AVMs (Arteriovenous malformations)
Arteriovenous malformations, or AVMs, are complex tangles of arteries and veins located within the substance of the brain. These result from abnormal development. They are usually present at birth but progressively enlarge during a person's lifetime. In an AVM there is shunting of arterial blood, which is under high pressure, directly to the venous system without the usual intervening capillaries which , in a normal person, dampen the pressure. As a result the veins dilate, can rupture and put pressure on the brain.
AVMs can occur anywhere in the brain. They are usually fed by one or more major arterial branches. The complexity of treatment depends on the location of the lesion, its size and the number of branches which feed it.
The shunting of arterial blood directly to the venous system may have several dangerous effects:
1. Hemorrhage
2. Seizures
3. Vascular "steal"
Hemorrhage from rupture of an AVM can cause major and catastrophic brain damage. Poorly controlled seizures (epilepsy) and the shunting of blood through the AVM starves the brain tissue for oxygen and nutrient. Both of these latter effects result in slow and insIDious decline of brain function.
Because of the increased pressure on the arteries and veins in an AVM aneurysms may develop on the feeding arteries and on the draining veins. This occurs in approximately 15% of cases. In such cases the aneurysm should be consIDered as an additional and separate lesion and can also be associated with catastrophic hemorrhage and neurologic deficit (stroke).
The best treatment for an AVM is surgical removal. This is much easier and safer earlier in the course of development when the AVM is small and has not yet had the opportunity to recruit additional brain arteries into its feeding blood supply.
Small AVMs (less than 2 cms - .75 inches- in diameter) can be treated non-invasively with stereotactic radiosurgery. Here finely focused beams of gamma radiation intersect to deliver a high dose of radiation to the AVM and minimal radiation to the surrounding brain.
Arterioveous malformations are readily noted on MRI studies. In addition, an MRI can detect whether or not an AVM has ever bled. Patients with headaches thought to be migraines occasionally have an AVM. MRIs should be done early in patients with Migraine headaches and migraine equivalent type symptoms.
