In medicine, a coma is a profound state of unconsciousness, which may result from a variety of conditions including intoxication (drug, alcohol or toxins), metabolic abnormalities (hypoglycemia, hyperglycemia, ketoacidosis, etc.), central nervous system diseases, stroke, head trauma, seizures, and hypoxia. The metabolic abnormalities are the most common causes of coma.
A temporary coma is sometimes deliberately induced (using drugs) to reduce swelling of the brain after injury.
In layman's terms
- "A coma is a profound state of unconsciousness. Patients are alive but unable to move or respond to their environment. There are several levels of coma and patients may, or may not, progress through them. The responsiveness of the brain lessens as the coma deepens and when it becomes more profound, normal body reflexes are lost and the patient no longer responds even to pain. The chances of recovery depend on the severity of the underlying cause. It is unclear whether a deeper coma alone necessarily means a slimmer chance of recovery because some people in deep coma recover well while others in a so-called milder coma sometimes fail to improve." 
Contrasts to other conditions
The difference between coma and stupor is that a patient with coma cannot give a suitable response to either noxious or verbal stimuli, whereas a patient in a stupor can give a rough response (like screaming) to a noxious stimulus.
Coma is also to be distinguished from the persistent vegetative state which may follow it. This is a condition in which the individual has lost cognitive neurological function and awareness of the environment but does have noncognitive function and a preserved sleep-wake cycle. Spontaneous movements may occur and the eyes may open in response to external stimuli, but the patient does not speak or obey commands. Patients in a vegetative state may appear somewhat normal. They may occasionally grimace, cry, or laugh.
Likewise, coma is not the same as brain death, which is the irreversible cessation of all brain activity. One can be in a coma but still exhibit spontaneous respiration; one who is brain-dead by definition cannot do so.
Coma is different from sleep: Sleep is always reversible.
The outcome for coma and vegetative state depends on the cause and on the location, severity, and extent of neurological damage: outcomes range from recovery to death. People may emerge from a coma with a combination of physical, intellectual, and psychological difficulties that need special attention. Recovery usually occurs gradually, with patients acquiring more and more ability to respond. Some patients never progress beyond very basic responses, but many recover full awareness.
A coma rarely lasts more than 2 to 4 weeks. Some patients who have gone into vegetative state may go on to regain a degree of awareness. Others may remain in a vegetative state for years or even decades. The most common cause of death for a person in a vegetative state is infection such as pneumonia.
There have been controversies and legal cases over whether to keep comatose patients alive for long periods using life support equipment. One such case is that of Karen Ann Quinlan, who fell into a coma after ingesting sedatives and alcohol at a party in 1975. Her parents, in opposition to the doctors caring for her, eventually won the legal right to have her removed from her ventilator. She was able to breathe on her own, and lived in a vegetative state until her death from pneumonia on 11 June 1985.
Glasgow Coma Scale
The Glasgow Coma Scale is used to quantify the severity of a coma. There are three components to the score: Eye opening response, Verbal response, and Motor response.
- E1 = None; E2 = To pain; E3 = To speech; E4 = Spontaneous
- V1 = None; V2 = Incomprehensible; V3 = Inappropriate; V4 = Confused; V5 = Oriented
- M1 = None to pain; M2 = Extension to pain (decerebrate posturing); M3 = Flexion to pain (decorticate posturing); M4 = Withdrawal to pain; M5 = Purposeful movement/localized response to pain; M6 = Obeys commands.
A score of E4V5M6 indicates the normal state; a score of E1V1M1 indicates complete un-responsiveness.
- Some of the information in this section is from the public domain resource provided by the National Institute of Neurological Diseases and Stroke at http://www.ninds.nih.gov/health_and_medical/disorders/coma_doc.htm