AAOS Comprehensive Orthopaedic Review

Section 7 - Spine

Chapter 67. Infections of the Spine

I. Introduction

A. Sequelae—Spinal infections have significant potential local and systemic sequelae.

 

B. Transmission and pathogens

 

1. Different types of spinal infections typically demonstrate distinct modes of transmission and are often associated with specific pathogens (

Table 1).

 

2. Postoperative infections generally arise from direct inoculation of a surgical wound.

 

3. Diskitis and epidural infections most commonly result from hematogenous spread.



II. Postoperative Infections

A. Incidence and microbiology

 

1. Postoperative infections occur almost exclusively from inoculation of a surgical site with skin flora (eg, Staphylococcus aureusStaphylococcus epidermidis) at the time of surgery, but may also develop as a result of hematogenous spread.

 

2. Multiple procedural and patient factors may influence the incidence of postoperative infections.

 

a. Longer surgical time

 

b. Immunocompromised state

 

c. Increased blood loss

 

d. Poor nutritional status

 

e. Posterior surgical approach

 

f. Obesity (body mass index >35)

 

g. Use of instrumentation or operating room microscope

 

   *Peter G. Whang, MD, or the department with which he is affiliated has received research or institutional support from Medtronic Sofamor Danek and is a consultant or employee for Paradigm Spine.

 

h. Prior spinal surgery or local radiation

 

i. Longer constructs or more extensive procedures

 

j. Tobacco or alcohol abuse

 

k. Multiple trauma

 

B. Clinical presentation

 

1. Most postoperative spinal infections are clinically evident.

 

2. Increased pain and tenderness occurs around the surgical site

 

3. Constitutional symptoms (eg, fever, chills, or malaise) are common but may be absent.

 

4. Incisional erythema, breakdown, or drainage may be present, although a tight fascial closure may allow deeper infections to develop without any obvious superficial manifestations.

 

5. A wound infection may take a week or more to declare itself after surgery.

 

C. Diagnostic studies

 

1. Laboratory studies

 

[Table 1. Risk Factors for Spinal Infections Caused by Specific Pathogens]

a. WBC, ESR, and CRP values are usually abnormal, but these markers may also be elevated secondary to the physiologic stresses of the surgical intervention.

 

b. ESR normally peaks within 5 days of surgery but may remain elevated for 40 days or more.

 

c. CRP typically reaches its maximum around postoperative day 2 but may be elevated for more than 14 days after surgery.

 

2. Imaging modalities

 

a. Plain radiographs of the surgical site may rule out underlying structural abnormalities that might account for a patient's clinical presentation.

 

b. CT scan and MRI are useful for identifying atypical fluid collections, but these studies frequently yield relatively nonspecific findings that may be difficult to differentiate from normal postoperative changes.

 

3. Cultures

 

a. Identifying the specific organism responsible for an infection is essential to direct appropriate antibiotic treatment.

 

b. Superficial wound cultures are generally not indicated because they are at significant risk for contamination.

 

c. Blood cultures should be drawn if a systemic infection is suspected.

 

d. In equivocal clinical scenarios, a needle biopsy may be required to access deep fluid loculations that cannot be differentiated from postoperative hematomas.

 

e. Intraoperative cultures remain the gold standard for confirming the presence of an active wound infection and isolating the causative pathogen.

 

D. Prophylactic antibiotics

 

1. Perioperative prophylactic antibiotics have been shown to reduce the incidence of postoperative wound infections by up to 60%, so it is widely recommended that a single parenteral dose of antibiotics be administered 30 to 60 minutes before incision to allow for systemic distribution and adequate tissue penetration.

 

a. First-generation cephalosporins (eg, cefazolin) provide good coverage of gram-positive organisms including S aureus and S epidermidis, two of the most common skin contaminants.

 

b. An alternative antibiotic such as vancomycin should be considered for patients with sensitivities to cephalosporins or those known to be colonized with methicillin-resistant S aureus (MRSA).

