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Severe sensory deficits with preserved motor function after removal of a spinal arteriovenous malformation: correlation with simultaneously recorded somatosensory and motor evoked potentials.

pinal cord function can be monitored during surgery using somatosensory evoked potentials S (SSEPs) which measure ascending conduction in dorsal columns (1). More recently, conduction in corticospinal tracts has been monitored using myogenic motor evoked responses to transcranial stimulation (24) . The development of techniques for monitoring motor function was prompted, in part, by the occurrence of false-negative results with SSEP monitoring (5,6). However, even when motor evoked response monitoring is available it may nonetheless still be important to monitor SSEPs. We report a case in which posterior tibial nerve somatosensory evoked potentials (PTN-SSEPs) and myogenic motor evoked potentials to transcranial electrical stimulation (tce-MEPs) were monitored throughout an operation for excision of an arteriovenous malformation (AVM) of the conus medullaris. Immediately before final removal of the AVM, the PTN-SSEPs were lost, whereas the tce-MEPs remained unchanged. Postoperatively, there was a severe sensory deficit in both lower extremities involving predominantly proprioception. Although no motor deficits were observed, the loss of proprioception resulted in severe gait impairment. This case confirms that PTN-SSEPs and tce-MEPs monitor distinctly different pathways in the spinal cord and reveals the