Brain Arteriovenous Malformations (AVMs) are a collection of abnormal vessels that can occur within any part of the brain tissue. They are created during fetal development and occur in less than 1% of the population. Bleeding is the most common symptom upon presentation. Additionally, an enlarging AVM can cause seizures, pressure, or steal phenomenon, which ultimately may lead to a neurological deficit.
To discuss how AVMs occur, it is helpful to understand how blood normally flows within the body.
Blood normally flows from the heart through arteries to supply all organs, including the brain. As blood flows from the heart into the large arteries, it is pumped at relatively high pressure and speed, both of which decrease as the blood flow reaches smaller and smaller arteries. Eventually, the blood reaches vessels smaller than the diameter of a human hair, which are known as capillaries. As the blood flows slowly through these tiny conduits, it gives up oxygen and nutrients to the brain tissue and receives carbon dioxide and other waste products in return.
The blood then enters tiny veins at very low pressure, eventually entering larger veins in its return to the heart and lungs. Thus, pressure and speed of blood flow in the veins is normally very low, compared to that of the arteries. Because of this, the walls of the veins are relatively thin and delicate compared to the walls of the arteries, which must be thicker in order to handle the higher pressure and speed of the blood flow.
AVMs occur when the relatively large arteries within the brain directly connect to veins without the capillary mesh that normally exists between them. Because of the abnormal “short circuit,” the blood is delivered at relatively high pressure with rapid flow directly into the veins, and because the thinner vein walls are not designed for this condition, they expand, pushing against the neighboring areas of normal brain tissue, which may result in damage to that tissue resulting in weakness, numbness, loss of vision, or seizures.
AVM’s are graded by size, type of venous drainage and whether they involve eloquent tissue of brain such as speech or motor cortex (Spetzler-Martin Classification). On average, there is a 1% per year likelihood of intracranial bleeding. Often the supplying arteries, the AVM itself, or the enlarged veins rupture, resulting in an intracranial hemorrhage, a type of stroke which is the most common presentation of an AVM. Depending on the location and hemorrhage size of the AVM, this type of stroke intracranial bleed may manifest itself by symptoms ranging from severe headache, paralysis, or even death. Patients can be asymptomatic or may present with headaches, seizures, or deterioration of neurologic function. If hemorrhage occurs patients may experience severe headache, stroke-like symptoms or even loss of consciousness. If patients get to the hospital in time, they undergo a work up including CT scan and rapid treatment to stabilize their condition followed by admission to the Neurointensive care unit.
AVMs are most often congenital (present at birth) and usually no specific cause can be identified. In the vast majority of cases, AVMs are not inherited and other members of the family are not at increased risk for an arteriovenous malformation.
AVMs are typically diagnosed by a CT scan or MRI upon presentation of symptoms that may include seizures, headache, or stroke-like episodes. However, an increased number of AVMs are diagnosed incidentally during evaluation for other conditions. Once an AVM is confirmed, an angiogram is performed to identify the vessels involved in the malformation.
Cerebral angiogram is the best diagnostic test for diagnosing AVMs, and provides sufficiently detailed information, which is useful in planning and implementing therapy for most AVMs.
Treatment of an AVM is directed toward preventing brain injury that could result from bleeding or re-bleeding. No treatment currently exists which can repair damage already done to the brain by the AVM. For example, seizures might continue after embolization or even complete removal of the AVM. Patients with neurological deficits resulting from AVM hemorrhage would likely still experience the same deficits after treatment of the AVM, although improvement may occur.