Ganglion cyst of Guyon's canal causing ulnar nerve compression

Description: A 48-year-old man presented complaining of pain in the right little finger for 2 months, associated with clawing and paresthesia in the ring and little fingers. Physical examination revealed muscular wasting of the interossei and the hypothenar eminence, and grade M3 (MRC classification) weakness isolated to the ulnar-innervated intrinsic muscles of the hand. Sensory function in the hand was normal.
Electromyographic nerve conduction studies revealed a marked reduction in amplitude of the right ulnar motor response to the abductor digiti minimi and the first dorsal interosseus muscles, with prolonged distal motor latency, indicating slow conduction distally through the wrist and hand. Ultrasonography demonstrated a ganglion cyst within Guyon’s canal arising from the pisiform-triquetral joint, impinging on the ulnar nerve.
Musculoskeletal images. Ganglion cyst of Guyon's canal causing ulnar nerve compression. Elias DA, Lax MJ, Anastakis DJ. Can J Surg. 2001 Oct;44(5):331-2.