An arteriovenous malformation is a localized or diffuse vascular lesion consisting of direct connections between arteries and veins, with the absence of the intervening capillary bed (network of tiny capillaries). This results in the abnormal flow of blood from the arteries (high flow) directly into veins (low flow). Localized lesions, most commonly seen on the head and neck, often appear as light vascular stains at birth. Diffuse lesions, commonly seen on the trunk or on a limb, may not be observed until later in childhood, as they enlarge with age.

Both localized and diffuse lesions are seen in internal organs such as the brain, lung, liver and intestines. Lesions within the brain are the most common arteriovenous malformation. They are initially silent, with symptoms depending upon the rapidity of enlargement.

While the precise cause of all arteriovenous malformations is unknown, they are generally thought to be caused by errors in the formation and development of the normal arterial-capillary-venous connections that occur very early in prenatal development. Some arteriovenous malformations can be associated with syndromes due to genetic abnormalities. RASA1 mutations and hereditary hemorrhagic telangiectasia (HHT) have been associated in some cases of brain AVMs. HHT also increases risk of lung and liver AVMs. PTEN mutations have been found in association with diffuse AVMs, particularly in combination with fatty overgrowth or mass.

There is no evidence that drugs or medications taken during pregnancy or environmental exposures during that time cause AVMs.