A stroke or a brain attack occurs when a blood clot blocks a blood vessel or when a blood vessel bursts, interrupting blood flow to an area of the brain. An ischemic stroke occurs when the blood vessel is blocked. This is the most common type of stroke. A haemorrhagic stroke (brain bleed) occurs when a blood vessel breaks. This type of stroke is less common.

The brain needs oxygen in the blood to survive, when the blood flow is disrupted, the cells in the part of the brain that were receiving this blood can die. These cells usually die within minutes to a few hours after the stroke starts. When cells die, chemicals are released that can cause even more cells to die. This is why there is a small window of opportunity for treatment of ischemic stroke.

When brain cells die, functions that were under control of the dying brain are lost. These include functions such as language, speech, movement, and sensation. The specific abilities lost or affected depend on where in the brain the stroke occurs and on the size of the stroke.