[A method of electrical cervical motor root stimulation: effect of stimulation parameters and normal values]

Following percutaneous stimulation over the cervical column, a compound action potential (CMAP) can be recorded from the muscles of the arm. The actual site of excitation is located on the spinal motor roots, probably near to their exit from the spinal column. This method allows the electroneurographical assessment of the proximal brachial plexus and is used to calculate a central motor conduction time following motor cortex stimulation. So far, the optimal placement of the stimulating electrodes is unknown, and normal values must be established. We measured onset latencies of CMAPs of Mm biceps brachii and abductor digiti minimi (ADM) after supramaximal cervical motor root stimulation in 31 healthy subjects. Distances between the stimulating electrodes varying from 3 to 12 cm were studied. With 6 cm interelectrode distance, the electrodes were then placed either in a longitudinal or in a transversal direction, with the cathode over the interspace C7/T1 or C6/7. In addition, for the longitudinal stimulation, the effect of cathodal versus anodal stimulation over the C7/T1 interspace was studied. The results of various stimulating electrode positions and directions over the lower cervical column were then compared statistically. With the longitudinal electrode position, the cathode over C7/T1 and the anode placed 6 cm cranially, it was always possible to excite muscles from both sides simultaneously. When a very proximal stimulation site at the nerve roots was required in both muscles after one single stimulus, this method was superior to the transversal electrode position with the cathode C6/7 or C7/T1, or to the longitudinal electrode position with the anode over C7/T1 and the cathode placed cranially. For the ADM (29 subjects, 58 sides), the mean onset latency was 13.795 +/- 1.307 ms (mean +/- 1 SD), the mean side-to-side difference (SSD) was 0.452 +/- 0.388 ms. The mean amplitude was 7.71 +/- 2.64 mV, the mean side-to-side quotient (SDQ) was 41.88 +/- 97.59%. For M. biceps (29 subjects, 58 sides), the mean onset latency was 5.138 +/- 0.582 ms, the mean SSD 0.166 +/- 0.193 ms. The mean amplitude was 7.91 +/- 3.45 mV, the mean SSQ was 55.42 +/- 67.94%.