A 62-year-old man develops severe low back pain radiating down both legs, saddle anesthesia, urinary retention with overflow incontinence, and bilateral flaccid weakness of the ankles with absent ankle jerks. Reflexes and power are asymmetric. MRI shows a large central disc prolapse. At what level has the compression occurred relative to the spinal cord itself?

• A Compression of the conus medullaris within the dural sac
• B Compression of the filum terminale at S2
• C Compression of the lumbosacral nerve roots below the termination of the cord ✓
• D Compression of the sacral segments of the cord at the L1 vertebral level

Explanation

The findings describe cauda equina syndrome: asymmetric radicular pain, saddle anesthesia, retention, and lower motor neuron signs. The cauda equina consists of lumbosacral and coccygeal nerve roots lying free within the lumbar cistern below the conus medullaris, so a lesion here produces root signs rather than cord signs. The key distractor, conus medullaris syndrome, tends to be symmetric, presents early with sphincter involvement and less severe radicular pain, and gives predominantly upper motor neuron signs with a patchy sacral sensory loss.

Reference: Snell's Clinical Neuroanatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.