Regional differences in ulnar nerve excitability may predispose to the development of entrapment neuropathy

To assess whether there are differences in nerve excitability properties between proximal and distal stimulation sites in the ulnar nerve in healthy controls, which may provide information on whether alteration in ion channel function predisposes to the development of ulnar neuropathy at the elbow.

Nerve excitability studies were undertaken in 11 healthy controls. Studies were undertaken with stimulation of the ulnar nerve at the elbow and wrist. Recordings were obtained from abductor digiti minimi in both sets of studies.

Recordings obtained following stimulation of the nerve at the elbow demonstrated significant differences to those obtained following stimulation of the nerve at the wrist. Specifically, there was a left shift in stimulus-response curves at the elbow compared to the wrist, with prolonged strength-duration time constant, and reduced rheobase (P<0.05). These changes were accompanied by increased refractoriness and reductions in superexcitability and late subexcitability (P<0.05).

The present findings may suggest relative depolarization of ulnar nerve axons at the elbow.

These changes may reflect regional differences in axonal Na(+)/K(+) pump function and thereby predispose the ulnar nerve to conduction failure and axonal degeneration when exposed to trauma.