The spinal cord is vulnerable to injury arising in the structures that surround it. More than in any other part of the nervous system, pathologic lesions impinging on the spinal cord often originate in the membranes or vertebral column that surround it. The neurologic clinician must, therefore, be very familiar with these structures and their relationship to the spinal cord.
Three membranes surround the spinal cord: The outermost is the dura mater (dura), the next is the arachnoid, and the innermost is the pia mater (pia) (Figs 6–1 and 6–2). The dura is also called the pachymeninx, and the arachnoid and pia are called the leptomeninges.
FIGURE 6–1 Schematic illustration of the relationships between the spinal cord, spinal nerves, and vertebral column (lateral view), showing the termination of the dura (dura mater spinalis) and its continuation as the filum terminale externum. (Compare with Fig 5–4.)
FIGURE 6–2 Drawing of a horizontal section through a vertebra and the spinal cord, meninges, and roots. Veins (not labeled) are shown in cross section. The vertebra and its contents are positioned as they customarily would be with CT and MR imaging procedures.
The dura mater is a tough, fibrous sheath that extends from the foramen magnum to the level of the second sacral vertebra, where it ends as a blind sac (see Fig 6–1). The dura of the spinal cord is continuous with the cranial dura. The epidural, or extradural, space separates the dura from the bony vertebral column; it contains loose areolar tissue and a venous plexus. The subdural space is a narrow space between the dura and the underlying arachnoid.
The arachnoid is a thin, transparent sheath separated from the underlying pia by the subarachnoid space, which contains cerebrospinal fluid (CSF).
The pia mater closely surrounds the spinal cord and sends septa into its substance. The pia also contributes to the formation of the filum terminale internum, a whitish fibrous filament that extends from the conus medullaris to the tip of the dural sac. The filum is surrounded by the cauda equina, and both are bathed in CSF. Its extradural continuation, the filum terminale externum, attaches at the tip of the dural sac and extends to the coccyx. The filum terminale stabilizes the cord and dura lengthwise.
The dentate ligament is a long flange of whitish, mostly pial tissue that runs along both lateral margins of the spinal cord between the dorsal and ventral rootlets (see Fig 6–2). Its medial edge is continuous with the pia at the side of the spinal cord, and its lateral edge pierces the arachnoid at intervals (21 on each side) to attach to the inside of the dura. The dentate ligament helps to stabilize the cord from side to side.
There are eight pairs of cervical nerves. The first seven emerge above each respective cervical vertebra; the eighth (C8) lies below vertebra C7 and above the first thoracic vertebra (see Fig 6–1). Each of the other spinal nerves (T1–12, L1–5, S1–5, and normally two coccygeal nerves, Co1 and Co2) emerges from the intervertebral foramen below the respective vertebra. The cauda equina is made up of dorsal and ventral roots that arise from lumbar and sacral segments of the cord. These roots sweep downward within the dural sac, below the termination of the cord, and give the appearance of a horse’s tail.
Investment of Spinal Nerves
As the ventral and dorsal roots (on each side) at each segmental level converge to become a spinal nerve, they are enclosed in sleeves of arachnoidal and dural tissue (see Fig 6–2). The dorsal root sleeve contains the dorsal root ganglion near the point at which both sleeves merge to become the connective tissue sheath (perineurium) of a spinal nerve. The dorsal root (with its ganglion) and the ventral root of the nerve (surrounded by fat and blood vessels) course through the intervertebral foramen, except in the sacral segments where the dorsal root ganglia lie within the sacrum itself.
SPINAL CORD CIRCULATION
A. Anterior Spinal Artery
This artery is formed by the midline union of paired branches of the vertebral arteries (Figs 6–4 and 6–5). It descends along the ventral surface of the cervical spinal cord, narrowing somewhat near T4.
FIGURE 6–3 Epidural tumor in Hodgkin’s disease, showing compression of the thoracic spinal cord (Weil stain). The illustration is positioned to conform with customary CT and MR imaging procedures.
FIGURE 6–4 Cross section of the cervical spinal cord. The diagram shows the anterior and posterior spinal arteries with their branches and territories. There are numerous variations in the vascular supply.
