MRI Findings in Suspected Cauda Equina Syndrome
MRI lumbar spine without IV contrast will show compression of the cauda equina nerve roots, most commonly from a large central disc herniation at L4-L5 or L5-S1, along with assessment of the degree of thecal sac effacement and spinal canal patency. 1, 2
Primary MRI Findings
Anatomical compression of the cauda equina nerve roots is the hallmark finding that MRI must identify, which can result from: 2
- Disc herniation (most common cause, typically massive midline herniation at L4-L5 or L5-S1 levels) 3, 2
- Neoplasm (tumor causing mass effect on nerve roots) 2
- Infection (epidural abscess or discitis-osteomyelitis with associated compression) 1, 2
- Spinal stenosis (severe narrowing of the spinal canal) 1, 2
- Hemorrhage (epidural hematoma compressing the cauda equina) 2
Specific Imaging Characteristics
Thecal sac effacement is a critical measurement, where ≥50% effacement predicts significant spinal stenosis requiring surgical intervention. 1, 2
Soft-tissue pathology visualization includes: 1, 2
- Direct visualization of compressed nerve roots within the thecal sac
- Assessment of vertebral marrow signal changes (indicating infection, tumor, or trauma)
- Evaluation of spinal canal patency and degree of stenosis
- Identification of epidural fluid collections or masses
When to Add IV Contrast
MRI without and with IV contrast should be obtained when clinical suspicion suggests underlying malignancy, infection, or inflammation as the etiology. 1, 2
The contrast enhancement helps: 1
- Identify and define the size and extent of epidural abscesses
- Distinguish between tumor types and characterize enhancement patterns
- Determine the presence and degree of spinal cord or cauda equina compression from infectious or inflammatory processes
- Evaluate marrow or paraspinal muscle edema associated with infection
Critical Diagnostic Points
MRI has 96% sensitivity and 94% specificity for identifying cauda equina pathology, making it the gold standard that cannot be substituted by CT imaging alone. 3, 2
The imaging must be performed emergently (ideally within one hour of clinical suspicion being raised) for any patient with red flag symptoms including: 3, 2, 4
- Bilateral radiculopathy
- New-onset urinary symptoms with low back pain or sciatica
- Saddle anesthesia
- Progressive neurologic deficits
Common Pitfalls to Avoid
Do not wait for complete urinary retention or complete saddle anesthesia before ordering MRI, as these are late signs indicating potentially irreversible damage. 3, 2
Do not rely on CT findings alone to rule out cauda equina syndrome, as CT has extremely low sensitivity (only 6%) for identifying epidural pathology and neural compression compared to MRI's 96% sensitivity. 3
Remember that MRI's role is to identify which patients with red flag clinical features have significant compression requiring urgent surgical decompression, not to make the clinical diagnosis of CES, which remains fundamentally a clinical diagnosis. 2, 4