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Arteriovenous Malformations of the Brain (AVM)

 

Arteries are thick-walled blood vessels designed to carry oxygen, bound to the red blood cells, to parts of the body where it will be used to fuel the activities of the organ's cells. Once the oxygen has been unloaded from the red cells the blood is carried back to the lungs in thinner blood vessels called veins so that it may collect more blood and begin the return jouney. An arteriovenous malformation (AVM) forms in the brain when an abnormal connection develops between its arteries and the veins carrying blood away. Instead the blood passes through a tangle of abnormal vessels called a nidus (from the latin for "nest"). The arteries often enlarge in response to increased amounts of blood being pushed through them and the veins, unused to the higher pressure, oxygen-laden blood, tend to become larger and more tortuous.

The high blood-flow through an AVM places stresses on the vessels which may cause bleeding. Aneurysms may develop on the arteries, veins or within the nidus which represent particularly weak area prone to haemorrhage. An AVM may develop in many parts of the body but in the nervous system are of particular concern because of the damage that bleeding within the brain or spinal cord may cause. Such AVMs are sometimes termed pial AVMs. Brain AVMs are thought to occur in about 0.15 of the population although it is difficult to be certain given that people may harbor one which does not cause any symtoms.

 

What causes an AVM?

CTA showing ruptured left middle cerebral artery aneurysm

It was believed that AVMs formed in-utero or in the period immediately after birth. In recent years cases have emerged where the AVM clearly developed later in childhood. This tells us that the development of an AVM can be a dynamic process. Most AVMs encountered in clinical practice are isolated lesions but there are uncommon conditions where an aberrent gene programs AVMs and other vascular malformations to develop throughout life. Examples of such conditions are:

Hereditary Haemorrhagic Telangectasia (HHT):

This is a collective term for a group of conditions characterised by the development of multiple AVMs in various locations including the brain, skin, eyes, mucus membranes and lungs. Telangectasia are small bright red vascular malformations in the skin. It is thought to affect between 0.1-0.2% of the population.

  • HHT-1 is caused by a mutation of the ENG gene. Patients develop AVMs in the lung and in the brain.
  • HHT-2 is caused by a mutation of the ACVRLI gene. AVMs of the brain and lung develop but patients are affected later in life than in HHT-1.
  • HHT-3 is assosciated with more frequent involvment of the liver than types 1 and 2. The exact gene abnormality is not known but thought to be on chromosome 5.
  • HHT/Juvenile Polyposis Syndrome is the result of a mutation of the SMAD4 gene and results in a propensity to develop polyps in the gastrointestinal tract as well as AVMs. Such polyps are not found in HHT types 1-3.

 

Capillary Malformation- Arteriovenous Malformation Syndrome(CM-AVM):

AVMs develop on the face, arms and legs but may also be found in muscle, bone, brain and spinal cord. CM-AVM is caused by mutations of the RASA 1 gene.

Craniofacial Arteriovenous Metameric Syndrome (CAMS)

is again a collective term for a group of related conditions, some of which have been recognised in isolation for a long time. The human body develops in discrete sections (called metameres). Vascular malformations of the head, neck and brain occur in this condition within the discrete metameres that make up the head and neck. A rare variety of spinal coloumn AVM (juvenile or metameric AVM) likely results from the same process ("SAMS"). This group of conditions may encompass the Sturge-Weber and Wyburn-Mason Syndromes.

  • CAMS1 sees AVMs develop in the region of the nose, the frontal part of the brain and the hypothalamus.
  • CAMS2 is characterised by AVMs of the mid-face, retina, visual pathways within the brain, the thalamus and the occipital lobes.
  • CAMS3 affects the hindmost parts of the brain (pons, cerebellum) and the jaw.

There is a great deal yet to be learned about why AVMs develop in tissues and through understanding these processes it is hoped new treatments may emerge.

 

What is the risk an AVM might bleed?

Arteriovenous malformations occur in different sizes and configurations. How the vessels are plumbed together within it is referred to as its angioarchitecture. While we recognise patterns in the occurence of AVMs each could also be thought as relatively unique in their architecture. Therefore one should be cautious in generalising population statistics to an individual's risk of haemorrhage. There is also the problem that many AVMs may be undiscovered because they do not cause symptoms. The medical literature most often estimates the risk of haemorrhage from an AVM at between 2 and 4% per annum. It is likely that some are at less risk and others are at considerably more. Unfortunately it is not currently possible to predict the risk with confidence. Not every AVM will haemorrhage however and a specialist will be able to discuss with you the architectural factors that may place an AVM into a higher risk group.

