Neurogenic motor evoked potentials: a prospective comparison of stimulation methods in spinal deformity surgery.

Neurogenic motor evoked potentials (NMEPs) elicited by spinal cord stimulation via the spinous processes (SP-NMEP) have been widely accepted as a sensitive method of monitoring motor tract function. SP-NMEP requires additional surgical dissection as well as electrodes within the wound, making the method somewhat inconvenient. A less invasive percutaneous method of spinal cord stimulation (PERC-NMEP) has more recently been described. We prospectively compared the SP-NMEP and PERC-NMEP methods in 184 patients undergoing 225 surgical procedures. Although SP-NMEP responses were more readily obtainable than PERC-NMEP, the reliability of the two methods was not significantly different. Both methods were found to be sensitive to neurologic deficit. The present study suggests that when responses are obtained, the percutaneous method is reliable enough to obviate the spinous process method of monitoring the motor function of the spinal cord.

Lack of specificity in electrophysiological identification of lower sacral roots during selective dorsal rhizotomy.

The authors investigated the efficacy of anal sphincter electromyography (EMG) in identifying the lower sacral roots during selective dorsal rhizotomy. In nine children undergoing selective dorsal rhizotomy for cerebral palsy (CP) spasticity, direct electrical stimulation of the L1-S5 dorsal and ventral roots was performed while monitoring EMG responses from the anal sphincter and lower-extremity muscles. Anal sphincter activation was seen with stimulation of lumbosacral roots at many levels. Stimulation of dorsal and ventral roots gave anal sphincter EMG responses in 100% of the dorsal and ventral roots from L-4 and caudally. Only at the L-1 level did a minority of nerve roots have anal sphincter response to stimulation. Patterns of extremity muscle and sphincter activation specific to the S3-5 roots, namely anal sphincter activation without activation of other muscle groups, were found in only five (22%) of 23 roots stimulated. The pattern of stimulation responses in the majority of S3-5 roots indicated that the pathophysiology of lower-extremity spasticity in CP may involve the anal sphincter and does not spare the lower sacral roots. Thus, this study indicates that electrophysiological mapping alone, without anatomical identification, cannot be used to identify the lower sacral roots during selective dorsal rhizotomy for CP spasticity, and it proposes a model for investigation of associated bowel and bladder symptoms.

Somatosensory evoked potentials fail to diagnose thoracic outlet syndrome.

Somatosensory evoked potentials (SEPs) are used in the diagnosis of thoracic outlet syndrome (TOS), even as an indication for surgery. The purpose of this study was to evaluate the use of SEPs in the diagnosis of TOS. Twenty-one patients (mean age, 37 years) with TOS and 23 control subjects (mean age, 34 years) were included. Somatosensory evoked potentials of median and ulnar nerves were measured bilaterally in patients in both a relaxed and arms-elevated provocative position. A three-way analysis of variance showed no significant difference between the interpeak latencies of the TOS and control groups (p = .352). Significant differences were found in testing positions (p = .0014) and nerve tested (p = .001) in both groups. Therefore, this study suggests that SEPs are not helpful in the diagnosis of TOS.

Validity and reliability of spinal cord monitoring in neuromuscular spinal deformity surgery.

Somatosensory evoked potentials (SSEPs) have become a standard of care in surgery for spinal deformity. Recent reports in the literature have suggested SSEP monitoring is not efficacious in surgeries for patients with neuromuscular disease. Electrophysiologic data were retrospectively analyzed from 74 patients with various neuromuscular disorders undergoing spinal-deformity surgery from 1989 through February 1995 at this medical center. The monitoring protocol included SSEPs recorded from multiple sites located cortically, subcortically, and peripherally. Neurogenic motor evoked potentials (NMEPs) were also employed. Anesthetic regimens were controlled for compatibility with evoked potential monitoring. Use of this intraoperative monitoring protocol resulted in reliable data for 95% of the patient population having baseline responses. Findings suggest that evoked potentials can be used effectively during surgery for neuromuscular spinal deformity. Use of a specific protocol allowed acquisition of reliable data intraoperatively, suggesting these methods are a valid means of monitoring neurologic status.