This condition is worthy of emphasis because the diagnosis is often missed or mistaken for another disease, sometimes with disastrous results. Children or adults may be affected. Infection of the epidural space has a wide variety of sources. Staphylococcus aureus is the most frequent etiologic agent, equally distributed between methicillin resistant and methicillin sensitive types, followed in frequency by streptococci, gram-negative bacilli, and anaerobic organisms. An injury to the back, often trivial at the time, furunculosis or other skin or wound infection, or a bacteremia may permit seeding of the spinal epidural space or of a vertebral body. This gives rise to osteomyelitis with extension to the epidural space. Occasionally, the spread is from an infected disc. Diabetes and recently treated cancer may be risk factors. Another source is bacteremia following the use of nonsterile needles or the injection of contaminated drugs. The thoracic spine is the most frequently affected, perhaps simply because of its greater length compared to the other portions of the spine.
Organisms may be introduced into the epidural space during spinal surgery or rarely via a lumbar puncture needle during epidural or spinal anesthesia or from epidural injections of steroid or other therapeutic agents; the localization is then over the lumbar and sacral roots. In these cases of cauda equina epidural abscess, back pain may be severe but neurologic symptomatology is minimal unless the infection extends upward to the upper lumbar and thoracic segments of the spinal cord. It must be acknowledged that some, even fulminant, cases have no clear source in the body for the bacterial abscess but most of these instances occur in patients who are diabetic or have cancer, even if the latter has been adequately treated.
At first, the purulent process in the cervical or thoracic region is accompanied only by low-grade fever and aching local back pain, usually intense, in most cases followed within a day or several days by radicular pain. Headache and nuchal rigidity are sometimes present; more often there are just persistent pain and a disinclination to move the back. After several days, there may be a rapidly progressive paraparesis associated with sensory loss in the lower parts of the body and sphincteric paralysis. In some cases, the evolution of the myelopathy is more gradual. Examination discloses the signs of a complete or partial transverse cord lesion, including at times the elements of spinal shock if paralysis has evolved rapidly. Percussion of the spine usually elicits tenderness over the site of the infection.
A distinction is often made between this typical acute to subacute form of abscess and a more indolent type that is due to a partially granulomatous lesion over several segment of the spine. This form does not always require surgical treatment, as detailed below.
Diagnosis The diagnosis can usually be ascertained from MRI (Fig. 42-2) but care must be taken to obtain images from levels rostral and caudal enough to detect the infected collection. There may be enhancement of the margins of the purulent collection after several days.