Spinal subdural and epidural abscesses

A spinal subdural and epidural abscess is a collection of pus in the subdural or epidural space that causes mechanical compression of the spinal cord.

Spinal subdural and epidural abscesses usually occur in the thoracic and lumbar regions. A focal point for infection can usually be identified. It may be distant (eg, endocarditis, furuncle, dental abscess) or proximal (eg, vertebral osteomyelitis, pressure ulcers, retroperitoneal abscess). They may occur spontaneously, spread hematogenously, and are often secondary to urinary tract infection that extends into the epidural space via Batson's plexus. Epidural abscesses most commonly occur after spinal cord instrumentation, including surgery and epidural neural blocks. The literature suggests that steroid injection into the epidural space results in immunosuppression and an increased incidence of epidural abscesses. Although theoretically plausible, statistical evidence (given that thousands of epidural injections are performed in the United States daily) leaves this opinion in question. In about 1/3 of cases, the cause cannot be determined. The most common cause of spinal subdural and epidural abscess is Staphylococcus aureus, followed by Escherichia coli and mixed anaerobic flora. Rarely, the cause may be a tuberculous abscess of the thoracic region (Pott's disease). It can occur in any part of the spine and skull.

Symptoms begin with local or radicular back pain, percussion tenderness, which gradually become more pronounced. Fever is usually present. Compression of the spinal cord and equine roots may develop, causing paresis of the lower extremities (cauda equina syndrome). Neurological deficit may progress over hours and days. Subfebrile temperature and general symptoms, including malaise and loss of appetite, progress to severe sepsis with high fever, rigidity and chills. At this point, the patient develops motor, sensory deficits, symptoms of bladder and bowel damage as a result of nerve compression. As the abscess spreads, the blood supply to the affected area of the spinal cord is disrupted, leading to ischemia and, if untreated, to infarction and irreversible neurological deficit.

The diagnosis is clinically confirmed by back pain that increases in the supine position, leg paresis, dysfunction of the rectum and bladder, especially when combined with fever and infection. It is diagnosed by MRI. It is necessary to study the bacterial culture from the blood and inflammatory foci. Lumbar puncture is contraindicated, as it can cause herniation of the abscess with increased compression of the spinal cord. Routine radiography is indicated, but it reveals osteomyelitis in only 1/3 of patients.

All patients with suspected epidural abscess should have laboratory tests including complete blood count, erythrocyte sedimentation rate, and blood chemistry. Also, all patients with pre-positive epidural abscess should have blood and urine cultures to initiate antibiotic therapy immediately while workup is ongoing. Gram stain and culture are necessary, but antibiotic therapy should not be delayed until these results are available.

Prompt initiation of treatment is essential to prevent sequelae such as irreversible neurologic deficits or death. The goals of treatment of epidural abscess are twofold: treating the infection with antibiotics and draining the abscess to relieve pressure on neural structures. Since most cases of epidural abscess are caused by Staphylococcus aureus, antibiotic therapy such as vancomycin, which is effective against staph, should be started immediately after blood and urine cultures have been obtained. Antibiotic therapy may be adjusted based on culture and sensitivity results. As noted, antibiotic therapy should not be delayed until a definitive diagnosis is made if epidural abscess is considered in the differential diagnosis.

Antibiotics alone are rarely effective, even when the diagnosis is made early in the disease; drainage of the abscess is required for effective recovery. Drainage of an epidural abscess is usually accomplished by decompressive laminectomy and evacuation of the contents. Recently, surgical radiologists have had success in draining epidural abscesses percutaneously using drainage catheters under CT and MRI guidance. Serial CT and MRI are useful in subsequent resolution; scanning should be repeated immediately at the first sign of neurologic deterioration.

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Differential diagnosis

The diagnosis of epidural abscess should be suspected and excluded in all patients with back pain and fever, especially if the patient has had spinal surgery or an epidural block for surgical anesthesia or pain control. Other pathological conditions that should be considered in the differential diagnosis include diseases of the spinal cord itself (demyelinating diseases, syringomyelia) and other processes that can compress the spinal cord and nerve root sites (metastatic tumor, Paget's disease, and neurofibromatosis). The general rule is that without an accompanying infection, none of these diseases usually causes fever, only back pain.

Failure to diagnose and promptly and thoroughly treat an epidural abscess can result in disaster for both the physician and the patient.

The asymptomatic onset of neurologic deficits associated with an epidural abscess may lull the physician into a sense of security that may cause irreversible harm to the patient. If an abscess or other cause of spinal cord compression is suspected, the following algorithm should be followed:

• Immediate collection of blood and urine for culture
• Immediate initiation of high-dose antibiotic therapy that covers Staphylococcus aureus
• Immediate use of available imaging techniques (MRI, CT, myelography) that can confirm the presence of spinal cord compression (tumor, abscess)
• In the absence of one of the above measures, immediate transportation of the patient to a highly specialized center is necessary.
• Repeat examination and surgical consultation in case of any deterioration in the patient's neurological status

Delay in diagnosis places the patient and physician at high risk for poor outcome. The physician should consider epidural abscess in all patients with back pain and fever until another diagnosis is confirmed and treat accordingly. Overreliance on a single negative or equivocal imaging result is a mistake. Serial CT and MRI are indicated for any deterioration in the patient's neurologic status.

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