The term vegetative state describes the clinical condition of an individual who as a result of severe brain injury has lost all evidence of conscious awareness or behavior but possesses cyclic patterns of wakefulness and arousable sleep. The vegetative state, acute or chronic, reflects the presence of severe structural brain injury or degeneration.
Unarousable coma is never permanent.
Acute coma reflects severe temporary dysfunction in subcortical arousal centers but comatose persons who do not die within a matter of days or weeks always regain arousal along with cyclic wake sleep cycles. PVS also can occur during the terminal phases of a number of degenerative brain diseases, such as Alzheimer's disease. Almost all such instances develop insidiously, preceded by progressive cognitive loss but no impairment of arousal associated with cyclic sleep wake activity. Patients in a vegetative state always retain at least some and often all brain stem reflex activity. If they survive early coma, nearly all regain ventilatory and cardiovascular independence and all maintain a normal body temperature in the absence of complicating infections or other thermogenic illnesses.