Cerebrovascular surgery – Arteriovenous malformations
Arteriovenous malformations are abnormal tangles of blood vessels. They can occur anywhere in the body, but in this discussion of cerebrovascular disease the focus will be on malformations in the brain.
The best way to think about this is to consider the normal vascular system. Arteries are thick walled blood vessels that carry oxygen rich blood way from the heart to the rest of the body. Veins are thin walled blood vessels that drain oxygen-depleted blood away from the body to the heart and lungs. Capillaries are smaller vessels between these two systems that allow a stepwise decrease in the pressure under which the blood is. In arteries blood is under high pressure, while in veins blood is under low pressure.
The cause of AVM’s remains largely undetermined. Early scientific opinion was that these abnormalities are congenital i.e. some you are born with. This is because this is a condition most commonly discovered in young adults. However it does not only occur in this age group.
Imaging is becoming more easily accessible to patients and for that reason abnormalities such as AVMs are increasingly diagnosed. These are incidental AVM’s that are asymptomatic. This means that the population incidence is uncertain, but this is not a common condition.
When AVM’s are symptomatic, they will either bleed resulting in stroke like symptoms that come on suddenly or they can cause a seizure disorder. AVMs can also cause vascular steal phenomenon. This occurs when blood rushes to the malformation away from the normal brain next to it. The normal brain is getting relatively little blood and this results in the brain function of this area being abnormal.
If an AVM is symptomatic treatment is usually recommended, especially if the AVM has bled. The risk or the AVM bleeding again once it has bled is much higher than the baseline rate of 2 to 4 % per year.
The decision making to decide to treat an asymptomatic malformation is much more complex. The main indication for treatment would be to prevent haemorrhage of the malformation. This requires a consultation with a neurosurgeon to discuss the pros and cons regarding treatment as they pertain to your specific case.
The diagnosis of a malformation is made on specialised brain imaging such as a CT angiogram, MRI and digital subtraction angiogram. The latter test is regarded as the gold standard and provided detailed information about the anatomy of your malformation and informs decision making regarding treatment.
Decision making on treatment of malformations is complex and requires advise from a specialist neurosurgeon. In general principles an AVM that is causing symptoms is more likely to need treatment, while an asymptomatic one may require observation instead. However, treatment has to be tailor made for individual patients with individual malformations. The main aim of treatment is to prevent new or recurrent bleeding.
Treatment options include microvascular surgery, focused radiotherapy (radiosurgery) and endovascular embolization. These treatments are not infrequently combined to provide the best solution. Microvascular surgery, where possible, will remove the malformation and provide immediate protection from risk of bleeding.
With radiosurgery, treatment is less invasive, but there is a delay before protection from bleeding is achieved. Endovascular embolization is only rarely used on its own to definitively treat malformations to prevent rebleeding,