Giant cell arteritis (GCA) is a form of vasculitis, a group of disorders that cause inflammation of blood vessels. GCA most commonly affects the arteries of the head (especially the temporal arteries, located on each side of the head), but arteries in other areas of the body can also become inflamed. The inflammation causes the arteries to narrow, resulting in poor blood flow. Symptoms when arteries in the head are involved may include a throbbing headache on one side or the back of the head, tenderness of the scalp, flu-like symptoms, and/or problems with eyesight. Symptoms when other arteries are involved depend on the location of those arteries. The cause of GCA is still being studied, but it is thought to involve the immune system mistakenly attacking the artery walls. Several genetic and environmental factors may increase a person's risk to develop GCA. GCA may develop with or after another inflammatory disorder known as polymyalgia rheumatica.