When the blood clot forms within an artery of the brain, it is called cerebral thrombotic stroke. These often occur at night or first thing in the morning. A wandering clot (embolus) or some other particle that forms away from the brain, usually in the heart, may also cause anischemic stroke.

This is called cerebral embolism. The clot is carried by the bloodstream until it lodges in an artery leading to or in the brain, blocking the flow of blood. The most common cause of these emboli is blood clots that form during atrial fibrillation.

Hemorrhagic strokes, which have a much higher fatality rate than strokes caused by clots, occur when a blood vessel inside the brain or on the brain's surface ruptures and bleeds into the space between the brain and the skull.

Hemorrhage (or bleeding) into the brain occurs when the wall of the artery is weakened by high blood pressure or deposition of amyloid. Amyloid is a protein complex related to Alzheimer's disease.

Hemorrhage on the brain's surface can be caused by a head injury or a burst aneurysm. Aneurysms are blood-filled pouches that balloon out from weak spots in the artery wall. They're often caused or made worse by high blood pressure.

Aneurysms aren't always dangerous, but if one bursts in the brain, they cause a hemorrhagic stroke.

The most common symptom of a stroke is sudden weakness or numbness of the face, arm or leg, most often on one side of the body. Other symptoms include: confusion, difficulty speaking or understanding speech; difficulty seeing with one or both eyes; difficulty walking, dizziness, loss of balance or coordination; severe headache with no known cause; fainting or unconsciousness.

About one third of all strokes are preceded by one or more "mini-strokes", or transient ischemic attacks (TIAs). The symptoms of TIAs are identical to those of stroke. However, TIAs last a few minutes to several hours, always with complete recovery within 24 hours, so it is easy to ignore them.

But the underlying problem continues and TIAs are often early warning signs of more serious strokes in the future. Recognising and responding to the warning signs gives the patient the best chance of recovery. Patients often do not seek help for a day or more after the first symptoms.

That reduces the effectiveness of treatment.

Sources: Department of Medicine at the University of HongKong, American Stroke Association, American Heart Association,World Health Organisation, National Stroke Association, StanfordStroke Centre

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