There are two common types of stroke; the most common is called an Ischaemic Stroke where a blood clot (thrombus) forms at a blockage of an artery or where a blood clot, air bubble or fat globule (embolus) has formed “upstream” within the arterial tree and moved “downstream” in the blood flow through progressively smaller arteries until it finds an artery too small to pass through. This embolus often forms in the heart or one of the large arteries that flow out of it. Ischaemic strokes are often associated with disorders that involve clotting of the blood or cause inflammation of the arteries such as vasculitis.

Blockage of the very small blood vessels deep within the brain is called a Lacunar Stroke. The second type is a Haemorrhagic Stroke; this maybe caused by an intracerebral bleed (bleed within the brain tissue) or a subarachnoid haemorrhage (bleed into the fluid filled space between the brain and the inner surface of the skull lined by the subarachnoid membrane or into the fluid filled cavities within the brain known as ventricles). Haemorrhagic strokes usually involve disorders that prevent or reduce normal clotting of the blood that result in an increased potential for bleeding; or they are associated with malformations of the blood vessels themselves such as a weakness causing a bulge of the walls known as aneurysms.

It is possible for blood vessels to deteriorate in very large ischaemic strokes so that they then allow bleeding and transform into haemorrhagic strokes.