Giant cell arteritis is an inflammation of the lining of your arteries. Most often, it affects the arteries in your head, especially those in your temples. For this reason, giant cell arteritis is sometimes called temporal arteritis. The cause of giant cell arteritis is still unknown but is thought to be from the immune system causing damage to the body's own blood vessels. Polymyalgia rheumatica is an inflammatory disorder that is closely related to giant cell arteritis and occurs in 40% to 60% of patients with giant call arteritis.