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Coma from WikiPedia:

In medicine, a coma (from the Greek ???? koma, meaning deep sleep) is a profound state of unconsciousness. A comatose person cannot be awakened, fails to respond normally to pain or light, does not have sleep-wake cycles, and does not take voluntary actions.

Coma may result from a variety of conditions, including intoxication, metabolic abnormalities, central nervous system diseases, acute neurologic injuries such as stroke, and hypoxia. A coma may also result from head trauma caused by mechanisms such as falls or car accidents. It may also be deliberately induced by pharmaceutical agents in order to preserve higher brain function following another form of brain trauma, or to save the patient from extreme pain during healing of injuries or diseases. The underlying cause of coma is bilateral damage to the Reticular formation of the midbrain, which is important in regulating sleep.

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