What is stroke?
Stroke occurs when the supply of blood to the brain is either interrupted or reduced. When this happens, the brain does not get enough oxygen or nutrients which causes brain cells to die. There are three main kinds of stroke; ischemic, hemorrhagic and TIA. This article will focus on ischemic and hemorrhagic strokes, as there is a separate Knowledge Centre article for TIAs, which goes into specific detail about them.
Stroke is also more likely to affect people if they are overweight, aged 55 or older, have a personal or family history of stroke, do not exercise much, drink heavily, smoke or use illicit drugs.
What causes stroke?
The different forms of stroke have different specific causes.
Ischemic stroke is the most common form of stroke, accounting for around 85% of strokes. This type of stroke is caused by blockages or narrowing of the arteries that provide blood to the brain, resulting in ischemia – severely reduced blood flow.
These blockages are often caused by blood clots, which can form either in the arteries connecting to the brain, or in other blood vessels before being swept through the bloodstream and into narrower arteries within the brain. Clots can be caused by fatty deposits within the arteries called plaque.
Hypertension can lead to rupturing of blood vessels and hemorrhagic stroke.
Hemorrhagic stroke are caused by arteries in the brain either leaking blood or bursting open. The leaked blood puts pressure on brain cells and damages them. Blood vessels can burst or spill blood in the middle of the brain or near the surface of the brain, sending blood into the space between the brain and the skull.
The ruptures can be caused by conditions such as hypertension, trauma, blood-thinning medications and aneurysms (weaknesses in blood vessel walls).
Intracerebral hemorrhage is the most common type of hemorrhagic stroke and occurs when brain tissue is flooded with blood after an artery in the brain bursts. Subarachnoid hemorrhage is the second type of hemorrhagic stroke and is less common. In this type of stroke, bleeding occurs in the subarachnoid space – the area between the brain and the thin tissues that cover it.
Transient ischemic attack (TIA)
TIAs are different from the aforementioned kinds of stroke because the flow of blood to the brain is only briefly interrupted. TIAs are similar to ischemic strokes in that they are often caused by blood clots or other debris.
TIAs should be regarded as medical emergencies just like the other kinds of stroke, even if the blockage of the artery is temporary. They serve as warning signs for future strokes and indicate that there is a partially blocked artery or clot source in the heart.
According to the Centres for Disease Control and Prevention (CDC), over a third of people who experience a TIA go on to have a major stroke within a year if they have not received any treatment. Between 10-15% will have a major stroke within 3 months of a TIA.