Why must a Martin-Gruber anastomosis be excluded before an ulnar conduction block is diagnosed in ulnar neuropathy?

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

The converse is true with ulnar nerve stimulation during recording over the hypothenar eminence (abductor digiti quinti) or the first digital interosseous muscle; median nerve fibers are innervating ulnar muscles in the hand, and the elbow response is smaller (see the images below). This could be mistaken for a conduction block. Accordingly, a Martin-Gruber anastomosis should be excluded before an ulnar conduction block is diagnosed.

First 3 traces correspond to ulnar compound muscle First 3 traces correspond to ulnar compound muscle action potential (CMAP) amplitude during recording at abductor digiti quinti (ADQ) and stimulating at wrist, below elbow, and above elbow, respectively. Fourth trace corresponds to stimulation of median nerve at elbow during recording at ADQ. Although CMAP amplitude is reduced markedly above elbow, this is compensated for by adding response seen after stimulation of median nerve; this represents Martin-Gruber anastomosis.
First 3 traces correspond to stimulation of ulnar First 3 traces correspond to stimulation of ulnar nerve during recording at first dorsal interosseous (FDI) muscle at wrist, below elbow, and above elbow, respectively. Fourth trace corresponds to stimulation of median nerve at elbow during recording at FDI muscle; this represents Martin-Gruber anastomosis.

These relations can be visualized even more clearly by considering the characteristic EMG patterns with respect to the corresponding anatomy (see the image below).

In those with Martin-Gruber anomaly who have no ot In those with Martin-Gruber anomaly who have no other significant neuropathy or nerve compression, stimulation of specific nerves at different sites yields differing results. With median nerve, stimulation at elbow yields larger compound muscle action potential (CMAP) at hypothenar muscles, first dorsal interosseous (FDI) muscle, or thenar muscles (or combination thereof) than does stimulation at wrist. With ulnar nerve, stimulation at wrist yields larger CMAP at hypothenar muscles, FDI muscle, or thenar muscles (or combination thereof) than does stimulation at elbow. In this context, "larger" and "smaller" generally refer to amplitude differences ≥1.0 mV.

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