The "Nail File Sign": A Report of Three Cases With Varied Etiologies.

Similar to the Wartenberg, Froment, and Duchenne signs, the more recently described "nail file sign" is also considered a diagnostic feature of ulnar nerve neuropathy. The "nail file sign" indicates a patient's inability to maintain the distal interphalangeal (DIP) joints of the small and ring fingers in a flexed position for filing the nail. Localizing the site of an ulnar nerve lesion is based on the topography of muscle weakness. The presence of the "nail file sign" points to a location at or proximal to the branch of the flexor digitorum profundus (FDP) muscle. We describe three patients who exhibited the "nail file sign," each with a different etiology. One patient had an ulnar nerve neuropathy at the elbow from compression by a cyst, another had ulnar neuropathy from entrapment at the cubital tunnel, and the third had it from an injury to the FDP tendons attached to the small and ring fingers. Testing for weakness of flexion at the DIP joints of the ring and small fingers should be an essential part of the clinical evaluation of patients with ulnar neuropathy, as it provides an important clue for anatomic localization of the lesion. Ultrasound studies complement the electrodiagnostic evaluation, not only to confirm the location but also to determine the cause of the ulnar nerve neuropathy.