The effect of ischaemia on nerve conduction in the carpal tunnel syndrome

Abstract

A study of conduction in motor fibers of the median nerve during ischemia was made in 14 patients with a clinical diagnosis of carpal tunnel syndrome and in 10 control subjects. In each case a pneumatic cuff around the upper arm was inflated to above arterial pressure for 30 minutes to produce ischemia. Studies were made of the effects of ischemia on the size of the thenar muscle action potentials, changes in latency, the recovery of nerve conduction following ischemia and the susceptibility to ischemia in the carpal tunnel syndrome. The amplitude and area of the muscle action potential decreased more rapidly during ischemia in two-thirds of the carpal tunnel patients than in the control subjects. When changes in latency were compared, differences between the two groups were less marked. Recovery of nerve conduction, after release of the cuff, occurred rapidly in both patients and controls. Susceptibility to ischemia in the carpal tunnel patients was closely related to the severity of pain and paresthesia in the days preceding the test. Both ischemia and direct pressure appear to contribute to the abnormality of nerve function. It is suggested, however, that in the carpal tunnel syndrome two mechanisms may be operable, (1) a rapidly reversible change in nerve fibers associated with ischemic attacks, and (2) a slowly developing structural change in fibers resulting from pressure on the nerve under the flexor retinaculum.