Muscle membrane polarisation after provocative tests, and after cooling: the normal CMAP changes to be expected

Abstract

Objective To know the range of changes of compound muscle action potentials (CMAPs) in the muscles innervated by the ulnar nerve after diverse provocative tests, 14 healthy patients were studied with the same protocol. Methods CMAPs were measured at rest, just after a short exercise test (SET), during short 5 and 10 c/s repetitive nerve stimulation (RNS) trains, at ∼32 and ∼20 °C. Results At 32 °C, the SET induced a significant but transient enlargement of the CMAPs (amplitude increased by 8.3%, duration decreased by 9%) that was only partially reproduced by RNS trains, except for a significant shortening of the CMAPs at 10 c/s. At 20 °C without exercise, CMAPs increased significantly by ∼30% in amplitude, duration and area, and after the SET the inverse of what has been seen at 32 °C was observed (amplitude decreased by 1.7% and duration increased by 9%). RNS at 20 °C produced a marked interpatient heterogeneity except for a significant shortening of the CMAPs at 10 c/s. In one pure autonomic failure patient, the infusion of norepinephrine induced potentiation of the responses at rest and a decrease in the expected changes after provocative tests. Conclusions CMAP amplitude and duration are significantly modified just after the SET at 32 °C, at rest at 20 °C and after RNS at 10 c/s but not at 5 c/s. Although providing indirect evidence, these findings indicate that provocative tests make the muscle membrane hyperexcitable by the way of a direct influence on the electrical events and by an indirect local catecholamine spillover.