FIGURE 6–5 Vascularization of the spinal cord (ventral view).
Abnormal masses (tumors, infections, hematomas) may occur in any location in or around the spinal cord. Tumors (eg, meningiomas, neurofibromas) are often located in the intradural extramedullary compartment. Epidural masses, including bone tumors or metastases, can displace the dura locally and compress the spinal cord (Fig 6–3). Spinal cord compression may progress rapidly and can result in paraplegia or quadriplegia. If diagnosed early, however, it may be readily treated. Thus, suspected spinal cord compression requires urgent workup. Intradural extramedullary masses, most often in the subarachnoid space, may push the spinal cord away from the lesion and may even compress the cord against the dura, epidural space, and vertebra. Intramedullary, and therefore intradural, masses expand the spinal cord itself (see Fig 5–24). An epidural mass is usually the least difficult to remove neurosurgically. Clinical Illustration 6–1 describes a patient with an epidural abscess.
B. Anterior Medial Spinal Artery
This artery is the prolongation of the anterior spinal artery below T4.
C. Posterolateral Spinal Arteries
These arteries arise from the vertebral arteries and course downward to the lower cervical and upper thoracic segments.
D. Radicular Arteries
Some (but not all) intercostal arteries from the aorta supply segmental (radicular) branches to the spinal cord from T1 to L1. The largest of these branches, the great ventral radicular artery, also known as the artery of Adamkiewicz, enters the spinal cord between segments T8 and L4 (see Fig 6–5). This artery usually arises on the left and, in most individuals, supplies most of the arterial blood supply for the lower half of the spinal cord. Although occlusion in this artery is rare, it results in major neurologic deficits (eg, paraplegia, loss of sensation in the legs, urinary incontinence).
E. Posterior Spinal Arteries
These paired arteries are much smaller than the single large anterior spinal artery; they branch at various levels to form the posterolateral arterial plexus. The posterior spinal arteries supply the dorsal white columns and the posterior portion of the dorsal gray columns.
F. Sulcal Arteries
In each segment, the branches of the radicular arteries that enter the intervertebral foramens accompany the dorsal and ventral nerve roots. These branches unite directly with the posterior and anterior spinal arteries to form an irregular ring of arteries (an arterial corona) with vertical connections. Sulcal arteries branch from the coronal arteries at most levels. Anterior sulcal arteries arise at various levels along the cervical and thoracic cord within the ventral sulcus (see Fig 6–4); they supply the ventral and lateral columns on either side of the spinal cord.
An irregular external venous plexus lies in the epidural space; it communicates with segmental veins, basivertebral veins from the vertebral column, the basilar plexus in the head, and, by way of the pedicular veins, a smaller internal venous plexus that lies in the subarachnoid space. All venous drainage is ultimately into the venae cavae.
THE VERTEBRAL COLUMN
The vertebral column consists of 33 vertebrae joined by ligaments and cartilage. The upper 24 vertebrae are separate and movable, but the lower 9 are fixed: 5 are fused to form the sacrum, and the last 4 are usually fused to form the coccyx. The vertebral column consists of 7 cervical (C1–7), 12 thoracic (T1–12), 5 lumbar (L1–5), 5 sacral (S1–5), and 4 coccygeal (Co1–4) vertebrae. In some individuals, vertebra L5 is partly or completely fused with the sacrum.
Figure 6–1 illustrates the relation of the spinal cord itself to the surrounding vertebrae. Recall that the spinal cord tapers and ends at the L1 or L2 level of the vertebral column. Below that level, the dural sac within the vertebral column contains the cauda equina.
CLINICAL ILLUSTRATION 6–1
A 61-year-old former house painter with a history of alcoholism was admitted to the medical service after being found in a hotel room in a confused state that was attributed to alcohol withdrawal syndrome. The patient did not complain of pain but said he was weak and could not get out of bed. He had a fever. The intern’s initial neurologic examination did not reveal any focal neurologic signs. The lumbar puncture yielded CSF containing a moderate number of white blood cells and protein of about 100 mg/dL (elevated) with normal CSF glucose. Despite treatment with antibiotics, the patient did not improve, and neurologic consultation was obtained.
