Classification

Although Strokes can be categorised by the position of the occlusion or infarct the most frequent method of classification is by the initial cause of the stroke.

Strokes can be classified into two major groups; ischemic and haemorrhage. Ischemia is due to the interruption of blood supply leading to cerebral infarction, while haemorrhage is due to rupture of a blood vessel. 80% of strokes are due to ischemia with the remainder due to haemorrhage although some haemorrhages develop within areas of ischemia. In ischemic strokes blood supply is decreased to a part of the brain, which leads to dysfunction and necrosis. Haemorrhagic strokes are due to the accumulation of blood anywhere within the skull and have specific symptoms such as headache.

Cerebral infarction results from an interruption in blood supply to the brain due to either:

• Emboli
• Thrombosis
• Small Vessel Disease

Atheroma is an important factor in embolic and thrombotic strokes. This is the accumulation of cell debris such as cholesterol, calcium or fibrous connective tissue. This commonly arises at junctions of arteries for example where the two vertebral arteries join to form the basilar artery. The build up of atheroma may lead to the stenosis (narrowing) of arteries at a point near the Circle of Willis and eventually leading to cerebral ischaemia. The atheromatous plaque can also fragment and act as emboli or act as a focus for thrombus formation. Haemorrhage into an ulcerated plaque can also produce a stenosis or occlusion.

MCA Stroke image courtesy of Wikipedia encyclopedia under the terms of the GNU free documentation licence.

Embolytic Strokes

An embolus is a particle that will travel through the blood stream and blocks a blood vessel in another part of the body. Emboli commonly arise from the heart or proximal arteries in the neck and will migrate up to the vessels of the brain. The most common sources of heart emboli include a thrombus from atrial fibrillation, mural thrombus from Myocardial Infarction and left ventricular dilation, with heart emboli accounting for about 10% of strokes. Emboli that arise from proximal neck vessels usually relate to atheromatous disease, which usually consist of plaque debris such as cholesterol or platelet debris.

Thrombotic Strokes

Thrombosis is the formation of a blood clot inside a vessel wall, obstructing the blood flow in the circulatory system.

Thrombosis arises in three main circumstances:

• Abnormality in vessel wall
  • Thrombosis usually occurs in context of atheroma, produces stenosis and irregularities in vessel walls
• Abnormal tendency of blood to thrombose
  • Predisposition to clotting eg polycythaemia and throbocythaemia
• Stasis of blood flow
  • Usually occurs in association with severe atheromatous stenosis
  • Most common cause of stroke in young adults

Thrombus image courtesy of Wikipedia encyclopedia under the terms of the GNU free documentation licence.

Small Vessel Disease

Intracerebral Small Vessel disease occurs in branches of major arteries and results in their occlusion. Patients usually have a predisposition to atheroma.

Intracerebral haemorrhage

Intracerebral haemorrhage is usually due to hypertension (high blood pressure). It can occur when small penetrating arteries within the brain rupture at sites of weakness. These are most frequent in the basal ganglia with primary intracerebral haemorrhage accounting for 15% of strokes and 5% due to subarachnoid haemorrhage.