al., 1996 ; Glassman et al., 1998 ). In some studies, chronic NSAID use for more than 3 months was associated with lower fusion and success rates ( Deguchi et al., 1998 ; Glassman et al., 1998 ; Harder and An, 2003 ).

The effects of NSAIDs are thought to principally affect the bone morphogenetic proteins (BMPs) ( Harder and An, 2003 ). The BMPs are a class of osteoinductive proteins that play an important role in bone growth and are essential for the growth and development of skeletal tissue and for bone regeneration during fracture repair ( Bostrom and Camacho, 1998 ; Khan et al, 2000 ). Martin and others (1999), using a rabbit model of spinal fusion, investigated the effects of ketorolac on graft healing. Compared with a saline control group, less than one half of the rabbits receiving ketorolac had a stable bone graft. However, the coadministration of a recombinant BMP with ketorolac to a third group of rabbits resulted in 100% successful fusion rate. The investigators concluded that the effects of the NSAID could be completely reversed by the administration of a BMP ( Martin et al., 1999 ). Clinical trials using recombinant BMPs to improve graft healing are only in the preliminary stage ( Khan et al., 2002 ;Sandhu and Khan, 2003 ). Initial studies evaluating the effects of cyclooxygenase-2 (COX-2)-specific inhibitors on bone healing have produced similar results ( Gilron et al., 2003 ). The healing of stabilized tibial fractures in COX-2-deficient mice was significantly delayed, as was intramembranous calvarial bone formation. These bone-healing deficiencies were reversed by the administration of prostaglandin E2 and by BMP-2 ( Zhang et al., 2002 ). In some studies, COX-2 inhibition seemed to have less severe effects on bone healing compared with nonspecific COX-1 and COX-2 NSAIDs (Gerstenfeld, 2003 ). In a retrospective analysis of more than 300 patients undergoing spinal fusion who received a COX-2 inhibitor, ketorolac, or no NSAIDs for the first 5 postoperative days, patients who received ketorolac had a threefold increase in nonunion rate compared with COX-2 recipients or controls ( Maxy and Glassman, 2001 ; Gajraj, 2003a, 2003b [70] [71]).

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Copyright © 2005 Mosby, An Imprint of Elsevier

▪ JOINT DISORDERS

▪ ARTHROGRYPOSIS MULTIPLEX CONGENITA

Arthrogryposis multiplex congenita (AMC) consists of a heterogeneous group of nonprogressive conditions which have in common fetal akinesia. This results in the birth of a baby with multiple, congenitally curved, rigid joints. The incidence is 1 in 3000 live births. Because of their extensive contractures, tense skin, and minimal muscle mass and subcutaneous tissue, these children have been described as looking like “thin, wooden dolls.”

Arthrogryposis is a physical sign resulting from many different medical conditions. In the past, an in utero, self-limited anterior horn cell disease was proposed ( O'Flaherty, 2001 ; Bonilla-Musoles et al., 2002 ). Efforts are being made to distinguish between myogenic and neurogenic causes of AMC. AMC has been associated with maternal myasthenia gravis, in which there is transplacental transfer of maternal anti-acetylcholine receptor antibody to the fetus early in gestation, limiting fetal movements ( Riemersma et al., 1996 ; Jacobson et al., 1999 ; Polizzi et al., 2000 ). AMC has also been associated with a mutation in rapsyn, a receptor-associated protein at the synapse that is responsible for the maintenance of specific structures at the synapse of the neuromuscular junction ( Burke et al., 2003 ). Neurogenic AMC can result from focal cerebral deficits and produce asymmetrical limb involvement ( Takano et al., 2001 ).

Most children with AMC have quadrimelic limb involvement. Joint involvement is symmetrical, increases distally, and results in severe contractures and joint rigidity. The most common orthopedic deformities include talipes equinovarus, dislocated hip, dislocated patella, and scoliosis. Surgical management is directed at correcting all lower extremity deformities that delay ambulation. These are ideally performed before the patient is 2 years old. Upper extremity surgery is designed to improve hand function.

The anesthetic management of children with arthrogryposis multiplex congenita (AMC) is complicated by associated congenital abnormalities, an abnormal upper airway, and positioning difficulties. Arthrogryposis is associated with congenital heart disease, pulmonary hypertension-cor pulmonale, and urogenital anomalies ( Oberoi et al., 1987 ). Case reports highlight associated renal tubular acidosis, cholestasis, gastroschisis, and Hirschsprung's disease ( Eastham et al., 2001 ; Sooriyabandara and Aluwihare, 2001 ). Decreased pulmonary reserve resulting from pulmonary hypoplasia and scoliotic restrictive lung disease may potentiate hypoxemia and may necessitate postoperative ventilatory support. Despite these associated anomalies, it must be emphasized that these children have normal intelligence and can expect to live a nearly normal life span.

Patients with arthrogryposis have micrognathia, a high arched palate, and a short and rigid neck making tracheal intubation difficult and at times impossible ( Oberoi et al., 1987 ; Szmuk et al., 2001 ). Direct laryngoscopy and intubation become more difficult as the patient ages because craniofacial involvement often progresses with growth. Alternatives to direct laryngoscopy and tracheal intubation, such as the use of the laryngeal mask airway, with or without the use of a tube exchanger, or fiberoptics, have been used successfully in this disorder ( Nguyen et al., 2000 ; Szmuk et al., 2001 ; Thomas and Parry, 2001 ). The extensive contractures, tense skin, and minimal muscle mass and subcutaneous tissue pose challenges for intraoperative positioning and intravenous access.

Children with arthrogryposis may have altered responses to neuromuscular relaxants and are akin to other patients with anterior horn cell diseases. Although hyperkalemic responses to succinylcholine have not been reported, prudence mandates avoiding the routine use of depolarizing muscle relaxants. Although the response to nondepolarizing relaxants has been reported to be extremely variable, the use of short-acting nondepolarizing agents in association with careful monitoring of neuromuscular function has been successful in these patients.

The association of intraoperative hyperthermic crises with AMC has been sporadically reported in the literature. Hopkins and others (1991) , in reporting three cases of hyperpyrexia, reviewed the literature of nine additional cases. In nine cases, hyperthermia, tachycardia, and hypercarbia were observed intraoperatively. Six episodes were assumed to be malignant hyperthermia, and dantrolene therapy was immediately instituted. Two cases responded rapidly to aggressive cooling. One patient received a nontriggering anesthetic. The halothane contracture test was not performed in any of these reported cases. Conversely, Baines and others (1986) reported no case of malignant hyperthermia among 67 patients with AMC anesthetized with triggering agents. In light of the absence of laboratory confirmation, the occurrence of a hyperthermic crisis without exposure to triggering agents, and a large number of affected patients without a hyperthermic response, Hopkins and others (1991) concluded that these hypermetabolic responses are distinct from malignant hyperthermia. The association between AMC and malignant hyperthermia may be unfounded.

▪ JUVENILE RHEUMATOID ARTHRITIS

Juvenile rheumatoid arthritis (JRA) is the most common rheumatic disease of childhood ( Jarvis, 2002 ; Schneider and Passo, 2002 ). It is defined as the presence of peripheral arthritis beginning before the patient is 16 years old and having a duration of at least 6 weeks. The estimated incidence is 14 cases per 100,000 children per year.

JRA is divided into three subsets based on the symptoms at onset. Oligoarthritis, representing approximately 50% of all JRA, is common in girls, with the peak age of onset at 2 years. During the first 6 months of the disease, fewer than five joints are affected, typically the knees and less commonly the ankles and wrists. Seventy-five percent of these patients have a positive antinuclear antibody (ANA) titer. Oligoarthritis is rarely destructive. Polyarticular JRA, representing 20% to 40% of JRA cases, is also more common in girls, with a peak age of onset of 3 years. In this variant, five or more joints are affected, commonly the small joints of the hand. Only 40% of these patients have a positive ANA result, and 10% have positive test results for rheumatoid factor. Polyarticular JRA has associated subcutaneous nodules and erosions. The arthritis is destructive in 15% of patients. Moderate systemic manifestations of anemia and growth retardation are seen. Systemic JRA, also known as Still's disease, manifests with fever, macular rash, leukocytosis, lymphadenopathy, and hepatomegaly. Symmetrical polyarthritis is seen in these children. Boys and girls are affected equally, and they can present for treatment at any age. The cervical spine, jaw, hands, hips, and shoulders can be involved. Only 10% of these children have positive ANA results. The arthritis can be destructive in 25% of cases. Patients with systemic JRA can have pericarditis, pleuritis, uveitis, splenomegaly, and abdominal pain. Common surgical procedures include various joint arthroplasties and synovectomies, correction of uveitis, scoliosis surgery, limb length-discrepancy surgery, and limb and mandibular osteotomies.

