Ulnar neuropathy caused by an accessory abductor digiti minimi muscle.

Peripheral neuropathies of the ulnar nerve at various locations in the upper extremity are common, but compression of the ulnar nerve at the level of the wrist due to an anomalous muscle is rare, though reported in the literature. Three cases of an abnormal abductor digiti minimi origin were first reported by Wood in 1868, but no mention was made of an associated ulnar neuropathy. To data, there have been 8 reported cases of an anomalous abductor digiti minimi, with sensory or motor abnormalities of the ulnar nerve the most common presentation. This case report describes a patient who had subjective paresthesiae in the ulnar nerve distribution for 2 years. He was found, at exploration, to have an anomalous muscle belly originating proximally from the deep forearm fascia with insertion into the abductor digiti minimi. We present the case, discuss the spectrum of compression neuropathies of the ulnar nerve at the wrist, and a review of the occurrence of anomalous muscles responsible.