Treating patients who suffer from a common condition that affects blood vessels in the brain increases their risk of stroke, a study has found. People with a condition known as arteriovenous malformation (AVM) -- which causes blood vessels in the brain to tangle -- have a better outcome if doctors treat their symptoms only and not the AVM. A team of doctors looked at the long-term outcome of patients with the condition, which is caused by abnormal connections between the arteries and veins in the brain.