Influence of the transverse distance between surface recording electrodes and sensory nerves.

The extent of the influence of the transverse distance between surface recording electrodes and the lateral antebrachial cutaneous and sural nerves when performing orthodromic recordings of sensory nerve action potential amplitudes was investigated on 60 healthy volunteers, using a stringent protocol. The recording electrodes were positioned on top of the studied nerves and per 5 mm to each side. A sensory nerve action potential was measured and recorded on every position until no action potential could be measured any more. The mean range varied from 65 mm ulnar to 60 mm radial for the lateral antebrachial cutaneous nerve and from 55 mm medial to 55 mm lateral for the sural nerve. Since amplitude decreases 16% every 5 mm for the lateral antebrachial cutaneous nerve and 12% every 5 mm for the sural nerve, it is better not to determine the location of recording electrodes merely in accordance anatomical charts, but to adjust the electrode placement individually in order to detect the location where maximal amplitude of a SNAP can be recorded.