Infections of the Spinal Cord

Epidemiology

The incidence of bacterial meningitis is 2 to 3 per 100,000. It occurs most commonly in newborns, in infants aged 3 to 8 months, and in adults at the second and sixth decades. Spinal meningitis occurs less frequently than intracranial meningitis.

Clinical Presentation

The nature and the course of spinal meningitis vary with the virulence of the infective agent and the response of the host. It may present with flu-like symptoms. The neurologic manifestations are frequently limited to irritability, paresthesias, and sphincter dysfunction. Generally, the cranial meninges are also involved, leading to the typical signs and symptoms of nausea, vomiting, photophobia, inactivity, irritability, somnolence, and meningismus (stiff neck).

Pathophysiology

The infection is transmitted to the spine by hematogenous spread from extraspinal foci of infection, contiguous spread from adjacent spondylodiscitis or spinal epidural abscesses, direct inoculation, and an unexplained source of infection. The course depends on the virulence of the organism, the number of inoculating organisms, and the immunologic status of the patient.

Spinal meningitis is classified in accord with the specific class of infective agent.

Pyogenic leptomeningitis is the most common intradural infection of the spine. In adult patients, Neisseria meningitidis, Staphylococcus aureus , and Streptococcus species are the leading causes of infection. In the newborn, group B Streptococcus , gram-negative bacilli, and Listeria monocytogenes are the main causes of infection.

Tuberculosis caused by Mycobacterium tuberculosis is the most common cause of granulomatous spinal meningitis. Tuberculous leptomeningitis commonly occurs together with tuberculous myelitis and tuberculous radiculitis. Frequently, intradural spinal tuberculosis presents with, or shortly after, intracranial tuberculosis. Less often, spinal tuberculous meningitis may be secondary to tuberculous spondylitis (uncommon) or may present as the sole primary infection (rare). Tuberculomas may form on the inner aspect of the dura. Tuberculomas tend to excavate into the cord, so leptomeningeal tuberculosis may be difficult to distinguish from dural disease or intramedullary tuberculoma.

Pathologically, the meninges, cord, and nerve roots demonstrate congestion and exudates. As the exudate organizes, the coated nerve roots adhere to each other. The subarachnoid space narrows, becomes loculated, or is focally obliterated, leading to compartmentalization of the thecal sac, secondary syringomyelia, and/or hydrocephalus.

Brucellosis is another cause of granulomatous spinal infection, with 2% to 30% spinal involvement. Inflammatory vasculitis may develop during the course of the disease and permit the organism to pass into the cerebrospinal fluid (CSF). The spondylitis and meningitis may occur simultaneously during the acute stage, with the symptoms of one masking those of the other.

Spinal cryptococcosis usually manifests as an infiltrating granulomatous mass that expands within the extra-dural or intradural extramedullary space to compress the spinal cord. Cryptococcal meningitis is frequently seen in AIDS patients, in whom the meningeal inflammatory reaction may be minimal and meningeal enhancement is uncommon.

Cysticercosis is the most common parasitic infection of the spinal leptomeninges. It is thought to result from direct CSF seeding of the larvae throughout the subarachnoid space from a source in the cerebrum. Intramedullary cysticercosis is much less common than subarachnoid infestation.

Viral spinal meningitis is extremely rare.

Pathology

Pyogenic and coccidioidal meningitides form a purulent exudate in the subarachnoid space. Fungal meningitides form small nodules (noncaseous granulomas) within thick, opacified meninges.

Pyogenic spinal meningitis induces an inflammatory infiltrate, first of polymorphonuclear leukocytes, then lymphocytes. These purulent exudates distend the subarachnoid space and penetrate into the perivascular spaces of the spinal cord. Bacteria then stimulate the production of cytokines and other inflammatory products.

Tuberculous meningitis causes an infiltration of lymphocytes and mononuclear cells and leads to formation of granulomas (tubercles). The tubercles show a central area of caseous necrosis, an intermediate layer of epithelioid cells, and a peripheral ring of lymphocytes. Perivascular inflammation may lead to vasculitis. The numbers of acid-fast bacilli vary, and the bacteria may become undetectable after treatment has begun.

