It is likely that most arteriovenous malformations in the brain do not bleed. The discovery of an AVM, especially when asymptomatic, presents numerous complex considerations. While living with the malformation entails certain risks, the potential hazards associated with treatment may surpass these risks. Although medical science has yet to fully address these challenges, consulting with a specialist can assist in navigating the uncertainties and devising a strategy tailored to your individual circumstances.
Accidental Discoveries
Increasingly brain AVMs are first discovered when an individual undergoes cross-sectional imaging of the brain with CT or MR for an unrelated reason. Access to high-quality imaging has become much more widespread and affordable. This is a double edged-sword as carrying out imaging in well individuals risks identifying lesions with the potential to harm them in the future but where that risk might well never have been realised. Treatment of asymptomatic lesions exposes one to risks associated with that treatment
It is estimated that AVMs are present in about 0.15% of the population and such discovery poses particularly difficult questions for someone who has no symptoms caused by it. Any risk might not be realised for many years in the future if indeed they are realised at all.
If for any reason you are concerned that you might harbour an AVM (or any other central nervous system disease) because a relative has been diagnosed then we strongly recommend you speak with a specialist about your concerns before undergoing imaging. If you are considering undergoing health-screening imaging pre-scan counselling regarding unexpected asymptomatic discoveries should form part of that process.
The Risk Of Haemorrhage
Arteriovenous malformations of the brain (AVM) are found in about 10 to 18 per 100000 people. Many remain undiscovered causing no symptoms throughout a person's life. They develop early in childhood or are present at the time of birth
Estimating the risk of bleeding is not straightforward. Assuming the prevalence of the condition described above is correct there is also the fact that AVMs form a very varied family. Some are large while others are small. They may have many or few arteries feeding blood to them. There may be many veins draining the malformation while others rely on a single vein. Consequently the stresses placed on these vessels are different from case to case and therefore the risk of bleeding probably differs, at least a little, from one malformation and individual to the next. These statistics are estimates and not "predictions" of what will happen in the future.
The best quality data currently suggest the risk of a first-ever haemorrhage from an AVM is about 1.3% per year. The risk of subsequent haemorrhages then increase, particularly in the ensuing 12 months before settling back to an elevated baseline of about 4.8% per year. The risks of bleeding do seem to slowly increase with each decade one lives with an AVM but in absolute terms the increase is small at less than one per cent per decade.
To Treat or Not?
Bleeding from an AVM has serious consequences and once it occurs further bleeding becomes much more likely than before. There is about a 10% risk of death with each haemorrhage as well as a risk of persisting disability. There is usually little doubt that a person will benefit from treating a ruptured AVM when it is feasible to do.
As doctors ideally we would like a decision to deliver treatment to be supported by the results of randomised controlled trials (RCTs). In the most definitive trials a treatment is proven safe and effective after comparison with a sham treatment (a placebo) or with no treatment at all. For example the carotid endartrectomy operation has been demonstrated in several RCTs to reduce the lifetime risk of stroke in selected patients with narrowings of the internal carotid artery in the neck. It is very difficult however to carry out RCTs in a rare condition such as BAVM as there is a smaller pool of patients willing to randomly undergo treatment or live with their condition. Another challenge is to ensure one is "comparing apples with apples" given the great variation in size and blood flow between individual AVMs.
The ARUBA Trial
An attempt has been made to do this for brain AVM and the results published as A Randomized trial of treatment for Unruptured AVM ( ARUBA). The trial was halted when it became apparent that neurological worsening or death was three times more prevalent in treated patients compared to those only observed. The interpretation of this result was that the trial did not support treatment of an unruptured AVM. At the time of writing that observation remained unchanged at follow-up and the ARUBA study remains the only completed, attempt at a randomised trial in this area.
Many specialists in the treatment of AVM had significant reservations regarding this interpreatation of the result. The end-point of complete obliteration of an AVM was not reached for many patients which may have explained some of the observed complications. Most patients were treated with radiosurgery or endovascular techniques when one would have anticipated from the reported characteristics of the AVMs that surgical treatment would have been an appropriate means of completely treating them. Yet only 22 of 106 treated patients had surgical removal.
