What is a stroke?

A stroke is a sudden disturbance of the normal functioning of the nervous system, caused by a disruption of the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. supply to the brain. There are two main possible underlying causes for this disrupted bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. supply:

  • A blockage in one or more of the arteries carrying bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. to the brain ('ischaemic' stroke)
  • A bleed in the brain ('haemorrhagic' stroke).[1,2]

There are many different possible neurological problems seen with a stroke. However, the most common are the paralysis, or inability to move, one side of the body (this is called 'hemiplegiaAlso called hemiparesis. Paralysis of one side of the body.'), and an inability to speak or understand speech ('aphasiaDifficulty in understanding and generating speech.').[1] Learn more about the symptoms of stroke.

While strokes are less common in people younger than 40 years old, it is important to remember that people of any age can have a stroke Any sudden neurological problem caused by a bleed or a clot in a blood vessel. - even babies and children

By definition, with a stroke, the disturbance in normal neurological functioning lasts for more than 24 hours, or results in death.[1]

If all the neurological problems settle down completely within a single day, the episode is called a 'transient ischaemic attack' (TIATransient ischaemic attack; any neurological problem caused by an interruption in the blood supply to the brain that resolves within 24 hours.) - commonly referred to as a 'mini-stroke'. A TIATransient ischaemic attack; any neurological problem caused by an interruption in the blood supply to the brain that resolves within 24 hours. may serve as a warning of a higher risk of stroke in the future: around 15 per cent of people who have a full stroke have experienced a TIATransient ischaemic attack; any neurological problem caused by an interruption in the blood supply to the brain that resolves within 24 hours. at an earlier time.[3]

Ischaemic strokes are much more common than haemorrhagic strokes, making up 85 per cent of all strokes.[1]

How common are strokes?

The World Health Organization estimates that 15 million people have a stroke every year. The yearly number of strokes is increasing as the average age of the world's population increases.[4-6]

The incidenceThe number of new episodes of a condition arising in a certain group of people over a specified period of time. of stroke Any sudden neurological problem caused by a bleed or a clot in a blood vessel. (the number of new strokes seen in a group of people over a period of time) varies in different parts of the world. For example, parts of France have been estimated to have a stroke incidenceThe number of new episodes of a condition arising in a certain group of people over a specified period of time. of 240 per 100,000 people per year, whereas some areas of Russia have been rated as having an incidenceThe number of new episodes of a condition arising in a certain group of people over a specified period of time. of 600 per 100,000.[7]

While strokes are less common in people younger than 40 years old, it is important to remember that people of any age can have a stroke Any sudden neurological problem caused by a bleed or a clot in a blood vessel. - even babies and children.[6]

What causes a stroke?

Ischaemic strokes - those caused by a blockage in the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. supply to the brain - may have a variety of causes: for example, atherosclerosisDisease leading to fatty deposits in the inner walls of the arteries, which reduce and may eventually obstruct blood flow. of the large arteries or a bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. clot Blood that has coagulated, that is, has moved from a liquid to a solid state.(embolismObstruction of blood flow by an embolus, a clot (or other material, for example, fat or air) that has become dislodged from elsewhere in the blood system.) originating in the heart.[2] Learn more about atherosclerosis.

Haemorrhagic strokes - in other words, those caused by bleeding rather than a blockage in an arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. - may be due to an intracerebral haemorrhage[Defined separately] (a bleed within the brain) or a subarachnoid haemorrhageBleeding into the subarachnoid space surrounding the brain, the area between two of the three layers (meninges) surrounding the brain and spinal cord (the arachnoid mater and the pia mater). (a bleed just outside the brain).[2]

Ischaemic and haemorrhagic strokes are discussed in more detail below.

Ischaemic strokes

The most common cause of a blockage in the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. supply to the brain is a bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. clot resulting from atherosclerosisDisease leading to fatty deposits in the inner walls of the arteries, which reduce and may eventually obstruct blood flow. ('furring up') of the arteries of the brain. Learn more about atherosclerosis.[1]

The heart is another important source of clots that can cause a stroke. For example, an abnormal heart rhythm, 'atrial fibrillationA common abnormal heart rhythm causing a rapid, irregular pulse and failure of the upper chambers of the heart (atria) to pump properly. Abbreviated to AF.', means that bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. can stagnate in the upper chambers of the heart, making the formation of clots more likely. Such clots can then break off and travel through the bloodstream to the brain.