Anesthetic considerations in the child with JRA are principally focused on airway management. The mandibular head and the temporomandibular joint (TMJ) can be destroyed in JRA, limiting mouth opening. TMJ disease can also affect mandibular growth, resulting in micrognathia. Cervical spine disease is commonly seen in the systemic and the polyarticular forms of JRA, sometimes resulting in spinal fusion, reducing cervical mobility. Cervical stiffness is reported in 46% to 60% of patients. Radiographic changes are usually seen in the late stages of the disease and only in children with severe involvement. As a component of atlantoaxial rotatory subluxation or as a solitary manifestation, torticollis can develop, further increasing the degree of difficulty of these patients—airway management (Subach et al., 1998 ; Uziel et al., 1998 ). Cricoarytenoiditis, an unusual manifestation of systemic JRA, can result in airway obstruction and in severe distortion of the glottic anatomy ( Jacobs and Hui, 1977; Vetter, 1994 ). Children with JRA should undergo a complete preoperative evaluation of the TMJ and the cervical spine, looking for evidence of limited range of motion. The assessment should include dynamic radiographs.

▪ MARFAN SYNDROME

Marfan syndrome is an autosomal dominant connective tissue disorder caused by mutations of genes on chromosome 15 that encode fibrillin, a complex glycoprotein that helps maintain the elastic properties of the soft connective tissues. These tissues are all lax, resulting in joint subluxation (Dietz et al., 1991a, 1991b [55] [56]; Dietz and Pyeritz, 1995 ). Marfan syndrome patients have abnormalities of their cardiovascular, skeletal, and ocular systems. The cardiovascular complications of Marfan syndrome, including aortic root dilatation and dissection, are thought to be consequences of the alterations in the fibrillin-rich aortic tunica media. Aortic disease is the major cause of morbidity and premature death of these patients ( Gott et al., 2002 ).

Aortic size appears to correlate with central pulse pressure, and children with Marfan syndrome maintained on β-blockade and calcium antagonist therapy have slower aortic root growth than do those who are unmedicated ( Jondeau et al., 1999 ; Rossi-Foulkes et al., 1999 ). Long-term medical therapy in combination with prophylactic cardiac surgery when the ascending aorta exceeds 50 mm has increased the long-term survival of patients with Marfan syndrome ( Finkbohner et al., 1995 ; Gott et al., 1999 ). The incidence of mitral valve prolapse is increased among persons with Marfan syndrome.

In addition to the cardiovascular anomalies, patients with Marfan syndrome have a high incidence of pectus excavatum and spontaneous pneumothoraces. The inward depression of the sternum that results from excessive growth of costochondral cartilages, in combination with severe scoliosis, produces a precarious physiologic state in which respiratory compromise can result from lung compression, or cardiovascular collapse can occur from impeded venous return or distortion of the great vessels or diminished coronary perfusion. Pizov and others (1997) observed that after inexplicable cardiovascular collapse during posterior spinal instrumentation, mild sternal compression was sufficient to disrupt coronary artery blood flow as visualized by echocardiography in his patient with severe, concurrent scoliosis and pectus excavatum.

Marfan syndrome affects the spine in several ways. Scoliosis has been reported in 40% to 70% of patients with the syndrome. These patients are at risk for atlantoaxial translation with neck flexion and extension ( Hobbs et al., 1997 ). Patients undergoing seemingly routine uneventful direct laryngoscopy and endotracheal intubation can develop atlantoaxial rotatory subluxation manifested as unresolved torticollis and neck pain in the postoperative period. These events are thought to result from abnormal bone morphology, from the abnormal shape of the atlantoaxial facet, or from the laxity of ligaments (Herzka et al., 2000 ). In a series of 100 patients with Marfan syndrome, more than 50% had evidence of increased atlantoaxial translation. The preadolescent population had a greater range of motion than did the adolescent or adult groups ( Hobbs et al., 1997 ). Atlantoaxial subluxation has been reported as a cause of sudden death of patients with Marfan syndrome ( MacKenzie and Rankin, 2003 ).

Anesthetic management of patients with Marfan syndrome starts with a preoperative cardiovascular and cervical spine evaluation. Patients who are destined to undergo prone procedures should have the effects of sternal compression evaluated by echocardiography preoperatively. The anesthesiologist should consult with the patient's cardiologist to determine whether the doses of β-blockade and calcium channel antagonism are appropriate. The anesthetic technique should aim to decrease myocardial contractility and avoid sudden increases in blood pressure, to minimize the risk for aortic dissection or rupture. In the setting of mitral valve prolapse, hypovolemia and tachycardia should be avoided. Nasotracheal fiberoptic intubation may be useful in patients with underlying disease and those with atlantoaxial instability.

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Copyright © 2005 Mosby, An Imprint of Elsevier

▪ SYNDROMES OF DISPROPORTIONATE SHORT STATURE: DWARFISM

Children with dwarfism are unified solely by their phenotype of disproportionate short stature and associated limb deformities. More than 35,000 dwarfs are estimated to live in the United States, and the individual osteochondrodysplasias and mucopolysaccharidoses that produce this phenotype number well over 100 ( Berkowitz et al., 1990 ). Achondroplasia, the most common form of dwarfism, has an incidence of 150 per 1 million live births ( Orioli et al., 1986 ). Many other forms are rare. The mucopolysaccharidoses (MPS) (i.e., Hurler's syndrome [MPS I-H], Hunter's syndrome [MPS II], Morquio's syndrome [MPS IV], and Scheie's syndrome [MPS V]) are few in number but pose significant anesthetic challenges. At one time, dwarfs were considered social curiosities who possessed an aura of mystery and played a prominent role in folklore and mythology. Improved societal support for this disability has resulted in an enormous number of orthopedic procedures that are performed on these patients. Limb-lengthening techniques ( Yasui et al., 1997 ; Aldegheri and Dall'Oca, 2001 ), cervical decompression, joint replacement, limb realignment, and bone marrow transplantation for patients with mucopolysaccharidoses are but a few of the procedures that are increasingly being performed on these patients. The number of diseases that constitute the dwarfing syndrome is enormous, and the limited anecdotal anesthetic experience for many of these precludes an encyclopedic review of all of the syndromes. Nevertheless, the common pathologic conditions to a large number of the dwarfing syndromes listed in Box 21-3 should be evaluated in the preoperative preparation of an affected patient.

BOX 21-3 

Systemic Manifestations of the Dwarfing Syndromes

Upper Airway Abnormalities—Mucopolysaccharidoses, Achondroplasia

Macroglossia

Micrognathia (mesomelic dysplasia, diastrophic dysplasia, dwarf with Russell-Silver syndrome)

Small oral opening

Temporomandibular joint immobility

Tonsillar and adenoidal hypertrophy

Narrow nasopharynx

Laryngomalacia

Difficult Laryngoscopy—Mucopolysaccharidoses

Abnormal glottic structures

Short neck, cervical scoliosis (Morquio syndrome, metatropic dysplasia, diastrophic dysplasia, spondyloepiphyseal dysplasia)

Copious secretions

Upper Airway Obstruction—Mucopolysaccharidoses

Abnormal neck position

Abnormal upper airway

Laryngomalacia

Copious secretions

Pulmonary Dysfunction—Mucopolysaccharidoses, Achondroplasia, Asphyxiating Thoracic Dystrophy

Restrictive lung disease

Thoracic dystrophy

Scoliosis

Pulmonary hypertension

Obstructive sleep apnea

Cardiovascular Dysfunction—Osteogenesis Imperfecta, Mucopolysaccharidoses, Asphyxiating Thoracic Dystrophy, Achondroplasia

