Brain Arteriovenous Malformations (AVMs)
Arteriovenous malformations, or AVMs, are a collection of abnormal vessels which may occur within virtually any part of the brain tissue. They are thought to arise during fetal development and occur in less than 1% of the population. AVMs can cause an array of problems, but bleeding is the most common presentation. Additionally, an enlarging AVM can cause seizures and pressure which ultimately may lead to a neurological deficit.
To understand AVMs, it is helpful to first discuss normal blood flow function within the body. In a healthy individual, blood normally flows from the heart through arteries to supply all organs including the brain. Four major arteries supply the brain including two carotid arteries in the front of the neck and two vertebral arteries in the back of the neck.
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 the smallest vessels called capillaries, vessels smaller than the diameter of a human hair. 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 (the vessels that return blood to the heart) are normally very low compared to that of the arteries (the vessels responsible to deliver or pump blood from the heart to the rest of the body). Because of this design, the walls of the veins are relatively thin and delicate compared to those of the arteries which must be thicker and more inflexible to handle higher pressure and speed.
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 higher pressure with more rapid flow directly into the veins. Because the veins are not designed for this condition, they expand, pushing against the neighboring areas of normal brain. The result may be damage to the normal brain tissue which can cause weakness, numbness, loss of vision, or seizures. Often the supplying arteries, the AVM itself, or the enlarged veins rupture, resulting in the most common presentation of an AVM, an intracranial hemorrhage which is a type of stroke.
AVMs are most often congenital (present at birth) and usually no specific cause for the AVM 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 having an AVM.