 

What symptoms might an AVM cause?

Apart from those caused directly by bleeding AVMs have been assosciated with a variety of other symptoms

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Brain Haemorrhage:

AVMs may bleed causing symptoms reminiscent of subarachnoid haemorrhage i.e. a sudden-onset, severe headache, nausea and vomtting. There may also be loss or impairment of the persons conciousness. If bleeding occurs into the substance of the brain there may be loss of the function which in the area served by that part of the brain e.g. paralysis of limb(s), impairment of the ability to speak or understand or loss of part of the visual field. Bleeding may also precipitate an epileptic seizure. Bleeding is the symptom which most often brings an AVM to attention. If bleeding has occurred studies indicate that the risk of further haemorrhage remians elevated to between 10 and 20% per annum for the subsequent 12 months or so. The risk of bleeding from AVMs is dificult to estimate given the variation in architecture between lesions and one should treat estimates cautiously. Features such as aneurysm on the feeding artery or nidus may mean it is more likely to present with haemorrhage. Narrowings in the veins drain blood from the AVM are also assosciated with increased bleeding risk.

 

Seizures:

Epileptic fits are the presenting symptom in about 15-20% of AVMs coming to a neurosurgeon's attention. It may occur in assosciation with an episode of bleeding. Alternatively the AVM's presence within the brain may serve as a focus from which seizures develop and spread to other parts of the brain. in the former the priority is medical control of seizures as with any other form of epilepsy. it is often possible to prevent seizures completely with medication. In a minority of cases where seizures are difficult to control treatment of the AVM may be considered. careful investigation is required to demonstrate that the AVM is the only source of the seizure activity and a team of epilepsy specialist may be consulted to ensure that is the case. Simply treating every AVM found without specialist investigation results in disappointing seizure control rates afterwards. Remember that both surgical removal of an AVM and stereotactic radiosurgery may "irritate" or injure the brain and trigger epileptic activity in their own right.

 

Headache

Headache is reported very commonly in people found to have an AVM. It may the reason the patient undergoes imaging, revealing an AVM. It is also the case that non-specific headache is a very common symptom in people who do not have AVMs. Therefore is often difficult to demonstrate a definite causal relationship between the headache and the malformation.

 

Bruits:

Occasionally large amounts of blood rushing from an AVM into the large veins contained the lining of the brain (the dura) will create a noise. This is frequently described as "whooshing" and may be audible to your doctor if they place a stethoscope on the head.

 

Steal:

Rarely patients report symptoms or headache that are not explained by bleeding, epileptic activity in the brain or by pressure on the brain from the enlarged blood vessels. It is sometimes postulated that an AVM can produce such symptoms by depriving adjacent brain tissue adjacent of enough brain oxygen and resulting in its malfunction. This is very uncommon and believed most likely to occur in larger malformations with a very high blood flow through them.

 

 

What tests are likely to be advised?

An AVM will often be first seen on either a computed tomography (CT) or magnetic resonance (MR) scans. These are imaging tools which allow the brain to be viewed in cross-section and they provide importance information about where the malformation is located within the brain when planning treatment. They are also very sensitivie ways of detecting bleeding from the AVM. To fully evaluate the angioarchitecture of the malformation a Digital Subtraction Angiography (DSA) is recommended. After a local anaesthetic injection a small tube is passed into a large artery at the top of the leg. Occasionally an artery at the elbow is used. Guided by x-rays the tube is directed into the blood vessls of the brain where dye is injected to produces very detailed pictures of the circulation. DSA is carried out by a specialist neuroradiologist who will discuss the potential risks of the procedure with the patient and/or their carers. It will identify characteristics which are assosciated with bleeding such as aneurysms and narrowing in the veins taking blood away from the nidus (stenoses).

Treatment is usually recommended if possible for brain AVMs which have haemorrhaged. There is presently a great deal of uncertainty as to whether the risks assosciated with treatment are justified in AVMs which have not yet caused symtoms. The decision needs to be carefully weighed on a case-by-case basis.