Congenital heart disease

Acquired valvular disease

Cor pulmonale

Restrictive lung disease

Obstructive sleep apnea

Neurologic Complications—Achondroplasia, Osteogenesis Imperfecta

Atlanto-occipital instability, odontoid hypoplasia

Cervicomedullary compression

Intracranial hypertension

Deafness

Hematologic Dysfunction—Osteogenesis Imperfecta

Disorder platelet aggregation

Other Manifestations—Osteogenesis Imperfecta

Propensity for bony fractures

Hypermetabolism

▪ AIRWAY ABNORMALITIES

Characteristics

The anesthetic management of dwarfs is frequently complicated by anatomic abnormalities of the upper airway and by difficulty in visualization of the larynx during direct laryngoscopy. Inability to intubate is the major cause of morbidity and mortality when anesthetizing dwarfs ( Berkowitz et al., 1990 ). Upper airway obstruction is frequently a result of thickened pharyngeal and laryngeal structures, narrowed nasal passages, micrognathia, copious secretions, pharyngeal hypoplasia, and tracheal narrowing. This is seen most frequently in patients with mucopolysaccharidoses, diastrophic dysplasia, camptomelic dysplasia, severe diastrophic dysplasia, and dwarfs with Russell-Silver syndrome. Some patients demonstrate upper airway obstruction even in the awake state. Airway patency can be severely affected by positional changes alone. Some patients with achondroplasia, Morquio's syndrome, and metatropic dysplasia maintain a patent airway with the neck extended but completely obstruct when the neck is flexed. Sedation and general anesthesia often result in complete upper airway obstruction. Direct laryngoscopy and tracheal intubation are extremely difficult to perform in many dwarfs. Endotracheal intubation is often hampered by inadequate laryngeal exposure, infiltration of the glottic structures with abnormal mucopolysaccharide, and tracheal and subglottic narrowing. In contrast, the patient with achondroplasia may have narrow nasal passages and pharyngeal hypoplasia because of dysplasia and angulation of the cranial base and hypoplasia of the maxilla, but the airway is rarely obstructed, and it can be easily managed with a facemask ( Monedero et al., 1997 ).

Anesthetic Management

Preoperative sedative drugs should be avoided in patients prone to upper airway obstruction. Before induction, intravenous access is obtained, and an antisialagogue is administered. If the potential for severe upper airway obstruction and difficult intubation is anticipated, the equipment and personnel required for establishment of an emergency airway should be present before the induction of anesthesia (see Chapter 10 , Induction of Anesthesia). Spontaneous ventilation is mandatory. Often, the only way to identify the glottis is by observation of the air bubbles during spontaneous ventilation. An inhalational induction with high concentrations of oxygen and sevoflurane or a continuous intravenous infusion of propofol or ketamine is an equally effective approach in this situation. After an adequate anesthetic plane is achieved, endotracheal intubation can be accomplished by direct laryngoscopy or by fiberoptic-guided bronchoscopy. Alternatives include the lightwand and fiberoptic intubation by means of a laryngeal mask airway. Neuromuscular relaxants are avoided until the airway is secured. Rarely, a tracheostomy performed while the patient is awake may be the safest approach. The examiner must avoid neck manipulation, particularly neck flexion during laryngoscopy, in patients with atlantoaxial instability or foramen magnum stenosis.

▪ PULMONARY DYSFUNCTION

Characteristics

The pulmonary dysfunction common to children with the dwarfing syndromes is multifactorial in origin. Restrictive lung disease is a consequence of thoracic cage dystrophy (e.g., Jeune's syndrome) or scoliosis, and it results in reduced lung volumes, ventilation-perfusion mismatching, progressive hypoxemia, and hypercarbia. Obstructive or central sleep apnea causes pulmonary hypertension, substantial morbidity, and sudden death ( Sisk et al., 1999 ). Structural abnormalities, particularly in MPS, may cause intrathoracic obstruction. The preoperative assessment of these patients must include an evaluation of pulmonary function and a sleep study. The finding of central apnea necessitates a neuroradiologic evaluation of the cervical spine and foramen magnum. The presence and severity of pulmonary hypertension can be determined by an electrocardiogram and echocardiography.

Anesthetic Management

The degree of pulmonary dysfunction discovered during preoperative evaluation has profound implications on intraoperative and postoperative management. Severe obstructive sleep apnea may preclude an inhalational induction of anesthesia or the use of premedicants. Significant restrictive lung diseases with attendant ventilation-perfusion mismatch, proclivity to atelectasis, and development of increased PAO2 - PaO2 may necessitate placement of an indwelling arterial catheter to ensure adequate gas exchange throughout the perioperative period. The severity of restrictive lung disease dictates the necessity for continued intubation and ventilation postoperatively.

▪ CARDIAC DYSFUNCTION

Characteristics

Children with dwarfing syndromes have a variety of causes for cardiac dysfunction. There is a high incidence of coexisting structural heart disease, such as atrial septal defects, in a number of the dysplasias. Acquired valvular heart disease is a common complicating feature of children with MPS and is occasionally found in osteogenesis imperfecta. Ischemic heart disease may result from infiltrative mucopolysaccharides or from the consequences of cor pulmonale and long-standing pulmonary hypertension. Physical examination, chest radiography, electrocardiography, and echocardiography are useful in diagnosing pulmonary hypertension. The most reliable indicators of pulmonary hypertension are the presence of tricuspid regurgitation and prolongation of the right systolic time interval. The electrocardiogram should be reviewed for evidence of myocardial ischemia, particularly in patients with Hurler's syndrome or Hunter's syndrome.

Anesthetic Management

The anesthetic management of a patient with dwarfism, impaired myocardial dysfunction, and pulmonary hypertension must be meticulously planned and executed. Invasive monitoring of arterial, central venous, and in severe cases, pulmonary artery pressures is frequently necessary. The use of intraoperative transesophageal echocardiography may be warranted in patients with severe valvular dysfunction or those who have evidence of ischemic heart disease. Modest hypoxemia and hypovolemia can exacerbate preexisting pulmonary hypertension and borderline right ventricular function, and they must be avoided. The need to maintain an adequate anesthetic depth, crucial in avoiding intraoperative pulmonary hypertensive crises, must be balanced by the limitations imposed by compromised ventricular function. Attention must also be directed to the effects of changes in heart rate, blood pressure, filling pressures, and systemic vascular resistance on myocardial function. In light of these challenges, an opioid-based anesthetic in combination with adrenergic-modulating agents may provide the most stable intraoperative milieu for this subset of patients. The use of opioids in patients at risk for postoperative airway obstruction may result in postoperative intubation and mechanical ventilation.

▪ NEUROLOGIC DYSFUNCTION

Characteristics

Cervicomedullary compression and hydrocephalus are the main neurologic concerns in patients with dwarfism. The causes of cervical cord compression include atlanto-occipital instability, foramen magnum stenosis, and, infrequently, cervical scoliosis. Foramen magnum stenosis is a frequent complication of achondroplasia. Physical findings or a history consistent with upper motor neuron weakness (e.g., progressive weakness, hyperreflexia, abnormal plantar response), sleep apnea, cyanosis, or respiratory distress is suggestive of cervicomedullary compression. Flexion and extension neck films should be obtained to determine the degree of cervical spine instability in affected patients.

Anesthetic Management

Improper positioning of the head, neck, and shoulders during the induction of anesthesia, laryngoscopy, and surgery may lead to catastrophic intraoperative cord ischemia. Patients with Morquio's syndrome, diastrophic dysplasia, and achondroplasia are at greatest risk. The maintenance of in-line traction during intubation or fiberoptic intubation may be advisable. Cervical stabilizing devices such as a halo cast or a Milwaukee brace may be applied before anesthesia to avoid cervical subluxation and dislocation. Unfortunately, these devices may make direct laryngoscopy impossible. Succinylcholine should be avoided if there is any evidence of pyramidal tract signs, muscle wasting, or paresis. Autonomic hyperreflexia may be a potential problem in patients with cervical cord compression or myelopathy.

