MR demonstration of an anomalous muscle in a patient with coexistent carpal and ulnar tunnel syndrome. Case report and literature summary.

An aberrant muscle is demonstrated by magnetic resonance (MR) imaging in a patient presenting with focal wrist swelling and compression neuropathy of median and ulnar nerves following 4 months of carpentry work. The muscle originated from the palmaris longus tendon and ulnar antebrachial fascia at the lower half of the forearm as a single belly, then diverged medially from palmaris longus tendon and bifurcated. Both portions of the split muscle extended into the distal ulnar tunnel or Guyon's canal. One segment joined with the abductor digiti minimi muscle and the other with the flexor retinaculum. MR was able to clearly delineate this hypertrophied, symptomatic muscle anomaly. It may be helpful when mass effect is suspected in either tunnel, or in patients with atypical work-related carpal tunnel syndrome with evidence of significant ulnar neuropathy for evaluation of underlying anomalous musculature. Normal MR images of the wrist are included for comparison and the literature is reviewed.