A stroke occurs when there is a sudden interference with the blood
supply to part of the brain. Brain cells must have a continuous supply of
oxygen-rich blood. If deprived of blood even for a few minutes, they cease
to function and then die. Brain cells control sensation and movement and
thinking. When nerve cells cannot function, the body function which they
control stops functioning too. The result may be difficulty in speaking or
seeing or using a limb, for example, or even loss of consciousness.
One of the commonest causes of a stroke is the blockage of a brain artery
by a blood clot (thrombosis). A clot is not likely to form in a healthy artery.
However, when an artery is damaged by atheroma, a clot is apt to form
around the rough deposit on the arterial wall. Sometimes a clot formed
somewhere else in the cardiovascular system can fly off and become wedged
in a vessel such as a brain artery with similar consequences. This is called a cerebral embolus. A stroke can also occur when an artery in the brain
springs a leak and bursts. This is called a cerebral haemorrhage. A cerebral
haemorrhage is more likely when the patient also has high blood pressure.
Once a stroke has happened and the brain cells have died they will not
recover but the neighbouring brain cells can be encouraged to take over the
work of the damaged cells; sometimes to remarkable extent.