The coexistence of intracranial hypertension and a difficult airway is particularly challenging to the anesthesiologist. The conventional approaches of inhalational or intravascular induction may have salutary effects on one element of management but catastrophic consequences on the other. Balancing these conflicting requirements must be done on a case-by-case approach. One approach is to use an intravenous infusion of propofol to place a laryngeal mask airway, through which a fiberoptic bronchoscope is passed. Once identification of the glottic structures is assured, an intubating dose of propofol is administered to blunt the intracranial pressure response to endotracheal intubation.

▪ OSTEOGENESIS IMPERFECTA

Osteogenesis imperfecta represents a subpopulation of dwarfs with unique problems. Osteogenesis imperfecta is a disease resulting from a defect in type I collagen, and this group of disorders manifests with abnormalities in bone, teeth, sclera, and ligaments ( Chevrel and Meunier, 2001 ; Cohen, 2002 ; Zeitlin et al., 2003 ). The four clinical subtypes are listed in Table 21-7 .

TABLE 21-7   -- Subtypes of osteogenesis imperfecta

Type

Features

I

Mildest form of osteogenesis imperfecta (OI); mild bone fragility; bimodal fracture curve (first peak between 1 year and puberty; second, smaller peak postmenopausally in women and after 70 years in men); normal stature; blue sclerae; deafness in some cases, most commonly occurring in second decade; dominant inheritance

II

Most severe form of OI; perinatal lethal form; one half do not survive day 1, and 90% are dead by 1 week; extreme short stature; short, bowed long bones, particularly lower limbs; ribs have beaded appearance from recurrent fractures; respiratory insufficiency; absence of calvarial mineralization; dominant mutation

III

Progressively deforming type; many fractures; thin ribs with discrete fractures; curvature of spine that may be severe enough to reduce pulmonary reserve; severe short stature; deafness; dentinogenesis imperfecta; dominant mutation; rare recessives unlinked to type I collagen genes

IV

Mild to moderate bone fragility; short stature; deafness in some cases; dentinogenesis imperfecta; dominant inheritance

Adapted from Cohen MM Jr: Some chondrodysplasias with short limbs: Molecular perspectives. Am J Med Genet 112:304–313, 2002.

 

 

 

The hallmarks of this disease are bony fragility and multiple fractures after even innocuous trauma. Scoliosis and kyphosis are common. Medical management includes the use of bisphosphonate drugs (Chevrel and Meunier, 2001 ; Glorieux, 2001 ; Zeitlin et al., 2003 ). These are anti-bone-resorptive drugs that may reduce fracture frequency, increase bone density, promote remodeling of previously crush-fractured vertebrae, reduce chronic pain, and improve mobility in children and infants with osteogenesis imperfecta.

The hallmark of anesthetic management is to handle these patients very gently. Fractures may occur from simple procedures such as applying a tourniquet or taking a blood pressure or while positioning the patient on the operating room table. Airway management may cause fractures, and the physician must pay particular attention to the teeth, mandible, and cervical spine. Occasionally, visualization of the airway is difficult, and the use of a laryngeal mask airway may be very helpful ( Kostopanagiotou et al., 2000 ). Osteogenesis imperfecta patients have a hypermetabolic state and become hyperthermic during anesthesia ( Porsborg et al., 1996 ). This is not malignant hyperthermia, even though a few case reports of true malignant hyperthermia have been reported in these patients ( Cole et al., 1982 ). The routine pediatric anesthetic practice of preventing intraoperative hypothermia, such as using warming blankets and heated, humidified gasses, should be tempered, and antimuscarinics such as atropine and glycopyrrolate should be used judiciously.

Some patients with osteogenesis imperfecta bruise easily as a result of a presumed platelet abnormality. Bleeding and hemorrhage are rare, but approximately 30% of these patients have abnormal bleeding times, capillary fragility, and reduced levels of factor VIII.

OSTEOPETROSIS

Osteopetrosis (i.e., marble bone disease) is an inherited disease in which bone cannot be resorbed or remodeled, and it is associated with significant physiologic derangements. The severity of osteopetrosis can be predicted by its inheritance pattern; the autosomal recessive form is very severe and predominately affects infants and children. Benign autosomal recessive osteopetrosis is associated with cerebral calcifications and renal tubular acidosis ( Gerritsen et al., 1994 ). Children with the malignant form of recessive osteopetrosis frequently suffer from airway compromise, intracranial hypertension, and pancytopenia with a compensatory hepatosplenomegaly ( Geyser et al., 1982 ; Fasth and Porras, 1999 ). Persons with all forms of osteopetrosis are at risk for pathologic fractures. Patients with osteopetrosis frequently require anesthetics for bone biopsies to assess the efficacy of interferon therapy and for treatment of their pathologic fractures ( Key et al., 1995 ).

Burt and others (1999) , in a series of 65 anesthetics for children with osteopetrosis, reported that the rate of airway management difficulties was much higher in this group of children compared with the other children anesthetized at their institution. Mandibular abnormalities and temporomandibular joint immobility contributed to the difficulty of orotracheal intubation, and abnormalities of the nasal turbinates made nasotracheal intubation difficult. Cervicomedullary stenosis limited optimal head positioning, and concurrent thrombocytopenia exacerbated airway instrumentation and limited the options of a regional anesthetic. No particular anesthetic technique was deemed superior, except for an emphasis on meticulous preoperative airway preparation with the ready availability of the resources for emergency airway management.

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▪ CEREBRAL PALSY

Cerebral palsy is a static encephalopathy that may be defined as a nonprogressive disorder of posture and movement. It is often associated with epilepsy and abnormalities of speech, vision, and intellect resulting from a defect or lesion of the developing brain ( Kuban and Leviton, 1994 ). It is the most common childhood motor disability, occurring in 2 of 1000 live births ( Grether et al., 1996 ; Nelson and Grether, 1999 ). Multiple causes of CNS damage result in the phenotype of cerebral palsy. In premature infants, periventricular leukomalacia is commonly associated with the development of cerebral palsy. In term infants, early antenatal insults are attributed to be the cause of the CNS injury and can manifest as events at the time of delivery. The most common etiologic factors are prematurity and birth weight above or below ideal weight for gestational age ( Jarvis et al., 2003 ).

Cerebral palsy may be classified by a description of the motor handicap in terms of physiologic (major motor abnormality), topographic (extremity involved), etiologic, and functional capacity ( Table 21-8). Although the CNS lesion is static, the degree of impairment can change with time. Cerebral palsy is commonly associated with a spectrum of developmental disabilities, including mental retardation, epilepsy, and visual, hearing, speech, cognitive, and behavioral abnormalities. The motor handicap may be the least of the child's problems. The need to treat each patient's individual problems uniquely and to avoid generalization cannot be overemphasized. Some children with cerebral palsy may be of normal intelligence but limited in their ability to communicate. Others with marked developmental delay may be difficult to separate from their parents because of natural fear and an inability to reason in a way expected of children with normal intelligence.

TABLE 21-8   -- Four classification systems for cerebral palsy

Physiologic

Topographic

Etiologic

Functional

Spastic

Monoplegia

Prenatal (e.g., infection, metabolic, anoxia, toxic, genetic)

Class I: no limitation of activity

Athetoid

Paraplegia

Perinatal (e.g., anoxia)

Class II: slight to moderate limitation

Rigid

Hemiplegia

Postnatal (e.g., toxins, trauma, infection)

Class III: moderate to great limitation

Ataxic

Triplegia

 

Class IV: no useful physical activity

Tremor

Quadriplegia

 

 

Atonic

Diplegia

 

 

Mixed

Double hemiplegia

 

 

Unclassified

 

 

 

 

 

Patients with cerebral palsy often require multiple surgical procedures. Orthopedic operations to improve function of the extremities are common, and some patients require surgical correction of progressive spinal deformities. Common procedures include surgical soft tissue procedures that reduce muscle spasm around the hip girdle, including an adductor tenotomy or psoas transfer and release.

Several drugs are commonly used in treating spasticity, athetosis, dystonia, and seizures in cerebral palsy patients, and many of these drugs have significant anesthetic implications. Drugs used to treat spasticity include dantrolene, benzodiazepines, and baclofen. Incapacitating athetosis is treated with levodopa and dystonia with carbamazepine and trihexyphenidyl. Seizures are commonly treated with phenobarbital, phenytoin, clonazepam, carbamazepine, and sodium valproate.

