Congenital arteriovenous fistula is a condition where an artery (a blood vessel that transports blood from the heart to the organs) and a vein (a blood vessel that takes blood back from the body to the heart) are fused together. The point where the two blood vessels meet is called a fistula. Normally, arteries transfer blood to the veins through small, root-like networks of blood vessels called capillaries. However, in a fistula, blood is transferred directly from an artery to a vein. Vein walls are not as strong as artery walls, so the direct transfer, as opposed to the gradual capillary transfer, can cause small veins to enlarge and bulge. This can cause fistulas to look similar to varicose veins. For larger fistulas, the skin above it may appear swollen and bruised. In addition to physical signs, larger fistulas can create abnormal blood pressure readings. A large fistula makes the heart work overtime all the time and increases a person's risk for heart failure. Medical imaging technology, such as ultrasound or MRI, is used to confirm the presence of a suspected fistula. Treating a fistula depends on the size and location of it. A small, surface level fistula can often be cut out or blocked using laser therapy. Fistulas in harder to reach places, such as those near the eye, brain or other major organs, require more precise care and/or surgery. These cases can also be treated with injection therapy, which carefully inserts substances to block blood flow.