Evaluation of motor function during thoracic and thoracolumbar spinal surgery based on motor-evoked potentials using train spinal stimulation

Study Design. Using compound muscle action potentials after train spinal stimulation, intraoperative motor functional monitoring was performed during thoracic and thoracolumbar spinal surgery. Objectives. This study was designed to clarify the clinical usefulness of train spinal stimulation and to determine the critical point of compound muscle action potential change at which neurologic injury during surgery occurs. of Background Data. In 1995 the authors reported that train spinal stimulation allows for the recording of compound muscle action potentials, even in animals and humans under general anesthesia. The facilitative effect of train stimulation overcomes the suppressive effects of anesthetics and allows potentials to pass through synapses, thereby enabling a reliable recording of lower extremity compound muscle action potential. Methods. Multisegmental recording of compound muscle action potentials after train spinal stimulation was conducted on 34 patients undergoing surgical treatment for thoracic or thoracolumbar lesions. During surgery, train stimuli (5 pulse, Interstimular Interval: 1 ms) were administered using an epidural electrode introduced transcutaneously. Compound muscle action potentials were recorded from a total of 128 muscles. Anesthesia was maintained using fentanyi and propofol or nitrous oxide with or without isoflurane. Muscle relaxation was attained mainly by controlled infusion of vecuronium bromide. The percent occurrence of recordable compound muscle action potentials was determined, and the potential changes were correlated with changes in muscle strength. Results. Compound muscle action potentials could be recorded from at least one muscle in 94% of the patients, even in most patients with severe motor dysfunction. The compound muscle action potential changes before and after surgical maneuver were divided into four grades. All compound muscle action potential changes in deteriorated muscles belonged to Grade 2 (a 10% latency delay) or Grade 3 (disappearance). Conclusions. The success rate in obtaining muscle potentials was greatly enhanced when all of the following methods were used: train spinal stimulation, anesthetic with weak suppressive effect, multiple muscle recording, and percutaneous introduction of epidural electrode. The critical point of compound muscle action potential change should be defined as a 10% latency delay or disappearance. Multisegmental muscle potential after train spinal stimulation is the most-appropriate method for thoracic and thoracolumbar spinal surgery.