2-A-D-4. A revisional study on intraoperative recording of motor potential evoked by transcranial electrical stimulation

We reviewed the monitoring condition of intraoperative motor evoked potential (MEP) clinically applied to neurosurgical field. Our study population was comprised of 256 patients who underwent intraoperative MEP monitoring during surgery for supratentorial lesions. We electrically stimulated the motor cortex transcranially. Electromyograms (EMGs) were recorded from the flexor carpi ulnaris (FCU), abductor pollicis brevis (APB), tibialis anterior (TA), gastrocnemius (GA), abductor hallucis brevis (AHB) muscles. We investigated the optimal interstimulus interval (ISI) and numbers of stimulus train. MEP amplitudes decreased inversely proportional to the ISI. However, there were 2 peaks of MEP amplitudes at around the ISI of 1.5 ms and 2.8 ms. MEP amplitudes increased in proportional to the numbers of train; however, were saturated with 4 trains in APB, FCU, and with 5 trains in TA, GA, and AHB. The former declination of MEP amplitude is from duration of EPSP at spinal anterior horn cell, and the 2 peaks are from the interaction of interneuron within motor cortex. MEP amplitude varies depending on ISI, and target muscle. Accordingly, to obtain maximal MEP amplitude as intraoperative monitoring, it is advisable to adjust the numbers of stimulus train or ISI.