A 54-year-old man with long-standing low back pain develops urinary retention, saddle anaesthesia, and markedly asymmetric flaccid weakness of both legs with absent ankle jerks. Which feature best distinguishes this presentation as cauda equina syndrome rather than conus medullaris syndrome?

• A Symmetric sensory loss confined to the perianal region
• B Early urinary retention with saddle anaesthesia
• C Extensor plantar responses bilaterally
• D Asymmetric lower motor neuron pattern weakness with depressed reflexes ✓

Explanation

The cauda equina consists of lumbosacral nerve roots, so compression produces an irregular, asymmetric, purely lower motor neuron pattern of weakness with wasted muscles and depressed reflexes. Conus medullaris lesions produce early, symmetric deficits with prominent sphincter disturbance and often extensor plantars from associated upper motor neuron involvement. Retention and saddle anaesthesia occur in both syndromes and therefore cannot discriminate between them, making option B the tempting but incorrect choice.

Reference: Bailey and Love's Short Practice of Surgery, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.