THE USE OF A CIRCUMFERENTIAL CATHODE IMPROVES AMPLITUDE OF INTRAOPERATIVE ELECTRICAL TRANSCRANIAL MYOGENIC MOTOR EVOKED-RESPONSES

Measurement of motor evoked responses to transcranial electrical stimu lation (tc-MER) is a technique for intraoperative monitoring of motor pathways. Since most anesthetics significantly reduce motoneuronal exc itability, optimal stimulation paradigms should be sought. We compared the efficiency of stimulus delivery using two different configuration s of the cathode component of the stimulating electrode pair (circumfe rential: F-z, F-3, F-4, A(1), and A(2) versus a single cathode at F-z) . The anode was positioned at C-z with both cathode configurations. Fo urteen neurologically normal patients undergoing spinal surgery were a nesthetized with sufentanil-N2O-ketamine. Partial neuromuscular blocka de (single twitch height 25%) was maintained with vecuronium. Compound action potentials to transcranial stimulation with both cathode confi gurations were recorded from the tibialis anterior muscle. All recordi ngs were completed before spinal manipulation. The median amplitude re sponse using the F-z cathode configuration was 256 mu V (10th-90th per centiles: 50-641 mu V). With the circumferential cathode configuration , tc-MER amplitude increased to 281 (87-1479) mu V (P < 0.01). There w as no significant difference in onset latency between electrode config urations. The observed tc-MER amplitude augmentation with the use of a circumferential cathode might allow tc-MER monitoring in those patien ts who do not have sufficiently reproducible responses when a single c athode is used. A possible explanation is that the circumferential cat hode alters the direction of electrical currents in the cortex, result ing in more efficient depolarization of cortical motor neurons.