On examination, the patient was confused and uncooperative. He stated he was weak and could not walk. Motor examination revealed flaccid paraparesis. Deep tendon reflexes were absent in the legs, and the plantar responses were extensor. The patient was not cooperative for vibratory or position sense testing. He denied feeling a pin as painful over any part of the body; however, when the examiner watched for a facial wince on pinprick, a sensory level T5–6 could be demonstrated. On gentle percussion of the spinal column, there was tenderness at T9–10.
Imaging of the spinal column revealed an epidural mass. The patient was taken to surgery, and an epidural abscess, extending over five vertebral segments, was found. The spinal cord under the abscess was compressed and pale, probably as a result of ischemia (vasospasm leading to inadequate perfusion with blood).
The motor status of this patient suggested a spinal cord lesion, which was confirmed on sensory examination. Percussion tenderness over the spine, which is often seen with epidural abscesses or tumors, provided additional evidence for disease of the spinal column. Epidural spinal cord compression is especially common in the context of neoplasms (eg, breast, prostate) that metastasize to the spine. The possibility of spinal cord compression should be considered, and the vertebral column gently percussed, in any patient with a known malignancy and recent-onset or worsening back pain. As noted earlier, epidural spinal cord compression can be effectively treated in many patients if recognized early in its course. However, if it is not diagnosed and rapidly treated, it can progress to cause irreversible paraplegia or quadriplegia. Any patient with suspected spinal cord compression must be evaluated on an urgent basis.
The vertebral column is slightly S-shaped when seen from the side (Fig 6–6). The cervical spine is ventrally convex, the thoracic spine ventrally concave, and the lumbar spine ventrally convex, with its curve ending at the lumbosacral angle. Ventral convexity is sometimes referred to as normal lordosis and dorsal convexity as normal kyphosis. The pelvic curve (sacrum plus coccyx) is concave downward and ventrally from the lumbosacral angle to the tip of the coccyx. The spinal column in an adult is often slightly twisted along its long axis; this is called normal scoliosis.
FIGURE 6–6 The vertebral column.
Most vertebrae share a common architectural plan. A typical vertebra (not C1, however) has a body and a vertebral (neural) arch that together surround the vertebral (spinal) canal (Fig 6–7). The neural arch is composed of a pedicleon each side supporting a lamina that extends posteriorly to the spinous process (spine). The pedicle has both superior and inferior notches that form the intervertebral foramen. Each vertebra has lateral transverse processes and superior and inferior articular processes with facets. The ventral portion of the neural arch is formed by the ventral body.
FIGURE 6–7 Computed tomography image of a horizontal section at midlevel of vertebra L4.
Articulation of a pair of vertebrae is body to body, with an intervening intervertebral disk and at the superior and inferior articular facets on both sides. The intervertebral disks help absorb stress and strain within the vertebral column.
CLINICAL ILLUSTRATION 6–2
A 74-year-old male, with a history of prostate cancer, complained of 3 weeks of lower back pain. He noted that he felt tingling in his feet and legs, extending all the way up to the waist. He did not complain of weakness, but admitted that he had fallen several times while walking stairs.
Physical examination revealed a sensory level (loss of pinprick and light touch sensation) in both legs, extending to just below the umbilicus. Vibration and position sense were present but impaired in the legs. There was mild (4+/5) weakness of the legs. Deep tendon reflexes (knee jerks, Achilles reflexes) were hyperactive in the legs, and the plantar response we extensor bilaterally.
Imaging revealed a tumor, most likely metastatic from the patient’s prostate cancer, that had infiltrated the vertebral body at T1, which was now compressing the spinal cord. The patient was immediately referred for treatment.
This case illustrates several important points: First, back pain and neurological complaints in the legs must always trigger consideration of spinal cord compression. Second, while sensory symptoms (such as numbness or tingling or pain) often occur early, patients may not complain of motor loss early in the course of disease—this patient did not explicitly complain of weakness, although he admitted to falling and was found on examination to have mild weakness of the legs. Third, because the spinal cord is shorter than the vertebral column, there is not perfect alignment between the segment of the bony vertebral column that is involved, and the affected segment of the spinal cord. In this case, a lesion of the T1 vertebra compressed the T4 spinal cord. The anatomic relationship between the segments of the bony vertebral column and the spinal cord within it are shown in Figure 5–3 and Table 5–1.
