Martin-Gruber Anastomosis

In routine ulnar nerve conduction studies (NCS) you can encounter a conduction block between the wrist and below-elbow stimulation sites. This finding should be interpreted cautiously, as it could be a pseudo-conduction block typically seen in Martin-Gruber Anastomosis (MGA). MGA is a common anomaly encountered in 15%-30% of patients. There is a major danger in misinterpreting this pseudo-conduction block as a sign of acquired demyelinating peripheral neuropathy, which is often treated with immunosuppressive therapy.

The anastomosis involves only the motor fibers, sparing the sensory ones. It occurs when fibers of the median nerve cross over (usually in mid-forearm) and run with the distal ulnar nerve to supply ulnar muscles. The most common muscle to be co-supplied is the first dorsal interosseous muscle (FDI).

In this case, the higher amplitude compound muscle action potential (CMAP) at the wrist is the result of stimulating nerve fibers from both ulnar and median nerves, while the lower CMAP at the below-elbow site ( proximal to the cross-over site) only involves the ulnar nerve fibers.

To confirm the presence of MGA, stimulation at the median wrist and antecubital fossa while recording from Abductor Digiti Minimi (ADM) should be performed.

MGA is NOT present when a small positive deflection is seen at both the median wrist and antecubital fossa reflecting a volume conducted potential from the median muscles.

MGA is PRESENT when a small positive deflection is seen at the median wrist, with a small CMAP when stimulating at the antecubital fossa.