Spinal epidural abscess
Spinal epidural abscess represents infection of the epidural space, located between the spinal dura mater and the vertebral periosteum. It can present with rapidly deteriorating neurological function due to compression. Imaging is best performed with MRI and emergency surgery is often required.
Spinal epidural abscess is an uncommon condition with an estimated incidence of 2-3 per 10,000 hospital admissions. It has a peak incidence in the fifth-to-seventh decades of life with a male predominance, which might be due to predisposing conditions and risk factors that are more prevalent in older people 2. Risk factors include 3, 4:
- comorbidities: diabetes mellitus; alcohol abuse; HIV infection
- spinal abnormality or intervention: degenerative joint disease; trauma; surgery or procedure
- potential local or systemic source of infection: IVDU
Many clinical features are non-specific, even in an acute case of spinal epidural abscess, particularly if there are no demographic or epidemiological clues. Classical symptoms include:
- spinal pain
- neurological deficit
This triad, however, is only present in 10-15% of the cases at first physician contact. Back pain and severe, circumscribed tenderness are by far the most frequent early findings 2.
Bacteria gain access to the epidural space by three mechanisms:
- haematogenous dissemination from a remote focal infection
- direct invasion from a neighbouring infected structure: discitis-osteomyelitis, septic arthritis
- iatrogenic inoculation: epidural/spinal anaesthetics, steroid injections, surgery
The source of infection is not always apparent, and cannot be identified in approximately one-third of the cases 2.
Spinal epidural abscess developing from direct spread from neighbouring structures are, not surprisingly, usually adjacent to the primary focus.
For example, discitis-osteomyelitis which is thought to be the primary source of infection in up to 80% of patients 3, usually results in anterior abscesses 4. In contrast, when facet joint septic arthritis is the primary infection, collections tend to be posterior or posterolateral. This is the same distribution as what are believed to be cases resulting from direct haematogenous spread, which is primarily located in the posterior/dorsal aspect of the spinal canal 4.
Plain films have little role to play in investigating a patient with suspected spinal epidural abscess, as no direct visualisation of the collection is possible. They can, however, be useful in visualising established discitis-osteomyelitis, particularly when MRI (and to a lesser extent CT) are not available.
Despite excellent bony anatomical details, CT even with contrast can struggle to demonstrate smaller collections. It may, however, reveal changes of discitis-osteomyelitis and facet joint septic arthritis, both by direct visualisation of eroded bone/joint, as well as identifying adjacent soft tissue stranding 3.
Gadolinium-enhanced MRI is the imaging choice for diagnosis of spinal epidural infection.
There are two main patterns, with distinct imaging appearances 5:
- phlegmonous stage of infection results in homogeneous enhancement of the abnormal area which correlate to granulomatous-thickened tissue with embedded micro-abscess without a significant pus collection
- liquid abscess surrounded by inflammatory tissue which shows varying degree of peripheral enhancement with gadolinium
The key to identifying liquid abscess, which is usually sufficient cause for surgical drainage, is the presence of a region of high T2 signal, with low T1 signal and without enhancement (usually surrounded by a rim of enhancement).
Conditions to be considered include 6:
- 1. Reihsaus E, Waldbaur H, Seeling W. Spinal epidural abscess: a meta-analysis of 915 patients. Neurosurg Rev. 2001;23 (4): 175-204. Pubmed citation
- 2. Sendi P, Bregenzer T, Zimmerli W. Spinal epidural abscess in clinical practice. QJM. 2007;101 (1): 1-12. doi:10.1093/qjmed/hcm100 - Pubmed citation
- 3. Darouiche RO. Spinal epidural abscess. N. Engl. J. Med. 2006;355 (19): 2012-20. doi:10.1056/NEJMra055111 - Pubmed citation
- 4. Mackenzie AR, Laing RB, Smith CC et-al. Spinal epidural abscess: the importance of early diagnosis and treatment. J. Neurol. Neurosurg. Psychiatr. 1998;65 (2): 209-12. doi:10.1136/jnnp.65.2.209 - Free text at pubmed - Pubmed citation
- 5.Chao D, Nanda A. Spinal epidural abscess: a diagnostic challenge. Am Fam Physician. 2002;65 (7): 1341-6. Pubmed citation
- 6. Diagnostic Imaging. (2004) ISBN:0808923153. Read it at Google Books - Find it at Amazon
- 7. Numaguchi Y, Rigamonti D, Rothman MI et-al. Spinal epidural abscess: evaluation with gadolinium-enhanced MR imaging. Radiographics. 1993;13 (3): 545-59. doi:10.1148/radiographics.13.3.8316663 - Pubmed citation