Coccidioidal meningitis causes a meningeal reaction similar to tuberculosis, with organisms surrounded by epithelioid cells, giant cells, lymphocytes, and plasma cells. There may be small abscesses with noncaseous necrosis and vasculitis. In the tissues, the fungi form large spherules.

Imaging

Imaging studies may be positive or negative, so negative imaging does not exclude meningitis. The diagnosis of meningitis usually depends on CSF analysis and clinical symptoms. Unfortunately, MRI has low specificity for differentiating infectious meningitis from a leptomeningeal neoplastic process.

MRI

Bacterial meningitis may extend to involve all segments of the spine. Precontrast T1-weighted (T1W) imaging may be normal or reveal nonspecific findings such as increased signal intensity of the CSF, irregular cord outline, and thickened dura. MRI commonly displays clumped nerve roots, an indistinct interface between the spinal cord and CSF, adhesions of the membranes, focal or diffuse obliteration of the subarachnoid space, and (multi)loculation of the subarachnoid space ( Figs. 19-1 to 19-3 ). T2-weighted (T2W) imaging has limited use, since the high signal intensity of CSF may obscure the meningeal structures, but it may show focal or diffuse cord enlargement. Postcontrast T1W imaging typically shows an inflamed pia-arachnoid, may show inflamed dura and nerve sheathes, and may display abnormalities within the spinal cord. The enhancement pattern may be linear (most common), nodular, or diffuse. No significant correlation has been found between the pattern or severity of enhancement and either the severity of symptoms or the specific infective agent. In acute meningitis, precontrast MRI may appear entirely normal whereas postcontrast MRI demonstrates the lesions in detail. In chronic healed meningitis, precontrast MRI usually shows the findings whereas postcontrast MRI detects no enhancement and adds no further information.

The MRI features of tuberculous leptomeningitis closely resemble those of pyogenic meningitis (see Fig. 19-2 ).

Fungal spinal meningitis is very uncommon. Coccidioidal meningitis preferentially involves the basal cisterns and upper cervical subarachnoid space (see Fig. 19-3 ), leading to thickened, inflamed cervical meninges, possible cord infarction, and hydrocephalus. Candidiasis and histoplasmosis may also present as leptomeningitis.

Spinal cryptococcosis may manifest as a granulomatous mass that infiltrates the extradural or intradural extramedullary spaces and compresses the spinal cord. In AIDS patients and other immunocompromised hosts, cryptococcal meningitis may cause only minimal meningeal inflammation and minimal meningeal enhancement.

Viral meningitis shows no specific imaging findings. In AIDS patients, cytomegalovirus (CMV) may cause lumbosacral polyradiculopathy with meningeal enhancement on postcontrast MRI.

Cysticercosis of the leptomeninges and CSF exhibits discrete cystic lesions with signal very close to that of CSF. These cysts are particularly well shown by T2W MRI sequences with very high spatial resolution (e.g., constructive interference in steady-state sequences [CISS]). The rims of the cysts may show ring-like enhancement on contrast-enhanced MRI ( Fig. 19-4 ). Racemose forms, with or without septal-capsular enhancement, are common in the CSF. Subarachnoid cysticercosis may cause homogeneous arachnoiditis that extends over the surfaces of the spinal cord in a sheet. Congenital lesions such as arachnoid and dermoid cysts should be considered in the differential diagnosis of the spinal cysticercosis.

Hydatidosis of the CSF may also manifest as solitary or multiple cysts with signal similar to CSF. The cyst walls rarely enhance. Multiple daughter cysts and sterile cysts may be found together ( Fig. 19-5 ).

KEY POINTS: SPINAL MENINGITIS

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  The MRI findings in patients with intradural extramedullary infections are quite similar to those of leptomeningeal tumor spread. Contrast agent administration increases the sensitivity but does not affect the specificity.

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  Fine linear enhancement of the cord surface also may be detected in sarcoidosis. Concurrent systemic manifestations and elevated angiotensin-converting enzyme level help in the diagnosis.