Another potential source of clots in the heart is from prostheticArtificial. heart valves.[1] People who have atrial fibrillationA common abnormal heart rhythm causing a rapid, irregular pulse and failure of the upper chambers of the heart (atria) to pump properly. Abbreviated to AF. or prostheticArtificial. heart valves are usually given the anti-clotting drug warfarin to help prevent the formation of bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. clots that can lead to a stroke.

A blockage within an arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. of the brain will not always cause symptoms; for example, sometimes other arteries may open up to maintain the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow to the brain. Once bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow to a particular part of the brain falls below a certain level, however, problems are seen in the function of the nervous system. In the early stages, if bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow is restored, then normal function will also be restored.[1]

Sometimes, the restoration of bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow to the area may actually result in a secondary bleed within the brain. This is called 'haemorrhagic transformation'. Certain medications such as 'clot-busting' drugs - thrombolyticsA medication that breaks up blood clots. - can increase the risk of this happening.[1]

Haemorrhagic strokes

A bleed in the brain is caused by the rupture of a bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. vessel. There are two types of bleed that can cause a stroke: an intracerebral haemorrhage[Defined separately], within the brain tissue itself, and a subarachnoid haemorrhageBleeding into the subarachnoid space surrounding the brain, the area between two of the three layers (meninges) surrounding the brain and spinal cord (the arachnoid mater and the pia mater). , just outside the brain. [1]

The bleeding interferes with the structure of the nerveBundle of fibres that carries information in the form of electrical impulses. cells and brain tissue, and can expand over minutes or hours. This expanding volume of bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. takes up room within the rigid skull - so the tissue of the brain becomes compressed.[1]

Factors that may lead to a haemorrhagic stroke include an abnormal growth of bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. vessels (vascular malformationAbnormalities in the blood vessels.), a tumour or an abnormal dilation in the wall of an arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. (aneurysmAn abnormal swelling in the wall of an artery.).

People whose bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. is unable to clot properly are also predisposed to a haemorrhagic stroke Any sudden neurological problem caused by a bleed or a clot in a blood vessel. - for example, because of haemophiliaA hereditary disorder that causes very slow clotting of the blood, due to deficiency of a coagulation factor (either Factor VIII or Factor IX). , liverA large abdominal organ that has many important roles including the production of bile and clotting factors, detoxification, and the metabolism of proteins, carbohydrates and fats. disease, kidney (renal) disease or anti-clotting medication (anticoagulants). High blood pressure also contributes to this risk.[8]

References: 
  1. Boon NA, Colledge NR and Walker BR. 'Davidson's Principles and Practice of Medicine'. Churchill Livingstone Elsevier. 2006; 20th edition.
  2. Grysiewicz RA, Thomas K and Pandey DK. Epidemiology of ischemic and hemorrhagic stroke: incidenceThe number of new episodes of a condition arising in a certain group of people over a specified period of time., prevalence, mortality, and risk Factors. Neurol Clin 2008; 26: 871-95.
  3. Wu CM, McLaughlin K, Lorenzetti DL et al. Early risk of stroke after transient ischemic attack: a systematic review and meta-analysis. Arch Intern Med 2007; 167: 2417-22.
  4. Zhang H, Thijs L and Staessen JA. Blood pressure lowering for primary and secondary prevention of stroke. Hypertension 2006;48;187-95.
  5. Editorial: Reducing the burden of stroke. Lancet 2005; 366: 1752.
  6. 'Global Burden of Stroke.' WHO. Link
  7. Donnan GA, Fisher M, Macleod M et al. Stroke. Lancet 2008; 371: 1612-23.
  8. Sahni R and Weinberger J. Management of intracerebralWithin the brain. hemorrhage. Vasc Health Risk Manag. 2007; 3: 701-9.