 

2. For prolonged surgical procedures or cases with significant blood loss or gross contamination, additional doses of antibiotics should be administered intraoperatively at intervals one to two times the half-life of the medication.

 

3. Although many surgeons continue prophylactic antibiotics postoperatively, this practice is not supported by the literature and may actually select for drug-resistant organisms with greater virulence.

 

E. Management

 

1. Medical management may initially be considered for a suspected superficial postoperative spinal infection in the absence of a pathologic fluid collection or a frank abscess.

 

a. Any suspected wound infection treated with antibiotics alone must be followed closely to rule out any progression or involvement of the deeper tissues.

 

b. In addition to the clinical appearance of the incision, the patient's response to medical treatment may also be monitored with laboratory studies (eg, ESR, CRP).

 

2. The mainstay of treatment is open irrigation and debridement.

 

a. When there is sufficient clinical evidence supporting the diagnosis, surgical intervention should be performed immediately on a presumptive basis instead of being delayed for confirmatory imaging studies.

 

b. Irrigation, debridement, and the administration of perioperative antibiotics should not be initiated until after superficial and deep wound cultures have been obtained.

 

c. Instrumentation is often retained because the increased stability is not only important for the proper treatment of the underlying spinal pathology but may also facilitate the eradication of infection (

Figure 1).

 

d. Loose bone graft is usually removed because it may act as a potential nidus of infection, but any material adherent to the surrounding bony structures may be left in place.

 

e. Although many surgeons elect to close the wound primarily over drains, a grossly infected wound may be left open for serial irrigation and debridement procedures.

 

i. Wound closure may subsequently be performed in a delayed fashion once there is no evidence of contamination and cultures are negative.

 

[Figure 1. Images of the spine of a patient with a postoperative spinal infection. A, Sagittal T2-weighted MRI demonstrates an infection involving the disk space. The spine was also unstable at this level and was treated with repeated posterior irrigations and debridements and then an anterior interbody fusion with autograft, followed by a posterior instrumented fusion. B, Radiograph of the spine after treatment.]

ii. The vacuum-assisted closure (VAC) system is also gaining popularity among many surgeons.

 

f. Initial treatment is with broad-spectrum parenteral antibiotics that should be modified according to the results of the intraoperative wound cultures.

 

i. Antibiotics are routinely continued for at least 6 weeks, and any subsequent changes in the medical regimen are based on the clinical response and laboratory profile of each patient.

 

ii. If appropriate, patients may later be converted to oral antibiotics.



III. Hematogenous Diskitis and Osteomyelitis

A. Incidence and microbiology

 

1. Pyogenic infections of the spine unrelated to a surgical procedure most frequently develop secondary to hematogenous seeding from distant sites, although direct extension from adjacent structures is also possible.

 

2. Hematogenous spinal infections represent approximately 2% to 7% of all cases of pyogenic osteomyelitis.

 

3. The age distribution is classically bimodal, with a small peak between 10 and 20 years of age and another, larger peak in the elderly.

 

4. The incidence of pyogenic spinal infections has been rising, most likely because of the escalating number of invasive medical procedures being performed as well as the increasing prevalence of immunocompromised patients.

 

5. With hematogenous spinal infections, pathologic organisms usually emanate from the vascular end plates into the relatively avascular disk space before spreading to adjacent vertebral bodies.

 

a. Infections were initially thought to disseminate to the spine in a retrograde fashion via the network of valveless venous channels in the epidural space known as the Batson venous plexus, but this mechanism has recently fallen out of favor.

 

b. Alternatively, it has been shown that the cartilaginous end plates contain multiple small, low-flow vascular anastamoses that provide an ideal environment for the inoculation and growth of microorganisms.

 

c. As the infection progresses, necrosis of the end plates allows these infectious agents to penetrate the avascular disk space where they are shielded from host immune defenses.