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  Arachnoiditis is another entity that must be taken into account in the differential diagnosis of enhancing nerve roots and meninges. Arachnoiditis is the inflammatory process of the spinal leptomeninges, seen as intradural clumping of the spinal nerve roots. Previous myelography, surgery, trauma, and subarachnoid hemorrhage may also be the predisposing factors of arachnoiditis. The exudative material around the nerve roots eventually causes adhesions, finally resulting in the obliteration of nerve root sheets and clumping of the roots. Early thickening of the nerve roots, later clumping, and dural adhesions appear with blunting of axillary root sleeves. Depending on the severity of the adhesions, stenosis or complete block of the spinal canal and also compartmentalization of the thecal sac with resultant mass effect on the cord may be seen. T2W imaging in the axial plane can display centrally clumped or peripheral adherent roots. Contrast enhancement of nerve roots is an infrequent finding that is often uncertain and useless for the correct diagnosis.

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  A rare chronic inflammatory hypertrophy of the dura, pachymeningitis hypertrophica of unknown origin, shows similar findings of meningitis.

Epidemiology

Infection of the spinal cord is a rare clinical entity. The lesion can start as spinal venous thrombosis with resultant ischemic infarction that is later complicated by bacterial involvement. Postvaccinal occurrence or preceding infection is common in children whereas the acquired immunodeficiency syndrome (AIDS) is one of the major causes in adults. Often the exact etiology of myelitis is unknown and bacteria, fungi, viruses, and parasites may be the cause.

Pyogenic infection occurs most often. Similarly, the incidence of granulomatous infection of the cord is relatively low with tuberculosis as the most common cause. Children and adults (mean age: 34) and males are frequently affected. No specific ethnicity is defined.

Clinical Presentation

Spinal cord abscess is difficult to diagnose clinically, because the major symptoms have an insidious onset. Acute infection of the spinal cord may first present as mild back pain, then advance to paresthesias with a rising level of sensory impairment, and culminate in complete transverse myelitis. The most common neurologic deficits are sensory, motor, and sphincter dysfunction. Subacute or chronic myelitis tends to mimic a neoplasm. There may be recurrent bouts of radiculomyelitis with intervals of normal neurologic function. Rapid development of flaccid paraparesis may eventuate in permanent diminution or loss of tone with reduced deep tendon reflexes. Overall, sensory impairment is less prominent than motor impairment. Proprioception and vibratory sensation are usually preserved with no spasticity.

Ultimately, paralysis develops in 58% of cases, usually as a late finding after irreversible neurologic impairment has already occurred.

Pathophysiology

In adults and children, bacterial myelitis and cord abscess usually reflect secondary hematogenous spread from a primary cardiopulmonary focus. Up to 40% of pediatric cases may be associated with spinal dysraphism or other congenital spinal malformations. Staphylococcus aureus, S. epidermidis, Bacteroides, Haemophilus species, and Listeria monocytogenes are the major organisms causing bacterial cord abscesses.

Tuberculous meningitis may lead to syringohydromyelia. This generally results from inflammatory edema and ischemia in the early period of the disease. Focal scarring in the subarachnoid space impedes free circulation of CSF, thus forcing CSF into the central canal of the spinal cord via Virchow-Robin spaces, finally giving rise to focal cystic dilatations that will eventually coalesce to form a syrinx. Later in the course of tuberculous meningitis, chronic arachnoiditis may loculate the subarachnoid space.

Pathology

The spinal cord is swollen and may discharge pus after myelotomy. The dura is tense owing to the underlying swollen cord.

Irrespective of the agent, the infected cord demonstrates edema, perivascular inflammation, and variable degrees of vascular compromise, which may lead to vascular thrombosis, cord ischemia, and infarction. Histopathology shows inflammation with polymorphonuclear leukocytes, monocytes, squamous epithelial cells, and histiocytes.

Tuberculomas are composed of a caseous center and a surrounding granulomatous reaction that includes giant cells, lymphocytes, and fibrosis. They may spontaneously become cystic, fibrous, or calcified.

Imaging

Obtaining the correct diagnosis is extremely difficult on imaging criteria alone. History, laboratory findings, and histology are usually required for understanding the lesions shown by imaging. It is important to note that an infected syrinx may simulate an intramedullary abscess, particularly in children with congenital spinal malformations.