Transcranial electrical stimulation as predictor of elicitation of intraoperative muscle-evoked potentials

Study Design. Preoperative electrophysiological and neurologic findings from patients with cervical myelopathy were evaluated statistically to determine their predictive value relative to the success of eliciting intraoperative motor-evoked potentials. Objectives. To determine which preoperative variables accurately predicted the success of eliciting an intraoperative muscle-evoked potential. Summary of Background Data. Motor-evoked potential recorded from the muscles after transcranial electrical stimulation is one of the most widely used methods for intraoperative spinal cord monitoring. However, motor-evoked potentials recorded from lower limb muscles are not detectable in patients with severe cervical myelopathy. Therefore, it is helpful to know the probability of the intraoperative transcranial electrical stimulation-motor evoked potential elicitation before the operation. Methods. There were 38 patients with cervical myelopathy. Before the operation, motor-evoked potentials following transcranial magnetic stimulation were recorded from the flexor hallucis brevis, and central motor conduction times were measured. Neurologic function was evaluated using the Japanese Orthopedic Association score. During the operation, transcranial electrical stimulation-motor evoked potential from the flexor hallucis brevis was recorded. The Japanese Orthopedic Association score, threshold intensity of magnetic stimulation, and central motor conduction times were statistically evaluated for their potential of being predictors. Results. The intraoperative transcranial electrical stimulation-motor evoked potential was detectable in all cases in which the preoperative transcranial magnetic stimulation-motor evoked potential was elicited by a lower intensity than 50% of the maximum output of the stimulator. Therefore, simultaneous use of other methods of monitoring should be considered in such cases that need higher output. However, the Japanese Orthopedic Association score or central motor conduction times were not useful criteria. Conclusions. The threshold intensity of the preoperative transcranial magnetic stimulation-motor evoked potential was helpful in predicting elicitation of the intraoperative transcranial electrical stimulation-motor evoked potential.