 

6. Diskitis most commonly occurs in the lumbar spine (50% to 60%), followed by the thoracic (30% to 40%) and cervical (10%) regions.

 

7. Up to 17% of affected individuals will present with neurologic deficits resulting from compression of the neural elements secondary to the progressive collapse of the vertebral column or from direct extension of the infection itself.

 

8. S aureus is the most common pathogen responsible for pyogenic spinal infections and is successfully isolated in up to 65% of patients.

 

a. Gram-negative enteric bacteria may be responsible for another 20% of vertebral osteomyelitis and diskitis cases.

 

b. The risk of infection with MRSA has also increased as the prevalence of drug-resistant organisms continues to rise.

 

B. Clinical presentation

 

1. These infections may be difficult to differentiate from degenerative spondylotic disease or sprain/strain injuries, so patients should be routinely questioned regarding a history of any constitutional symptoms as well as any recent illnesses, spinal procedures, or travel that may support the diagnosis of an infection.

 

2. Often these various clinical findings are not present; eg, only approximately one third of patients with diskitis will report a history of fever.

 

[

Figure 2. Images of the spine of a neurologically intact, immunocompromised patient with diskitis of the thoracic spine. A, Sagittal T2-weighted MRI. The infecting organism was identified by a CT-guided biopsy (B). The patient was treated with antibiotics and bracing.]

3. Because the clinical presentation may be so non-specific, most patients exhibit signs and symptoms of the spinal infection for more than 3 months before the correct diagnosis is made.

 

C. Diagnostic studies

 

1. Laboratory studies

 

a. Because of the indolent nature of these infections, the WBC may be normal, but the ESR and CRP are elevated in 90% of patients with diskitis.

 

b. The CRP typically increases more acutely than the ESR with the onset of infection and normalizes more quickly than the ESR as it resolves.

 

2. Imaging modalities

 

a. As with all suspected spinal infections, the initial imaging studies should be plain radiographs, which are useful for assessing the structural stability of the spine and may reveal evidence of a more aggressive pathologic process such as end-plate erosions or sclerosis, destruction of disk spaces, or collapse of the vertebral bodies; unfortunately, these findings may not be apparent for weeks or even months.

 

b. CT scan provides more information about bony cross-sectional anatomy and may demonstrate pathologic changes earlier in the disease course. With the addition of contrast, CT scans also facilitate the visualization of fluid collections within the psoas muscle or epidural space.

 

c. Radiographs or CT scans may exhibit a "vacuum disk," characterized by the presence of air within the disk space; this phenomenon is associated with degenerative disease rather than infection, which is more likely to give rise to fluid within the disk space.

 

d. MRI is the optimal imaging study for confirming the diagnosis of infectious diskitis.

 

i. In the setting of an active infection, the fluid-filled disk and any edema in the adjacent vertebral bodies are bright on T2-weighted images (Figure 2, A).

 

ii. With gadolinium, paraspinal and epidural enhancement also may be observed.

 

e. In contrast to most malignancies, which primarily involve the vertebral body, the nidus of an infection is typically located within the disk space, a distinction that may be used to differentiate these two pathologic processes.

 

f. If MRI is not available or is inconclusive, nuclear medicine studies such as technetium Tc 99m bone scans and indium-111 tagged white blood scans may be useful for detecting diskitis. These nuclear medicine techniques are extremely sensitive for detecting infections, but they are not as specific; eg, these studies may also be positive in patients with degenerative spondylosis.

 

3. Cultures

 

a. Blood cultures are routinely obtained in cases of suspected hematogenous osteomyelitis or diskitis.

 

i. These cultures are positive in approximately 33% of patients.

 

ii. The likelihood of successfully isolating a specific organism is greater if the cultures are drawn before the administration of antibiotics when the patient is actively febrile.

 

b. CT-guided biopsy of the lesion may also be necessary to confirm the diagnosis and provide tissue for culture and sensitivity testing (Figure 2, B).

