Clinical manifestations

Presentation and course

The presenting features of spinal subdural empyema are fever, back or neck pain, and rapidly developing signs of spinal cord compression (01; 21; 13; 32; 45). Radicular pain is frequently present and is often referable to multiple dermatomal levels (01; 21; 13). Systemic symptoms of fever and malaise preceding spinal cord involvement may be present for up to 4 weeks (01; 21; 13). In occasional patients, symptoms may develop over a period of 1 to 4 weeks (42; 19; 38), and spinal subdural empyema may rarely present as a chronic mass lesion (33; 11). Typically, however, spinal cord signs and symptoms progress rapidly once the subdural space is involved, resulting in severe cord injury within 48 to 72 hours after symptoms begin (01; 21; 13; 32). In some patients, the clinical course of subdural empyema may be so rapid as to suggest cord infarction. Characteristic initial findings include back pain and nuchal rigidity, so the initial presentation may be that of acute bacterial meningitis (01; 21; 13). Absence of spine tenderness to percussion has been suggested as a means to differentiate spinal subdural empyema from spinal epidural abscess, but severe back pain, including focal pain to percussion, may be present in both conditions (13; 32). Rarely, spinal subdural empyema may be chronic, and in this setting, the presentation may be suggestive of spinal cord or parameningeal neoplasm (33).

Prognosis and complications

Prognosis in spinal subdural empyema depends heavily on the duration of subdural infection and the severity of the neurologic deficit at the time of presentation; for this reason, immediate diagnosis and institution of therapy are essential if severe neurologic injury is to be avoided. Abbott, in his 1953 review of the literature, found that seven of 20 patients with spinal subdural empyema had a fatal outcome, 10 had partial recovery with residual neurologic findings, and three had complete recovery (01). In Fraser's 1972 study, five of 10 patients died and only two had full neurologic recovery (21). Death from spinal subdural empyema still occurs, however, as does severe neurologic disability. In Brock and Bleck's series of 17 patients, two patients died (one secondary to problems not directly related to the empyema), seven were left with moderate or severe neurologic deficits, four were left with mild deficits or back pain, and four patients achieved complete recovery (13). Levy's review indicates an overall mortality of 20% (32).

Clinical vignette

A 10-year-old girl was in excellent health until the day of her admission, when she developed a fever, severe back and neck pain, and confusion. Examination by her pediatrician revealed her to be febrile to 40°C with flat discs, but with severe nuchal rigidity, Kernig and Brudzinski signs, and bilateral Babinski signs. Lumbar puncture at L4 to L5 revealed frank pus. The patient was placed on intravenous ampicillin and transferred for evaluation. On examination, the patient was comatose with severe nuchal rigidity. General physical examination was otherwise unremarkable, and optic discs were flat. The patient had periodic respirations with increasingly longer pauses during the initial examination. Cranial nerves, including pupillary responses, oculocephalic reflexes, corneal reflexes, and gag reflex were intact. Cold water calorics produced conjugate ocular deviation without nystagmus. Motor examination revealed flaccid lower extremities. The patient did not withdraw to pinprick or deep pain. Reflexes were 1+ in the left biceps, triceps, and brachioradialis, and absent in the right upper extremity and in both legs. Babinski sign was present bilaterally. Myelography revealed multiple indentations of the dye column, consistent with subdural pus, extending from S1 to C3. While being taken to surgery, the patient suffered a cardiorespiratory arrest and could not be resuscitated. Autopsy confirmed the presence of a subdural empyema extending from the lower lumbar spine to the foramen magnum.

Material from the initial lumbar puncture and from autopsy grew out Staphylococcus aureus.

