Multiple measures of axonal excitability: a new approach in clinical testing

From measurements of nerve excitability and the changes in excitability produced by nerve impulses and conditioning currents, it is possible to infer information about the membrane potential and biophysical properties of peripheral axons. Such information cannot be obtained from conventional nerve conduction studies. This article describes a new method that enables several such measurements to be made on a motor nerve quickly and reproducibly, with minimal operator intervention. The protocol measures stimulus-response behavior using two stimulus durations (from which the distribution of strength-duration time constants can be estimated), threshold electrotonus to 100-ms polarizing currents, a current-threshold relationship (indicating inward and outward rectification), and the recovery of excitability following supramaximal activation. The method was tested on 30 healthy volunteers, stimulating the median nerve at the wrist and recording from the abductor pollicis brevis. The results were comparable with previously published normal data, but the recordings took less than 10 min. The convenience and brevity of the new method make it appropriate for routine clinical use.