A 28-year-old motorcyclist sustains a complete brachial plexus injury. At exploration, roots C5 and C6 are found to be avulsed from the spinal cord (preganglionic injury), while C7 is in continuity. The most reliable electrodiagnostic finding distinguishing preganglionic from postganglionic injury at C5 is:

• A Absent SNAP (sensory nerve action potential) of the lateral antebrachial cutaneous nerve
• B Preserved SNAP of the lateral antebrachial cutaneous nerve with absent motor response (CMAP) ✓
• C Absent CMAP (compound motor action potential) of the biceps on nerve conduction study
• D Fibrillation potentials in the rhomboids on EMG

Explanation

In preganglionic (avulsion) injury, the dorsal root ganglion (DRG) remains intact in the intervertebral foramen — sensory axons peripheral to the DRG are still connected to their cell body and do not undergo Wallerian degeneration. Therefore the SNAP of the lateral antebrachial cutaneous nerve (C5-derived, via musculocutaneous nerve) is PRESERVED despite complete clinical sensory loss, because peripheral sensory axons remain viable. Motor axons, however, are disconnected from anterior horn cells and degenerate, giving absent CMAP. In postganglionic injury, the DRG is disconnected, SNAP is absent. Rhomboid fibrillations on EMG suggest dorsal scapular nerve (C5 root) involvement but are not specific for preganglionic vs postganglionic.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.