Truth be told, no one really knows. The answer is probably more complicated that we could explain simply. But in general, polymyalgia rheumatica and giant cell arteritis are inflammatory diseases, and steroids are extremely potent anti-inflammatory agents. Nobody has ever done a study on giving steroids to some patients and placebo to others to demonstrate what the effect of steroids is. This would not be ethical. But, we know from several experiences, that a delay in starting steroids can cause a huge amount of debility in patients with PMR and blindness or stroke in patients with GCA.