Arteriovenous Malformations - AVM

AVMs 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 (ie. 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.

What problems do AVMs cause?

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 (fig 1 and 2). 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.

What are the consequences of an AVM bleeding?

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 an AVM to 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).

Treatment options include observation alone, microsurgery, focused irradiation and embolisation. Management by observation alone may be appropriate. The decision to refrain from intervention depends on an understanding of the risks of treatment and the risks of non-treatment. The risk of hemorrhage (a bleed into the brain known as a "stroke") averages 2 to 4 % per year. This means that over the next 10 years the risk of hemorrhage approximates 30%, over the next 20 years approximates 50% and over the next 30 years approximates 66%. If a hemorrhage occurs the chance of death or permanent deficits affecting brain function is close to 50%.

Because the cause for hemorrhage is wear-and-tear changes the chance of hemorrhage varies according to the presence or absence of these wear-and-tear changes. An example of these changes is if a hemorrhage has occurred. If hemorrhage has occurred the risk of another hemorrhage during the next year may be higher than 10%. Other signs of wear-and-tear are the presence of small "blow-outs" (known as aneurysms). In the presence of these aneurysms (seen by angiography) the risk of hemorrhage each year is 7%. This means that over the next 10 years the risk of hemorrhage approximates 50%, over the next 20 years approximates 77% and over the next 30 years approximates 90%. In the absence of hemorrhage or signs of wear-and-tear on angiography the risk of hemorrhage is about 1.7% per year. This means that over the next 10 years the risk of hemorrhage approximates 16%, over the next 20 years approximates 30% and over the next 30 years approximates 40%.

Given the variable risk for hemorrhage assessment of individual risks may not be possible without a number of investigations, including MRI scan and cerebral angiography. Furthermore, the wear-and-tear changes may take time to develop and repeat investigations may be needed from time to time in order to see if the risks of hemorrhage have changed.

Apart from hemorrhage, other problems can occur. The most common problems that can arise other than hemorrhage are seizures (or epilepsy) and neurological deficits (loss of some form of body function). These problems account for a decline in function in about one quarter of all people with AVMs over a 20 year period from diagnosis. For an AVM to cause epilepsy it must be in a part of the brain that when neurons (nerve cells) are irritated a chain reaction results in many neurons firing to produce a seizure. Not all AVMs are capable of producing seizures. A loss of function in the absence of hemorrhage is usually associated with large AVMs.

Treatment by microsurgery may be recommended as the preferred method of management. The aim of surgery is the complete removal of the AVM without damaging critical brain. A number of considerations need to be made in order to estimate risk of surgery. Location, size and blood vessel supply and drainage have a large impact on these risks.

A grading system is used that allows a rough guide to risks. This grading system (Spetzler-Martin grading) assigns 1 point to AVMs smaller than 3 cm in largest diameter, 2 points to AVMs between 3 and 6 cm in largest diameter, and 3 points for AVMs larger than 6 cm. In addition, a point is added if the AVM is located in critical brain, and a point if the veins drain deeply (into the veins in the centre of the brain).

Therefore, AVMs can be graded from 1 (the simplest surgical cases) to grade 5 (the most difficult AVM). Beside these features, AVMs that are grade 3 with artery supply from small deep arteries (ie. lenticulostriate arteries) are more difficult than grade 3 AVMs that do not have such an arterial supply.

In surgery on more than 550 cases of AVMs performed by Professor Morgan the risk of death was 2% and a further 12% developed a new deficit. These new deficits were serious (ie. having to become dependent on others for at least some aspects of self-care) in 1.6% and mild (self-caring) in 10.4%. However, these risks were strongly influenced by grade. For AVMs that were grade 1 or 2 (more than 250 cases) death occurred in 0.5% of cases and mild deficits occurred in a further 1% of cases. For grade 3 AVMs (more than 100 cases) 2.7% died, 2.7% developed serious deficits and 16.2% developed mild deficits. For AVMs that were grade 4 or 5, death occurred in 7.7%, serious deficits developed in a further 5.5% and mild deficits in 12.4%. Obliteration of AVMs occurred in all cases and no patient has suffered a hemorrhage from AVM after discharge from hospital.

Being able to classify AVMs into their grade allows a comparison to treatments by observation, focused irradiation and embolisation. This allows recommendation of the most appropriate treatment.

Focused irradiation (sometimes called gamma knife or x-knife treatment) is where irradiation is precisely delivered to the AVM without the need to expose the brain in surgery. It works by slowly blocking off the blood vessels within the AVM. The higher the dose of radiation delivered to the margins of the AVM the quicker obliteration is achieved. However, the dose of irradiation that is absorbed by normal brain must be kept below critical levels in order to minimize the risks of radiation damage. AVMs that are best suited to Focused irradiation that allows the maximum rate of cure with the least damage to critical brain are small AVMs. Cure takes on average 2 to 3 years to occur. Therefore, the risks of Focused irradiation are a combination of the risk of radiation damage to the brain and the risk of hemorrhage occurring before complete obliteration.

For AVMs that are small (<3 cm maximum diameter) obliteration can be achieved in approximately 75% of cases in 2 to 3 years. The average risk for these lesions is 8% of some permanent complication (from death to mild deficits). For larger lesions the obliteration rate becomes considerably less.

Microsurgery is generally recommended for grade 1 and 2 AVMs. Focused irradiation may be best suited to small deeply placed grade 3 AVMs fed by deep arteries.

Embolisation involves occluding all or part of the AVM by injecting glue or particles. The embolisation is performed during angiography by the interventional radiologist. Angiography is a procedure that involves inserting a catheter (a fine tube) into the femoral artery (an artery by the hip) and threading the catheter through the arteries of the body into the arteries supplying the AVM. Angiography then is performed by injecting an iodine containing fluid whilst X-ray pictures are taken. This shows the AVM.

Embolisation is infrequently used in attempting to cure AVMs but is often used to reduce bleeding at the time of surgery. Because the risk of embolisation at Royal North Shore and Dalcross Hospitals is 4.4% (as reported in publications) it is reserved for more difficult surgical cases.