The cause of abnormal blood vessel development in the brain is unknown.
Arteriovenous malformations vary greatly from person to person.
The size varies, from massive lesions involving multiple vessels to lesions so small they are difficult to distinguish on testing. Large malformations may have enough blood flow through them to stress the pumping ability of the heart, especially in young patients. There are often no symptoms until complications occur. In many cases, symptoms are related to bleeding from the abnormal vessels, which are often fragile and lack the normal supportive structure of arteries and veins. While there is some controversy regarding this, in general, small lesions are more likely to bleed than large ones.