Lumbrical-interossei motor studies localize ulnar neuropathy at the wrist

Ulnar nerve entrapment at the wrist (UNW) is uncommon and often difficult to localize electrophysiologically. The difference between the motor latencies to the median-innervated second lumbrical (2L) and ulnar-innervated palmar interosseous (Pl) (Diff 2L-Pl) has been shown to be of localizing value in patients with median neuropathy at the wrist. In the last year, we evaluated 2 patients with clinically definite ulnar neuropathy at the wrist. We performed motor studies to the 2L-Pl on the 2 patients and 12 disease controls with ulnar neuropathy at the elbow as follows: Using the same electrodes to record both the 2L and Pl, the median and ulnar nerves were each stimulated supramaximally above the wrist using identical distances. In the disease control subjects, the Diff 2L-Pl was essentially the same as normal controls (mean [0.13], range [(-0.3)-0.4]). In both patients with UNW, the Diff 2L-Pl clearly supported the routine electrophysiological studies in localizing the lesion (ulnar latencies were 1.1 and 1.8 ms longer than the median latencies). We conclude that the lumbrical-interosseous latency difference is useful in localizing ulnar nerve entrapment to the wrist.