Factors Affecting AVMs
Once formed, various external factors may affect the development, size and shape of AVMs.
Normally, before a baby is born and starts using the lungs to breathe, blood from the heart headed for the lungs in the pulmonary artery is shunted (diverted) away from the lungs and returned to the heart. Other arteries become smaller and smaller as they reach the tissues they supply. Then they break into capillaries and finally they drain into veins.
When there is direct arterial shunting from the artery to the vein, the result can be a tangle of vessels that make up an AVM. This shunting causes low blood pressure in the arterial vessels feeding the AVM and neighboring areas of the brain that they normally supply with blood.
An intracranial hemorrhage is bleeding within the cranium (skull) which can be caused by an AVM. This can also be caused by a stroke or an aneurysm in the brain. Abnormal changes in blood pressure make AVMs vulnerable to causing an intracranial hemorrhage.
There are a small number of syndromes that people have from birth that are cause AVMs. These syndromes are very rare.
Sturge–Weber syndrome results in the deformity of blood vessels on the surface of the brain. In rare cases it may cause a severe form of epilepsy, mental retardation or progressive shrinkage of the brain.
Osler-Weber-Rendu syndrome is a genetic disorder that leads to the development of abnormal blood vessels in the skin, mucous membranes, and often in the lungs, liver or brain. Patients with Osler-Weber-Rendu syndrome usually suffer from constant nosebleeds or strokes.
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