To understand why brain AVMs are dangerous it is important to first understand the way that normal blood vessels are connected with one another:
The Normal Artery-to-Vein ConnectionThere are two main types of blood vessels: the arteries and the veins. Arteries bring oxygen-rich blood from the heart and lungs into organs such as the muscles, bones and brain, and veins bring it back to the heart and lungs where it is re-oxygenated. As the arteries travel deeper and deeper into the tissues, they become thinner and thinner, until they reach a point of maximal narrowing - this area is known as the capillary bed. Blood flow slows down in order to be transferred from arteries into veins.
Thus, one of the most important functions of the capillary bed is to relieve some of the pressure generated by blood as it flows through the larger arteries into the tissues.
Veins spring out of the capillary beds, and they get progressively larger as they exit the organs on their way to the heart and lungs, where blood is finally replenished with oxygen.
Abnormal Artery-to-Vein ConnectionsBrain AVMs are made up of arteries and veins which are connected in such a way that there is no capillary bed at all. This causes the pressure of the arteries to be transmitted directly into the veins within the AVM. This unusual flow of blood creates an area of high pressure and high turbulence that causes the AVM to become larger over time, and to affect the function of the surrounding brain tissue.
What Does a Brain AVM Look Like?Brain AVMs vary in size. Some are tiny and go unnoticed for many decades. Others form large and tortuous channels of arteries which pulsate strongly as they connect directly to their corresponding veins. AVMs can be found anywhere in the brain, including the brain cortex, the white matter, and the brainstem.
Who Develops Brain AVMs?Brain AVMs affect about 0.1% of the population, and are present at birth, but they rarely affect more than one member of the same family. They happen roughly equally in men and women. People typically begin to feel symptoms between the ages of 10 and 30, but it is not uncommon for people to be diagnosed after the age of 50.
What Are the Symptoms of an AVM?About half of the patients find out they have an AVM only after they suffer a subarachnoid hemorrhage. The other half are affected by seizures, headaches, and stroke symptoms such as hemiplegia or hemiparesis.
How Is an AVM Diagnosed?Often, the diagnosis of an AVM can be made by an expert radiologist with just CAT scan of the brain. Most physicians, however, feel more comfortable diagnosing AVMs after performing an MRI. In cases when bleeding has occurred, the AVM can be completely obscured by intracerebral bleeding, requiring a cerebral angiogram to establish a final diagnosis.
How Are AVMs Treated?The most common types of treatment available include surgical resection, endovascular embolization, and stereotactic radiosurgery -- all of which might be used either alone or in combination. In general, these treatments aim to lower the risk of bleeding, or re-bleeding.
An issue that is presently under intense research is whether doctors should treat AVMs that are discovered before they cause bleeding. As it turns out, no studies have assessed the risk of bleeding when these unruptured and sometimes symptom-free AVMs are left untreated. Some people even believe that AVMs that are discovered before they bleed have a lesser risk of bleeding than those which are discovered after they bleed. Nonetheless, the uncertainty causes many surgeons and patients to decide in favor of surgery, in spite of the risk that the treatment itself could cause permanent brain injury.
A study is now in place to investigate this very question. The study's name is ARUBA or "A Randomized Trial of Unruptured Brain AVMs".
What Is the Prognosis?The prognosis of an AVM depends on several factors, beginning with whether the AVM is discovered before or after bleeding. More than 90% of those who bleed survive the event. In those whose AVM is discovered before bleeding, the prognosis is directly related to the AVM's size, symptoms, proximity to vital areas of the brain and, as stated above, whether or not the AVM is treated.
Source:Allan H. Ropper, Robert H. Brown, Adams and Victor's Principles of Neurology, 8th Edition (McGraw-Hill)
Hartmann A, Mast H, Choi JH, Stapf C, Mohr JP. Treatment of arteriovenous malformations of the brain. Curr Neurol Neurosci Rep. 2007 Jan;7(1):28-34.