The first definition of CFS by Holmes et al (1988) was only really a dilution of the definition of ME - but it was a significant one. To quote Byron Hyde :
The first move to start shifting the meaning of the term markedly away from ME came from Sharpe (et al) in 1991. Known as the "Oxford Criteria", the definition of CFS proposed by Sharpe and now in use in the UK states that :
"psychiatric disorders (including depressive illness anxiety disorders, and hyperventilation syndrome) are not necessarily reasons for exclusion."
Byron Hyde had this to say on the subject :
"The UK definition of CFS was developed by a panel of physicians who were primarily psychiatrists, with few if any clinicians who had ever looked at an epidemic of M.E."
And, to quote Ellen Goudsmit :
"the currently used criteria for CFS are selecting a much more heterogeneous population, including patients with burn-out, primary sleep disorders, depression and a host of other conditions characterised by fatigue."
Schluerdeberg's modifications in 1992 further broadened the meaning so as not to exclude patients suffering from depression, panic disorder, generalised anxiety disorder or even Somatoform disorder. So if a patient is diagnosed as having psycho-somatic symptoms, they can be classified as suffering from CFS.
Nowadays, you can conduct research on any group of such patients and publish the results as research into "CFS/ME". This makes a nonsense of the growing pile of CFS reseach; it is not research into ME.
As Action for ME put it :
Disability Living Allowance Briefing, AfME 1993/4.
and the award-winning essay by Ellen Goudsmit at David Axford's website