@darkarta, The way my Mayo Rheumatologist explained it to me it's like arthritis all over the body. I had swelling with some fluid in the joints when I was first diagnosed with PMR. The prednisone is supposed to help with the inflammation. I have no medical background but I'm thinking with inflammation can mean you can have more fluid buildup. Here's some information that explains it much better than I can...

"How is PMR diagnosed? - Diagnosis. There's no specific test to diagnose polymyalgia rheumatica. Your doctor will make a diagnosis after listening to you talk about the history of your symptoms and by carrying out a physical examination. You'll also have blood tests to check for any inflammation in your body, and to rule out other conditions." from Polymyalgia rheumatica (PMR) | Causes, symptoms, treatments: https://www.versusarthritis.org/about-arthritis/conditions/polymyalgia-rheumatica-pmr/