The use of transcranial magnetic stimulation for monitoring descending spinal cord motor function

This report describes our initial clinical experience using transcranial magnetic stimulation for monitoring spinal cord motor function during surgical procedures. Motor evoked potentials were elicited using a cap shaped coil placed on the scalp of 27 patients while recording peripheral motor responses (compound muscle action potentials--CMAPs) from the upper (N = 1) or lower limbs (N = 26). Wherever possible, cortical somatosensory responses (SEPs) were also monitored by electrically stimulating the left and right posterior tibial nerve (N = 25) or the median nerve (N = 1). The judicious choice of anesthetic regimens resulted in successfully obtaining motor evoked responses (MEPs) in 21 of 27 patients and SEPs in 26 of 27 patients. Single pulse TMS resulted in peripheral muscle responses having large variability, whereas, the variability of SEPs was much less. Criteria based on response variability for assessing clinically significant changes in both MEPs and SEPs resulted in two false negative predictions for SEPs and none for MEPs when evaluating postoperative motor function. We recommend monitoring both sensory and motor pathways during procedures where placing the spinal cord at risk of damage.