Ulnar nerve compression in Guyon's canal: MRI does not always have the answer
A right-handed 37-year-old man presented with paraesthesia and altered sensation on the volar aspect of his left ring and little fingers. Although Froment's sign was negative, he had generalized weakness affecting the middle finger and thumb compared with the right hand. This had been present for 2 years. He had had previous surgery for a ganglion on the radial aspect of his left wrist. Clinical examination was otherwise normal.\nNerve conduction studies demonstrated no definite peripheral ulnar nerve lesion at the wrist or elbow. However, ulnar mixed nerve responses were slightly reduced in amplitude.\nA T1-weighted and STIR (short T1 inversion recovery) MRI scan of the wrist, reconstructed in axial and coronal planes was performed (Fig. ). This showed no obvious deformity in Guyon's canal and no abnormality along the ulnar nerve. However, it did demonstrate a multi-loculated ganglion along the volar aspect of the distal radius closely abutting the radio-carpal ligament and a fatty mass superficial to the roof of Guyon's canal. As the patient had persistent hand symptoms, we proceeded to surgically explore his wrist.\nThe ganglion was first excised from the radial aspect of the volar wrist and the median nerve was then released in the carpal tunnel. The incision was extended medially and Guyon's canal was explored. A multi-lobulated piece of fatty tissue was found within the canal measuring 3 cm × 2 cm × 1.5 cm (Fig. ). This was compressing the ulnar nerve. This was removed and histological analysis showed mature adipose tissue consistent with lipoma.\nThe ulnar nerve was intact throughout its course in the canal with no macroscopic pathology. The ulnar artery was also intact with no evidence of thrombosis.\nThe patient made a good postoperative recovery and his symptoms resolved completely over the following 4 months.