Pathogenesis of Spinal Epidural Abscess
Spinal epidural abscess develops through three primary mechanisms: hematogenous spread from distant infection sites (most common), direct extension from adjacent vertebral osteomyelitis or discitis, and iatrogenic inoculation from spinal procedures. 1
Hematogenous Spread
The most frequent pathway involves bacterial seeding of the epidural space through bloodborne dissemination from remote infection sources 1:
- Skin infections serve as a primary source, particularly in patients with IV drug use 1
- Urinary tract infections represent another common origin 1
- Oral cavity and dental infections can seed the epidural space hematogenously 1
- Mastoid and middle ear infections may spread via the bloodstream 1
- Pulmonary infections occasionally serve as the source 1
Direct Extension from Adjacent Structures
Contiguous spread occurs when infection erodes from neighboring tissues into the epidural space 1:
- Vertebral osteomyelitis frequently extends into the epidural space 1
- Discitis can progress to involve the epidural compartment 1
- Paraspinal soft tissue abscesses may penetrate into the spinal canal 1
Iatrogenic and Traumatic Inoculation
Direct bacterial introduction represents an increasingly recognized mechanism 1, 2, 3:
- Spinal surgery carries a 2-3% infection risk with instrumentation 1, 2
- Epidural anesthesia and pain injections can introduce pathogens directly 1, 3
- Lumbar puncture rarely causes epidural abscess 1
- Spinal biopsy procedures pose infection risk 1
- Deep paravertebral or intragluteal injections can track to the epidural space 3
- Contaminated intravenous catheters may seed infection 3
- Penetrating trauma provides direct inoculation 1
High-Risk Patient Populations
Certain conditions dramatically increase epidural abscess risk 2, 4, 5:
- IV drug abuse was present in 33% of cases in one major series 2
- Diabetes mellitus occurred in 27% of patients 2, 5
- Prior spinal surgery was documented in 17% of cases 2
- Immunocompromised states including cancer, HIV/AIDS, and chronic alcoholism 5
Common Causative Organisms
The microbiology reflects the mechanism of spread 1, 2:
- Staphylococcus aureus predominates (67% of cases), with 15% being methicillin-resistant strains 2
- Streptococcal species account for approximately 8% 2
- Aerobic gram-negative bacilli occur particularly after procedures 1
- Anaerobes are frequently involved, especially in polymicrobial infections 1
- Nocardia, mycobacteria, and fungi represent rare causes 1
Critical Clinical Pitfall
The "classic triad" of back pain, fever, and neurologic deficit appears in only 10-15% of cases at initial presentation, making early diagnosis extremely challenging. 4 Back pain and low-grade fever are the most consistent early findings, but these nonspecific symptoms often delay diagnosis until irreversible neurologic damage occurs 2, 4. The erythrocyte sedimentation rate is elevated in 95% of cases and should prompt urgent MRI in at-risk patients with localized spinal pain 2.