The muscle spasticity of cerebral palsy is thought to be caused by inadequate release of the inhibitor γ-aminobenzoic acid (GABA) in the dorsal horn of the spinal cord, resulting in a relative excess of excitatory glutamate on the alpha motor neurons that produces simultaneous contraction of agonist and antagonist muscle groups ( Albright et al., 1991 ; Albright, 1996 ; Albright and Shultz, 1999 ). The symptoms of spastic diplegia can be treated surgically with rhizotomy, a procedure in which the roots of the spinal nerves are divided. Spasticity can also be treated medically by the administration of a continuous intrathecal infusion of baclofen or by local intramuscular injection of botulinum toxin. Baclofen is a GABA agonist on the receptors located in the dorsal horn of the spinal cord, and it can reduce the pain and muscle spasms. Continuous intrathecal baclofen infusions have been successfully used to treat ambulating children with inadequate leg strength and patients with upper and lower extremity spasticity, improving their function and comfort levels ( Albright et al., 1991 ; Albright, 1996 ; Albright and Shultz, 1999 ). Botulinum toxin is taken up in the presynaptic terminal, and it inhibits acetylcholine release into the neuromuscular junction, functionally denervating muscle fibers within 2 to 3 cm of the injection for up to 2 to 6 months ( Denislic and Meh, 1995 ; Forssberg and Tedroff, 1997). Tight heel cords may be treated surgically by tenotomy of the Achilles tendon.

Preoperative concerns for children with cerebral palsy focus on their pulmonary, gastrointestinal, and neurologic problems. Children with cerebral palsy are prone to aspiration pneumonia from gastroesophageal reflux and nasopharyngeal aspiration. Many have chronic lung disease with a reactive component, and they suffer from frequent respiratory infections ( Toder, 2000 ). Obstructive sleep apnea is seen in 20% to 50% of this patient population ( Kotagal et al., 1994 ). Obstructive sleep apnea has many causes, including bulbar dysfunction, neurogenic laryngomalacia, and laryngeal dystonia (Worley et al., 2003 ).

Airway management can be complicated by restricted TMJ range of motion and by poor and malpositioned dentition. Cerebral palsy patients commonly have gastroesophageal reflux disease, dysfunctional swallowing, and severe food refusal, all contributing to suboptimal nutritional status that portends increased perioperative complications. Children with cerebral palsy have a higher incidence of latex allergy than the general population ( Nakamura et al., 2000 ). Thirty percent of children with cerebral palsy require antiepileptic medications ( Singhi et al., 2003 ).

Perioperative seizure management in the patient with cerebral palsy is the same as for any patient with a seizure disorder. Most anesthetic agents are anticonvulsants, and they can be used safely in patients with underlying seizure disorders. A few anesthetic agents, such as enflurane, etomidate, ketamine, methohexital, EMLA (i.e., eutectic mixture of local anesthetics, 2.5% lidocaine and 2.5% prilocaine), and normeperidine (i.e., active metabolite of meperidine), are proconvulsants and should be avoided if alternatives are available. Most of these proconvulsant anesthetic agents actually raise the seizure threshold in normal patients and are proconvulsant only in patients with underlying seizure disorders. Patients should take their chronic anticonvulsants on the day of surgery. Virtually all anticonvulsants have long half-lives of elimination (24 to 36 hours), and if blood anticonvulsant levels are within the therapeutic range, a 24-hour period can elapse without taking the anticonvulsant and without increasing the risk of a seizure.

In most orthopedic surgical procedures performed in cerebral palsy patients, almost any anesthetic technique and combination of drugs can be used. Potent inhaled anesthetics, muscle relaxants (including succinylcholine), hypnotics, sedatives, opioids, and local anesthetics have been used safely. Children with cerebral palsy appear to have a lower minimum alveolar concentration (MAC) than unaffected children ( Frei et al., 1997 ). Succinylcholine does not cause hyperkalemia, and it can be safely administered to cerebral palsy patients. These patients have a slightly increased sensitivity to succinylcholine compared with normal children ( Dierdorf et al., 1985 ; Theroux et al., 1994 ). Resistance to nondepolarizing muscle relaxants and rapid recovery from neuromuscular blockade have been reported in cerebral palsy patients, which may be explained by the increase in extrajunctional acetylcholine receptors ( Theroux et al., 2002 ).

Postoperative pain management is important in the care of cerebral palsy patients. The surgical procedures, particularly those for relieving spasticity, are extremely painful. The severely affected child with cerebral palsy may be unable to communicate his or her pain, and health care providers are often unable to accurately assess the severity of postoperative pain. Parents and other routine caretakers are invaluable in assessing the pain of these patients. The Non-communicating Children's Pain Checklist—Postoperative Version has been validated for children with intellectual disabilities (Breau et al., 2002, 2003 [24] [23]). Postoperative pain is treated with continuous epidural (caudal or lumbar) infusions. Lidocaine (1.5 to 2.0 mg/kg per hour) plus fentanyl (0.5 mcg/kg per hour), chloroprocaine (3 mg/kg per hour) plus fentanyl (0.5 mcg/kg per hour), and bupivacaine (0.625 to 1 mg/mL, 0.2 to 0.4 mg/kg per hour) with or without fentanyl (0.5 mcg/kg per hour) or hydromorphone (2 to 4 mcg/kg per hour) have all been used. Muscle spasms are virtually universal and are treated prophylactically with intravenous diazepam. The addition of clonidine to the postoperative epidural infusion at a dose of 0.08 to 0.12 mcg/kg per hour is also effective at relieving muscle spasms ( Nolan et al., 2000 ). The management of posterior rhizotomy requires special attention. Often used in severe spasticity, this surgical procedure requires stimulation of the dorsal roots intraoperatively and observation of muscle response ( Peacock and Staudt, 1990 ; Abbott et al., 1993 ) (see Chapter 18 , Anesthesia for Neurosurgery).

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▪ ILIZAROV METHOD

HISTORY

Professor G.A. Ilizarov introduced the concept of distraction osteogenesis in the 1950s. Working as a general practitioner in Siberia, he found himself treating many patients with chronic osteomyelitis associated with bone loss and many World War II veterans who had developed fracture nonunions. Using materials from the metal factories at which many of his patients were employed, he fashioned external fixators and transosseous wires to induce the formation of new bone between freshly cut osseous surfaces that are gradually pulled apart. Using this technique, he was able to salvage limbs that otherwise would have been amputated. For many years, he worked in isolation, with his techniques remaining unknown in most of the world ( Fig. 21-5 ).

 
 

FIGURE 21-5  A, Wedge osteotomy (right) after application of an Ilizarov apparatus to correct a rotational deformity. With the external fixator in place (left), the rotational deformity has been corrected, and the bone is ready for lengthening by slow adjustment of the diaphyseal rings. B, Children with external fixators are encouraged to resume as much normal function as possible.

 

 

Over the past 20 years, this method has undergone significant refinements, and it is now employed in the treatment of congenital limb and other skeletal deformities, acquired short limbs, and angular deformities and the reconstruction of large bony defects resulting from trauma, tumor excision, infection, and fracture nonunions ( Herbert et al., 1995 ). Distraction osteogenesis has also been used to treat the numerous syndromes associated with micrognathia and retrognathia, such as the Pierre Robin sequence, Treacher Collins syndrome, Nager's syndrome, velocardiofacial syndrome, and Pfeiffer's syndrome, that can result in airway obstruction ( McCarthy et al., 2001 ; Sidman et al., 2001 ).

The success of the Ilizarov technique of distraction osteogenesis depends on adherence to the principles of tension-stress phenomena. These include a low-energy osteotomy to preserve periosteal blood supply; a slow, incremental distraction rate to preserve soft tissue blood supply; and maintenance of full function of the extremity. Bone healing is promoted by the biologic stress of walking on or flexing a broken limb, causing a trampoline-like effect of pulling and contracting that stimulates bone growth and healing. Using a corticotomy that preserves blood flow to the periosteum and the medullary canal, a gap between healthy, vascular-sufficient bone is created. Wires are inserted into the bone above and below the osteotomy and are attached under tension to an external fixator at 90-degree angles to the plane of the deformity. After an initial latency period, the osteotomy is gradually distracted at a rate of 1 mm per day in four incremental steps. The external fixator serves as the distracting device and provides optimal mechanical stability so that weight bearing and range of motion on the operative limb are possible on the second hospital day. The early functional use of the affected limb stimulates callus formation and osteoblastic activity ( Aronson, 1997 ).

