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CONDITIONS TREATED

Brain Arterial Venous Malformations (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. These appear 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 harbour 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 life, the common age at presentation is between 20 and 40 years. The majority of AVMs become symptomatic by age 50.

How do AVMs present?

  • Hemorrhage: most common, 50% of patients with AVMs will present with a hemorrhage

  • Seizures: 25% of patients have this as their presenting symptom

  • Pressure on the surrounding brain (mass effect)

  • Headaches

  • Bruit

What is the risk of hemorrhage from AVMs?

The cumulative risk of hemorrhage from an AVM is estimated at 2 to 4 percent per year. Each hemorrhagic episode carries a 30% risk of death and about 25% risk of significant long- term neurological deficits (morbidity).

How are AVMs evaluated?

AVMs have distinguishing characteristics on neurological imaging. CT scan and MRI may be helpful but all patients require formal brain vessel study with an angiogram. The MRI and the angiogram give important information regarding the location and anatomy AVM (i.e. what arteries and veins are involved), which have implications for the treatment recommendations. In addition, old hemorrhagic episodes that may have not resulted in symptoms may be seen on the MRI.

How are AVMs graded?

AVMs are graded by a grading system that uses three important characteristic of the AVM to arrive at the grade.

  • 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 result 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 risks associated with them.

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?

Microsurgery has been considered the treatment of choice for AVMs that are considered operable based on the grade and expected risks of surgery. Other factors taken into account include the age of the patient, and therefore the years of hemorrhage risk the patient is exposed to, and the past behaviour of the malformation.

Other treatment options include:

  • Stereotactic radiosurgery (Gamma Knife): 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". The success rate is approximately 80%. The risks may be lower than surgery, especially for AVMs in sensitive locations.

  • 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 by careful evaluation of the patient and the diagnostic studies that are available. Based on the patients expectations and needs and the grade of the AVM the optimal treatment modality can be formulated.


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