Classification of strokes
There are two types or forms of stroke.
- Ischaemic (blockage of a blood vessel supplying the brain)
- Haemorrhagic (bleeding into or around the brain)
Ischaemia is the term used to describe inadequate blood flow to an organ or part of the body. This results in loss of oxygen and nutrition. Ischaemia ultimately leads to Infarction, which is the term used to describe cell death.
Large vessels and small vessels may be affected, and different clinical pictures will result. A large vessel stroke will produce dramatic and catastrophic results, whereas a small vessel stroke will be silent and/ or transient. Silent strokes can “eat away” at the brain, causing progressive damage, with stepwise deterioration. A person who has a large stroke often has coincidental small vessel disease in the deep white matter of the brain.
Ischaemic stroke
An ischaemic stroke is the commonest form of stroke, accounting for 80 to 85 percent of all strokes. It occurs when an artery supplying a part of the brain with blood becomes blocked causing a sudden reduction or complete cessation of blood flow. This will ultimately lead to a brain infarction.
Blood clots are the commonest cause of artery blockage in the brain. Blood clots cause strokes in one of two ways:
Embolic stroke
In this type of stroke the blood clot forms in another part of the body, most commonly in the heart due to turbulent blood flow in a heart chamber. The clot then becomes dislodged and travels in the bloodstream until it becomes stuck in an artery in the brain, blocking the blood flow. This free roaming clot is called an embolus.
Thrombotic stroke
In this type of stroke the blood clot forms in the artery itself. This commonly occurs over a patch of fatty tissue called atheroma (this is often called furring up or hardening of the arteries). Atheroma is common in older people. If a patch of atheroma becomes large enough it can trigger the blood passing over it to clot. The blood clot so formed stays attached to the wall of the artery until it grows big enough to block the flow of blood. This type of fixed blood clot is called a thrombus.
There are other rare causes of ischaemic stroke, which are outside the scope of this guidance.
Haemorrhagic stroke
In the normal healthy brain, neurones do not come into direct contact with blood. Oxygen and nutrients pass through the thin walls of tiny blood vessels called capillaries to supply the brain cells.
A damaged or weakened artery may burst and bleed into the surrounding brain tissue. This not only reduces the blood supply to more distant parts of the brain, it also upsets the delicate chemical balance the neurones require in order to function. Affected neurons become damaged or die. This is called a haemorrhagic stroke and accounts for about 15 to 20 percent of all strokes.
