of the brain (or arteriovenous malformations) are congenital (but
usually not inherited) lesions that are composed of a tangle of abnormal
arteries and veins joined together without the presence of the smallest
blood vessels of the brain (i.e. capillaries). Within the tangle of
blood vessels there is little, if any, functioning brain. They can occur
anywhere in the brain or spinal cord. It is unusual for their to be
more than one AVM.
How do AVMs cause trouble?
In the normal circulation, blood is ejected from the heart into
the arteries. These branch and get smaller. Eventually the smallest
blood vessel of the body is reached, the capillary, that is so small that the
red cells have to bend out of shape a little to fit through. When the
blood is through the capillaries it reaches the veins that become larger and
take the blood back to the heart. However, in the region of the AVM there
are no small blood vessels. The artery type vessels join directly to the
vein type vessels. Within the AVM, blood rushes from the artery-type
vessel to the vein-type vessel without being slowed down by the very small
vessels normally separating artery from vein. Blood rushing more quickly than
normal is like a road with more traffic than planned. The surface develops
wear-and tear and may ultimately break. This is the cause of bleeding into the
brain. Half of all patients present with bleeding (or hemorrhage). Of the
remainder, seizures (or epilepsy), headache and loss of function are ways in
which AVMs can declare themselves.
When an AVM bleeds into the brain there is approximately a 50% chance that either death or a permanent loss of some brain function will occur.
How likely is it that an AVM will bleed?
The risk varies from person to person and also with each person over time. On average, the risk of bleeding over the 10 years from diagnosis is 30%. But if signs of wear-and-tear are present (such as aneurysms) this risk might be greater than 50%. Over 30 years from diagnosis the risk of bleeding averages 66% but may be as high as 90% (if wear-and-tear changes are present). Two well conducted studies of a large number of patients are illustrated on the graph below. The strength of the study from Toronto lies in the large number of patients for the early years following diagnosis. The strength of the study from Helsinki lies in the length of follow-up. Both studies are useful when considering the natural history of brain AVM. The presence of aneurysms (the wear-and-tear changes) increases the likelihood of rupture in the Toronto study (not quantified in the graph below).