Ischaemic stroke

A stroke can occur when the blood supply to the brain is blocked. This is called an ischaemic stroke (is-key-mick) and is the most common type of stroke. Usually a blood clot is responsible. Clots help stop bleeding from wounds but can also form inside blood vessels, the tubes that carry blood around the body. Blood carries oxygen and nutrients to your brain cells; if it cannot get through, the brain can be injured. Blood vessels include arteries, which carry blood from the heart to the rest of the body, and veins, which return blood to the heart. A clot that blocks an artery to the brain causes an ischaemic stroke. Clots that cause strokes often form in the heart or in one of the large vessels supplying the brain, then travel and block a blood vessel in the brain. This is known as thromboembolism.

Large artery disease

A common cause of ischaemic stroke is atherosclerosis, when cholesterol builds up as plaques on the inner walls of arteries. When these plaques grow, a clot can form. Such clots often form in the carotid arteries in the neck, which are the brain’s main blood supply, or in arteries within the brain. Blockage of a large artery can affect a large area of the brain. Many of these strokes can be prevented by managing risk factors such as high blood pressure and high cholesterol.

Heart conditions

Clots often form in the heart. Causes include atrial fibrillation (AF), an irregular heartbeat where one of the heart’s chambers does not pump in a coordinated way, so blood can stagnate and clots may form. Another cause is patent foramen ovale (PFO), a connection between the two sides of the heart that can allow clots from veins (e.g. deep vein thrombosis) to reach the arteries supplying the brain. Damage to the heart from infections or heart attacks can also affect pumping and allow clots to form where blood flow is sluggish.

Small blood vessels

A stroke that blocks narrow arteries deep in the brain is called small vessel disease. Risk factors are similar to those for large-artery clots. Small vessel disease can also be caused by rare genetic conditions such as CADASIL, which affects muscle cells in blood vessel walls.

Other causes

A dissection or tear in a neck artery can create a clot at the site of the tear. Although uncommon, dissections occur more often in younger people and can result from neck injury or genetic factors such as fibromuscular dysplasia and vascular Ehlers–Danlos syndrome. A sudden drop in blood pressure can rarely reduce blood supply enough to cause a stroke (watershed stroke). Inflammation of blood vessel walls (cerebral vasculitis), some blood disorders, and rare conditions such as reversible cerebral vasoconstriction syndrome can also lead to ischaemic stroke.