F97. Interaction of cathodal and anodal stimulations in nerve conduction studies

Introduction In nerve conduction studies (NCS), it has long been believed that the phenomenon of anodal block occurs under the anode during bipolar stimulation. However, recent studies demonstrated that anodal block hardly occurs, whereas anodal stimulation actually occurs instead, i.e. the action potential is generated from the anode itself. During our previous investigation on anodal stimulation, we unexpectedly discovered a phenomenon that the anodal stimulation was easy to occur when the cathode was placed away from the nerve trunk. Furthermore, careful observation in ordinary NCS reveals that the cathodal stimulation becomes difficult to elicit when the anode is moved away from the nerve during the effort to make the baseline flat. These findings suggest that the cathodal stimulation and anodal stimulation may interact with each other. In this study, we aimed to investigate the interaction of cathodal and anodal stimulations. Methods Subjects were 7 healthy volunteers. The ulnar nerve was stimulated at the wrist using two bipolar surface electrodes. Both stimulating electrodes were placed perpendicular to the nerve course. The cathode of one electrode was placed distally, whereas the anode of the other electrode was placed proximally separated by the inter-electrode distance of 23 mm. In this way, we reproduced the setting of ordinary NCS. Furthermore, we were able to change the stimulus intensity at cathode and anode independently. The ascending volley from the anode was evaluated by the antidromic mixed nerve action potential (MNAP) at the elbow, and the descending volley from the cathode was evaluated by the compound muscle action potential (CMAP) from the abductor digiti minimi muscle. In this study, we performed two experiments. In Experiment 1, we variously changed the stimulus intensity at the cathode, and investigated how the current required for the anodal stimulation changes. In Experiment 2, we variously changed the stimulus intensity at the anode, and investigated how the current required for the cathodal stimulation changes. Results In Experiment 1, when the stimulus intensity at the distal cathode was set at 0, 5, 10, 15, and 20 mA, the threshold of MNAP by anodal stimulation was 8.8 ± 2.0, 10.2 ± 1.8, 11.5 ± 2.3, 12.6 ± 2.8, and 13.6 ± 3.0 mA, respectively. In Experiment 2, when the stimulus intensity at the proximal anode was set at 0, 2, 4, 6, and 8 mA, the threshold of CMAP by cathodal stimulation was 2.9 ± 0.3, 2.8 ± 0.3, 2.6 ± 0.3, 2.5 ± 0.3, and 2.3 ± 0.3 mA, respectively. Conclusion During bipolar stimulation, the cathodal stimulation suppresses the anodal stimulation, whereas the anodal stimulation assists the cathodal stimulation. As a mechanism of such inter-electrode interaction, we postulated that the hyperpolarized zone formed around the depolarized cathode suppresses the anodal stimulation, whereas the depolarized zone formed around the hyperpolarized anode assists the cathodal stimulation.