Anodal Block

When depolarization under the cathode occurs, there is also hyperpolarization of the nerve beneath the anode. The hyperpolarization is without significance in NCS unless the cathode and anode are inadvertently reversed. When this happens, depolarization occurs 2 to 3 cm proximal to the point that has been marked for depolarization, resulting in apparent prolonged

Fig. 1. Virtual cathode. From top to bottom, the first waveform shows a slightly submaximal motor response. The second stimulus produces a maximal response. The third stimulus is of 25% higher intensity and generates a supramaximal response that is unchanged in amplitude or latency. A further increase in stimulus intensity produces a response that is of slightly faster latency without a change in amplitude (virtual cathode). The virtual cathode effect is demonstrated again in the bottom tracing after another increase in intensity.

Fig. 1. Virtual cathode. From top to bottom, the first waveform shows a slightly submaximal motor response. The second stimulus produces a maximal response. The third stimulus is of 25% higher intensity and generates a supramaximal response that is unchanged in amplitude or latency. A further increase in stimulus intensity produces a response that is of slightly faster latency without a change in amplitude (virtual cathode). The virtual cathode effect is demonstrated again in the bottom tracing after another increase in intensity.

distal latencies, and the area of hyperpolarization may block some of the impulses coming from the proximal depolarized zone (anodal block).

Reversing the orientation of the stimulator (cathode proximal) is routinely used to avoid anodal block during F-wave measurements. However, a distal compound motor action potential (CMAP) is still generated despite the hyperpolarized zone, although there may be a subtle reduction in amplitude that goes unnoticed. The corollary is that F-waves are readily obtained even if the operator fails to reverse the position of the stimulator.

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