The effects of mexiletine on excitability properties of human median motor axons

Objective To investigate the effects of mexiletine, an analog of lidocaine, on excitability of human axons in vivo. Methods Threshold tracking was used to measure multiple excitability indices (strength-duration time constant, rheobase, refractoriness, supernormality, and threshold electrotonus) in median motor axons of 20 patients with neuropathic pain or muscle cramping, before and 3 months after treatment with oral 300 mg mexiletine per day. Results After treatment, there was a reduction in pain/muscle cramps, associated with decreased strength-duration time constants (P=0.01), increased rheobasic currents (P=0.06), and lower refractoriness (P=0.02), all of which were consistent with reduced nodal Na+ currents. Supernormality and threshold electrotonus did not change significantly. The changes in strength-duration properties suggest a decrease in persistent Na+ conductance. The lowered refractoriness after treatment might result from reduced transient Na+ currents, but the lack of change in supernormality and threshold electrotonus was not consistent with this hypothesis. Conclusions Oral mexiletine in a dosage of 300 mg daily suppresses persistent Na+ currents in human motor axons. Significance Measurements of the excitability indices can be used for non-invasive assessment and monitoring of the effects of mexiletine in patients with neuropathic pain or muscle cramps.