Ischaemic strokes

Ischaemic strokes occur when blood clots block the flow of blood to the brain. Blood clots typically form in areas where the arteries have been narrowed, or blocked, by fatty cholesterol -containing deposits, known as plaques. This narrowing of the arteries is known as atherosclerosis.

As we get older, our arteries become narrower, but certain risk factors can dangerously accelerate the process. Risk factors include:

• smoking,
• high blood pressure (hypertension),
• obesity,
• high cholesterol levels (often caused by a high-fat diet), and
• a family history of heart disease, or diabetes.

Diabetes is also a risk factor, particularly if it is poorly controlled because the excess glucose in the blood can damage the arteries.

Another possible cause of ischaemic stroke is an irregular heartbeat (atrial fibrillation) which can cause blood clots that become lodged in the brain. Atrial fibrillation can be caused by:

• high blood pressure,
• coronary artery disease,
• mitral valve disease (disease of the heart valve),
• cardiomyopathy (wasting of the heart muscle),
• pericarditis (inflammation of the bag surrounding the heart),
• hyperthyroidism (overactive thyroid gland),
• excessive alcohol intake, and
• drinking lots of caffeine - for example tea, coffee, and energy drinks.

Haemorrhagic strokes

Haemorrhagic strokes occur when a blood vessel in the brain bursts. The main cause of this is high blood pressure (hypertension) which can weaken the arteries in the brain and make them prone to split, or rupture.

The risk factors for high blood pressure include:

• being overweight,
• drinking excessive amounts of alcohol,
• smoking,
• a lack of exercise, and
• stress, which may cause a temporary rise in blood pressure.

A person's ethnic group can also be a risk factor for high blood pressure. As many as half of all people of black African, or Caribbean origin, who are over 40 years of age, are likely to have high blood pressure. Research has suggested that the reason for this is that people of African origin have an increased sensitivity to the effects of salt which can cause their blood pressure to rise.

A haemorrhagic stroke can also sometimes occur as a result of a traumatic head injury.

Other causes

Around 5% of strokes are caused by a condition known as a subarachnoid hemorrhage. This condition differs from other causes of stroke because it involves defects in the brain that are present at birth.