A 52-year-old man with type 2 diabetes mellitus and hypertension is scheduled for elective total knee replacement under spinal anesthesia. During the procedure, 2 hours after subarachnoid injection of 3 mL of 0.5% bupivacaine, he develops sudden onset of severe bilateral leg weakness, urinary retention, and loss of anal sphincter tone. On examination, he has a sensory level at T10 and absent lower limb reflexes. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Cauda Equina Syndrome ### Presentation Pattern **Key Point:** Cauda equina syndrome (CES) is a rare but devastating complication of spinal anesthesia characterized by bilateral lower limb weakness, saddle anesthesia, and urinary/fecal dysfunction occurring during or immediately after the procedure. ### Distinguishing Features in This Case | Feature | Cauda Equina Syndrome | Transient Neurological Symptoms | Anterior Spinal Artery Syndrome | Epidural Hematoma | |---------|----------------------|--------------------------------|--------------------------------|-------------------| | **Onset** | During/immediately after SA | 24–48 hrs post-procedure | Immediate | Hours to days post-procedure | | **Bilateral leg weakness** | Yes (symmetric) | Rare | Yes (lower limbs) | Variable | | **Urinary retention** | Yes (hallmark) | No | Possible | Possible | | **Sensory level** | Saddle distribution | Stocking/glove | Suspended sensory loss | Variable | | **Prognosis** | Often permanent | Usually reversible | Often permanent | Depends on timing of decompression | | **Recovery** | Poor without urgent intervention | Spontaneous recovery | Poor | Good if decompressed early | **High-Yield:** The combination of **bilateral leg weakness + urinary retention + anal sphincter dysfunction occurring during/immediately after spinal anesthesia** is pathognomonic for CES. ### Mechanism CES results from direct neurotoxicity of local anesthetic agents on the cauda equina nerve roots, particularly when: - High concentration of local anesthetic is used - Prolonged contact with nerve roots occurs - Microcatheters are employed (now largely abandoned) - Patient is in lithotomy position (pooling effect) **Clinical Pearl:** Modern spinal anesthesia using single-dose, large-bore needles with dilute local anesthetic solutions has significantly reduced CES incidence. ### Management 1. Immediate MRI of lumbar spine to rule out compressive lesions 2. Urgent neurosurgical consultation 3. Supportive care (catheterization, bowel management) 4. Rehabilitation and long-term follow-up **Warning:** Unlike transient neurological symptoms, CES typically does NOT resolve spontaneously and often results in permanent neurological deficit.