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 vasculopathies. Pathological features 1. Gross view a. Intracranial AVMs resemble a sphere formed by a mass of red and blue threads, which Gusing and Bailey call a tangled mass of vessels; b. AVMs are mostly conical, with the bottom located at or parallel to the cortical surface and the tip pointing to the ventricles; c. Vascular nests can be dense or diffuse, ranging from a few centimeters to the entire hemisphere; d. Adjacent vascular nests can be dense or diffuse. d. 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 a. Arteries (1) The arteries in AVM are abnormally dilated, with wall thinning, degeneration or lack of mid-membranes and elastic plates in some areas; (2) There is degenerative 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. b. Vascular nests (1) Vascular nest vessels may have hypertrophic mesothelium, making it more difficult to identify whether they are arteries or veins; (2) Aneurysms and sclerotic island-like brain tissue may be present within the vascular nests. c. Veins (1) “Arterialized” veins with thickened walls due to cellular hyperplasia; (2) Although AVM veins have thickened walls and resemble arteries in general, they lack organic elastic plates and are not true arterial structures. d. Even in diffuse lesions, there will be normal brain tissue between the AVM vessels, but the brain tissue within the AVM is generally not functional.