Arteriovenous malformations (AVMs)
Arteriovenous malformations (AVMs) are abnormal masses of blood vessels — arteries and veins called the nidus that develop while an unborn baby is growing inside the mother's uterus. The blood in the arteries dumps directly into the veins with no network of tiny capillaries in between, which normally occurs. And because the blood vessels that make up these masses are abnormally fragile, the malformations may bleed, leading to serious complications.
Although AVMs can occur anywhere (arms, hands, legs, feet, lungs, heart, liver and kidneys), about half of them are found in the brain, brainstem and spinal cord. Because they hemorrhage easily, AVMs can cause strokes, paralysis and the loss of speech, memory, or vision. There is about a 10% fatality rate associated with an AVM hemorrhage.
How often do AVMs occur?
Brain AVMs occur in approximately three of every 100,000 people, and about 40 to 80% of them experience some bleeding. It has been estimated that the annual risk of bleeding is about 1 to 4%. After age 55, the risk of bleeding decreases. Pregnancy and labor has been associated with an increased chance of AVM rupture. Although people are born with AVM's, the malformations are rarely discovered before age 20.
The causes of AVMs
Some researchers suggest there is a genetic link for some AVMs, but no one is sure. The majority of AVMs are discovered in people ages 20 to 40. Some evidence indicates that the malformations develop when the unborn fetus is 45 to 60 days old, but other information suggests that AVMs are primitive structures left over after fetal blood-circulating systems develop.
Symptoms
Typically, the first symptom of an AVM hemorrhage in the brain is either a sudden severe headache or seizures. The headache may be focused in one specific area or it may be more general. Sometimes the headache is mistaken for a migraine. Other symptoms may include vomiting, stiff neck, confusion, irritability, sleepiness, lethargy, or weakness anywhere in the body.
Vision may also be affected; a person may experience decreased, double, or blurred vision. About 25% of patients experience loss of vision, weakness, or mental changes, depending on the exact location of the AVM.
Additional signs of a bleeding AVM in the brain are impaired speech or smell, dizziness, paralysis of the facial muscles, drooping eyelids, fainting, and ringing or buzzing in the ears.
Diagnosis
Based on the clinical symptoms such as severe headache or neurological problems, the following tests may be used:
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A complete neurologic exam
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A computed tomography (CT/CTA) scan of the head
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Functional MRI
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A variety of tests of language comprehension, speech production, sensation, and other tasks to help determine the risk of treatment.
Evaluation and treatment
The treatment team typically consist of a neurologist, neuroradiologist, neuropsychologist, neurosurgeon, and anesthesiologist; a careful evaluation is key to successful treatment. The following are treatment options to be considered:
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Observation - There is some debate over whether or not to treat AVMs that have not ruptured and are not causing any symptoms. The risks and benefits of proceeding with treatment need to be measured on an individual basis, taking into account factors such as the person's age and general health, as well as the AVM's size and location and risks of treatment.
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Symptomatic treatment - Antiseizure medications, pain relievers for
headaches, and migraine medications may provide adequate symptom control for many patients.
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Surgery - Removing the AVM is the surest way of preventing it from causing future problems. Often surgery is recommended for AVMs that are close to the surface, but it may be too difficult or risky for deeply located or very large ones.
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Targeted radiation – Stereotactic radiosurgery is particularly useful for treating small malformations deep within the brain. This approach can often destroy the AVM without damaging the surrounding tissue. However, the success or failure of radiation often takes time to be evident called the latency period which is often 2-3years. During that time the patient is at risk for hemorrhage.
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Embolization – This approach involves blocking the blood flow to the malformation through the placement — via a catheter — of a hardening polymer. In the event that embolization is being used in advance of surgery, at least two or three embolization procedures may be necessary delivered at intervals of 2 to 6 weeks. The procedure alone can in some circumstances provide complete blockage of the AVM and potential cure when the AVM is small.
Prognosis
Approximately 10% of AVM hemorrhages are fatal. Seizures and neurological changes may be permanent in another 10 to 30% of cases of AVM rupture. If an AVM bleeds once, it is about 6% likely to bleed again in the next year if it is not treated. As time passes from the initial hemorrhage, the risk for further bleeding drops to about 3 to 4%, the baseline natural history risk. If the AVM has not bled, it is possible that it will bleed at a natural history risk rate of 3 to 4%/year. Some AVM's harbor features that make them more likely to bleed. Untreated AVMs can grow larger over time and rarely go away by themselves.
Once an AVM is surgically removed, it is likely curative in adults. In children, surgical removal can cure the AVM, but long-term angiographic followup is recommended since the AVM can recur in rare cases. In AVM's that have been treated radiosurgically and have been documented as obliterated on 2-to-3-year post-treatment angiograms, long-term follow-up angiography is also recommended.