How does the wrist response in nerve conduction studies vary with and without a Martin-Gruber anastomosis?

Updated: Jun 08, 2018
  • Author: Charles F Guardia, III, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

In a patient who does not have a Martin-Gruber anastomosis, stimulating the median nerve at the wrist produces a CMAP amplitude at the thenar eminence (eg, abductor pollicis brevis) that is essentially the same size as the thenar CMAP produced by elbow stimulation. (The CMAP produced by wrist stimulation could be a bit larger because stimulating further away from the ultimate target muscle gives a little more temporal dispersion of the signal.)

In a patient who does have the anomaly, however, the wrist response is smaller because many axons from the median nerve have crossed already. Contributions from now median-innervated ulnar intrinsic hand muscles falsely increase the elbow response.


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