The most significant factor with patients diagnosed with these two conditions that I have found is; in general, if you have a majority of CFS symptoms, you are more likely to have a higher load of heavy metals such as Mercury, Lead, Chromium, Copper etc.
On the FMS side in general, there is often little or very low metal toxins in the bodies’ organs but a significantly much higher amount of environmental chemical toxins such as Foreign Estrogens...
Link: https://en.m.wikipedia.org/wiki/Xenoestrogen
...or Xenobiotics such as Toluene, Chlorine, Insecticides, etc.
These are primary nerve and hormonal disrupters and are a good explanation for neurological symptoms related to pain syndromes in patients with this condition.
https://www.wddty.com/magazine/1997/december/chemical-sensitivity.html
Mercury exposure: watch Dr Chris Shade’s story here:
Extensively researched medicines are now being prescribed resulting in a vastly improved outcome even in the most difficult of cases. Eminent fellow doctors who have vast experience in the field, such as, Dr Amy Yasko, Dr Rich Van Konynenburg, Dr Sarah Myhill and many others.