Loss and Recovery of Consciousness Have Distinct Neurobiology

Jim Kling

October 21, 2010

October 21, 2010 (San Diego, California) — New research suggests that there are distinct patterns of neurobiology that occur during loss of consciousness and recovery following anesthesia. The work was presented here at the American Society of Anesthesiologists 2010 Annual Meeting.

The descent into and the return from consciousness during general anesthesia remains a mysterious process; researchers disagree whether the transitions are continuous or sudden jumps between binary states. Information integration theory suggests that consciousness loss and recovery occur in a graded fashion, but other approaches support nonlinear phase transitions.

"In the past 5 to 10 years, based on neuroimaging data and EEG [electroencephalogram] data, people have started to think about how different areas of the brain communicate with one another, and how that's interrupted during general anesthesia. We're trying to get a more refined sense of what happens to networks in the brain during general anesthesiology," George Mashour, MD, PhD, assistant professor of anesthesiology and neurosurgery at the University of Michigan in Ann Arbor, who participated in the research, told Medscape Medical News.

The researchers used mathematical graph theory to assess network alterations and connection strengths, to see if they changed gradually or abruptly. They studied 20 normal male volunteers who were anesthetized using propofol. A 21-channel EEG was measured at baseline, during anesthesia, and during recovery. The researchers constructed a network comprised of cross-correlations among channels that significantly deviated from randomized data.

Dr. Mashour's team used a measure of global efficiency based on shortest path length to evaluate the optimal information transfer throughout the brain network. Across different states, the researchers measured the effects of changes in the structural network and connection strength on optimal information transfer.

The analysis revealed that the optimal network structure in the baseline conscious state experienced a global disruption at loss of consciousness, but underwent a gradual restoration about 5 minutes before recovery, Dr. Mashour reported.

Connection strength revealed a different picture. There was no change in its effect on optimal information transfer during loss of consciousness, but there was a precipitous effect at the point of recovery, and it then returned quickly to baseline. "Alterations of network structure were associated with induction, whereas alterations of connection strength were associated with emergence," the authors write in the abstract.

Dr. Mashour described the sudden increase of connection strength during recovery as a point of cognitive "ignition" within the optimal brain network structure.

"There have been data in the past few years suggesting that there's a distinct neurobiology in the induction of anesthesia and the emergence from anesthesia. What we've done is support that notion by showing network level effects that were distinguishable," he said.

The team also found some interesting effects in the parietal regions of the brain. "There was a more marked disruption of parietal processing than frontal processing. That's interesting, because we normally use electrodes on the forehead. [It could be that] we should be looking at different areas of the brain, even though they may be less accessible. We need to explore if there are ways of monitoring more critical regions of information processing in the brain that may tell us more about anesthetic-induced unconsciousness," said Dr. Mashour.

The change in the parietal cortex "is potentially predictive of the way consciousness dissipates and reintegrates. A key question is how specific those findings are to the parietal cortex," Max Kelz, MD, PhD, assistant professor of anesthesiology at the University of Pennsylvania in Philadelphia, who attended the presentation, told Medscape Medical News. "I think we're a long way off from not just having empirical monitors [of consciousness], which are used today, but rationally designed monitors that would be predictive of events," Dr. Kelz added.

The study did not receive commercial support. Dr. Mashour and Dr. Kelz have disclosed no relevant financial relationships.

American Society of Anesthesiologists (ASA) 2010 Annual Meeting: Abstract 783. Presented October 18, 2010.

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