Small arteriovenous fistulas in your legs, arms, lungs, kidneys or brain often won't have any signs or symptoms and usually don't need treatment other than monitoring by your doctor. Large arteriovenous fistulas may cause signs and symptoms.

Arteriovenous fistula signs and symptoms may include:

• Purplish, bulging veins that you can see through your skin, similar to varicose veins
• Swelling in the arms or legs
• Decreased blood pressure
• Fatigue
• Heart failure

An arteriovenous fistula in your lungs (pulmonary arteriovenous fistula) is a serious condition and can cause:

• A bluish tinge to the skin
• Clubbing of fingers
• Coughing up blood

An arteriovenous fistula in your gastrointestinal tract can cause bleeding in your digestive tract.

Causes of arteriovenous fistulas include:

• Cardiac catheterization. An arteriovenous fistula may develop as a complication of a procedure called cardiac catheterization. During cardiac catheterization, a long, thin tube called a catheter is inserted in an artery or vein in your groin, neck or arm and threaded through your blood vessels to your heart.

If the needle used in the catheterization crosses an artery and vein during your procedure, and the artery is widened (dilated), this can create an arteriovenous fistula. This rarely happens.

• Injuries that pierce the skin. It's also possible to develop an arteriovenous fistula after a piercing injury, such as a gunshot or stab wound. This may happen if your wound is on a part of your body where a vein and artery are side by side.
• Being born with an arteriovenous fistula. Some people are born with an arteriovenous fistula (congenital). Although the exact reason why isn't clear, in congenital arteriovenous fistulas the arteries and veins don't develop properly in the womb.
• Genetic conditions. Arteriovenous fistulas in the lungs (pulmonary arteriovenous fistulas) can be caused by a genetic disease (Osler-Weber-Rendu disease, also known as hereditary hemorrhagic telangiectasia) that causes blood vessels to develop abnormally throughout your body, but especially in the lungs.
• Surgical creation (AV fistula procedure). People who have late-stage kidney failure may have an arteriovenous fistula surgically created to make it easier to perform dialysis. If a dialysis needle is inserted into a vein too many times, the vein may scar and be destroyed.

Creating an arteriovenous fistula widens the vein by connecting it to a nearby artery, making it easier to insert a needle for dialysis and causing blood to flow faster. This AV fistula is usually created in the forearm.

Beyond genetic or congenital conditions, your risk of developing an arteriovenous fistula may increase due to certain factors, including:

• Cardiac catheterization, especially if the procedure involves blood vessels in your groin
• High blood pressure (hypertension)
• High body mass index (BMI)
• Certain medications, including some blood thinners (anticoagulants) and medications used to control bleeding (anti-fibrinolytics)
• Older age
• Being female

Left untreated, an arteriovenous fistula can cause complications, some of which can be serious. These include:

• Heart failure. This is the most serious complication of large arteriovenous fistulas. Since your blood flows more quickly through an arteriovenous fistula than it would if your blood flowed through a normal course of arteries, capillaries and veins, your heart pumps harder to compensate for the drop in blood pressure (called high-output heart failure). Over time, the increased intensity of your heart's pumping can weaken your heart muscle, leading to heart failure.
• Blood clots. An arteriovenous fistula in your legs can cause blood clots to form, potentially leading to deep vein thrombosis, a painful and potentially life-threatening condition if the clot travels to your lungs (pulmonary embolism). Depending on where your fistula is, it can lead to a stroke.
• Leg pain. An arteriovenous fistula in your leg can also cause you to develop pain in your legs (claudication), or can worsen pain you already have.
• Bleeding. Arteriovenous malformations may lead to bleeding, including into your gastrointestinal system.