An arteriovenous fistula is an abnormal channel between an artery and a vein.
Rarely, a large fistula may divert enough blood to cause symptoms of reduced blood flow in the affected arm or leg (steal syndrome).
Although doctors may be able to hear the distinctive sound of blood flow though a fistula by using a stethoscope, imaging tests are often needed.
Fistulas can be cut out or eliminated with laser therapy, or sometimes substances are injected into the fistula to block the blood flow.
(See also Overview of the Venous System.)
Arteries carry blood with oxygen and nutrients away from the heart to the rest of the body. Veins return blood to the heart from the rest of the body. Capillaries are tiny, extremely thin-walled vessels that act as a bridge between arteries and veins. The thin walls of the capillaries allow oxygen and nutrients to pass from the blood into tissues and allow waste products to pass from tissues into the blood.
Normally, blood flows from arteries into capillaries and then into veins. An arteriovenous fistula is an abnormal connection directly between an artery and a vein. With an arteriovenous fistula, some blood flows directly from an artery into a vein, bypassing the capillaries.
Arteriovenous fistulas may be
Congenital, meaning the person was born with one
Acquired, meaning that it developed (or was created) after birth
Congenital arteriovenous fistulas are uncommon.
Acquired arteriovenous fistulas can be caused by any injury that damages an artery and a vein that lie side by side. Typically, the injury is a piercing wound, as from a knife or bullet. The fistula may appear immediately or may develop after a few hours. The area can swell quickly if blood escapes into the surrounding tissues.
Intentional creation of arteriovenous fistulas for medical treatment
Some medical treatments, such as kidney dialysis, require that a vein be pierced for each treatment. With repeated piercing, the vein becomes inflamed and clotting can develop. Eventually, scar tissue may develop and destroy the vein. To avoid this problem, doctors may deliberately create an arteriovenous fistula, usually between an adjoining vein and artery in the arm. This surgical procedure widens the vein, making needle insertion easier and enabling the blood to flow faster. Faster flowing blood is less likely to clot. Unlike some large arteriovenous fistulas, these small, intentionally created fistulas do not lead to heart problems, and they can be closed when no longer needed.
Symptoms of Arteriovenous Fistula
When congenital arteriovenous fistulas are near the surface of the skin, they may appear swollen and warm.
If a large acquired arteriovenous fistula is not treated, a large volume of blood flows under high pressure from the artery into the vein network. Vein walls are not strong enough to withstand such high pressure, so the walls stretch and the veins enlarge and bulge (sometimes resembling varicose veins). In addition, blood flows more freely into the enlarged veins than it would if it continued its normal course through the arteries. As a result, blood pressure can fall, sometimes causing fatigue, light-headedness, or, rarely, fainting spells.
To compensate for this fall in blood pressure, the heart pumps more forcefully and more rapidly, thus greatly increasing its output of blood. Eventually, the increased effort may strain the heart, causing heart failure. The larger the fistula, the more quickly heart failure can develop, causing shortness of breath and swelling of the legs.
Rarely, a large fistula may divert enough blood from the affected arm or leg (steal syndrome) and cause numbness, pain, and cramping, bluish or grayish discoloration, and in severe cases, skin sores. When present, the skin discoloration may be difficult to appreciate on dark skin.
Diagnosis of Arteriovenous Fistula
Imaging tests, usually ultrasonography
With a stethoscope placed over a large acquired arteriovenous fistula, doctors can hear a distinctive “to-and-fro” sound, like that of moving machinery. This sound is called a machinery murmur.
Doppler ultrasonography is used to confirm the diagnosis and to determine the extent of the problem. For fistulas between deeper blood vessels (such as the aorta and vena cava), magnetic resonance angiography (MRA) or computed tomography (CT) angiography is more useful.
When a fistula is serious enough to require treatment, doctors may do angiography, in which they inject a liquid contrast agent into a blood vessel. The contrast agent (sometimes inaccurately called dye) helps the fistula show up more clearly on x-rays so that doctors can choose the best treatment option.
Treatment of Arteriovenous Fistula
For congenital fistula, endovascular treatment
For acquired fistulas, surgery
Congenital arteriovenous fistulas usually need treatment only if they cause symptoms. When necessary, doctors usually do an endovascular procedure to block the abnormal connection between the artery and vein. This procedure is done using a small flexible tube (catheter) inserted into a vein to inject coils or plugs into the abnormal connection. This procedure must be done by a skilled vascular surgeon because the fistulas are sometimes more extensive than they appear to be on the surface. Arteriovenous fistulas near the eye, brain, or other major structures can be especially difficult to treat.
Acquired arteriovenous fistulas usually have a single, large connection that can be corrected by a surgeon as soon as possible after diagnosis. The surgeon cuts the connection and then sews shut the holes in the artery and vein.
