| Acute | Chronic | |
|---|---|---|
| Duration of symptoms | Less than 2 wk | More than 2 wk |
| Fever | Often present | Low grade or absent |
| Systemic toxicity | Sometimes | Infrequently |
| Source | Hematogenous (often from minor skin infection) | Direct extension from vertebral osteomyelitis |
| Back pain | Always | Always |
| Localized spinal tenderness | Very common | Nearly universal |
| Root weakness | Common | Common |
| Peripheral leukocytosis | Usually present | Usually absent |
| Erythrocyte sedimentation rate | Greatly elevated | Greatly elevated |
| CSF leukocytesa (per mm3) | Usually 50–1000 | Often <50 |
| CSF protein >100 mg/dL | Almost always | Almost always |
| Anatomic location | Usually posterior to spinal cord | Commonly anterior to spinal cord |
| Gross pathology | Purulent exudate | Granulation tissue |
Abbreviation: CSF = cerebrospinal fluid.
a Frank pus may be encountered if a lumbosacral epidural abscess is entered during attempted lumbar puncture. If this occurs, the spinal needle must not be further advanced because introducing the needle into the subarachnoid space may precipitate meningitis. The aspirated purulent material should be sent immediately in the airless capped syringe for appropriate studies, including Gram stain and aerobic/anaerobic cultures.
Another differential feature with therapeutic implications concerns the source of epidural infection. Microbes most commonly access the epidural space by hematogenous dissemination from a distant, sometimes trivial, infectious focus. A substantial minority of cases arise from contiguous spread, usually from vertebral osteomyelitis. Epidural abscesses of hematogenous origin are typically located in the dorsolateral thoracic or lumbar area, where the epidural space is widest. Abscesses that form secondary to adjacent osteomyelitis usually involve the epidural space anteriorly or circumferentially. In some cases, it is hard to determine whether the epidural space represents a primary or secondary site of infection.
Clinical presentation and course
The classical manifestations of spinal epidural abscess were partitioned by Heusner in 1948 as progressing through four sequential but overlapping stages: (1) spinal ache (or back pain); (2) root (or radicular) pain; (3) weakness; and ultimately (4) paralysis. The actual time between the onset of back pain and development of neurologic deficits can be highly variable. The unpredictable but potentially rapid evolution from backache to neurologic tragedy forces physicians to consider this entity in the differential diagnosis of all patients with new or changing back pain, particularly when fever and localized spinal tenderness coexist. Presenting complaints include paresthesias (sometimes described as “electric” in character), paresis, incontinence, constipation, or urinary retention. Atypical presentations include headache or meningismus (with cervical involvement), pleuritic or abdominal pain (with thoracic infection), and hip pain (with lumbar disease).
In patients with epidural abscesses, an inapparent primary source of infection, such as endocarditis, adjacent osteomyelitis, or a distant visceral abscess, may be present. Initially occult infectious foci may ultimately dictate the length of antimicrobial treatment or mandate additional procedures. Not unexpectedly, bacteremia is more often documented in acute hematogenous than chronic locally advancing infections. Especially when Staphylococcus aureus is the pathogen, the infection may be multifocal due to seeding of distant sites during a primary or secondary bacteremia.
The dreaded neurologic complications of epidural abscess can arise from either pressure causing compression of the spinal cord or vascular compromise causing ischemic necrosis. Cord compression may be more common, but septic thrombophlebitis is likely responsible for sudden unforeseen deterioration.
Risk factors
Patients with a history of back injury are predisposed to seed the injured area during transient bacteremia and therefore constitute a special risk group for vertebral osteomyelitis and/or epidural abscess. Penetrating trauma may seed the adjacent bone and epidural space. Suspicion of epidural infection should be raised when a patient with diagnosed osteomyelitis or after recent back surgery, epidural injection, or lumbar puncture reports worsening localized back discomfort.
All patients with bacteremia or candidemia incur some risk of metastatic seeding. Patients with cutaneous infections, infected catheters, dental manipulations, decubitus ulcers, urinary tract infections, or endocarditis can develop a secondary epidural focus through hematogenous spread, even in the absence of previously recognized back injury. The risk appears highest in the aftermath of S. aureus bacteremia and is not totally eliminated by the 2 to 4 weeks of antibiotic therapy usually given to such patients. Epidural infection may manifest itself weeks to months later. Under most circumstances, back pain developing or worsening in the year following an episode of S. aureus bacteremia should be presumed to represent metastatic infection until proved otherwise.
Injection drug users may develop infections of the epidural space. Diabetic patients, patients receiving long-term parenteral nutrition, and patients undergoing hemodialysis also appear to be at increased risk for epidural infection. Infections can arise from epidural injections or catheters due to breach of the anatomic barriers or even contamination of the injected material.
Microbiology
Staphylococcus aureus remains the predominant pathogen recovered from all types of epidural abscesses, often originating from an unnoticed and otherwise inconsequential primary skin focus. Injecting drug use, chronic hemodialysis, and indwelling vascular catheters predispose to S. aureus bacteremia associated with metastatic seeding.
A comprehensive clinical history including the epidemiologic circumstances may provide the only clues to otherwise unsuspected pathogens. Gram-negative osteomyelitis, discitis, and epidural infection can complicate injection drug use, where Enterobacteriaceae and Pseudomonas aeruginosa need to be considered among the possible pathogens. Gram-negative rods and occasionally enterococci can spread from urinary tract or pelvic infections to the lumbar spine and/or epidural space through vascular anastomoses in Batson’s plexus. Less commonly isolated bacterial species include streptococci (especially the S. milleri group), coagulase-negative staphylococci (usually postoperatively or after open trauma), anaerobes (from either the oral or intestinal flora), Brucella, and Salmonella species.
Tuberculous spondylitis (Pott’s disease) is frequently associated with epidural abscess and
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