Anterior spinal cord injury with preserved neurogenic 'motor' evoked potentials

Objective: To describe two cases in which intraoperative monitoring of neurogenic 'motor' evoked potentials (NMEPs) did not identify a spinal cord injury that resulted in paraplegia. Methods: Bilateral tibial nerve somatosensory evoked potential (SEP) and NMEP testing was performed in two patients during spinal deformity corrective surgery using standard stimulation and recording parameters. These potentials were obtained repetitively throughout the primary procedures and were performed again during a subsequent procedure that took place after the discovery of paraplegia. Results: SEP and NMEP signals were preserved in both patients and no adverse events were identified during the initial procedures. Postoperatively, paraplegia was identified immediately upon recovery from anesthesia and preserved posterior column function was apparent on clinical exam. In the procedures following the discovery of paraplegia, SEP and NMEP signals remained comparable with signals elicited in the initial surgeries. Conclusions: Based on these cases and previously published experimental evidence, we conclude that while 'NMEPs' remain a useful second test of spinal cord function, they are not reliable indicators of motor tract function. An alternate term, such as 'spinally-elicited peripheral nerve responses' should be used.