Electrophysiological diagnosis of interdigital neuropathy of the foot.

We have devised a method of sensory nerve conduction in the interdigital nerves of the foot using the near-nerve needle and signal averaging techniques. In 40 normal controls between the ages of 20 and 59, there was no difficulty in recording sensory compound nerve action potential (CNAP) in the various interdigital nerves. The amplitude of sensory CNAP ranged from 0.4-8.8 muV. The mean maximum nerve conduction velocities ranged from 33.3-40.5 M/second. In the group between 50-59 years old, nerve conduction changes due to aging were noted. In five patients with interdigital neuropathy (IDN) of the foot, a selective marked decrease in the amplitude of the CNAP in the involved interdigital nerve ("abnormal dip phenomenon") was the most characteristic electrophysiological finding. The present technique is capable of recording the sensory CNAPs in the interdigital nerves of the foot and confirming the diagnosis of IDN electrophysiologically.