Double peak sensory responses at submaximal stimulation

Objective: The objective of the study was to obtain knowledge about the different physiological situations where a double peak sensory response normally occurs and to better understand the significance of this particular sensory response. Methods: In 14 healthy subjects, conventional orthodromic sensory nerve conduction studies were performed on the median and ulnar nerves using submaximal stimulation. Various stimulus strengths, polarity, electrode positions and local anaesthesia were used to clarify the generation of the two peaks. Results: When the cathode and the anode were independently moved in distal direction, the first and the second peaks moved distally, respectively. This occurred for conventional and reversed position of the electrode. Anodal stimulation was ineffective after local skin anaesthesia. Conclusions: Our experiments seem to indicate that the double response represents the two stimulation sites, under the cathode and the anode, respectively. Obviously the double response can only occur if different axons are stimulated under the two poles. The cathode and the anode do not seem preferably to stimulate fast or slow axons. Studies with superficial anaesthesia may indicate that the cathode stimulates the sensory nerve directly while the anode mainly stimulates superficial structures, skin sensory receptors or intradermal nerve terminals.