Recovery of memory after general anaesthesia: clinical findings and somatosensory evoked responses.

BACKGROUND Mid-latency somatosensory evoked responses are used to monitor the integrity of the sensory pathways intra-operatively. They can quantify the effects of anaesthetics on the central nervous system. Mid-latency auditory evoked responses have been related to cognition during anaesthesia, but there are no detailed studies using median nerve somatosensory evoked responses (MnSSER). METHODS We studied 49 patients during recovery from general anaesthesia (isoflurane/nitrous oxide or propofol) to assess implicit and explicit memory function in relation to mid-latency MnSSER. The MnSSER recordings were made before anaesthesia, during steady-state anaesthesia, and at the end of the recovery period. The patients were interviewed 24 h later about their memory for the immediate wake up phase. Statistical analysis was by multivariate analysis of variance. RESULTS Out of 49 patients, 23 recalled the recovery period, 11 had implicit memory for an object shown to them during the recovery period, and 15 did not have any memory for the recovery period. At RECOVERY the patients with recall had significantly shorter MnSSER latencies N45 and P50 and inter-wave conduction times LatN35-LatP45 than patients without memory (P<0.05). CONCLUSIONS We conclude that MnSSER components warrant further investigation for studying the effects of anaesthetic drugs on cognitive function.