What are the characteristic findings for each type of Martin-Gruber anastomosis in nerve conduction studies for 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 distinctions between the three major types of the Martin-Gruber anastomosis, the tests performed to confirm them, and the possible areas of clinical confusion are summarized in the Table below.

Table. Types of Martin-Gruber Anastomosis (Open Table in a new window)

Type

Anatomy

Most Characteristic Finding

Confirmation

Additional Verification

Potential Clinical Confusion

I

Crossover fibers innervate hypothenar muscles

Ulnar stimulation at wrist* produces larger hypothenar CMAP than stimulation at elbow

Stimulation of median nerve at elbow† produces response at hypothenar muscles

Hypothenar CMAP from ulnar stimulation at wrist is equal to hypothenar CMAP from ulnar stimulation at elbow plus hypothenar CMAP from median stimulation at elbow

Smaller response from proximal stimulation could be mistaken for conduction block

II

Crossover fibers innervate FDI muscle

Ulnar stimulation at wrist produces larger FDI CMAP than stimulation at elbow

Stimulation of median nerve at elbow produces response at FDI

FDI CMAP from ulnar stimulation at wrist is equal to FDI CMAP from ulnar stimulation at elbow plus FDI CMAP from median stimulation at elbow

Usually none, because FDI muscle is not usually recording site; if it is used, conduction block could be suspected, as in type I

III

Crossover fibers innervate thenar muscles (typically ADP and FPB)

Elbow stimulation of median nerve produces greater thenar response than wrist stimulation

Ulnar stimulation produces thenar CMAP with initial positive deflection; it is higher with wrist stimulation than with elbow stimulation

For thenar CMAP amplitudes, median elbow stimulation amp is equal to median wrist stimulation amplitude plus ulnar wrist stimulation amplitude minus ulnar elbow stimulation amplitude

Can complicate median nerve studies, especially when carpal tunnel syndrome is involved

ADP—adductor pollicis; CMAP—compound motor (or muscle) action potential; FDI—first dorsal interosseous; FPB—flexor pollicis brevis.

*Ulnar stimulation at wrist yields larger CMAP at hypothenar muscles, FDI, or thenar muscles (or sometimes combination of these) than does stimulation at elbow.

†Median stimulation at the elbow yields larger CMAP at hypothenar muscles, FDI, or thenar muscles (or sometimes combination of these) than does stimulation at wrist.

Note: “Larger” and “smaller” generally mean amplitude difference ≥1.0 mV.


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