What happens during a stroke?

A clot, (an ischaemic stroke)

Four out of five strokes are ischaemic; this means they occur when a blood clot blocks blood flow to the brain. There are two types of blood clot that can cause a stroke:

Cerebral thrombosis
A blood clot that develops in an artery that supplies the brain.

Cerebral embolism
A blood clot that travels from elsewhere in the body to an artery in the brain. People who experience blood clots may have chronic health problems that affect the normal flow of blood such as:

• Hardening of the arteries (atherosclerosis) as a result of high blood pressure, diabetes, high cholesterol or smoking.

• Atrial fibrillation or other irregular heart rhythms.

• Heart disease (coronary artery disease, heart failure, heart valve problems).

A bleed (a haemorrhagic stroke)

This occurs as a result of bleeding into or around the brain. It can be caused by:

An intracerebral haemorrhage: this is where a blood vessel inside the brain bursts.

A subarachnoid haemorrhage: this occurs when a blood vessel on the surface of the brain bleeds into the area between the brain and the skull.

Strokes of this type are usually caused by longstanding high blood pressure or a ruptured aneurysm (a blood-filled balloon-like bulge in the wall of a blood vessel). They are often more severe than an ischaemic stroke and, although the symptoms are similar, early treatment is very different.

Why do the effects vary?

A stroke may cause only temporary symptoms such as facial palsy or a weakness in your arm that disappears within hours. This is known as a TIA (transient ischaemic attack, or ‘mini-stroke’). In more severe cases, it could take months of therapy before you stand unaided, move your arms easily or recover your powers of speech.

This variation depends on the part of your brain that has been injured. The right half controls the left side of the body and vice versa. Common symptoms such as weakness or paralysis usually happen on the opposite side of the body to the side of the brain damaged by the stroke. If it is the left half of the brain that is affected, you’re more likely to have problems with language (talking, understanding, reading and writing). If the right half is affected, you may have difficulties with perceptual skills (making sense of what you see, hear and touch) and spatial skills (judging size, speed, distance or position in space).

The severity of symptoms depends on the extent of the damage and whether it is caused by a clot or bleeding. Symptoms are also likely to be worse in those whose health is already poor.