Ulnar tunnel syndrome caused by two anomalous muscle bellies: a rare etiology of dynamic ulnar nerve compression.

Ulnar tunnel syndrome, or Guyon's canal syndrome, is an uncommon compressive neuropathy of the ulnar nerve at the wrist. While most cases result from trauma or ganglion cysts, anomalous muscles are rare. This case describes an ulnar tunnel syndrome caused by two anomalous muscle bellies crossing the ulnar neurovascular bundle in a 64-year-old man with recurrent ulnar neuropathy and intrinsic hand atrophy. Surgical excision and decompression resulted in durable symptom resolution at 5-year follow-up. This case highlights the importance of considering anatomical variations in persistent or recurrent ulnar symptoms, particularly after prior proximal decompression.