Anodal block: can this occur during routine nerve conduction studies?

The median nerves of five normal subjects were electrically excited at the wrist with fine-tipped stimulating electrodes in a bipolar fashion. Compound sensory nerve action potentials (CSNAPs) were recorded from the index finger and compound muscle action potentials (CMAPs) from the thenar muscles. Both the cathode and the anode were positioned over the length of the nerve. Recordings were performed with different cathode-to-anode distances of 5, 10, 20, and in some cases, 30 mm. Just supramaximal CSNAPs and CMAPs were obtained initially with the cathode situated distal to the anode and then with the stimulus polarity reversed. There were no significant differences in the amplitude, duration, and morphology of the CSNAPs or CMAPs that were recorded by using different stimulus polarities. There was a consistent increase in the onset latency of the responses when the stimulus polarity was reversed (cathode located proximal to anode). This increase in latency was proportionate to the increase in distance from the cathode to the recording electrode. The effect of anodal block could not be observed from the above experiment.