▪ ANESTHETIC CONSIDERATIONS

Because the Ilizarov procedure is applied to children with a wide spectrum of diseases, including those with complex congenital musculoskeletal anomalies, the anesthetic implications of their coexisting diseases often take precedence over those of the operative procedure. These operative procedures are often long and complex, but they are plagued with few hemodynamic perturbations and only modest blood loss. The perioperative complications of the Ilizarov procedure that are affected by anesthetic management include nerve injury and the need for intact motor function and optimal analgesia in the postoperative period. Nerve injury can occur during pin placement and during the distraction process. To recognize inadvertent surgical trauma to nerves in the operative field, neuromuscular relaxation is avoided so that muscle contractions can be recognized. Postoperative surgical pain can be intense in the first 48 hours, and these patients are encouraged to begin physical therapy on the first postoperative day with an emphasis on passive range of motion and weight bearing ( Paley, 1990 ).

Children undergoing an Ilizarov procedure are often anesthetized with a general anesthetic supplemented with an epidural catheter. In most cases, endotracheal intubation is accomplished with short-acting neuromuscular blockade or with deep inhalational or intravenous anesthesia. Children with concurrent airway or cervical spine anomalies are intubated with a fiberoptic bronchoscope or with a laryngeal mask airway-guided approach during spontaneous breathing. After the induction of general anesthesia, an epidural catheter is placed, and a continuous epidural infusion of 0.8 to 1 mg/mL of bupivacaine with 1 mcg/mL of fentanyl is begun in the operating room at an infusion rate of 1 mL/kg per hour. Anesthesia is maintained with small amounts of opioids and a low concentration of inhaled anesthetic agents. The epidural infusion of bupivacaine and fentanyl is continued for the first 24 to 36 hours postoperatively and augmented with acetaminophen. Postoperative analgesia is maintained with the epidural catheter. In patients without an epidural catheter in place, intravenous PCA is used.

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TOURNIQUETS

Pneumatic tourniquets are commonly used to provide a dry operative field and limit intraoperative blood loss during extremity surgery ( Kam et al., 2001 ). Modern pneumatic tourniquets consist of three basic components: a cuff, similar to a blood pressure cuff, which is wrapped around a patient's limb and then inflated; a compressed gas source; and a mechanism with a pressure gauge, designed to maintain pressure in the cuff at a set value. After elevation and application of an Esmarch bandage to exsanguinate the limb, the tourniquet is applied over smooth padding and inflated to a pressure of 100 mm Hg above systolic pressure for the lower extremity and 50 mm Hg above systolic pressure for the upper extremity ( Patterson and Klenerman, 1979 ). To prevent accidental injury, the cuff should have a width that is greater than one half of the limb's diameter and an accurate pressure gauge. The duration of inflation should be carefully monitored.

How long the tourniquet can remain safely inflated is controversial. The most common recommendation, 2 hours, is based on the finding that cellular ischemic changes, such as mitochondrial swelling, myelin degeneration, glycogen storage depletion, and Z-line lysis, are reversible if the tourniquet is inflated for no more than 1 to 2 hours ( Patterson and Klenerman, 1979 ). The deleterious effects of tourniquet inflation include pain while the tourniquet is inflated (“tourniquet pain”), metabolic and hemodynamic changes that occur during tourniquet inflation and deflation, and nerve injury and damage to blood vessels and muscle if the tourniquet is inflated for excessive periods.

In awake patients undergoing extremity surgery under regional anesthetic blockade, tourniquet pain is described as a dull, ill-defined ache that occurs approximately 45 to 60 minutes after a tourniquet is inflated. Over time, this pain becomes unbearable, but it subsides immediately after tourniquet deflation. This pain occurs despite adequate regional anesthesia for the surgical procedure itself. The cause of tourniquet pain remains uncertain. Early intervention in the treatment of tourniquet pain is imperative. Intravenous opioids have limited efficacy, and induction of general anesthesia or deflation of the cuff is the only effective solution to this problem. Prophylaxis may be possible. The addition of opioids to local anesthetic solutions at the time of neural blockade appears to decrease the incidence of tourniquet pain in patients undergoing a regional anesthetic.

The hemodynamic consequences of tourniquet application include increases in blood and central venous pressures. Kaufman and Walts (1982) reported an overall 30% increase in blood pressure during tourniquet inflation. The blood pressure response is more exaggerated in patients under general anesthesia than in those undergoing regional blockade ( Valli et al., 1987 ). Tourniquet-induced hypertension can be prevented by the preoperative administration of 0.25 mg/kg of ketamine ( Satsumae et al., 2001 ). Limb exsanguination and tourniquet inflation can redistribute 15% of the total blood volume to the general circulation rapidly. Central venous pressure increases of up to 14 mm Hg have been reported in adults with the application of bilateral tourniquets. The clinical significance of central venous pressure reduction that accompanies tourniquet deflation primarily depends on the presence of preexisting cardiac dysfunction.

The metabolic changes that accompany tourniquet use include an increase in core temperature during tourniquet inflation and the development of transient metabolic acidemia and hypercarbia after tourniquet release ( Dickson et al., 1990 ; Kam et al., 2001 ). The increased output of carbon dioxide and lactic acid from the ischemic limb causes a transient decrease in arterial pH, with a maximum decrease within 4 minutes, returning to baseline values within 10 to 30 minutes. The sudden release of carbon dioxide into the circulation when lower limb tourniquets are released can markedly increase intracranial pressure in head trauma patients ( Eldridge and Williams, 1989 ; Kam et al., 2001 ). Normocapnia maintained by hyperventilation after tourniquet deflation can prevent increased cerebral blood flow velocity and intracranial pressure.

Tourniquet-induced hyperthermia, usually 1°C to 2°C, occurs within 90 minutes of tourniquet inflation and appears to be the result of decreased cutaneous heat loss from skin distal to the tourniquet ( Bloch et al., 1992 ). The metabolic acidosis and hypercarbia that occurs after tourniquet release is the result of reperfusion and the washing out and reentry into the central circulation of lactic acid, potassium, and other toxic substances that accumulated in tissues during tourniquet-induced limb ischemia. Accompanying hemodynamic effects include hypertension and hypotension, tachycardia, bradycardia, and rarely, ventricular arrhythmias. These effects are self-limiting and usually resolve over a few minutes. Other than increasing minute ventilation in patients who are being mechanically ventilated, most pediatric patients rarely or never require specific therapy for tourniquet deflation.

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CLUBFOOT

Clubfoot (i.e., talipes equinovarus) is a relatively common congenital deformity that occurs in 1 of 1000 live births. Most clubfoot deformities are bilateral and can occur in otherwise normal children who have no syndrome, cytogenetic abnormality, or extrinsic cause for the deformity ( Drvaric et al., 1989 ; Cummings et al., 2002 ). Clubfoot is commonly seen in patients with neuropathies and myopathies such as myelodysplasia, cerebral palsy, arthrogryposis, spinal muscular atrophy, and muscular dystrophy ( Drvaric et al., 1989 ; Cummings et al., 2002 ). There are degrees of severity of clubfoot and the treatment is individualized in each patient. In some patients, manipulation and casting can restore the bony architecture. In others, surgery is required. When to perform surgery is controversial. Some surgeons prefer to operate on neonates; others operate when the child is 3 months, 6 months, or older than 1 year.

The patient is positioned prone (Cincinnati and two-incision technique) or supine (Turco incision), and the procedure is performed with a tourniquet. The surgery involves soft tissue release, including posterior, medial, plantar, and lateral releases; tendon transfer and lengthening; and pin fixation ( Drvaric et al., 1989 ; Cummings et al., 2002 ). At the completion of surgery, the foot and the calf to the middle thigh are well padded and casted. Postoperatively, patients experience intense pain. Virtually any general anesthetic technique can and has been used for this surgery. Because postoperative pain is such an important aspect of the care of these patients, a combined regional (epidural or sciatic nerve block) and general anesthetic technique is commonly used. The epidural catheter is used intraoperatively and postoperatively.

