Intraoperative changes in transcranial motor evoked potentials and somatosensory evoked potentials predicting outcome in children with intramedullary spinal cord tumors

Abstract

Object: Intraoperative dorsal column mapping, transcranial motor evoked potentials (TcMEPs), and somatosensory evoked potentials (SSEPs) have been used in adults to assist with the resection of intramedullary spinal cord tumors (IMSCTs) and to predict postoperative motor deficits. The authors sought to determine whether changes in MEP and SSEP waveforms would similarly predict postoperative motor deficits in children.

Methods: The authors reviewed charts and intraoperative records for children who had undergone resection for IMSCTs as well as dorsal column mapping and TcMEP and SSEP monitoring. Motor evoked potential data were supplemented with electromyography data obtained using a Kartush microstimulator (Medtronic Inc.). Motor strength was graded using the Medical Research Council (MRC) scale during the preoperative, immediate postoperative, and follow-up periods. Reductions in SSEPs were documented after mechanical traction, in response to maneuvers with the cavitational ultrasonic surgical aspirator (CUSA), or both.

Results: Data from 12 patients were analyzed. Three lesions were encountered in the cervical and 7 in the thoracic spinal cord. Two patients had lesions of the cervicomedullary junction and upper spinal cord. Intraoperative MEP changes were noted in half of the patients. In these cases, normal polyphasic signals converted to biphasic signals, and these changes correlated with a loss of 1-2 grades in motor strength. One patient lost MEP signals completely and recovered strength to MRC Grade 4/5. The 2 patients with high cervical lesions showed neither intraoperative MEP changes nor motor deficits postoperatively. Dorsal columns were mapped in 7 patients, and the midline was determined accurately in all 7. Somatosensory evoked potentials were decreased in 7 patients. Two patients each had 2 SSEP decreases in response to traction intraoperatively but had no new sensory findings postoperatively. Another 2 patients had 3 traction-related SSEP decreases intraoperatively, and both had new postoperative sensory deficits that resolved. One additional patient had a CUSA-related SSEP decrease intraoperatively, which resolved postoperatively, and the last patient had 3 traction-related sensory deficits and a CUSA-related sensory deficit postoperatively, none of which resolved.

Conclusions: Intraoperative TcMEPs and SSEPs can predict the degree of postoperative motor deficit in pediatric patients undergoing IMSCT resection. This technique, combined with dorsal column mapping, is particularly useful in resecting lesions of the upper cervical cord, which are generally considered to be high risk in this population. Furthermore, the spinal cord appears to be less tolerant of repeated intraoperative SSEP decreases, with 3 successive insults most likely to yield postoperative sensory deficits. Changes in TcMEPs and SSEP waveforms can signal the need to guard against excessive manipulation thereby increasing the safety of tumor resection.

Keywords: CUSA = cavitational ultrasonic surgical aspirator; IMSCT = intramedullary spinal cord tumor; MRC = Medical Research Council; SSEP = somatosensory evoked potential; TcMEP = transcranial motor evoked potential; dorsal column mapping; neuromonitoring; pediatric; spinal cord tumor; spine; transcranial motor evoked potential.

Fig. 1

Intraoperative photograph of dorsal column mapping with a microstimulator.

Fig. 2

Representative illustration of the 3 waveforms observed during tumor resection. Transition from polyphasic (A) to biphasic (B) or no signal (C) predicted postoperative motor deficits in all cases.

Fig. 3

Representative case of an intramedullary thoracic tumor (A) with intraoperative TcMEP waveform degradation from polyphasic (B) to biphasic signal (C and D). Linear correlation of the percent change in MEP signal versus the change in motor grade on postoperative Day 1 (p < 0.001, R² = 0.8533).

Fig. 4

Six patients demonstrated TcMEP signal changes intraoperatively. Among these patients, 1 had complete loss of waveform, whereas 5 changed to biphasic signals. Changes in motor grade are depicted at the postoperative, discharge, and last follow-up evaluation.