A 52-year-old man presents with severe band-like back pain, difficulty voiding, and numbness over the perineum. Examination shows symmetrical saddle anaesthesia, urinary retention with overflow incontinence, absent ankle jerks, and preserved knee jerks. MRI shows a lesion centred at the L1 vertebral level. Which structure is most likely involved?

• A Lumbosacral plexus
• B Cauda equina
• C Conus medullaris ✓
• D Femoral nerve

Explanation

The conus medullaris ends at approximately the L1 vertebral level and contains the sacral segments. Its compression produces early, symmetrical saddle anaesthesia with rapid urinary retention and overflow, and ankle jerk loss with knee jerks characteristically preserved. Cauda equina compression, which lies below L1, tends to produce asymmetric radicular pain, unilateral or patchy sensory loss, and later sphincter involvement, often with wasting of leg muscles.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.