coma, in astronomy: see comet.

coma, in medicine, deep state of unconsciousness from which a person cannot be aroused even by painful stimuli. The patient cannot speak and does not respond to command. Coma is the result of damage to the brain stem and cerebrum that may be caused by severe head or brain injury, cardiac arrest, stroke, diabetes, drug overdose, shock, or hemorrhage. It occurs just before death in many diseases. There are various depths of coma; the nature of the injury determines the level of supportive treatment necessary (see artificial life support). Survival and prognosis depend upon the cause, extent of damage, and duration of the coma.

The term persistent vegetative state was coined in 1972 to describe an unconscious state in which sleep and wake cycles remain and eyes may open, but there is no thinking, feeling, or awareness of one's surroundings (although one may react reflexive to certain stimulations). The brain stem is usually relatively intact but the cerebral cortex is severely impaired. It is this state that sometimes results from resuscitation and life support of people who otherwise would have died; partial emergence from such a state sometimes occurs with a year or two, but not after that.

Complete lack of consciousness, with loss of reaction to stimulus and of spontaneous nervous activity. It is usually associated with cerebral injury of metabolic or physical origin. Simple concussions cause short losses of consciousness. Coma from lack of oxygen may last several weeks and is often fatal. Coma caused by stroke can be sudden, while that caused by metabolic abnormalities (as in diabetes mellitus) or cerebral tumours comes on gradually. Treatment depends on the cause.

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