Arteriovenous Malformations 

Hemorrhage from cerebral arteriovenous malformations (AVMs) represents 2% of all hemorrhagic strokes. A clear understanding of the diagnostic and treatment algorithms involved with AVM management is imperative, because AVMs are a cause of hemorrhage in young adults.

AVMs are congenital lesions composed of a complex tangle of arteries and veins connected by one or more fistulae. The vascular conglomerate is called the nidus. The nidus has no capillary bed, and the feeding arteries drain directly to the draining veins.^[1] The arteries have a deficient muscularis layer. The draining veins often are dilated owing to the high velocity of blood flow through the fistulae. How the abnormal vessels appear or exactly when the process begins is unknown. Deranged production of vasoactive proteins is under investigation as the angiogenetic link to pathophysiology.

AVMs produce neurological dysfunction through 3 main mechanisms.^[2] First, hemorrhage may occur in the subarachnoid space, the intraventricular space or, most commonly, the brain parenchyma. Second, in the absence of hemorrhage, seizures may occur as a consequence of AVM: approximately 15-40% of patients present with seizure disorder. Finally, but rarely, a progressive neurological deficit may occur in 6-12% of patients over a few months to several years. These slowly progressive neurological deficits are thought to relate to siphoning of blood flow away from adjacent brain tissue (the "steal phenomenon"), a concept that has been recently challenged. Neurological deficits may be explained alternatively by the mass effect of an enlarging AVM or venous hypertension in the draining veins.

Frequency

United States

The detection rate in the general population based on prospective data from the New York Islands AVM Study is approximately 1.34 per 100,000 person-years.^[3] The prevalence of cerebral AVM in the United States is not known. Given the low threshold for MRI neuroimaging, many patients' conditions are now discovered before they experience a brain hemorrhage.^[2]

International

Reported detection rates range between 0.89 and 1.24 per 100,000 person-years according to reports from Australia, Sweden, and Scotland. The prevalence of cerebral AVMs in Scotland has been estimated to be 18 per 100,000 person-years.

Mortality/Morbidity

Although 300,000 persons in the United States may harbor AVMs, only 12% of AVMs are estimated to become symptomatic. Death occurs in 10-15% of patients who have hemorrhage, and morbidity of various degrees occurs in approximately 30-50%.

• Hemorrhage: In population-based studies, 38-70% of brain AVMs present initially with hemorrhages. The overall risk of intracranial hemorrhage in patients with known AVM is 2-4% per year. Patients presenting with a hemorrhage are at increased risk for rebleeding, particularly during the first year after the initial hemorrhage (recurrent hemorrhage rate within 12 months after initial hemorrhage: patients with hemorrhagic presentation 7-33%; patients with nonhemorrhagic presentation 0-3%).^{[4, 5]} Hemorrhage rates progressively converge with time for both patients groups after 1 year.^[5] Clinical and angiographic features associated with the risk for hemorrhagic presentation are male gender, small AVM size, location in the basal ganglia or posterior fossa, deep venous drainage, single or only few draining veins, high pressure in the feeding arteries as measured during angiography, and intranidal and flow-related feeding artery aneurysms.
• Although the initial presentation of a cerebral hemorrhage may be indistinguishable from those of other causes of hemorrhage, the neurological deficit in AVM-related hemorrhage tends to be less severe compared with a non–AVM-related hemorrhage. Recovery of AVM-related hemorrhage tends to be better, partly because of the relatively younger age of patients with AVM and partly because of functional cerebral reorganization in patients with cerebral AVMs.
• Seizures and epilepsy: Seizures unrelated to hemorrhage occur as the presenting symptom in 15-40% of patients with brain AVM. These may be focal or become secondarily generalized. Satisfactory treatment of seizures is usually possible with standard anticonvulsants. Presentation with seizures is associated with young age, large AVM size, lobar location (especially temporal lobe), and feeders mainly from the middle cerebral artery. Patients with brain ruptured AVM, especially if it is of cortical or subarachnoid location, are at increased risk to develop seizures and epilepsy similar to patients with this type of hemorrhages of other causes unrelated to brain AVM.
• Headache and migraine: In the general population, headache due to a brain AVM is an extremely uncommon cause. Headache unrelated to hemorrhage occurs in 4-14% of patients with AVM and may be the presenting symptom. The headache may be typical for migraine or may be present with a less specific complaint of more generalized head pain.

Race

No racial predilection has been identified.

Sex

Both sexes are affected equally.

Age

• Despite the presumed congenital origin of AVMs, the clinical presentation most commonly occurs in young adults.
• AVM hemorrhage or seizure as an incident event may occur in young children or adults older than 40 years; however, childhood migraine is common.
• A history of subtle learning disorder is elicited in 66% of adults with AVMs. This suggests early effects that are largely subclinical and do not come to medical attention.