A 26-year-old man has complete anaesthesia of the entire upper limb after a motorcycle accident. Nerve conduction studies show normal amplitude sensory nerve action potentials recorded from the affected digits despite clinically absent sensation. This finding localises the lesion to which site?

• A Distal to the dorsal root ganglion, confirming postganglionic injury
• B Proximal to the dorsal root ganglion, confirming preganglionic injury ✓
• C Within the sympathetic chain, confirming avulsion
• D At the peripheral nerve trunk, confirming neurapraxia

Explanation

Sensory nerve action potentials are generated by the cell bodies in the dorsal root ganglion. If the lesion is preganglionic, proximal to the ganglion, the peripheral sensory axons remain in continuity with their cell bodies, so sensory action potentials persist even though the patient has complete anaesthesia. Preserved SNAPs with clinical anaesthesia is therefore a hallmark of root avulsion and predicts poor spontaneous recovery.

Reference: Campbell's Operative Orthopaedics, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.