 

D. Management

 

1. Most pyogenic spinal infections are managed nonsurgically with antibiotics, immobilization, and other supportive care.

 

a. Broad-spectrum parenteral antibiotics should be administered empirically until a specific pathogen is identified, at which time the antibiotic regimen should be modified appropriately.

 

b. Treatment usually consists of intravenous antibiotics for a minimum of 4 to 6 weeks, followed by a variable course of oral antibiotics.

 

c. The response to antibiotics should be monitored with serial clinical evaluations and laboratory values because any changes in the status of the infection may not be immediately evident on imaging studies.

 

d. Spinal immobilization may also provide symptomatic relief and limit the development of deformity.

 

2. Surgical intervention may be indicated for patients who have failed medical management or those with neurologic deficits or progressive deformity, as well as in situations in which a definitive diagnosis was unable to be established.

 

a. The primary objectives of surgical treatment are debridement of the infected region, decompression of the neural elements, and stabilization of any resultant spinal deformity or instability that may be present.

 

b. Vertebral osteomyelitis and diskitis typically affect the anterior column, so these lesions are generally addressed anteriorly, but involvement of the posterior elements may also necessitate a posterior approach.

 

c. Any debridement resulting in a significant anterior column defect can be supported with a strut graft or interbody implant at the time of the intervention. Patients with significant instability or severe deformities may also require supplementary posterior instrumentation placed concurrently or in a staged fashion to achieve a solid arthrodesis.

 

d. Autogenous bone remains the gold standard graft material for fusion, especially in the setting of infection; however, allograft and/or metallic implants may also be acceptable in these patients.



IV. Granulomatous Infections

A. Incidence and microbiology

 

1. Granulomatous infections of the spine, also known as atypical or nonpyogenic infections, are caused by atypical bacteria, fungi, or spirochetes.

 

a. Although these types of infections involve a variety of disparate organisms, they are often classified together because of their similar clinical and histologic features.

 

b. Even though atypical infections are relatively rare compared to pyogenic infections in the United States, their incidence has increased dramatically as the number of immunocompromised hosts has grown.

 

2. Mycobacterium tuberculosis remains the most common cause of granulomatous spinal infections.

 

a. Approximately 15% of tuberculosis cases are associated with extrapulmonary disease, and at least 5% affect the spine, which is the most frequent site of bony involvement.

 

b. Colonization generally occurs by hematogenous spread, but these infections may also develop as a result of direct extension from visceral lesions.

 

3. Granulomatous infections may also be triggered by a variety of fungal species such as AspergillusBlastomycesCoccidioidesHistoplasma, and Cryptococcus, all of which are endemic to different regions of the United States.

 

4. Atypical bacterial species and spirochetes (eg, Actinomyces isrealii and Treponema pallidum) are responsible for an even smaller proportion of these types of infections.

 

5. Atypical infections most commonly originate in the peridiskal metaphysis of a vertebral body and propagate under the anterior longitudinal ligament to include adjacent levels.

 

a. Unlike pyogenic infections, the nidus is located in the middle of the vertebral body, so the disks are relatively spared.

 

b. Because of their predilection for the vertebral body, these lesions are often mistaken for tumors, and progressive deformity may be observed as the diseased segment becomes increasingly unstable.

 

B. Clinical presentation

 

1. Patients typically present with discomfort in the thoracic region, which is the most common site of involvement, but the correct diagnosis may be delayed because pain tends to be a relatively late finding that may become apparent only after significant vertebral collapse and focal kyphosis have already occurred.

 

2. Many individuals will report a history of systemic complaints. These constitutional signs and symptoms may be difficult to interpret in immunocompromised patients, who are at greatest risk for developing these infections.

 

C. Diagnostic studies

 

1. Laboratory studies

 

a. The WBC, ESR, and CRP may be elevated in these patients, but these values are generally nonspecific, and they are normal in as many as 25% of cases.

