jeff97 gave a pretty detailed explanation. Google all you can on it. As jeff mentioned the fact that prednisone works is sometimes a diagnosis. As a previous pmr sufferer I was diagnosed by rheumatologist at Mayo. Prednisone at the right dose(20mg to start) is a miracle. Dont get fooled by the steroids though. Prednisone can become the enemy. PMR can last for years and most people cannot get off the steroids. google prednisone and adrenal insufficiency.