An arteriovenous malformation (AVM) is a complex tangle of abnormal arteries and veins linked by one or more direct connections called fistulas or shunts. This tangle of abnormal arteries and veins is referred to as a nidus. Normally, as the high-pressure arterial blood is pumped through a capillary bed there is a gradual decrease in blood pressure before reaching the venous system. With an AVM, the capillary bed is absent and the high-pressure arterial blood bypasses normal brain tissue and is pumped directly into the normally low-pressure venous system.
Most people with neurological AVMs experience few, if any, significant symptoms, and the malformations tend to be discovered only incidentally, usually either at autopsy or during treatment for an unrelated disorder. But for about 12 percent of the affected population (about 36,000 of the estimated 300,000 Americans with AVMs), these abnormalities cause symptoms that vary greatly in severity. For a small fraction of the individuals within this group, such symptoms are severe enough to become debilitating or even life-threatening. Each year about 1 percent of those with AVMs will die as a direct result of the AVM.
The epidemiology and natural history of cerebral arteriovenous malformations (AVMs) remains incompletely elucidated. Several factors are responsible. With regard to the incidence and prevalence of AVMs, the results of prior studies have suffered because of the retrospective design, the use of nonspecific ICD-9 codes, and a focus on small genetically isolated populations. Recent data from the New York Islands AVM Hemorrhage Study, an ongoing, prospective, population-based survey determining the incidence of AVM-related hemorrhage.