A 38-year-old woman develops sudden-onset severe bilateral leg weakness and urinary retention 4 hours after spinal anesthesia for cesarean section. MRI spine is being considered. Which investigation should be performed FIRST to confirm the suspected diagnosis of cauda equina syndrome?

## Clinical Scenario: Cauda Equina Syndrome Post-Spinal Anesthesia The patient presents with: - Bilateral leg weakness (motor deficit) - Urinary retention (autonomic dysfunction) - Acute onset 4 hours post-spinal anesthesia This constellation is classic for **cauda equina syndrome (CES)**, a surgical emergency requiring urgent imaging and possible decompression. ## Investigation of Choice **Key Point:** MRI lumbar spine with T1, T2, and STIR (Short Tau Inversion Recovery) sequences is the investigation of choice for diagnosing cauda equina syndrome. It is the fastest, most sensitive, and most specific imaging modality. ### Why MRI is Optimal | Feature | MRI | CT Myelography | EMG/NCS | |---------|-----|-----------------|----------| | **Sensitivity for CES** | 95–100% | 85–90% | Low (functional, not structural) | | **Specificity** | Excellent | Good | Poor | | **Speed** | 15–20 min | 30–45 min (requires LP + contrast) | 45–60 min | | **Invasiveness** | Non-invasive | Invasive (requires LP) | Non-invasive but time-consuming | | **Identifies cord compression** | Yes (T2 hyperintensity) | Yes (contrast pooling) | No | | **Detects hemorrhage/edema** | Yes (STIR shows edema) | Limited | No | | **Contraindications** | Metallic implants | Contrast allergy, renal failure | None | **High-Yield:** STIR sequences are particularly valuable in CES because they highlight cord edema and intramedullary signal changes, which indicate severity and guide prognosis. ## MRI Findings in Post-Dural Puncture CES **Clinical Pearl:** MRI may show: 1. **Cauda equina compression** — crowding of nerve roots in the thecal sac 2. **Spinal cord edema** — T2/STIR hyperintensity (indicates ischemia or inflammation) 3. **Subdural hematoma** — if bleeding is the cause (hypointense on T1, hyperintense on T2) 4. **Arachnoiditis** — enhancement of nerve roots on post-contrast T1 ## Timing and Urgency **Warning:** CES is a neurosurgical emergency. Neurological recovery is time-dependent; outcomes worsen significantly if decompression is delayed >48 hours. MRI must be performed **within 1–2 hours** of symptom onset. **Tip:** If MRI is contraindicated (metallic implant), CT myelography is the next best choice, but it requires LP and carries infection risk in an already compromised patient. ## Why This Is Post-Dural Puncture CES **Key Point:** Mechanisms include: - Spinal hematoma (from epidural venous puncture) - Arachnoiditis (from needle trauma or infection) - Cord compression from abscess or granuloma (rare) - Intrinsic cord infarction MRI will identify the structural lesion and guide surgical intervention (decompression, evacuation of hematoma).