 

b. Individuals with active tuberculosis or previous exposure to Mycobacterium will normally exhibit a positive tuberculin purified protein derivative (PPD) skin test, although false-negative results may occur in anergic patients.

 

2. Imaging modalities

 

a. A chest radiograph should be obtained for any patient suspected of having tuberculosis.

 

b. Plain radiographs of the spine often demonstrate only subtle abnormalities such as peridiskal erosions and scalloping of the anterior vertebral bodies, but because the onset of back pain and other clinical symptoms may be delayed in these patients, these initial screening radiographs may reveal extensive bony destruction with focal kyphosis.

 

c. MRI remains the imaging modality of choice for evaluating granulomatous infections, which normally result in relative sparing of the intervertebral disks. Gadolinium is useful for distinguishing between an abscess, which displays only peripheral enhancement, and granulation tissue, which is characterized by a more global increase in signal intensity.

 

3. Cultures

 

a. Sputum specimens collected from subjects with pulmonary disease may reveal acid-fast bacilli (AFB).

 

b. A definitive diagnosis of tuberculosis may require a biopsy of the spinal lesion itself, which should be tested for AFB.

 

c. The detection of other atypical bacteria and fungal species also necessitates special stains and tissue preparations.

 

D. Management

 

1. Pharmacotherapy directed at the causative pathogen is the most effective treatment of granulomatous diseases.

 

a. The standard empirical treatment of tuberculosis consists of isoniazid, rifampin, pyrazinamide, and either streptomycin or ethambutol.

 

i. Patients with active disease usually undergo a minimum of 6 to 12 months of pharmacotherapy, although the final duration of treatment will ultimately be dictated by their subsequent response to these medications.

 

ii. Some of these antimicrobials may be discontinued, depending on the results of culture and sensitivity testing.

 

b. Most fungal species are adequately covered with antifungal agents such as amphotericin B and ketoconazole.

 

c. The medical management of these infections has become more difficult with the emergence of more atypical organisms and worsening drug resistance patterns.

 

2. Surgical treatment of granulomatous infections should be considered for abscesses, significant deformity, or patients who have failed nonsurgical therapies.

 

a. Urgent surgical intervention is rarely required except when there is evidence of a progressive neurologic deficit.

 

b. As with all spinal infections, surgical procedures normally involve a thorough debridement of the lesion, followed by reconstruction of the spinal column as needed.

 

i. Because anterior column support is critical for maintaining normal alignment and limiting the development of kyphosis, the stability of the vertebral column may be restored using a strut graft with or without supplementary internal fixation.

 

ii. Persistent colonization of metal implants is far less common with granulomatous organisms than it is with pyogenic infections.



V. Epidural Infections

A. Incidence and microbiology

 

1. An epidural abscess consists of a focus of infection that is contained within the bony spinal canal but remains extradural.

 

2. These lesions most often extend from adjacent vertebral osteomyelitis or diskitis, but hematogenous seeding and direct inoculation during spinal procedures are also potential mechanisms of infection.

 

3. Epidural abscesses currently account for approximately 7% of all spinal infections, but their incidence has continued to rise because of the growing population of immunocompromised patients and the escalating number of invasive spinal procedures that are being performed.

 

4. Epidural abscesses usually develop in adults 60 years of age or older and affect both sexes equally.

 

5. Most of these infections occur in the thoracic (51%) and lumbar (35%) regions, where they typically involve the posterior epidural space. When these lesions are present in the cervical spine, they are usually located anterior to the dura.

 

6. Although focal infections may be observed, epidural abscesses regularly include three or more motion segments.

 

7. S aureus is the causative agent in more than 60% of these cases; gram-negative rods are responsible for another 20%.

 

B. Clinical presentation

 

1. As with other types of spinal infections, the clinical presentation of epidural infections is highly variable; these lesions are initially misdiagnosed in up to 50% of cases.

 

2. Intractable neck or back pain is the most common symptom. In addition, these patients generally report more constitutional symptoms than do those with vertebral osteomyelitis or diskitis.

