Arteriovenous malformations (AVMs) are congenital vascular malformations that can occur in various parts of the central nervous system. They include direct arteriovenous traffic without an intermediate capillary bed. The incidence is about 1/10 of intracranial vascular disease. Pathological features 1. General view (1) Intracranial AVMs resemble a sphere formed by a mass of red and blue threads, which Gusing and Bailey call a tangled mass of blood vessels. (2) AVMs are mostly tapered, with the bottom located at or parallel to the cortical surface and the tip pointing to the ventricles. (3) Vascular nests can be dense or diffuse, ranging from a few centimeters to the entire hemisphere. (4) The adjacent brain parenchyma is stained with iron-containing heme left over from previous hemorrhage, and the surface meninges may be thickened and fibrotic, or collagenous, fibrotic, and calcified. 2. Histological features (1) Arteries ①The arteries of AVM are abnormally dilated, with thinning of the walls in some areas, degeneration or lack of the mesentery and elastic plates. ② Presence of degeneration, presumably by high flow affecting all shear forces in the vessel wall. Localized irregular thickening of the vessel wall, endothelial hyperplasia, mesothelial hypertrophy, substrate stratification, and thickening. (2) Vascular nests (1) Vascular nests may have hypertrophic mesothelium, making it more difficult to identify arteries or veins (2) Vascular nests may have aneurysms and sclerotic insula-like brain tissue (3) Veins (1) “Arterialized” veins with thickened walls due to cellular proliferation (2) Although AVM veins have thickened walls and resemble arteries in general, they lack organic elastic plates and all are not true arterial structures (3) Vascular nests (4) Vascular nests (4) Even in diffuse lesions, there is normal brain tissue between the AVM vessels, but the brain tissue within the AVM is generally nonfunctional.