Effect of a subdural air collection on the sensory evoked potential during surgery in the sitting position

Fifty-three cases with posterior fossa surgery were monitored using sensory evoked potentials (SEPs) and auditory evoked potentials (AEPs) simultaneously. Nine of them operated in the sitting position showed typical SEP changes during intracranial manipulations that were not attributable either to surgical or to anaesthetic handling. The SEP amplitude rapidly recovered when the patients were laid supine after operation. Skull X rays revealed intracranial air collections beneath the recording electrodes. The SEP changes were considered to be caused by conduction hindrance by the subdural air. In 1 patient, a way to overcome the air problem was demonstrated: bitemporal recording (T3-T4 montage) revealed little change in the SEP despite subdural air while the simultaneously recorded parieto-frontal record showed a drastic change.