Inverse volume-conductor method for noninvasive assessment of nerve injuries

A standard method for characterizing focal nerve lesions is to estimate the fraction of nerve fibers that cross the lesion. In animals, this fraction can be computed electrophysiologically by excising and crushing the nerve to record monophasic compound action potentials (MCAPs) whose areas are proportional to the number of active fibers. Under in situ conditions, however, CAPs are triphasic, and their area is not proportional to the number of active fibers because of phase cancellation. Volume conduction theory is used to compute the excised MCAP (EMCAP) from the triphasic CAP recorded on the skin surface. Treating the nerve as a line source in a semi-infinite uniform volume conductor, the integral equation relating the source and the surface CAP is obtained. Then the EMCAP is computed in the time domain using the method of collocation. In experimental studies, calculated EMCAP area ratios agreed accurately with actual excised MCAP area ratios, provided that the stimulus artifact was eliminated