Apparent conduction block in patients with ulnar neuropathy at the elbow and proximal Martin-Gruber anastomosis.

A Martin-Gruber anastomosis (MGA) commonly results in an abnormal decline in amplitude across the forearm segment when ulnar motor nerve conduction studies are performed. A recent report described a proximal MGA resembling partial conduction block in a patient with ulnar neuropathy at the elbow (UNE). As a result, we screened patients with similar findings. We detected a proximal MGA in three patients over a period of 2 years, which suggests that this may be an under-recognized anomaly. We conclude that a proximal MGA must be excluded in all cases of UNE showing apparent partial conduction block across the elbow segment.