An AVM is an abnormal collection of blood vessels where an arterial blood flow is passing directly into draining veins without the normal interposed capillary bed and tissue. This is a congenital anomaly (i.e. the patient is born with this) and it may never present and only be discovered at post mortem or can present with haemorrhage, seizures or rarely due to its size or by stealing blood from surrounding brain tissue.
These are rare lesions occurring in approximately 0.15% of the population. The average risk of haemorrhage from an AVM is 2-4% per year, but the bleeds can often be very small and the risk of serious morbidity or harm to the patient is approximately 20% over 20 years due to haemorrhage.
This is according to the Spetzler Martin grade. Depending on the size of the AVM, the site of the AVM within the brain and the pattern of venous draining. This gives a score out of 5. 1 is a small AVM in a non-eloquent area with superficial venous drainage and 5 being a large AVM in a very eloquent area with deep and difficult venous drainage.