Stroke can be classified in two categories. 80% of strokes are ischaemic, which is due to interruption of the blood supply; it is usually caused by a blockage of a blood vessel or “artery” in the brain. If an artery is blocked, the brain cells (neurons) cannot make enough energy and will eventually stop working. If the artery remains blocked for more than a few minutes, the brain cells may die.

Ischemic stroke can further be divided into two main types: thrombotic and embolic. A thrombotic stroke occurs when the damaged cerebral arteries become blocked by the formation of a blood clot within the brain. The total blockage may be due to clumping together of blood cells called platelets, or other substances normally found in the blood. This blockage of a narrowed artery will stop the blood supply to that portion of the brain. An embolic stroke is also caused by a clot within an artery, but in this case the clot (or emboli) was formed somewhere other than in the brain itself. These materials could be blood clots (e.g. from the heart) or fatty materials (e.g: from another artery in the neck). This naturally restricts the flow of blood to the brain and results in almost immediate physical and neurological deficits.

Less commonly, when a weakened blood vessel in the brain ruptures, a haemorrhage, or bleeding from the blood vessel, occurs suddenly. It can be also divided into two categories. The bleeding is most often due to severe high blood pressure, in the case of intracerebral haemorrhage (ICH). The sudden increase in pressure within the brain can cause damage to the brain cells surrounding the blood. A subarachnoid haemorrhage occurs when a blood vessel just outside the brain ruptures. The area of the skull surrounding the brain (the subarachnoid space) rapidly fills with blood. Subarachnoid haemorrhage is most often caused by abnormalities of the arteries at the base of the brain, called cerebral aneurysms. These are small areas of rounded or irregular swellings in the arteries, where the blood vessel wall becomes weak and prone to rupture.