The researchers explain that a fully anesthetized brain is much closer to the deeply unconscious low-brain activity seen in coma patients, than to a person asleep. Essentially, general anesthesia is a coma that is drug-induced, and, as a consequence, reversible. The states operate on different time scales - general anesthesia in minutes to hours, and recovery from coma in hours to months to years, if ever. The study of emergence from general anesthesia and recovery from coma could help to better understand how both processes occur.
Understanding that these states have more in common with each other than differences - that they represent a continuum of activity with common circuit mechanisms being engaged across the different processes of awakening from sleep or emerging from coma or general anesthesia - "is very exciting, because it gives us new ways to understand each of these states," says study co-author, Doctor Nicholas D. Schiff, a professor of neurology and neuroscience at Weill Cornell Medical College and a neurologist at NewYork-Presbyterian Hospital/Weill Cornell Medical Center. Co-authors of the study are Doctor Emery Brown of Massachusetts General Hospital, Massachusetts Institute of Technology and Harvard Medical School, and Doctor Ralph Lydic from the University of Michigan.
Knowing more about the brain circuit mechanisms may also help researchers develop therapeutic agents to "tweak the circuits as needed, to help us in the areas where we don't do well, such as abnormalities of sleep and, especially, emergence from a coma," Schiff says. "And while use of general anesthesia is an incredibly safe technique, it can have effects on the elderly, such as slower recovery time and impaired cognitive function afterwards."
In their review, which took three years to develop, the researchers synthesized the newest studies in these three areas, including work of their own. Among their other specialties, Dr. Brown's expertise is general anesthesia, Lydic's is sleep, and Schiff's is recovery 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," Schiff says. "By reframing these phenomena in the context of common circuit mechanisms, we can make each of these states understandable and predictable."
"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," explains Brown. "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."