Somatosensory evoked potentials (SEPs) were monitored during 20 operations, 13 for tumors in the posterior fossa and upper cervical region and seven for the treatment of aneurysms or carotid cavernous fistulae. Monitoring was technically satisfactory in 16 patients (80%). In two patients, monitoring was compromised by contamination with electrical noise. Two patients with mass lesions in the upper cervical region and posterior fossa were operated in the park bench position and SEP wave forms disappeared, probably because of the positioning, which included head flexion and rotation. Three of the 16 patients (19%) showed alterations in SEP wave forms. The changes were transient in two patients, who had no postoperative neurological deficits, and permanent in one, who exhibited motor palsy postoperatively. Intraoperative SEP alterations corresponded to the postoperative neurological findings in 14 cases (88%). Two patients with no intraoperative SEP alteration were found to have pyramidal tract damage postoperatively (false negative results). Intraoperative SEP monitoring appears useful in terms of minimizing the risk of neurological damage during occlusion of major cerebral arteries and also during manipulation in the upper cervical region. However, it appears necessary to develop a new monitoring method that will reflect the functional integrity of the motor pathway itself. SEPs and brainstem auditory evoked potentials (BAEPs) were simultaneously monitored during 10 posterior fossa operations, and SEP monitoring appeared to be a less sensitive indicator of brainstem function than BAEP monitoring. The authors' results suggest that alterations in the SEP wave form are reversible, reverting to normal if surgical manipulation is stopped immediately after the disappearance of the wave form.
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