50 per cent of strokes in children are haemorrhagic. Between 30 and 50 per cent of these are caused by arteriovenous malformations (AVMs). An AVM is a rare malformation of blood vessels where arteries (taking blood to the brain) become tangled with veins (draining blood from the brain), often appearing as a tangle of abnormal vessels. They can occur anywhere in the body but most commonly in the brain. This means that the high pressure of blood in arteries is transmitted directly into veins, which are not built to take this pressure they may therefore burst.
An aneurysm is a bulge in an artery wall. If the bulge grows too big, it can burst and cause bleeding into the brain. Aneurysms may arise as a result of an infection or without warning.
Cavernous malformations are thought to account for 20 to 25 per cent of haemorrhagic strokes in children. A cavernous malformation is a small cluster of abnormal, enlarged blood vessels, often resembling a blackberry shape.
They are mainly found in the brain and around the spine but they can occur anywhere in the body. Research is currently underway to understand why these occur. Evidence suggests that structural changes (mutations) in genes may trigger the abnormality in the vessels in some people.
Some of the disorders that have already been mentioned, moyamoya syndrome, types of vasculitis, SCD and clotting disorders (such as a lack of vitamin K, which helps with clotting) are also known risk factors for haemorrhagic stroke in children.