Cerebrovascular - Brain Arterial Venous Malformation (AVM)
What is an AVM?

An AVM is an abnormal collection of blood vessels, where arterial blood flows directly into the draining veins without the normally interposed capillaries. This appears as a "tangle" of vessels.

How common are AVMs?

AVMs are uncommon. The frequency of intracranial AVMs is about one-seventh that of aneurysms. This would indicate that about 0.14% of the population harbours one of these lesions.

In what age group are AVMs found?

AVMs are congenital lesions. Although about one-quarter of all AVMs hemorrhage within the first 15 years of a patient's life, the common age of persons at presentation is between 20 and 40 years. The majority of AVMs become symptomatic by the time a person reaches 50.

How do AVMs present?
  • hemorrhage: the most common presenting symptom, occurring in 50% of patients
  • seizures: the presenting symptom of 25% of patients
  • mass effect (pressure on the surrounding brain)
  • bruit (an abnormal sound detected when listening to blood flow)
What is the risk of hemorrhage from AVMs?

The risk of hemorrhage from an AVM is estimated at 2 to 4 percent per year; thus, a young patient faces a greater cumulative risk in the years ahead than does an older patient. Each hemorrhagic episode carries a 30% risk of death and approximately 25% risk of significant long-term neurological deficits.

How are AVMs evaluated?

Neurological imaging can reveal the distinguishing characteristics of AVMs. Angiograms provide the formal brain vessel study which is critical for all patients, identifying the arteries and veins that are involved in the AVM, which in turn helps determine treatment recommendations. The MRI can augment this information about the location and anatomy of the AVM and can alsoreveal old hemorrhagic episodes which may not have resulted in symptoms. CT scanning may provide further helpful details.

How are AVMs graded?

AVMs are graded by a system that assesses three important characteristic of the AVM.

  • Size: the larger the size, the higher the grade

  • Location: eloquent areas of the brain result in a higher grade (sensory, motor, language, visual cortex, hypothalamus and thalamus, internal capsule, brainstem)

  • Pattern of venous drainage: venous drainage into the deep structures of the brain results in a higher grade

In general, the higher the grade of an AVM, the more difficult it is to treat it surgically and the higher the associated risks.

Graded Feature Points
Size
Small (<3cm)
Medium (3-6 cm)
Large (>6cm)
1
2
3
Importance (eloquence) of adjacent brain
Non-eloquent
Eloquent
0
1
Pattern of Venous Drainage
Superficial only
Deep
0
1

How are AVMs treated?

For AVMs considered operable (based on the grade and expected risks of surgery), microsurgery is the treatment of choice. Other factors taken into account when considering surgery include the age of the patient (and therefore the number of years ahead in which the patient is exposed to hemorrhage risk) and the past behaviour of the malformation.

Alternate treatment options include:

  • Stereotactic radiosurgery (Gamma Knife Surgery or GKS): GKS is a good treatment option for small AVMs (<3cm) in surgically inaccessible locations. The drawback is that the risks of hemorrhage are not decreased until approximately 2 years after the treatment, as it takes this long for the radiation to make the AVM "disappear'. Nevertheless, the level of risk may be lower than that for microsurgery, especially for AVMs in sensitive locations. The success rate is approximately 80%.

  • Endovascular treatment: Some AVMs may be treated partially by embolization: clogging the malformation with a foreign material; however, this rarely provides a cure. If endovascular therapy is treatment considered, it is usually performed in conjunction with surgery or radiosurgery.

The optimal treatment method needs to be determined through careful consideration of the diagnostic tests that are available, the grade of the AVM, and evaluation of the patient's expectations and needs.