A stroke occurs when there is a reduction in blood supply to the brain causing permanent damage and resulting in a neurological deficit, e.g. limb weakness or facial drooping. There are two types of stroke depending on how they occur. Ischaemic stroke is the most common type, accounting for 80% of cases. It can be likened to 'a heart attack of the brain' with shared underlying mechanisms. The artery to the brain is blocked suddenly by a cholesterol plaque or blood clot. The brain tissue distal to this occlusion then dies. The source of the clot may be in the main arteries in the brain (cerebral thrombosis) or the clot may have formed elsewhere in the body e.g. the heart, and swept into the arteries supplying the brain, lodging in a vessel small enough to block its passage and therefore causing a stroke (cerebral embolism). Atrial fibrillation, an irregular heartbeat, is a major risk factor for clot formation in the heart and subsequent cerebral embolism causing a stroke. Over 15% of strokes are thought to be related to the heart.

The term transient ischaemic attack (TIA) is used if the episode of brain dysfunction is short-lived and the neurological deficit e.g. limb weakness, lasts just a few minutes before returning to normal function. This is usually a warning sign that all is not well with the blood supply to the brain, rather like angina is a warning sign that there is a reduction of blood to the heart. TIAs were traditionally diagnosed if the neurological weakness lasted less than 24 hours. More sophisticated brain-scanning techniques however, have led to a revision of the diagnosis- if there is any evidence of brain damage on the scan, the term stroke is used, even if the symptoms have resolved clinically.

Haemorrhagic stroke is the second type of stroke accounting for 20% of cases. This is also known as bleeding caused by a blood vessel in the brain rupturing suddenly. The leakage of blood around the area compresses the brain tissue and also starves brain cells of oxygen and nutrients. Hypertension is the main cause of haemorrhagic stroke.

Strokes usually result from healthy arteries clogging up with cholesterol. This process is called arteriosclerosis and results in a narrowing of the blood vessel. Arteriosclerosis is accelerated by hypertension, diabetes and high levels of blood cholesterol with the overall effect of a reduction in the blood supply to the brain and increased risk of stroke. Atrial fibrillation also causes strokes as outlined above.