Other Evidence in Support of a Conservative Approach
ARUBA has not however been the only research to suggest observation is safer than proactive treatment for unruptured BAVM. The New York Islands AVM study identified the uncertainty as to whether treatment was necessarily superior to observation and was an inspiration to conduct the ARUBA trial. The Scottish Intracranial Vascular Malformation Study (SIVMS) suggested a similar conclusion to ARUBA although it was an observational population study not designed to compare treatments. Again only a minority of participants received surgical treatment so these studies could be regarded as primarily reflecting the outcome of radiosurgical and endovascular treatment.
Determining whether to treat an unruptured AVM is a complex scientific challenge due to the rarity and variability of the condition. These studies highlight that any treatment approach for an unruptured AVM inherently involves risk, regardless of the method employed. Currently, the question remains unresolved.
Other Illness Linked To Unruptured AVM
Epilepsy (Seizure Disorders)
The term Epilepsy refers to a family conditions sharing a tendency to experience seizures. Our brain is made up of a complex network of cells communicating through electrochemical signals. A seizure occurs when nerve cells begin to discharge in an uncontrolled way and the effect on the individual is determined by which particular nerve cells begin the discharge as well as how far it spreads through the electrochemical network.
Seizures associated with brain AVM begin in the brain adjacent to the malformation where a thin layer of nerve cells suffer damage as a result of relatively reduced blood flow and the presence of large pulsating vessels nearby. The development of seizures is what leads to the diagnosis of an AVM in 16-53% of cases.
As one of the causes of epilepsy, treatment of an AVM may sometimes improve seizure control. Medication is the first line treatment to prevent more seizures. It is relatively rare for someone with AVM-related epilepsy to be rendered completely seizure-free and requiring no medication after successful AVM obliteration so it is not usually the sole reason to consider treatment.
Intervention too may initiate seizures in those without epilepsy. There are cases of intractable AVM-related epilepsy, resistant to medication where definitive treatment if feasible can help. In such cases the input of the epilepsy-surgery multidisciplinary team is valuable. Microsurgery and stereotactic radiosurgery have both been shown to have potential to improve such seizure control providing the AVM is completely obliterated.
Seizures will occasionally occur at the time of a haemorrhage from an AVM and may complicate its recovery. It is very important to remember that intervention too may initiate seizures in those without epilepsy.
Headache
Headaches lead to the diagnosis of an in AVM in 7 to 48% of cases. The precise mechanism by which unruptured AVMs might cause headache are not always clear.
Blood clotting (thrombosis) within the AVM circulation may precipitate swelling around it resulting in headache. Such swelling may lead to bleeding but not always. Conditions that might predispose to such thrombosis include, fever, dehydration or haematological disease.
Of all people presenting with headache only 0.2% are ultimately diagnosed with an AVM. Incidental benign headaches are far more common than BAVM. Migraine headaches may be more common in those with AVM compared to those without and a particular association with AVMs of the occipital lobes has been suggested. One cannot say that migraine headaches will predictably improve with the removal of the AVM however.
Tinnitus
Of all people presenting with headache only 0.2% are ultimately diagnosed with an AVM. Incidental benign headaches are far more common than BAVM. Migraine headaches may be more common in those with AVM compared to those without and a particular association with AVMs of the occipital lobes has been suggested. One cannot say that migraine headaches will predictably improve with the removal of the AVM however.
Focal Neurological Deficits
Focal neurological deficit (FND) is a collective term for symptoms that mimic a stroke. e.g. a weakness or loss of feeling. Such symptoms may be reversible. FND associated with AVM is very infrequent and the mechanism difficult to demonstrate. It is frequently attributed to a cerebrovascular steal phenomenon where the demand for blood flow through the nidus reduces blood flow in adjacent functional brain tissue. However the scientific basis for such a phenomenon is weak and if steal systems exist at all they are likely to be extremely rare.
