Coma

Coma, in medicine, state of unconsciousness in which a person is unresponsive to external stimuli. In the deepest coma, spontaneous respiration ceases, and a mechanical respirator must be used (see Artificial Respiration). Coma may last for a few days or, in rare cases, for years, usually progressing after the first month to a persistent vegetative state. Coma in which electrical activity can no longer be detected in the brain is called brain-death syndrome (see Death and Dying).

Coma results from decreased metabolic activity in the brain, which may be caused by cerebral hemorrhage, inflammation of the brain due to meningitis or encephalitis, drug overdose, oxygen deprivation (as in cardiac arrest), or abnormal metabolism. Among the conditions that can cause metabolism abnormalities are diabetic ketoacidosis, in which the blood is too acidic; a high blood level of ammonia, which often follows liver damage caused by alcohol; or uremia, in which damaged kidneys cannot process the toxic waste products of metabolism. Most people recover. Some develop “respirator brain,” a poorly understood condition in which the body cannot resume breathing on its own.