Coma is a deep and prolonged state of unconsciousness resulting from disease, injury or poisoning. The word coma usually refers to the state in which a person appears to be asleep but cannot be awakened.
Persistent vegetative state refers to another form of altered consciousness in which the person appears to be awake but does not respond meaningfully to the outside world. In this condition, the person's eyes may be open and there may be some yawning, grunting or other vocalizations. In both cases, the patient is alive, but the brain does not function fully.
Some causes of coma include:
Head trauma, such as may be sustained in a car accident, sports injury or falling injury
Complication of an underlying disease, such as seizure disorder, diabetes or liver or kidney failure
Poisoning, usually involving an overdose of drugs that depress the nervous system, such as narcotics, tranquilizers or alcohol
Coma rarely lasts more than a month and usually ends sooner. Coma may worsen to become a persistent vegetative state.