A 22-year-old man sustains a deep laceration at the wrist dividing the median nerve. At 6 weeks post-repair, which electrodiagnostic finding would MOST reliably indicate early axonal regeneration?

• A Presence of nascent motor unit potentials (polyphasic, low-amplitude MUPs) on needle EMG in thenar muscles ✓
• B Return of normal nerve conduction velocity across the repair site
• C Absence of fibrillation potentials and positive sharp waves on needle EMG
• D Return of two-point discrimination to < 5 mm in the index finger

Explanation

At 6 weeks post-repair of a Sunderland Grade 5 (neurotmesis/complete transection) injury repaired primarily, axons begin to reinnervate the most proximal denervated muscles at a rate of approximately 1 mm/day. The earliest EMG sign of reinnervation is the appearance of nascent motor unit potentials — polyphasic, low-amplitude MUPs detected on needle EMG during voluntary effort, signifying nascent neuromuscular junctions. These appear in proximal muscles before any clinical twitch is visible. Fibrillation potentials and PSWs are signs of ongoing denervation, not regeneration. NCV is not reliable across an acute repair site at 6 weeks.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.