Each disk (Fig 6–8) contains a core of primitive gelatinous large-celled tissue, the nucleus pulposus, surrounded by a thick annulus fibrosus. The disks are attached to the hyaline cartilage, which covers the superior and inferior surfaces of the vertebral bodies. The water content of the disks decreases with age, resulting in a loss of height in older individuals.
FIGURE 6–8 Computed tomography image of a horizontal section through L4 at the level of the L3–4 intervertebral disk. (Reproduced, with permission, from deGroot J: Correlative Neuroanatomy of Computed Tomography and Magnetic Resonance Imaging. 21st ed. Appleton & Lange, 1991.)
The spinal cord in adults ends at the level of L1–2. Thus, a spinal (lumbar) puncture can be performed below that level—and above the sacrum—without injuring the cord. Indications and contraindications for lumbar puncture are discussed in Chapter 24. Lumbar puncture and careful CSF analysis should be carried out as quickly as possible (although increased intracranial pressure or intracranial mass should first be ruled out; see Chapter 24) in any patient in whom meningitis is a possibility, since a delay in treatment can reduce the likelihood of good outcome.
Lumbar puncture is usually performed with the patient in the lateral decubitus position with legs drawn up (Fig 6–9); in this position, the manometric pressure of CSF is normally 70–200 mm of water (average, 125 mm). If the puncture is done with the patient sitting upright, the fluid in the manometer normally rises to about the level of the midcervical spine (Fig 6–10). Coughing, sneezing, or straining usually causes a prompt rise in pressure from the congestion of the spinal veins and the resultant increased pressure on the contents of the subarachnoid and epidural spaces. The pressure subsequently falls to its previous level.
FIGURE 6–9 Decubitus position for lumbar puncture. (Reproduced, with permission, from Krupp MA et al: Physician’s Handbook. 21st ed. Appleton & Lange, 1985.)
FIGURE 6–10 Lumbar puncture site with the patient in sitting position. The approach to the sacral hiatus for saddle-back anesthesia is also indicated.
After the initial pressure has been determined, three or four samples of 2–3 mL each are withdrawn into sterile tubes for laboratory examination. Routine examination usually includes cell counts and measurement of total protein. Cultures and special tests, such as those for sugar and chlorides, are done when indicated. The pressure is also routinely measured after the fluid is removed.
Herniated nucleus pulposus (also termed a ruptured or herniated disk) may be asymptomatic or can compress a neighboring spinal root (or, less commonly, may compress the spinal cord). These effects tend to occur most commonly at lower cervical and lumbar or upper sacral levels. When root compression occurs at lumbosacral levels, it can cause sciatica. Note that, as a result of the anatomic relation between the spinal roots and the vertebral column (see Fig 6–1), a herniation of the L4–5 disk will tend to compress the L5 root. The symptoms of nerve root compression may include pain, sensory loss (in an appropriate dermatomal pattern), weakness (of a lower-motor-neuron type in muscles innervated by the root in question), and diminution or loss of deep tendon reflexes mediated by the compressed root. Nerve root compression resulting from herniated disks often responds to conservative therapy. In certain cases, surgery may be needed.
Spina bifida results from the failure of the vertebral canal to close normally because of a defect in vertebral development. Associated abnormalities may be caused by defective development of the spinal cord, brain stem, cerebrum, or cerebellum. Other developmental defects, such as meningoceles, meningomyeloceles, congenital tumors, or hydrocephalus, may also occur.
There are two main types of spina bifida: spina bifida occulta, involving a simple defect in the closure of the vertebra, and spina bifida with meningocele or meningomyelocele, involving sac-like protrusions of the overlying meninges and skin that may contain portions of the spinal cord or nerve roots. Simple failure of closure of one or more vertebral arches in the lumbosacral region (spina bifida occulta) is a common finding on routine examination of the spine by radiography or at autopsy. There may be associated abnormalities, such as fat deposits, hypertrichosis (excessive hair) over the affected area, and dimpling of the overlying skin. Symptoms may be caused by intraspinal lipomas, adhesions, bony spicules, or maldevelopment of the spinal cord.
