Arteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. They are comprised of snarled tangles of arteries and veins. Arteries carry oxygen-rich blood away from the heart to the body’s cells; veins return oxygen-depleted blood to the lungs and heart. Seizures and headaches are the most generalized symptoms of AVMs, but no particular type of seizure or headache pattern has been identified.
AVMs are congenital lesions composed of a complex tangle of arteries and veins connected by one or more fistulae. The vascular conglomerate is called the nidus. The nidus has no capillary bed, and the feeding arteries drain directly to the draining veins. The arteries have a deficient muscularis layer. The draining veins often are dilated owing to the high velocity of blood flow through the fistulae. How the abnormal vessels appear or exactly when the process begins is unknown. Deranged production of vasoactive proteins is under investigation as the angiogenetic link to pathophysiology.
Hemorrhage: In population-based studies, 38-70% of brain AVMs present initially with hemorrhages. The overall risk of intracranial hemorrhage in patients with known AVM is 2-4% per year. Patients presenting with a hemorrhage are at increased risk for rebleeding, particularly during the first year after the initial hemorrhage (recurrent hemorrhage rate within 12 months after initial hemorrhage: patients with hemorrhagic presentation 7-33%; patients with nonhemorrhagic presentation 0-3%). Hemorrhage rates progressively converge with time for both patients groups after 1 year.