Biological basis

Etiology and pathogenesis

Staphylococcus aureus, which may be either methicillin-sensitive or methicillin-resistant, is isolated in approximately 50% of cases of spinal subdural empyema (01; 21; 13; 49). Aerobic and facultative streptococci, including Streptococcus milleri, are found in 18% of cases (01; 21; 22; 13; 32). Staphylococcus epidermidis, Streptococcus pneumoniae, Streptococcus intermedius, Bacteroides, Fusobacterium species, Peptococcus, Escherichia coli, viridans Streptococci, and non-tuberculous mycobacteria have been isolated in individual cases (01; 42; 44; 47; 34; 23; 32; 39; 02; 03). Pseudomonas aeruginosa has been isolated in a case occurring postsurgically (19), as has Mycoplasma hominis (51; 26). Salmonella species has been isolated in the case of a 14-year-old with systemic lupus erythematosus in whom an intracranial subdural empyema extended into the cervical subdural space (08). One case of extensive Nocardia subdural empyema has been reported in a patient with known preexisting Nocardia brain abscess who had received epidural steroids (07). A single case of recurrent spinal subdural empyema associated with Lawsonella clevelandensis has been reported (30). This agent, which may mimic Nocardia infection, is difficult to culture, and diagnosis may rely on molecular methods (30). More than one organism may be present (01; 21; 13; 45). Historically and in underdeveloped countries, tuberculosis has been a significant cause of spinal subdural empyema, and worldwide, tuberculosis remains the most common cause of subdural empyema in children or young adults (36; 43; 02; 03). Alessi and colleagues have reported a patient with HIV who developed a tuberculous subdural empyema as well as an intraspinal tuberculoma (06).

The spinal subdural space, like its intracranial counterpart, forms a continuous, potential space. Within the spinal canal, this potential space extends from the foramen magnum throughout the length of the cord and is interrupted only where the spinal dura and arachnoid are joined by penetrating vessels and nerves, such that it readily permits extensive longitudinal spread of infection (12). Organisms most frequently enter the spinal subdural space as a complication of bacteremia (01; 21; 13). Spinal subdural empyema may also develop as a complication of spinal epidural abscess, disc space infection, retropharyngeal abscess, decubitus ulcers, or osteomyelitis (01; 21; 13; 09; 17; 48). Spinal subdural empyema may occur as a complication of lumbar puncture, discography, epidural anesthesia, epidural blood patch, acromial bursa injection, acupuncture, accidental injury to the dura during surgery, or a persistent dermal sinus (01; 41; 34; 53; 10; 15; 05; 16; 52; 18; 26; 29; 40). Occasionally, spinal subdural empyema may coexist with or follow spinal epidural abscess (12; 40). In two cases, spinal subdural empyema due to Streptococcus milleri occurred in association with cranial infection and meningitis (23; 22). As noted, a single case has been reported in which an intracranial subdural empyema associated with Salmonella sp. extended into the spinal canal to produce a cervical subdural empyema (08). Spinal subdural empyema may occur at any level. Of 50 cases reviewed by Levy and colleagues, 17 involved the thoracic spine, 15 the cervical spine, 15 the lumbar spine, and one the sacral spine only. In the series by Sandler and colleagues, the thoracic spine was most frequently involved in children as well (43). The empyema may rapidly expand over several lumbar segments and will occasionally extend virtually the entire length of the cord (32; 07). Rapid expansion of the empyema within the narrow confines of the spinal canal may result in cord necrosis early in the course of infection, and longitudinal spread of infection can result in cord necrosis extending over multiple vertebral segments (01; 21; 13). Spinal cord infarction may also occur as a consequence of vasculitis and septic thrombophlebitis of spinal vessels (01; 21; 38; 46). Occasionally, spinal subdural empyema may be accompanied by spinal epidural abscess, spinal cord abscess, or intracranial parameningeal infections (09; 35; 39; 20; 11; 37).

Epidemiology

Spinal subdural empyema is, as mentioned above, an unusual condition that occurs at virtually any age and affects men and women more or less equally. In children, spinal subdural empyema has been reported more frequently in boys (43). In infants and younger children, spinal subdural infection may result from spinal dysraphism or other congenital defects (43).

Prevention

There are no specific measures to prevent the occurrence of spinal subdural empyema. It should be kept in mind that spinal subdural empyema has been associated with back trauma or surgery, diabetes mellitus, pregnancy, and intravenous drug abuse (01; 21; 13; 32; 07). Cases have developed after seeding of bacteria through dermal sinus tracts and have occurred as a complication of meningitis (41; 23; 22). As noted above, spinal subdural empyema may occur as a complication of discography (34; 53), epidural anesthesia, or blood patch (10; 26; 29). Six cases have been reported in association with lumbar puncture (32).

