Natural History of Intracranial AVMs

Epidemiology

The prevalence of intracranial arteriovenous malformations (iAVMs) in the general population is thought to be around 0.02%–0.2%, making them much less common than cerebral aneurysms (around 5%), ischemic stroke, and many other cerebrovascular disorders. The majority of iAVMs are sporadic and not related to any particular genetic predisposition, though they can be associated with certain conditions such as Osler-Weber-Rendu (also known as hereditary hemorrhagic telangiectasia or HHT), Wyburn-Mason, Sturge-Weber, and other syndromes. The clustering of AVMs among family members or the association of iAVM with other presenting features outside the brain (particularly AVMs in other parts of the body) should prompt thorough evaluation, since the somatic aspects of these syndromes often herald significant morbidity by themselves.

Intracranial AVMs are generally considered to be congenital lesions, resulting from developmental events that occur in utero. Rare reports document that de novo formation after birth is also possible, however, particularly when there is an underlying genetic or vascular susceptibility. Similarly, the presence of multiple iAVMs within the same individual is uncommon, and such an occurrence suggests genetic predisposition. There is no definite sex predominance.

Evolution of Anatomy

Angioarchitecture is a term often used to describe the pattern of arterial input, nidus anatomy, and venous drainage within an iAVM. In some patients, these elements remain static throughout life, while in others there can be considerable dynamic changes, despite the congenital basis of the lesions. Unlike neoplasms, the true nidus of an iAVM does not typically enlarge, but it can recruit additional arterial input from surrounding collateral pathways, which often leads to an appearance of growth in imaging studies. Similarly, the veins that drain the nidus may acquire interval recruitment, thrombosis, dilatation, or narrowing.

The hemodynamic stress due to extra flow within an iAVM may precipitate the development of an aneurysm within the feeding arteries, the nidus, or the draining vein. When affecting venous elements, these dilatations are often termed varices (singular: varix ). Feeding artery aneurysms may be physically remote from the nidus and can be a separate source of rupture that leads to intracranial bleeding. Empirically, some of these feeding arteries regress in size once the AVM nidus is cured or a significant reduction in flow is accomplished by embolization, surgical removal (resection), or radiation.

These dynamic processes may account for changes in symptoms over the life of an individual. For instance, acquired restrictions in venous outflow may produce audible signs of turbulent flow, resulting in tinnitus (a noise in the ear), or they can aggravate edema in the surrounding parenchyma, leading to headaches, seizures, or focal neurologic deficits. Spontaneous occlusion of a draining vein due to thrombosis or acquired stenosis from intimal hyperplasia may also precipitate hemorrhage due to the buildup of pressure within the nidus, since venous egress is blocked.

The effects of puberty and growth, as well as hormonal changes associated with the menstrual cycle, on iAVM anatomy and flow remain poorly characterized. Pregnancy, however, is considered to increase the risk of rupture.