Meningocele is herniation of the meningeal membranes through the vertebral defect. It usually causes a soft, cystic, translucent tumor to appear low in the midline of the back.
In meningomyelocele, nerve roots and the spinal cord protrude through the vertebral defect and usually adhere to the inner wall of the meningeal sac. If the meningomyelocele is high in the vertebral column, the clinical picture may resemble that of complete or incomplete transection of the cord.
Some patients may have a mild or severe headache after the procedure. The headache may be caused by the loss of fluid or leakage of fluid through the puncture site; it is characteristically relieved by lying down and exacerbated by raising the head. Injection of the patient’s own blood in the epidural space at the puncture site (blood patch) may give partial or complete relief. Serious complications, such as infection, epidural hematoma, uncal herniation, or cerebellar tonsil prolapse, are rare.
CSF examination is discussed in Chapter 24.
IMAGING OF THE SPINE AND SPINAL CORD
Imaging methods have great value in determining the precise site and extent of the involvement of pathologic processes in the spine and neighboring structures. (The methods themselves are discussed in detail in Chapter 22.)
Because roentgenograms (plain films) demonstrate the presence of calcium, various projections (anteroposterior, lateral, and oblique) of the affected area show the skeletal components of the spine and foramens (Figs 6–11 and 6–12). Fractures or erosions of the vertebral column’s bony elements are often easily seen, but the films provide little or no information about the spinal cord or other soft tissues.
FIGURE 6–11 Roentgenograms through the neck (lateral view).
FIGURE 6–12 Roentgenogram of lumbar vertebrae (left lateral view). (Compare with Fig 6–6, right side.)
A 49-year-old dock worker was reasonably healthy until a heavy piece of equipment fell high on his back, knocking him down but not rendering him unconscious. He was unable to move his arms and legs and complained of shooting pains in both arms and tingling in his right side below the axilla.
In the emergency room, flaccid left hemiplegia, right triceps weakness, and left extensor plantar response were noted. Pain sensation was lost on the right side from the shoulder down, including the axilla and hand but not the thumb.
What is the tentative diagnosis? What imaging procedure would you request to localize the lesion?
The patient underwent neck surgery. A few days postoperatively, he regained strength in his right arm and left leg, but the left arm continued to be weak. Pain sensation was not tested at this time.
Neurologic examination 3 weeks later disclosed fasciculations in the left deltoid, marked weakness in the left arm (more pronounced distally), mild spasticity of the left elbow, and minimal spasticity in the left knee on passive motion. Some deep tendon reflexes—all on the left side—were increased: biceps, triceps, quadriceps, and Achilles tendon. There was a left extensor plantar response. Position and vibration senses were intact, and pain sensation was absent on the right half of the body up to the level of the clavicle.
What is the sequence of pathologic events? Where is the lesion, and which neural structures are involved? Which syndrome is incompletely represented in this case? Which components of the complete syndrome are not present?
Two months before presentation, a 40-year-old camp counselor sustained a minor injury while playing baseball, feeling a snap and a stab of pain to his lower back when he slid feet first into third base. Shortly after this, he noticed dull pains in the same region. Several weeks later, he began to feel electric shock-like pain shooting down the back of his right leg to his toes on the right side. The pain seemed to start in the right buttock and could be precipitated by coughing, sneezing, straining, or bending backward. The patient had also noticed occasional tingling of his right calf and some spasms of the back and right leg muscles.
Neurologic examination showed no impairment of muscle strength, and normal deep tendon reflexes in the upper extremities. The Achilles tendon reflex was absent on the right and normal on the left, and there were flexor plantar responses on both sides. All sensory modalities were intact. There was spasm of the right paravertebral muscles and local tenderness on palpation of the spine at L5–S1 and at the sciatic nerve in the right buttock. Straight leg raising was limited to 308 on the right but was normal on the left. Radiographs of the lumbar spine were normal. MRI revealed a lesion. The patient was treated with nonsteroidal anti-inflammatory medications, together with bed rest. His pain resolved.
