(Science: anatomy, embryology) a tangled collection of abnormal blood vessels where there is an abnormal communication between the arterial and venous systems. The afferents flow directly into the venous efferents without the usual resistance of an intervening capillary bed.
They are mostly congenital. If large enough, they may produce a shunt of sufficient magnitude to raise the cardiac output.
Common sites include; skin, liver, brain, brainstem and spinal cord, where they may cause headaches, seizures or bleeding (subarachnoid haemorrhage).
See: arteriovenous fistula, cerebral arteriovenous malformations.