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        The brain is supplied by numerous blood vessels to perform its multifarious functions. Oxygen and sugar are the two most important nutrients carried to the brain through the blood. A sudden stoppage of blood flow to the brain depriving it of blood supply and delivery of oxygen and sugar can lead to failure of the functioning of brain, says prof R S Hariharan, director, Institute of Cardio-Diabetology & Diabetologist, Diabetes & Heart Care Hospital (P) Ltd, Nanganallur, a suburb in Chennai.

        Stroke is one important cause for sudden failure in the functions of brain. It is called 'stroke' because the individual may be totally in good health one moment, yet develop total failure of the functions of brain the next moment, almost like a 'bolt from the blue'. It is also referred to as 'cerebro-vascular accident' because like a road traffic accident, it is sudden in its development: only thing is here the accident referred to takes place in the blood vessels of the brain.

        What are the causes for such a stroke or cerebro-vascular accident?

        There are three main causes for stroke. They are (a) a clot forming in the blood vessels (b) a blocking agent arising from some other area in the body that blocks the blood vessel(s) in the brain and (c) a rupture of one of the blood vessels in the brain that leads to bleeding into the brain substance. Whatever is the cause, the immediate effect on the brain is similar: there is an abrupt cessation in the functions of the brain.

        Let us now look into certain important aspects of the functions of brain. By and large, the left side of the brain controls the functions of our right half of the body, while the right side of the brain controls the left half of the body. Therefore, any defect in the left side of the brain will cause defective functioning of the right side of body and a defect in the right side of the brain will cause defective functioning of the left side of the body.

        Further, the centre for speech is situated in the left side of the brain in all right-handed and most left-handed individuals. Therefore, any defect in the left side of the brain will not only result in loss of function on the right side of the body, but also result is loss of function in the left side of the body, which results from defect in the right side of the brain.

        Yet another important point is that level of consciousness is intimately connected with the speech centre. Therefore, loss of consciousness or coma is more common in patients who develop loss of speech along with inability to use right half of the body.

        In the immediate post-stroke period, swelling of the brain that results as a result of block in the blood vessels or bleeding, is responsible for depressing the functions of brain. As the size of the swelling increases, the various parts of the brain are compressed against the skull bones, and suffer failure in function. Such a progressive increase in swelling can involve vital centres for respiration and heart situated lower down in the brain. When this occurs, the patient develops failure of respiratory and heart functions and dies of cardio-respiratory arrest.

        What does coma mean?

        Coma means advanced brain failure, just as for example uraemia means renal failure.

        What are the causes of coma?

        There are many numbers of causes for coma. Head injury, infections of the brain and its covering called brain fever (encephalitis and meningitis), brain tumours, abnormal levels of metabolites in blood like sugar, urea, bilirubin, etc. lack of oxygen supply, for instance, following asphyxiation under water or in mines and the use of sleeping pills and other poisons all cause coma. But, the main interest now is only coma resulting from stroke or cerebro-vascular accident.