AVMs (Arteriovenous Malformations)

An AVM is a collection of abnormal blood vessels consisting of one or more dilated arteries feeding into a tangled mass of smaller, fragile vessels and thence into a series of enlarged veins, before returning blood back to the heart without it having performed its normal function. They can occur at any site in the body, but are of particular concern when they appear in the brain because of the consequences of haemorrhage, which may occur if the AVM ruptures. AVMs can indicate their presence by exhibiting all or any of the following features:

• Haemorrhage
• Epilepsy
• Neurological deficit
• Headache

  Treatment methods

  The prevention of haemorrhage is usually the main aim of treatment, and this is only guaranteed by completely obliterating or removing the AVM. The main treatment methods are outlined below.

  Surgery

  Complete excision is a proven effective treatment. It has the advantage that with complete removal the benefits are realised immediately. The disadvantages are the relatively high risk of producing neurological impairment and, as with any surgery, the recovery time, and the risks of general anaesthesia and infection. Furthermore, because of the former consideration, some AVMs are considered to entail such a high risk of neurological damage as to be deemed inoperable.

  Gamma knife

  Gamma knife has the advantage of being minimally invasive, thereby avoiding operative damage and a lengthy recovery period. It is most effective on small lesions; commonly a 90% obliteration rate (or higher) can be quoted for smaller AVMs, but this will depend on the location of the AVM within the brain. However, for larger lesions the success rate for complete obliteration decreases. A potential disadvantage of gamma knife is that a small amount of normal brain tissue directly surrounding the AVM may be at risk of radiation damage. Increasing the dose delivered to the AVM increases the chance of obliteration, but also increases the relative risk of the procedure. Careful consideration is therefore required when deciding upon dose. Obliteration of AVMs following gamma knife treatment can take up to two years (or even longer), and follow up is required during this time by means of MRI scanning at intervals. A final angiogram is then performed to confirm obliteration of the AVM.

  Embolisation

  Embolisation involves injecting resinous material, which then solidifies, into the feeing vessels of an AVM. It tends to be used for small AVMs, and alone is unlikely totally to obliterate a larger lesion. However, it may sometimes assist by reducing the size of an AVM, thereby facilitating later radiosurgery designed to 'finish off' the remainder.