Differential diagnosis

Associated or underlying disorders

Spinal parameningeal infection should be suspected in any patient presenting with acute, severe spinal or radicular pain, particularly if there is evidence of radicular involvement at multiple levels or if there is a history of back trauma, intravenous drug abuse, diabetes mellitus, or pregnancy (01; 21; 13). Although most cases of spinal parameningeal infection involve the epidural space, spinal subdural empyema is an even more urgent condition and is impossible to distinguish from spinal epidural abscess on clinical grounds. Spinal osteomyelitis may duplicate the findings of focal pain but will not, by itself, produce radicular or long tract findings. Rarely, spinal subdural empyema may develop chronically and may mimic a spinal or paraspinal malignancy (33).

Diagnostic workup

Hematological studies frequently reveal polymorphonuclear leukocytosis and an elevated erythrocyte sedimentation rate (01; 21; 13), although these abnormalities may be absent in chronic cases (33). Lumbar puncture is of limited value and should be avoided if possible because it carries with it real risk of seeding the subarachnoid space. Spinal fluid findings frequently show polymorphonuclear leukocytosis with depressed glucose and at times extreme elevation of protein. Bacteria may be isolated, although the question always exists as to whether the bacteria were carried into cerebrospinal fluid from the overlying empyema. The majority of cases of spinal subdural empyema reported prior to the mid-1990s had been diagnosed by myelography, usually supplemented by CT scanning (38; 47; 24; 28; 14; 34; 22; 13). Myelography shows spinal cord compression of various degrees of severity extending over multiple vertebral segments. At present, however, MRI, because of its ability to image multiple vertebral bodies and its ability to detect osteomyelitis and joint space infection, is almost invariably the initial procedure of choice, although it may then be followed by CT myelography (32; 10; 49; 31). An older case reported by Gelfand and colleagues described a patient in whom MRI failed to detect a subdural empyema detected by myelography, suggesting that emergent myelography should be considered if spinal subdural empyema is suspected and MRI is negative (22). The advent of more advanced MRI methods, in particular employing 3-Tesla MRI, however, may allow not only detection of spinal parameningeal infection but also differentiation between epidural and subdural abscesses (27; 33).

Myelography, like lumbar puncture, may introduce the offending organism into the subarachnoid space to cause meningitis. For this reason, the spinal puncture, if indicated, should be conducted at a level distant from that believed to be occupied by the empyema, employing a lateral cervical puncture if lumbar infection is suspected, and a lumbar approach should be used if the cervical or high thoracic spine is involved. Blood and material from infected sites should be cultured for aerobic and anaerobic organisms. Recovery of bacteria has been accomplished from blood in occasional cases in which cultures from the empyema were sterile (04; 28).

Management

Spinal subdural empyema is an absolute indication for immediate surgery, and no confirmed case of spinal subdural empyema has yet been reported in which a cure was achieved by medical management alone. Although patient survival and return to some degree of ambulation were achieved in one case using CSF drainage and antibiotic treatment, the patient was left with significant deficits (50).

If the causative agent is unknown, initial antibiotic therapy should be directed against S aureus and the streptococci and should include vancomycin until sensitivity to nafcillin or oxacillin is determined. Vancomycin should be continued if there is a history of penicillin allergy. Ceftazidime or cefepime and gentamicin should be added if there is any question of Pseudomonas aeruginosa or other gram-negative organisms, and metronidazole should be used if infection by Bacteroides fragilis is suspected. The possibility of tuberculous subdural empyema should be kept in mind in individuals from areas where the disease is still prevalent. Consideration should be given at surgery to extensive irrigation of the abscess cavity (46; 35; 27). Consideration may be given to irrigating with a solution including bacitracin and a second antibiotic such as vancomycin or gentamicin (46). Final selection of antibiotics is based on isolation of organisms from abscess material, cerebrospinal fluid, or blood and determination of antibiotic sensitivity.

Special considerations

Pregnancy

Spinal subdural empyema has been reported during pregnancy (25; 19). There are too few reported cases to determine whether pregnancy actually constitutes a risk factor for spinal subdural empyema.

Anesthesia

Spinal subdural empyema is not a contraindication to the use of general anesthesia. Use of spinal anesthesia of any sort should be approached with caution to avoid introducing infectious organisms into the subarachnoid space. As noted, spinal subdural empyema may arise after epidural anesthesia (10) and has been reported as a complication following acupuncture (16).