An arteriovenous malformation (AVM) 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) that normally connects these vessels.

Localized lesions, most commonly seen on the head and neck, often appear as light vascular stains at birth and generally do not enlarge until early childhood or adolescence.

Diffuse lesions, commonly seen on the chest and abdomen 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 bowel. 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 thought to be caused by errors in the formation and development of the normal arterial-capillary-venous connections that occur very early in embryonic life. However, the differing patterns of growth and microscopic presentation in localized and diffuse lesions indicate other additional causes for the latter.

Some arteriovenous malformations can be associated with genetic abnormalities such as PTEN mutations and Rasa.1 mutations.

The association of diffuse AVMs with PTEN mutations has been documented in numerous case reports in the literature. The PTEN gene is involved in normal vascular development. A mutation in PTEN would, therefore, result in abnormal angiogenesis, thus explaining the incidence of AVMs.

There is no evidence that drugs or medications taken during pregnancy or environmental exposures that may have occurred during that time can cause arteriovenous malformations.