Spinal epidural abscess
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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.
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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 5th-7th 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: intravenous drug use
Many clinical features are non-specific, even in an acute case of a spinal epidural abscess, mainly if there are no demographic or epidemiological clues. Classical symptoms include:
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:
hematogenous dissemination from a remote focal infection
direct invasion from a neighboring infected structure: discitis-osteomyelitis, septic arthritis
iatrogenic inoculation: epidural/spinal anesthetics, steroid injections, surgery
The source of infection is not always apparent, and cannot be identified in approximately one-third of the cases 2. Paralysis may result via spinal compression from a mass effect exerted by the abscess or secondary ischemia from septic thrombosis 8.
Spinal epidural abscesses develop from direct spread from neighboring structures and are, unsurprisingly, 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 a direct hematogenous 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 visualization of the collection is possible. They can, however, be useful in visualizing 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 visualization of eroded bone/joint, as well as identifying adjacent soft tissue stranding 3.
Gadolinium-enhanced MRI is the imaging choice for the diagnosis of spinal epidural infection. Diffusion-weighted sequences are useful to confirm infection 8.
There are two main patterns, with distinct imaging appearances 5:
phlegmonous stage of infection results in a homogeneous enhancement of the abnormal area which correlates to granulomatous-thickened tissue with embedded micro-abscess without a significant pus collection
liquid abscess surrounded by inflammatory tissue which shows a varying degree of peripheral enhancement with gadolinium
The key to identifying liquid abscesses, which is usually a 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). DWI/ADC commonly demonstrates restricted diffusion of the abscess content.
Conditions to be considered include 6,7:
vertebral metastases: may have an extradural extension and mimic phlegmonous epidural infection
lumbar disc disease: extruded or migrated disc
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