Evidence of altered motor axon properties of the ulnar nerve in carpal tunnel syndrome

Objective To analyse recruitment properties of ulnar nerve motor axons in 60 CTS patients with negative ulnar nerve electrodiagnostic tests. Methods Recruitment properties of the ulnar nerve were studied by analysing the relationship between the intensity of electrical stimulation and the size of motor response, i.e. the stimulus–response curve. Parameters of the curve (threshold, slope and plateau) were compared with those of the corresponding curve of the median nerve and both with parameters of 30 control curves. Results The ulnar nerve stimulus–response curve was strikingly abnormal and, except for severity, closely resembled that of the median nerve. The slope of the curve was significantly less than that of controls and decreased with increasing abnormalities of the median nerve. This suggested that the pathological process involving the ulnar nerve was contingent with the severity of median nerve involvement. Conclusions We propose that the ulnar nerve may be subject to compression in Guyon’s canal as a consequence of high pressure in the carpal tunnel of CTS patients. Significance Ectopic activity from ulnar axons may contribute to clinical spread of symptoms outside the median nerve territory in CTS. This does not exclude possible involvement of central plasticity mechanisms in producing extra-median symptoms in CTS patients.