Causes
Phlebitis occurs in people who develop blood clots in their veins, or when the veins are damaged from intravenous medications or an intravenous catheter. There are two kinds of veins that can be affected: superficial veins and deep veins.
Superficial veins are found in the fatty layer right under the skin and are visible as thin blue lines on the skin surface. The deep veins are not visible and are found in the muscles. The deep veins in the legs are squeezed by the surrounding muscles, which helps to move the blood from the legs upward to the heart. Since superficial veins aren't surrounded by muscles, there is no squeezing effect and the blood moves more slowly. You're more likely to get phlebitis in veins where the blood flows more slowly than normal, such as varicose veins.
Phlebitis is often caused by an injury to a vein. A blood clot, called a thrombus, can form and stick to the side of the vein. Since there are no muscles to squeeze the clot, it stays stuck inside the vein and blocks blood flow. In general, blood clots that form in the superficial veins do not move or break off. On the other hand, blood clots that form in the deep veins can break off and travel to the lungs, where they can cause difficult breathing.
Phlebitis can also be a complication of connective tissue disorders such as lupus, or of pancreatic, breast, or ovarian cancers. Phlebitis can also result from certain medications that irritate the veins.