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I. Definition

McCormick in 1966 and Russell and Rubenstein described four types of vascular malformations, and this is now accepted as the current nomenclature. [62-64] Cerebrovascular malformations are classified according to their histopathologic features as : arteriovenous malformation (AVMs), venous angioma, cavernous malformation, and capillary telangiectasia. The focus of this chapter is on true AVMs. A possible fifth category is a direct fistula, or arteriovenous fistula (AVF). These conditions are regarded as acquired lesions involving single or multiple dilated arterioles that connect directly to a vein without a nidus. They are high-flow, high-pressure lesions that have a low incidence of hemorrhage (examples include vein of Galen aneurysmal malformation, dural AVF, and carotid cavernous fistula). [62-64]

AVMs are vascular anomalies that may occur in any region of the brain. They are composed of a nidus with feeding arteries, and draining veins that form an anomalous mass of blood vessels in the pia matter, with direct arteriovenous shunts and a poor or absent capillary bed, and consequently a high-flow shunt that predisposes to arterialization of veins, vascular recruitment, and gliosis of brain tissue adjacent to the lesion. [30, 48]

Originally it was thought that the nidus lacked a true capillary bed. However, Sato et al. [90] analyzing the relationships between perinidal vessels and the nidus in 22 resected specimens from patients with cerebral AVMs, observed that all nidus were accompanied by a perinidal dilated capillary network that was connected not only to the nidus, feeding arteries, and draining veins, via arterioles and venules, but also to normal capillaries, arterioles, and venules. There is the hypothesis that these capillaries may contribute to the intraoperative and postoperative bleeding, growth, and even recurrence of surgically treated cerebral AVMs, as these perinidal capillaries may fuse to become part of the nidus [90].

Nidal vessels are structurally ambiguous resembling both arteries and veins, with high risk of intracranial hemorrhage, which is the main and the most feared manifestation of AVMs, followed by epileptic seizures. AVM-high-flow shunts cause great dilation of the drainage veins that can result in a mass effect. At the same time, they can provoke a reduction of the perfusion pressure in the adjacent brain, that being known as “brain vascular steal”, which is an important mechanism in the pathogenesis of focal neurological deficits. [18, 54, 56, 58]

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