A 68-year-old man presents with saddle anaesthesia, bilateral radicular leg pain, retention of urine, and loss of anal tone developing over two days. MRI shows compression below the L1 vertebral level. Which combination localises the lesion correctly?

• A Conus medullaris: symmetric sacral signs, early sphincter involvement, absent ankle jerks late
• B Conus medullaris: asymmetric radicular pain with preserved sphincters
• C Cauda equina: asymmetric radicular pain, saddle sensory loss, late flaccid paralysis with sphincter involvement ✓
• D Cauda equina: spastic paraplegia with extensor plantar responses

Explanation

Below the L1 vertebra the spinal canal contains only the cauda equina, made of lumbosacral nerve roots. Root compression produces severe asymmetric radicular pain, saddle anaesthesia, flaccid areflexic weakness and sphincter disturbance, typically evolving over days. Conus medullaris lesions present earlier with prominent symmetric sphincter dysfunction and less radicular pain. Spastic paraplegia with upgoing plantars indicates an upper motor neuron cord lesion above the lumbosacral enlargement, which excludes the cauda equina entirely.

Reference: Bailey and Love's Short Practice of Surgery, 28th ed.

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Written and medically reviewed by the StethoPrep medical team.