The percutaneous Ponseti approach to the clubfoot involves weekly stretching of the deformity, followed by application of a long leg cast. By 4 to 5 weeks, all components of the deformity are corrected, with the exception of the equinus. The equinus is addressed with a percutaneous Achilles tenotomy, followed by a final long leg cast ( Herzenberg et al., 2002 ; Ponseti, 2002 ; Ippolito, 2003; Lehman et al., 2003 ).

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▪ DEVELOPMENTAL DYSPLASIA OF THE HIP

Developmental dysplasia of the hip (i.e., congenital hip dislocation) is a spectrum of abnormalities of the developing hip joint that ranges from shallowness of the acetabulum to capsular laxity and instability to frank dislocation ( Bauchner, 2000 ; Eastwood, 2003 ; Scherl, 2004 ). Developmental dysplasia of the hip is relatively common, occurring in 1 of 1000 live births. Previously known as congenital hip dislocation, it is now understood to be a condition that is not purely congenital but develops over time. It is common in children born by breech delivery. Screening in the newborn period consists of looking for asymmetries in skin folds, range of abduction, and height of the knees, as well as using provocative testing. The latter, known as the Ortolani test, elicits a click or clunk as the femoral head is moved in and out of the acetabulum. In the absence of other developmental disabilities, developmental dysplasia of the hip does not cause significant functional disability even if the diagnosis is missed or delayed, but untreated, it can lead to degenerative hip arthritis.

Treatment is designed to relocate and stabilize the femoral head in the acetabulum. Bracing (with the Pavlik harness) ( Mubarak and Bialik, 2003 ) and body casting are used for the first 6 months to 1 year of life, after which most patients require surgery to reseat the hip. Virtually any general anesthetic technique can and has been used for casting and surgery.

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▪ SLIPPED CAPITAL FEMORAL EPIPHYSIS

Slipped capital femoral epiphysis (SCFE) is a displacement of the femoral head in relation to the femoral neck through the growth plate during a period of rapid growth in adolescence ( Loder et al., 2001 ;Scherl, 2004 ). SCFE is common in obese teenagers and manifests with pain localized to the groin or to the knee or distal thigh ( Kocher et al., 2004 ). On physical examination, these children limp, and the diagnosis made by obtaining anteroposterior and frog leg lateral radiographs of the pelvis. In 20% of patients, SCFE is bilateral on presentation, although only one side may be symptomatic.

Surgical management consists of placing one or two screws across the growth plate of the affected hip to prevent further slip. The pinning is made in situ, meaning that no attempt is made to reduce the epiphysis back to its original position; such maneuvers damage the blood supply to the femoral head and lead to avascular necrosis ( Boero et al., 2003 ). Virtually any general anesthetic technique can and has been used for this surgery. Many of these patients have full stomachs when they present emergently and are therefore at risk for pulmonary aspiration of gastric contents.

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FRACTURES

Children with fractures are among the most common patients seen by orthopedic surgeons. Most fractures are simple and treated without an anesthesiologist present. However, major blunt trauma often involves fractures of the long bones in children. These patients should be carefully examined for evidence of trauma involving other organ systems, particularly the cervical spine. In very young infants, fractures are rare. When present, child abuse must be considered in the differential diagnosis.

How to anesthetize a patient with a fracture depends on the urgency of the procedure, the risk of vomiting and aspiration, the child's maturity, and the wishes of the parents and surgeon. Regional or general anesthesia is possible. Regional anesthesia may make it impossible to evaluate motor function even if dilute concentrations of local anesthetics are used. When using general anesthesia, “full stomach” precautions should be used to minimize the risks of vomiting and pulmonary aspiration of gastric contents. This necessitates the use of rapid-sequence induction and airway protection with an endotracheal tube.

▪ UPPER EXTREMITY BLOCKS

The upper extremity is innervated by the brachial plexus. The nerves are enclosed in a perineurovascular sheath that is divided into a supraclavicular (interscalene) and an infraclavicular (axillary) compartment. This fascial compartmentalization limits the spread of local anesthetics. An interscalene block provides more extensive blockade than an axillary approach, even if the amounts of local anesthetics used are identical ( Dalens et al., 1987 ; Ross et al., 2000 ; Suresh and Wheeler, 2002 ). This occurs because more nerve fibers are blocked with a supraclavicular approach than with an axillary approach. If the upper arm is fractured, the axillary approach to the brachial plexus may not provide sufficient analgesia.

The axillary approach is a preferred technique because it is virtually free of all complications, does not require paresthesias to provide superior blockade, and is simple to perform ( Yaster and Maxwell, 1989 ). In contrast to the effects in adults, this technique blocks the radial and musculocutaneous branches of the brachial plexus in children. Several techniques have been described. Because the axillary artery lies within the fascial sheath of the brachial plexus and the spread of local anesthetic within the fascia results in neural blockade, local anesthetic solutions deposited anywhere near the axillary artery produce a nerve block.

One technique is shown in Figure 21-6 ( Yaster and Maxwell, 1989 ). The patient's arm is abducted at a right angle, with the forearm and hand supinated. The elbow is flexed and the hand held in a “saluting” position. The axillary artery is used as the landmark. After aseptic preparation of the skin, the anesthesiologist's nondominant hand is positioned such that the middle finger lies directly over the artery and compresses it and the brachial plexus against the humerus. A wheal of local anesthetic is deposited directly over the pulsating artery. Using a 25-gauge needle, local anesthetic is deposited on each side of the artery, staying as close as possible to the border of the artery. On the initial pass on each side of the artery, the needle is inserted slowly, aspirating constantly for arterial blood. If no blood is aspirated, 1 mL of local anesthetic is injected as the needle is withdrawn. In a fanlike manner, 1 mL of local anesthetic is injected on each sweep away from the artery. Usually, 5 mL of local anesthetic is deposited on each side of the artery (see Chapter 14 , Regional Anesthesia).

 
 

FIGURE 21-6  The axillary block. The anesthesiologist's nondominant hand is positioned such that the middle finger lies directly over the brachial artery (A), compressing the brachial plexus against the humerus. After placement of a skin wheal directly over the artery, fanlike injections of local anesthesia are made on each side of the artery as close as possible to the lateral border of the artery. Approximately five sweeps are made on each side of the artery, and 1 mL of local anesthetic is deposited on each sweep.

 

 

▪ LOWER EXTREMITY BLOCKS

The femoral nerve block is the quickest, easiest, and most effective technique of relieving the pain of a femoral shaft fracture ( Yaster and Maxwell, 1989 ). When bupivacaine without epinephrine is used in a femoral nerve block, patients with fractures are provided with adequate anesthesia for the application of traction and for the necessary (and usually painful) manipulations that occur during radiologic examinations. The duration of analgesia is approximately 3 hours, and peak plasma levels of bupivacaine average less than 1 mcg/mL.

The femoral nerve, composed of nerve roots from L2, L3, and L4, is a mixed sensory and motor nerve located just under the inguinal ligament, lateral to the femoral artery in the femoral sheath ( Fig. 21-7 ). After aseptic preparation of the skin, a 25- or 22-gauge, short-bevel needle is inserted perpendicular to the surface approximately 3 to 5 mm lateral to the artery and 1 to 2 cm inferior to the inguinal ligament. The needle is advanced until a pop is felt, indicating penetration of the fascia of the femoral sheath. This occurs at a depth that is clearly deeper than the artery. After a negative aspiration for blood, the needle is immobilized, and 1 to 5 mL of local anesthetic is deposited slowly. The needle is withdrawn to the skin and then redirected laterally and deep to the arterial pulsation. After aspirating for blood on each pass, more local anesthetic is injected in a fanlike manner away from the artery. This ensures adequate local anesthetic deposition in the fascia surrounding the nerve and thereby provides effective sensory blockade. The maximum dosage of 0.25% bupivacaine is 1 mL/kg. Higher concentrations of local anesthetic are rarely used because they produce motor blockade. The resulting anesthesia usually lasts 3 to 6 hours. This block can be repeated as often as is needed.

