Coma and General Anesthesia Demonstrate Important Similarities

A new study claims that the brain under general anesthesia (GA) is not in a state of sleep but rather in a state that is a reversible coma.

Researchers at the Weill Cornell Medical College (New York, NY, USA), Massachusetts General Hospital (MGH, Boston, USA) and the University of Michigan (Ann Arbor, USA) have published an extensive review article of GA, sleep, and coma, explaining that a fully anesthetized brain is much closer to the deeply unconscious low-brain activity seen in coma patients than to a person asleep, and that essentially, GA is a coma that is drug-induced, and, as a consequence, reversible. The two states operate on different time scales--GA in minutes to hours, and recovery from coma in hours to months to years, if ever. The study of emergence from GA and recovery from coma could help to understand better how both processes occur.

One critically important circuit the authors describe is between the cortex, the thalamus--a node of neural tissue at the center of the brain--and the basal ganglia. These regions are connected via two feedback loops; one is a negative feedback release on behavior, which is always active when overall brain activity is reduced, such as stopping a sleeping person from physically acting out their dreams. The second feedback loop, however, releases the brake imposed by the first feedback loop, which certain anesthetic drug scan trigger to function, producing what is known as "paradoxical excitation.” Eventually the brake is switched back on in these three states -- giving way to sedation and deeper sleep, or in the case of the severely traumatized brain patient, the return to a state of diminished responsiveness.

General anesthesia, according to the review is functionally equivalent to brainstem death, and perhaps explains why some patients do not fully recover consciousness for several hours after GA, as well as why postoperative cognitive dysfunction could persist in elderly patients for several months afterward. The review was published in the December 30, 2010, issue of the New England Journal of Medicine (NEJM).

"These findings show that general anesthesia is a reversible coma, and learning about the different ways we can safely place the brain into this state, with fewer side effects and risks, could be an important advance in general anesthesiology,” said co-author Emery Brown, MD, PhD, from the department of anesthesia, critical care, and pain medicine at MGH.”Also, in a scientific sense, monitoring brain function under general anesthesia gives us new insights into how the brain works in order to develop new sleep aids and new ways for patients to recover from coma.”

"We think this is, conceptually, a very fresh look at phenomena we and others have noticed and studied in sleep, coma, and use of general anesthesia,” added coauthor Nicholas Schiff, MD, a professor of neurology and neuroscience at Weill Cornell Medical College. "By reframing these phenomena in the context of common circuit mechanisms, we can make each of these states understandable and predictable.”

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Weill Cornell Medical College
Massachusetts General Hospital
University of Michigan