Normally blood is pumped from the heart into the large arteries at relatively high pressure and speed which slows down as the blood flow reaches smaller and smaller arteries. Eventually the blood reaches the smallest vessels called capillaries. Capillaries are smaller than the diameter of a human hair and as the blood flows through these tiny vessels, it gives up oxygen and nutrients to the brain tissue and receives carbon dioxide and other waste products. The blood then enters tiny veins at very low pressure, joining larger veins to return to the heart and lungs. Pressure and speed of blood flow in the veins are normally very low compared to that in the arteries. The walls of the veins are fairly thin and delicate compared to those of the arteries.

Arteriovenous malformations (commonly known as AVMs) arise because of abnormal communications, which directly connect arteries to veins, without a capillary bed in between. These abnormal communications are like short circuits, and are known as ‘fistulas’. Therefore, where capillaries are missing, blood is delivered at high pressure from arteries directly into the veins. Veins were not designed to withstand blood flowing at high pressure, so they expand causing the tangled mass of blood vessels of an AVM. The tangle would not normally exist if capillaries were present.

AVMs arise in the brain, spine, lungs, kidneys and the skin. Brain AVMs are the most common type, and may involve parts of the blood supply to the brain, head and neck. Four major arteries supply the brain; two carotid arteries in the front of the neck and two vertebral arteries in the back of the neck. Two external carotid arteries, also situated in the neck, supply other structures of the head. AVMs vary in size and location within the skull. Brain AVMs occur on the surface of the brain, or within it, but are separate from brain tissue. Occasionally AVMs lie within the outer, fibrous, protective coating of the brain called the dura, and these are known as ‘dural fistulas’.

In comparison to other conditions, AVMs of the brain are rare. Each year, 1 person in every 100,000 is newly diagnosed with a brain AVM. Although brain AVMs may come to medical attention at any age, the average at which they present is in one’s 40s.

The cause of brain AVMs is not known. Although they are thought to be present from birth, in some cases there is evidence that they have developed in adult life. Very occasionally they are inherited as part of a syndrome called Hereditary Haemorrhagic Telangiectasia, in which AVMs may occur in the lungs, gut, nose, and brain.

What symptoms can they cause?

Incidental

Sometimes brain AVMs cause no symptoms at all. One fifth of adults are diagnosed with a brain AVM during investigation of another problem altogether. These brain AVMs that have caused no symptoms are referred to as ‘incidental’ or ‘asymptomatic’.

Brain haemorrhage

The most common problem caused by brain AVMs is a sudden bleed from their blood vessels, known as brain haemorrhage, which is a form of stroke. When adults first come to medical attention with a brain AVM, half of them present with a brain haemorrhage. The symptoms caused by a brain haemorrhage depend on the location of the AVM within the brain and the severity of the bleeding. The haemorrhage can be disabling and sometimes even fatal. The first symptoms of a bleed may be:
• Headache (sudden & severe)
• Drowsiness, or loss of consciousness
• Other symptoms of a stroke, such as disturbance of vision, muscle weakness or paralysis, loss of sensation or numbness, tingling, confusion, problems in using or understanding language, dizziness, loss of coordination
• Seizures

Seizures / epilepsy

One fifth of brain AVMs are incidental when diagnosed, one half present with brain haemorrhage, and most of the rest present with one or more epileptic seizure(s). These are caused by the AVM pressing on the brain, which causes excessive electrical activity. In a seizure, people experience unusual feelings or their muscles may move uncontrollably in the parts of the body controlled by the area of the brain where the AVM lies, in which case the seizures are said to be ‘focal’. People may or may not become unconscious during focal seizures. A seizure is ‘generalised’ if it spreads to involve the whole of the body and consciousness is lost.

Other symptoms

Whether or not brain AVMs cause headache (and in particular migraine) is debated. Rarely, people experience symptoms of a stroke due to a lack of blood flow to an area of the brain (known as ‘ischaemia’), without having had a brain haemorrhage.

Future risks

The future risks of brain haemorrhage, epileptic seizure, and disability are not entirely predictable. Your investigations may show up other features suggesting a higher or lower risk of these complications. Because further research is vital to our understanding of the future risks, the following figures are only estimates.

The risk of bleeding from an AVM of the brain for the first time is approximately 1 in 50 (2%) every year, and the risk of another bleed following a previous one is probably somewhat higher. The risk of dying from a bleed can be as high as 1 in 5 (20%), and the risk of disability following a bleed may be as high as 1 in 3 (30%). For people with an AVM of the brain, each year there is a 1 in 100 risk of developing epileptic seizures for the first time, although they can often be controlled with anticonvulsants.