An arteriovenous malformation (AVM) is a specific term used to describe a tangle of blood vessels with abnormal connections between arteries and veins.
High pressure arteries containing fast flowing blood are directly connected to low pressure veins, which normally only contain slow flowing blood. This means that blood from the arteries drains directly into the veins without stopping to supply the normal tissues in that part of the body with essential substances like oxygen and nutrition. Over time this can lead to the normal tissues becoming painful or fragile.
It also means that the AVM gets progressively larger over time as the amount of blood flowing through it increases, and it can cause problems due to its size. Finally, it may also mean that the heart has to work harder to keep up with the extra blood flow.
One of our specialist doctors describes an AVM as a ring road that bypasses the high street of a town. Traffic (or blood) will use the bypass rather than the high street which suffers as a result.
However, many AVMs are small and do not cause any significant problems during a person’s life. An AVM usually has many arteries supplying it and several draining veins. The centre of the AVM is often called the nidus.
AVMs may occur anywhere in the body, including near the surface of the skin. AVMs in the brain and spinal cord have different symptoms and treatment.
What causes an AVM?
AVMs are almost always congenital (present at birth), although they are often not very obvious at birth.
It is thought that they arise from an error in blood vessel formation very early in pregnancy. In most cases we do not know exactly what causes this error. In a small proportion of cases, AVMs arise as part of an inherited condition.
There is no evidence to suggest that AVMs are the result of something you did or did not do during pregnancy.
How common are AVMs?
The incidence of all types of AVM taken together is thought to be approximately 1.4 in every 100,000.
What are the signs and symptoms of an AVM?
An AVM involving the skin may cause blue, purple or red discolouration. They often cause swelling and sometimes the blood vessels are visible close to the skin surface. The area usually feels warm compared to the surrounding skin, due to the high rate of blood flow through it. A pulse or buzz may be felt. The skin over the AVM may be delicate and more likely to break down or develop an ulcer, which can be painful. Over time, the increased blood flow may affect heart function.
Unlike haemangiomas, AVMs do not shrink. They tend to increase slowly in size during childhood and adolescence, and may become more prominent during pregnancy.
How is an AVM diagnosed?
As an AVM can be confused with other birthmarks, the diagnosis is best confirmed by a specialist centre.
The doctors there will carry out a physical examination and take details of the child’s medical history and usually this is enough to make a diagnosis, although ultrasound scans and/or magnetic resonance imaging (MRI) scans may be carried out to confirm it. Other imaging techniques, such as catheter angiograms, may be suggested to better define the AVM and plan treatment.
What day to day care do AVMs need?
Generally, AVMs do not need any special care on a daily basis. If the blood vessels in an AVM are near the surface of the skin, they can bleed if they are knocked or scratched.
If the AVM starts to bleed, apply pressure over it with a clean handkerchief, cloth or tissue for at least five minutes. If blood soaks through the handkerchief, cloth or tissue, put another one on top and keep up the pressure. Do not take it off to have a look as this could start the bleeding again. If the bleeding continues, even after pressing down on the AVM for five minutes, go to your nearest NHS walk-in centre or Accident and Emergency department. The team at Great Ormond Street Hospital can usually be contacted for advice if needed.
Occasionally, the skin over an AVM can form an open sore or ulcer, which is painful. Ulcers can become infected, so a visit to your doctor is important, as infected ulcerated areas may need treatment with antibiotics.
How is an AVM treated?
Whether and how an AVM is treated depends on its location and what problems it is causing. Some AVMs are small and do not cause any significant problems during a person’s life, so can be safely left untreated.
If an AVM is troublesome because of its size, location or other symptoms, there are several treatment options available.
Some small AVMs can be removed surgically but the majority of AVMs are too complex to treat in this way and are treated with embolisation instead. This is a minimally invasive procedure usually performed by an interventional radiologist.
It aims to slow down or block off blood flow through the AVM by closing some or all of the abnormal blood vessels from the inside. Various substances can be used to block the blood vessels, including medical glue, medical putty, tiny metal coils or plastic beads. The substance used depends on the area being embolised, the speed of the blood flow in that area and the size of the blood vessels. Sometimes two or more substances are used together to get the best result.
Embolisation of high flow AVMs is a very specialised procedure and is only performed in specialist centres. Planning such a procedure is often complex and the options and risks will be discussed in detail before any decision is made. Embolisation is a good way of controlling the symptoms of an AVM but may not offer a complete cure.
If the blood flow through the AVM is successfully reduced, surgery may be suggested to improve the cosmetic appearances afterwards or to remove what is left. Any surgery has a risk of bleeding and infection and some scarring will result as well. In some cases, it may be preferable to embolise the AVM and not carry out any further surgery afterwards.
What happens next?
If some of the AVM remains after treatment, blood can start to flow again through the AVM. This will be monitored over time, by seeing your child in clinic and sometimes performing repeat scans. Repeat embolisation procedures may be required if the blood flow through the AVM becomes troublesome again.
Vascular malformation research is an area of medicine that is continually advancing; studies have already given us improved options for treating AVMs and continue to tell us more about how and why they develop.