Electrophysiologic analysis of snap amplitude in orthodromic and antidromic studies

In 1966, Buchthal and Rosenfalk established that the sensory nerve action potential (SNAP) amplitude increased with increasing number of nerve fibers. Lambert and Dyck's in vitro study of the sural nerve, however, found the SNAP amplitude of a single fascicle was equivalent to the SNAP amplitude of the entire nerve. The current study confirmed Buchthal and Rosenfalk's original findings, and discovered an in vivo equivalent to Lambert and Dyck's findings. Ten subjects were used as their own controls. Orthodromic stimulation of both the index and middle fingers stimulated individually versus simultaneously revealed a significant increase in amplitude (p < 0.001). When the recording and stimulating electrodes were reversed, without physically changing the electrodes on the subject, there was no significant difference between the antidromic SNAP amplitudes recorded from separate fingers, or from both fingers simultaneously. Although in both cases the number of recorded nerve fibers approximately doubled, the effect on the SNAP amplitude was very different. This seemingly paradoxical result can be explained by electrophysiologic differences in the recording methods, without using the concept of fiber density.