Arteriovenous Malformations: How We Changed Our Practice

Brain arteriovenous malformation (AVM) is a cluster of direct connections of arteries to draining veins without an intervening capillary bed [4]. The three main components of an AVM are one or more feeding arteries, the nidus as the site of the arteriovenous shunt, and the draining venous structures. Arteriovenous malformations are high-flow, low-resistant shunts due to a significant pressure difference between the arterial and venous side. The pressure gradient and resultant high flow trigger remodeling of both arteries and draining veins. Arteries may be dilated and thin walled due to degeneration of the media and elastic lamina or thickened from endothelial proliferation, hypertrophy of the media, and changes in the basal lamina. Remodeling of the venous system is referred to as arterialization and includes thickening of the wall due to cellular proliferation without an organized elastic lamina [4, 10]. The draining veins commonly coalesce and form a major draining vein that eventually drains into a dural venous sinus. The pathogenesis of the AVM has not been fully elucidated. The predominant theory is that AVMs are congenital in nature and result from incomplete or abnormal resolution of a primitive vascular plexus that occurs during early embryogenesis [2]. One explanation for why they are rarely detected in utero or in infants is that they first appear in utero but then continue to grow after birth. There is also growing evidences for postnatal de novo formation of these lesions [7, 17, 21]. Recent studies have identified some of the factors that may be involved in the formation of AVMs. One of them is endothelin-1, found throughout the normal cerebral vasculature, and a potent vasoconstrictor that plays a role in vascular cell growth. Local repression of endothelin-1 within the AVM has been implicated in the pathophysiology underlying AVMs [23]. Endothelial cell-specific tyrosine kinases that are normally found in developing embryonic blood vessels and vascular endothelial growth factor have been shown to be increased in association with AVMs [12].