Nerve conduction · Advanced
Recognize anomalous innervation
Anomalous innervation
An anatomical variant can explain a CMAP pattern that otherwise looks pathological.
01Upper-limb communications
- Martin–Gruber communication commonly carries motor fibers from median to ulnar nerve in the forearm and can create amplitude discrepancies.
- Riche–Cannieu communication in the hand may alter thenar innervation.
02Lower-limb variants
- An accessory deep fibular branch may arise from the superficial fibular nerve and supply EDB, so anterior ankle stimulation may not activate all fibers.
- A larger proximal than distal CMAP should prompt review of technique and additional supply.
03Confirm systematically
- Change stimulating nerve and site while keeping the recording muscle fixed and comparing morphology.
- Document the observations and evidence rather than diagnosing block from one ratio.
Related nerves
Pause and explain
Explain in your own words: “A pattern that does not fit conventional anatomy is a reason to verify further.” Then think of a situation in which it could be misinterpreted.
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A variant can coexist with genuine entrapment or neuropathy.
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References and further reading
- Mid-palm recording technique, a new electrodiagnostic approach in Martin–Gruber anastomosis (2020) ↗Clinical Neurophysiology Practice · Original case research · Link accessed 2026-09-08
- Pitfalls in Using Electrophysiological Studies to Diagnose Neuromuscular Disorders (2012) ↗Journal of Clinical Neurology · Peer-reviewed review · Link accessed 2026-09-08
Educational synthesis; independent neuromuscular specialist review is pending. Publication dates and compilation dates are distinct. This is an original teaching synthesis; consult the source for complete methods and criteria.