Coma, from the Greek word koma, meaning deep sleep, is a state of extreme unresponsiveness, in which an individual exhibits no voluntary movement or behavior. Furthermore, in a deep coma, even painful stimuli (actions which, when performed on a healthy individual, result in reactions) are unable to effect any response, and normal reflexes may be lost.

Coma lies on a spectrum with other alterations in consciousness. The level of consciousness required by, for example, someone reading this passage lies at one extreme the spectrum, whereas complete brain death lies at the other end. In between are such states as obtundation (dullness), drowsiness, and stupor. All of these are conditions which, unlike coma, still allow the person to respond to stimuli, although such a response may be brief and require stimulus of greater than normal intensity.

Consciousness is defined by two fundamental elements: awareness and arousal. Awareness allows people...