Deep branch ulnar neuropathy due to giant cell tumor: report of a case.

Electrophysiologic studies indicated a lesion of the deep branch of the ulnar nerve in the palm of a 42-year-old man with painless weakness and wasting of ulnar-innervated intrinsic hand muscles. Magnetic resonance imaging of this region demonstrated a soft-tissue mass in the palm distal to Guyon's canal. Surgical exploration disclosed a giant cell tumor of the tendon sheath, compressing the deep branch of the ulnar nerve.