What is the most likely diagnosis?
Cases are discussed further in Chapter 25. Questions and answers pertaining to Chapters 5 and 6 are found in Appendix D.
Computed Tomography (CT)
Information about the position, shape, and size of all the elements of the spine, cord, roots, ligaments, and surrounding soft tissue can be obtained by a series of transverse (axial) CT images (or scans) (see Fig 6–7). CT myelography is done after contrast medium is injected into the subarachnoid space (Figs 6–13 and 6-14).
FIGURE 6–13 Computed tomography image of a horizontal section at the level of vertebra T12 in a 3-year-old child. The subarachnoid space was injected with contrast medium.
FIGURE 6–14 Reformatted CT image of midsagittal section of the lumbar spine of a patient who fell from a third-floor window. There is a compression fracture in the body of L1, and the lower cord is compressed between bony elements of L1 (arrows). The subarachnoid space was injected with contrast medium. (Reproduced, with permission, from Federle MP, Brant-Zawadski M (editors): Computed Tomography in the Evaluation of Trauma. 21st ed. Lippincott Williams & Wilkins, 1986.)
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging can be used in any plane. It has been used, especially with sagittal images, to demonstrate the anatomy or pathology of the spinal cord and surrounding spaces and structures (Figs 6–15 to 6–18). Because the calcium of bone does not yield a magnetic resonance signal, MRI is especially useful in showing suspected lesions of the soft tissues in and around the vertebral column (Figs 6–16and 6–19).
FIGURE 6–15 Magnetic resonance image of a coronal section through the body and (curved) lumbar spine. (Reproduced, with permission, from deGroot J: Correlative Neuroanatomy of Computed Tomography and Magnetic Resonance Imaging. 21st ed. Appleton & Lange, 1991.)
FIGURE 6–16 Magnetic resonance image of a coronal section through the neck at the level of the cervical vertebrae. Because of the curvature of the neck, only five vertebral bodies are seen in this plane. (Reproduced, with permission, from Mills CM, deGroot J, Posin J: Magnetic Resonance Imaging Atlas of the Head, Neck, and Spine. Lea & Febiger, 1988.)
FIGURE 6–17 Magnetic resonance image of a midsagittal section through the lower neck and upper thorax of a patient with AIDS. Multiple masses are seen in the vertebral bodies at several levels (arrows): Pathologic examination showed these to be malignant lymphomas.
FIGURE 6–18 Magnetic resonance image of a sagittal section of the lumbosacral spine. The arrow heads point to an intervertebral disc herniation at the L3–L4 level. (Reproduced, with permission, from Aminoff MJ, Greenberg DA, Simon RP: Clinical Neurology. 6th ed. McGraw-Hill, 2005.)
FIGURE 6–19 Magnetic resonance image of a midsagittal section through the lumbosacral spine. The mass visible in the body of L4 represents a metastasis of a colon carcinoma (arrow).
Byrne T, Benzel E, Waxman SG: Diseases of the Spine and Spinal Cord. Oxford University Press, 2000.
Cervical Spine Research Society: The Cervical Spine. 2nd ed. JB Lippincott, 1989.
Crock HV, Yoshizawa H: The Blood Supply of the Vertebral Column and Spinal Cord in Man. Springer-Verlag, 1977.
Newton TH, Potts DG (editors): Computed Tomography of the Spine and Spinal Cord. Clavadel Press, 1983.
Norman D, Kjos BO: MR of the spine. In: Magnetic Resonance Imaging of the Central Nervous System. Raven, 1987.
Rothman RH, Simeone FA: The Spine. WB Saunders, 1975.
White AA, Paujabi MM: Clinical Biomechanics of the Spine. JB Lippincott, 1978.
BOX 6-1 Essentials for the Clinical Neuroanatomist
After reading and digesting this chapter, you should know and understand:
• The structure of the meninges (pia, arachnoid, dura)
• Principles of spinal cord circulation
• Anatomy of the spine, including overall structure of the vertebral column (Fig 6–6) and the structure of the vertebrae (Figs 6–7 and 6–8)
• The principles underlying lumbar puncture
• Principles of imaging of the spine and spinal cord