People in a persistent vegetative state have suffered brain damage that has destroyed higher-brain functions, leaving them with no conscious awareness of their surroundings.
Because the more primitive parts of the brain, such as the brain stem, continue to function, they maintain many bodily functions, such as the ability to breathe and to cycle between sleep and wakefulness. Their eyes open and close, and they can have reactions to noise and movement -- including facial expressions such as grimaces, crying or even laughter -- that make it appear as if they have some consciousness.
But such patients do not speak and are unable to respond to commands. If this vegetative state continues for more than a month, it is classified as persistent.
Neurologists make the diagnosis by conducting repeated examinations, interviewing family members and questioning caretakers to determine whether a patient meets certain criteria, including:
No evidence of awareness of self or surroundings and an inability to interact with others.
No evidence of sustained, reproducible, purposeful or voluntary responses to visual, auditory, tactile or noxious stimuli.
No evidence of language comprehension or expression.
Intermittent wakefulness and sleep-wake cycles.
Enough activity in primitive parts of the brain to enable them to breathe and survive with nursing care.
Bowel and bladder incontinence.