The coma - A state of unconsciousness and unresponsiveness to external stimuli (for example, pinching) or internal stimuli (such as a full bladder).

Coma results from disturbance or damage to areas of the brain involved in conscious activity or maintenance of consciousness  in particular, parts of the cerebrum, upper parts of the brainstem, and central regions of the brain, especially the limbic system.

Conditions that can produce coma include severe head injury; disorders such as stroke or cardiac arrest, in which part or all of the brain tissue is deprived of blood; or infectious disorders that affect the brain, such as meningitis and encephalitis. In addition, excessively high or low blood levels of certain substances may result in coma; for example, a person with diabetes mellitus may become comatose if his or her blood level of glucose (sugar) rises or falls to an abnormal degree.

There are varying depths of coma. In less severe forms, the affected person may make small movements and respond to certain stimuli. In a deep coma, the person does not make any movements or respond to any stimulus. However, even people in deep comas may show some automatic responses, for example breathing unaided and blinking. If the lower brainstem is damaged, however, vital functions are impaired, and artificial ventilation and maintenance of the circulation are required.

If brain damage is minor and reversible, the person may make a full recovery, but deep coma due to severe trauma may result in long-term neurological problems such as muscle weakness or changes in behaviour. A person in a deep coma (a persistent vegetative state) may be kept alive for years provided the brainstem is still functioning. Complete and irreversible loss of brainstem function leads to brain death (the permanent cessation of all brain functions).


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