Abnormal, tangled collections of dilated blood vessels that result from congenitally malformed vascular structures in which arterial afferents flow directly into venous efferents without the usual resistance of an intervening capillary bed. A common type is the congenital arteriovenous fistula (ARTERIOVENOUS FISTULA /congenital). For arteriovenous malformations in the brain, INTRACRANIAL ARTERIOVENOUS MALFORMATIONS is available. (From Merck Manual, 15th ed).