 

3. Less virulent organisms may give rise to chronic infections that may not be associated with any obvious clinical signs or symptoms, especially in immunocompromised hosts.

 

4. A high risk of progressive neurologic deficits exists. These deficits can result from direct compression of the neural elements in conjunction with any accompanying ischemic injury brought about by the focus of infection.

 

C. Diagnostic studies

 

1. Laboratory studies—WBC, ESR, CRP

 

2. Imaging modalities

 

a. Plain radiographs and CT scans of the spine will often appear unremarkable unless there is a concurrent vertebral osteomyelitis or diskitis that is sufficiently advanced to produce radiographic abnormalities.

 

b. MRI with gadolinium is the most sensitive and specific imaging technique for identifying epidural abscesses because it facilitates the visualization of the soft tissues and any associated fluid collections (

Figure 3).

 

3. Cultures

 

a. Blood cultures are positive in 60% of these cases.

 

b. A definitive diagnosis is best established by obtaining tissue or fluid directly from the abscess; cultures of these specimens have been shown to have a sensitivity of at least 90%.

 

D. Management

 

1. As with all spinal infections, the primary goals of treatment of epidural infections are the eradication of the infection, preservation or improvement of neurologic function, relief of axial pain, and maintenance of spinal stability.

 

2. Certain neurologically intact patients may be candidates for nonsurgical treatment with antibiotics alone, but this option is generally reserved for patients who are poor surgical candidates. Regardless, any evidence of progressive neurologic decline or lack of response warrants emergent surgical intervention.

 

[Figure 3. Axial T1-weighted post-contrast MRI demonstrates a posterior epidural abscess of the lumbar spine (arrow). The patient was treated with laminectomy and with irrigation and debridement, followed by a course of antibiotics.]

3. With few exceptions, epidural abscesses are treated with surgical decompression of the affected levels, followed by long-term antibiotic therapy.

 

a. The optimal surgical approach is determined by the anatomic location of the infection.

 

i. Because most of these lesions are based posteriorly, a laminectomy is often required to adequately decompress the infection.

 

ii. When an abscess arises secondary to vertebral osteomyelitis, an anterior or circumferential decompression may be necessary.

 

iii. Fusion with or without instrumentation is also indicated if spinal stability is compromised either by the infection itself or as a result of any subsequent decompression or debridement.

 

b. As with other types of spinal infections, the wound may be closed primarily over drains or left open for serial debridements.



Top Testing Facts

1. The most common presenting symptom of a postoperative spinal infection is pain, although its onset may be delayed.

 

2. Most postoperative spinal infections are clinically evident, but laboratory and imaging studies also may be useful for confirming this diagnosis.

 

3. The mainstay of treatment of postoperative infections is surgical irrigation and debridement in conjunction with an appropriate course of antibiotics.

 

4. With hematogenous spinal infections, pathologic organisms usually emanate from the vascular end plates into the relatively avascular disk space before spreading to adjacent vertebral bodies.

 

5. Most patients with diskitis can be treated successfully with immobilization and antibiotics, but surgical intervention is indicated for infections recalcitrant to nonsurgical management or those resulting in any type of neurologic deficit or progressive deformity.

 

6. Granulomatous infections may be caused by tuberculosis, fungal species, or other atypical organisms.

 

7. Because of the indolent nature of granulomatous diseases, patients with these infections may already exhibit significant destruction of the vertebral column and focal kyphosis at the time of the diagnosis.

 

8. Pharmacotherapy directed at the causative pathogen is the most effective treatment of granulomatous diseases.

 

9. Epidural abscesses are associated with a high risk of neurologic compromise secondary to direct compression of the neural elements, as well as an associated ischemic injury caused by the infection itself.

 

10. With few exceptions, epidural infections should be treated surgically with decompression with or without fusion, followed by local wound care and an adequate course of antibiotic therapy.



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