 
 

FIGURE 21-7  The femoral nerve block. After placing a skin wheal, the anesthesiologist's nondominant hand compresses the femoral artery and nerve against the underlying tissue and bone immediately below the inguinal ligament. A 22- to 25-gauge needle is then inserted perpendicularly, approximately 0.05 to 1 cm lateral to the pulsation of the femoral artery (A), into the femoral nerve's fascial sheath.

 

 

Alternatively, a fascia iliaca block can be performed ( Dalens et al., 1989 ). This 3-in-1 block provides outstanding analgesia and provides coverage of the areas innervated by the femoral nerve, the lateral femoral cutaneous nerve, and the obturator nerve. A line is drawn outlining the skin projection of the inguinal ligament from the anterior superior iliac spine to the spine of the pubic bone. This line is then measured and divided into thirds. A short-bevel needle is inserted perpendicularly to the skin, 0.5 to 1 cm below at the outer third skin mark. A first give (with loss of resistance if gentle pressure is exerted on the barrel of the syringe) is felt as the tip of the needle crosses the fascia lata, and a second one occurs as the fascia iliaca is pierced. After aspirating for blood, the anesthetic solution is injected. A catheter can also be inserted and thereby provide continuous analgesia (see Chapter 14 , Regional Anesthesia).

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

▪ FAT EMBOLISM SYNDROME

FES is a collection of respiratory, hematologic, neurologic, and cutaneous symptoms and signs associated with trauma and other disparate surgical and medical conditions such as sickle cell acute chest syndrome and acute pancreatitis (Vichinsky et al., 1994, 2000 [268] [269]). The incidence of the clinical syndrome is low (<1% in retrospective reviews), and embolization of marrow fat appears to be an almost inevitable consequence of long bone fractures ( Mellor and Soni, 2001 ; Parisi et al., 2002 ). It is characterized by pulmonary insufficiency (i.e., hypoxemia), petechiae, and neurologic dysfunction (Box 21-4 ). The challenge to the pediatric anesthesiologist is to recognize the intraoperative manifestations of FES in a multiple-trauma patient or in a patient with an isolated long bone fracture.

BOX 21-4 

Diagnostic Criteria for Fat Embolism Syndrome

Major Criteria

Pulmonary insufficiency

  

 

PaO2 < 60 mm Hg

Neurologic dysfunction

  

 

Confusion

  

 

Disorientation

  

 

Lethargy

  

 

Focal deficits

  

 

Seizures

  

 

Coma

Petechiae

Minor Criteria

Fever

Thrombocytopenia

Anemia

Tachycardia

Elevated erythrocyte sedimentation rate

Retinal changes

PATHOPHYSIOLOGY

The physiopathologic mechanisms that produce the FES remain controversial. In the most accepted mechanical hypothesis, bone injury results in adipose tissue and blood vessel disruption and in hematoma formation. Increasing tissue pressure from hematoma expansion drives fat globules into the peripheral circulation. Chylomicrons are destabilized by the effects of fat intravasation and form very large, circulating fat globules. Fat can be detected in pulmonary arterial samples in up to 70% of patients with long bone or pelvic fractures, especially if the pulmonary artery catheter is wedged ( Byrick et al., 1989 ). In the pulmonary vascular bed, these fat particles are hydrolyzed to free fatty acids, which produce pulmonary vasculitis and hemorrhagic pneumonitis. Surfactant activity is compromised, functional residual capacity is decreased, and endothelial integrity is violated. As a result, a large PAO2 - PaO2 value is generated, pulmonary vascular resistance is increased, and pulmonary compliance is decreased. Fat particles and free fatty acids can enter the systemic circulation through pulmonary arteriovenous shunts to generate the central, renal, and cutaneous manifestations of this syndrome.

PRESENTATION

FES can be demonstrated in 90% of patients with long bone fractures, but symptomatic FES occurs in only 10% to 22% of patients with long bone or pelvic fractures. Classically, it develops 24 to 72 hours after an injury and is characterized by acute respiratory insufficiency with diffuse pulmonary infiltrates, global neurologic dysfunction, and petechiae. The pulmonary compromise is usually followed by the neurologic changes. If a petechial rash develops, it occurs 48 to 72 hours after the onset of FES. This complete presentation is seen in less than 10% of cases. Respiratory insufficiency may be the only manifestation of this syndrome, and it may occur in only one third of patients. This unexplained hypoxia is how FES manifests during general anesthesia ( van Besouw and Hinds, 1989 ). Few patients with FES have a fulminant course in which severe pulmonary hypertension and progressive right heart failure develop within hours of the injury. Vasopressor infusions are frequently required when this occurs.

The diagnosis of FES is a clinical one, and it may be difficult to establish ( Georgopoulos and Bouros, 2003 ). Supportive laboratory tests include an inexplicable drop in PaO2, hematocrit, platelet counts, and fibrinogen levels. The characteristic radiograph of bilateral fluffy pulmonary infiltrates may not be apparent for 24 to 48 hours after the onset of FES. Fat is seen in the urine in 50% of patients within 3 days. The utility of serum and urinary measurements of fat and lipase activity are limited by their poor sensitivity and specificity and by a lack of availability. Identification of fat droplet cells in bronchoalveolar lavage is the only rapid and specific method of identifying the development of this syndrome ( Chastre et al., 1990 ; Mimoz et al., 1995 ). The retinal changes of bilateral cotton-wool spots and intraretinal hemorrhages are seen in 60% of patients with FES.

In the operating room, the time of highest risk for the development of FES occurs during the transfer of the fracture patient from the stretcher to the operating room table. Fulminant FES can manifest within 30 minutes of this transfer, and it is recognized by inexplicable, progressive oxygen desaturation and increasing peak inspiratory pressures. A high index of suspicion is necessary to make the diagnosis. Other manifestations of FES during anesthesia include sudden hypotension, tachycardia, bradycardia, dysrhythmia, decreased lung compliance, pulmonary edema, and severe, unexplained surgical bleeding or oozing from multiple sites resulting from disseminated intravascular coagulation.

End-tidal CO2 monitoring does not seem to be as sensitive to fat emboli as it is in other embolic states. Although end-tidal CO2 does change with a massive fat embolism, end-tidal CO2 monitoring has not been as effective as echocardiography in detecting smaller emboli. Transesophageal echocardiography can detect fat emboli during surgical manipulation of the operative bone, and it can demonstrate the regional wall motion abnormalities and right ventricular dilatation that are harbingers of the FES physiologic perturbations ( Capan and Miller, 2001 ).

After FES develops, the treatment is nonspecific and supportive. It consists of early resuscitation and stabilization, administration of 100% oxygen, application of positive end-expiratory pressure, and the use of inverse-ratio ventilation. Bronchoscopy and bronchoalveolar lavage are useful in establishing a diagnosis and in removing the intraluminal debris and hemorrhagic exudate that accompany a fulminant presentation. An adequate intravascular volume must be maintained, and inotropic infusions and red blood cell transfusions are often required. Historically, advocated therapies have included intravenous alcohol, heparin, low-molecular-weight dextrans, and steroids. Limited data support the efficacy of any of these therapies once FES has begun. Early administration of methylprednisolone may decrease the incidence of FES ( Lindeque et al., 1987 ). A 10% mortality rate has been reported for all patients; among children, the mortality rate is 33%.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

SUMMARY

The orthopedic patient presents multiple challenges to the anesthesiologist. In many instances, the perioperative anesthetic plan for pediatric orthopedic patients depends more on the child's age and on the site and emergent nature of surgery than on the underlying disease or the specifics of the surgical procedure. In other cases, the underlying medical condition, associated anomalies, pathophysiology, and surgical procedure dictate the anesthetic plan. The anesthetic plan must address these issues and the recurring themes of positioning, airway management, blood loss and fluid replacement, conservation of body temperature, and postoperative pain management. In the future, the continued technologic, physiologic, and pharmacologic advances in our specialty will allow for longer, more extensive, and more innovative operations on younger and sicker patients than was possible in the past.

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Motoyama & Davis: Smith's Anesthesia for Infants and Children, 7th ed.

Copyright © 2005 Mosby, An Imprint of Elsevier

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