Arteriovenous malformation (AVM), which is like a tangled ball of arteries and veins. These malformations already occur very early in the pregnancy (in the 2nd to 3rd week), when the vascular system starts developing. They are therefore present from the very beginning and in the course of the person's life can give rise to a brain haemorrhage. Another way in which they can be discovered is due to an epileptic seizure. Alternatively, they can go completely unnoticed. Approximately three quarters of all cases are operable; the remainder is either located so deep inside the brain or is so large or difficult to access that surgery would be impossible without causing significant damage to normal tissue. In those cases, a special type of radiation may be a possibility. An AVM can also be invisible on an angiography, but visible on a CT scan or MRI.