Arterial and venous vascular malformations can occur in any site in the body. As the brain is exquisitely sensitive to small disturbances, AVMs come to notice there more than anywhere else. It is possible, even likely, that AVMs can occur silently in other parts of the body of many people and give no trouble.

Why do they form?

This question cannot be fully answered for any particular case. In general terms, the blood vessels form in embryonic life as a mass of tangled channels, and only later become organised into arteries and veins. If the organisation process goes wrong, then some tangled masses of blood vessels could be left behind.
Possibly in some cases, abnormalities of endothelial cells, which usually line blood vessels, perhaps triggered by viral infection, can cause growth of these cells outwards, forming an abnormal blood-containing mass.

What trouble can they give?

This depends on type, size and location of the AVM. The types of AVM are classified according to how much of an arterial flow passes through the AVM. Venous AVMs consist mostly of large veins, with big “lakes” of veins; capillary AVMs are a tangle of very small, low flow vessels; arteriovenous AVMs contain high flow connections between the arterial and venous vessels, and may be pulsating. Very occasionally, an abnormal connection between arterial and venous vessels may form in adult life, as a result of penetrating trauma. This is known as an AV fistula.
Bleeding is the most feared complication, but is not so serious in most peripheral AVMs as in the brain. Even if the amount of blood loss is small, a brain AVM bleed can result in serious damage; this is not so often the case for a peripheral AVM.
AVMs affecting the face can present with cosmetic problems. Large AVMs anywhere in the body can cause problems with deformity.

AVMs can occasionally become painful. This can occur for a variety of reasons, most commonly that blood clots form within low flow parts of the AVM. This is not thought to be dangerous except in rare cases, but pain in an AVM should be assessed by a specialist.

Large high flow arteriovenous AVMs act as a short circuit within the circulatory system; this may strain the heart, as it has an extra load to pump. This complication is fortunately very rare.

What can be done?

In most cases no action is required. The main medical reasons for action are expansion or bleeding of the AVM. Some AVMs, particularly if they are outwardly visible, may also be treated to improve appearance.

The treatments available are similar to those available for brain AVMs. Embolisation, using a thin catheter fed through an artery in the groin and manipulated into the AVM under X-ray control is the treatment of choice for arteriovenous AVMs.

Venous and capillary AVMs cannot be accessed from the arterial side of the circulation. Direct injection of alcohol or other material to clot up the venous lakes is sometimes used in venous AVMs. Laser treatment is often effective in capillary AVMs which are on the skin surface.
Surgery is generally reserved for exceptional cases. If surgery is advised for your AVM, I would recommend requesting a second opinion, excepting immediate emergencies.

What advice should I look for?

Capillary AVMs are also known as port wine stains, or simply as birthmarks. They are flat, reddish purple irregular shapes on the skin. They do not often need treatment, unless there is a deeper AVM, or for cosmetic reasons. Assessment for treatment can be made by a consultant dermatologist or plastic surgeon.
Venous and arteriovenous AVMs can present externally as a soft, blueish or even pulsating mass, below the skin. If they are located in a deeper part of the body, they may never come to light at all. Again most of these AVMs will probably need no treatment. Enlarging, bleeding, painful or pulsatile AVMs should be assessed by a vascular specialist; a vascular surgeon, vascular radiologist, or preferably both.

All types of AVMs in children should initially be assessed by a paediatrician. Children requiring treatment can then be referred on to the appropriate specialist.

There are a small number of specialist units in the UK with particular expertise in assessment and treatment of AVMs, which your local vascular specialist can refer to if they think it appropriate.

Acknowledgements
AVM Support UK would like to thank Dr Chan for his invaluable assistance in providing much needed information..