On the Serious side (for once!):
I tuned in to a fascinating edition of BBC Radio 4's 'You and Yours' programme this morning (26 September 2017) on the above topic. Although PMR / GCA wasn't mentioned specifically, various listeners and experts gave their input around the causes and symptoms of, and treatments for CFS / M.E. which will ring bells with many of us, I'm sure.
One of the main discussion points was around whether M.E. / Chronic Fatigue Syndrome is 'all in the mind': i.e. psychosomatic / imagined / laziness: and how quite a few medics dismiss it as a mental / psychological illness for which controversial therapies like 'Graded Exercise' and CBT are the usually recommended treatment according to the UK's N.I.C.E. guidelines. In other words: 'Get more exercise and change how you think about your illness'. There was a lot of strong reaction to that one!
That said, one expert did explain that M.E. absolutely DOES affect brain function (i.e. chemically / hormonally), and the term Brain Fog (impaired Cognitive Function) was mentioned several times. His opinion was that feelings of Anxiety and Depression as a result BOTH of having this chronic illness AND its Life changing / limiting consequences are real, natural, and very common. So, although therapies like CBT might help in the coping process for some sufferers, at least - they are not usually a cure. 'Graded Exercise' also came in for some criticism because many M.E. patients reported that ANY exercise left them feeling worse than if resting, and one listener used the term 'My energy bucket is always empty'.
Another interesting message in the programme was that many people with M.E. previously had some kind of Viral / Bacterial Infection prior to the onset, Lyme Disease being just one. Familiar to some of us with PMR / GCA?
Finally, another expert mentioned the possibility that an OVER-active Immune System and excessive inflammation in the body and brain can wreak havoc with physical and mental energy levels as much as an under-active immune system.
There was another BBC radio series recently, 'Brain On Fire' which I seem to remember followed the same thinking / research: and if I remember correctly, it was suggested that many conditions like Severe Anxiety and Depression can be linked as much to Inflammation in the Brain causing an imbalance in Brain Chemistry, as to external factors like Life experiences / traumas, etc.
I know this is a complex topic and I'm not taking a particular 'position' on it. I've just posted it in case any of You Lot here can relate to the Fatigue / Anxiety / Depression 'trilogy' especially. I think you can re-listen to these BBC programmes as 'podcasts' - but don't ask me how!
Happy Days, and try to keep smiling on the Journey..