Monitoring the integrity of somatosensory pathways with evoked electroencephalographic bursts

During isoflurane-induced electroencephalographic (EEG) suppression, external stimuli evoke high-amplitude cortical responses (bursts). We tested whether bursts evoked by somatosensory stimuli would reliably distinguish intact somatosensory pathways from pathways in which peripheral nerve conduction had been blocked by local anesthetic. Ten subjects were anesthetized with isoflurane until burst suppression was achieved. During EEG suppression, they were given somatosensory stimulation, consisting of 3-s episodes of 60 electric pulses (20 mA, 0.2 ms), to the tips of the left and right fifth fingers alternately for 10 min. One finger was then anesthetized at the base of the proximal phalanx with prilocaine and the other finger was injected with saline in a double-blind manner. The stimulation was continued for 20 min. In nine patients, the disappearance of bursts in response to stimuli applied to the anesthetized finger clearly indicated the side of the conduction block. After the injection of local anesthetic, there was a predominance of offset bursts over onset bursts in response to stimuli applied to the anesthetized finger (P < 0.05) before the responses disappeared. We conclude that evoked bursts merit further investigation for potential use in monitoring the integrity of neural pathways.