Compression of the Ulnar Nerve in Guyon's Canal Caused by a Large Hypothenar Cyst

Objective: We report the case of a 77-year-old man who presented with a long-standing, large swelling of the left hypothenar eminence. This was associated with recent-onset paresthesia and numbness of the ring and little fingers. Magnetic resonance imaging demonstrated a cystic lesion that occupied almost the entire bulk of the hypothenar eminence. Methods: Surgical exploration revealed a 7-cm, encapsulated, yellow-brown cyst, around which were stretched the superficial sensory branches of the ulnar nerve. The hypothenar musculature lay flattened against the deep border of the mass. Results: The cyst was removed and Guyon's canal was released. Histologic examination confirmed a large cyst containing proteinaceous debris and blood breakdown products. It might have resulted from hemorrhage into a long-standing ganglion. Removal of the cyst led to full resolution of the patient's symptoms. Conclusions: This represents an unusual cause of ulnar tunnel syndrome. It is rare to encounter such a large cyst in the hand and interesting in the sense that the resulting symptoms were relatively mild and took many years to develop.