Editing Cerebral arteriovenous malformation (section)

==Prognosis==
{{See also|Intracranial hemorrhage}}

The main risk is intracranial hemorrhage. This risk is difficult to quantify since many patients with asymptomatic AVMs will never come to medical attention. Small AVMs tend to bleed more often than do larger ones, the opposite of [[cerebral aneurysm]]s.<ref>{{cite journal|last=Crawford|first=PM|journal=J Neurol Neurosurg Psychiatry|year=1986|volume=49|pages=1–10|pmid=3958721|doi=10.1136/jnnp.49.1.1|title=Arteriovenous malformations of the brain: natural history in unoperated patients|issue=1|pmc=1028639|display-authors=etal}}</ref>  If a rupture or bleeding incident occurs, the blood may penetrate either into the brain tissue ([[cerebral hemorrhage]]) or into the [[subarachnoid space]], which is located between the sheaths (meninges) surrounding the brain ([[subarachnoid hemorrhage]]). Bleeding may also extend into the [[ventricular system]] ([[intraventricular hemorrhage]]). Cerebral hemorrhage appears to be most common.<ref name="pmid11571210" />
One long-term study (mean follow up greater than 20 years) of over 150 symptomatic AVMs (either presenting with bleeding or seizures) found the risk of cerebral hemorrhage to be approximately 4% per year, slightly higher than the 2–4% seen in other studies.<ref>{{cite journal|last=Ondra|first=SL|journal=J Neurosurg|year=1990|volume=73|pages=387–391|pmid=2384776|doi=10.3171/jns.1990.73.3.0387|title=The natural history of symptomatic arteriovenous malformations of the brain: A 24-year follow-up assessment|issue=3|display-authors=etal}}</ref><ref name=":1" /> The earlier an AVM appears, the more likely it is to cause hemorrhage over one's lifetime; e.g. (assuming a 3% annual risk), an AVM appearing at 25 years of age indicates a 79% lifetime chance of hemorrhage, while one appearing at age 85 indicates only a 17% chance.<ref name=":1">{{cite book|last=Greenberg|first=Mark|title=Handbook of Neurosurgery|url=https://archive.org/details/handbookofneuros0006gree/page//|edition=6th|year=2006|orig-year=1990|publisher=[[Thieme Medical Publishers|Thieme]]|pages=835–837}}</ref> Ruptured AVMs are a significant source of morbidity and mortality; following a rupture, as many as 29% of patients will die, with only 55% able to live independently.<ref name=":0" />