An unusual location of ulnar nerve pathology: a perineurioma of the ulnar nerve in the upper arm.

A young man presented with progressive motor weakness and atrophy of the ulnar muscles of his left hand over a period of more than 2 years. Electrodiagnostic studies indicated an ulnar nerve lesion, but it was not localized. High-resolution sonography of the ulnar nerve revealed an enlarged and hypoechogenic ulnar nerve at an unusual location, namely 12.5 cm proximal to the medial epicondyle. Histology showed that this was an intraneural perineurioma. High-resolution sonography of the ulnar nerve is very useful in the discovery of this unusual location of nerve pathology and may assist in its early detection.