What is a coma?

A coma is a deep unconsciousness. The word comes from a Greek word meaning "state of sleep." But being in a coma is not the same as being asleep. A comatose person is alive and breathing, but so unconscious that he or she can't respond to pain, the sound of a voice or any other stimuli. Nor can he perform voluntary actions. The brain still functions, but at its most basic level.

To understand more about comas, we need to look at the brain and how it works.

The cerebrum is the largest part of the brain, controlling cognitive and sensory functions such as intelligence, reasoning, emotions and memory. The cerebellum in the back of the brain controls movement and balance. The brain stem links the two hemispheres of the cerebrum to the spinal cord, and it controls body functions like breathing, consciousness, sleep cycles and blood pressure. The thalamus, a large mass of neurons beneath the cerebrum, relays sensory impulses to the cerebral cortex.

Scientists believe that consciousness depends on the steady transmission of chemical signals from the brain stem and thalamus to the cerebrum. Any interruptions to the messages can lead to an altered state of consciousness, including comas.

One type of coma is a vegetative state, which represents an awake but unresponsive level of consciousness. Many of these patients were comatose previously and after days or weeks emerge to an unconscious state in which their eyelids are open, giving the impression they're awake. Family members sometimes falsely believe such patients are becoming awake and communicative, because he or she grunts, yawns or moves limbs or the head. However, these patients don't actually respond to any stimuli and evidence of extensive brain damage persists.

Coma is a state of unarousable unresponsiveness. Patients who have suffered severe head injury, near-drowning, poisoning, or stroke may enter coma. Careful neurological assessment is crucial in this group of patients because even though this is a very serious state, it is important to realize that the cause of coma needs to be determined if there is any hope for treatment and recovery. There have been recent cases of individuals who have emerged from coma-like states after many years. Some of these patients report awareness of sound, recognition of voices, and even the perception of pain, even while they were thought to be unaware by caregivers.