Intra-operative recording of cortical somatosensory evoked potentials as a method of spinal cord monitoring during spinal surgery.

The experience with cortical somatosensory evoked potential (SEP) recording during 13 cases of spinal neurological surgery is described. Good quality cortical SEPs were obtained in eight patients with a variety of intradural and extradural spinal disorders. The short latency components of these waveforms were stable during anaesthesia with nitrous oxide, 0.5% halothane and fentanyl. Fluctuations in signal amplitude were, however, common. In the one patient in whom the cortical SEP waveform was distorted intra-operatively, there was an increased neurological deficit. In one normal and four patients with impairment of dorsal column function, no intra-operative cortical SEP was recorded. In these five patients spinal SEPs were recorded rostral to the level of spinal disease. Monitoring spinal cord function using cortical SEPs can provide useful neurophysiological information, however, there are limitations to its utility. These relate to difficulties in signal acquisition, the low signal amplitude, attenuation of the signal during intramedullary surgery and uncertainties in signal interpretation. All these problems are exacerbated if the patient has a pre-operative clinical somatosensory deficit. Although most of these problems can be overcome using spinal SEP monitoring, intra-operative SEP recordings are not an infallible guide to spinal cord integrity since they reflect the functional status of only the dorsal column-medial lemniscus pathway.