* The S-ME/CFS group is not selected on severity — but does it have any useful meaning? *
S-ME/CFS is not a selection of the most severe patients (it could have been)
S-ME/CFS defines individuals who meet more than one case definition. In practice, this means excluding ME/CFS patients who only meet Fukuda criteria.
The S-ME/CFS group was not significantly more fatigued not had worse SF-36 physical health [composite score] than regular MEcfs, though its DSQ score was significantly lower.
The authors have the data to select on severity
A "severe" category can only be created by classifying each patient on the basis of severity. While there is no universally agreed definition of severity in ME/CFS, all the ones I know of are based on levels of functioning (e.g. the SF 36 physical function scale, housebound/bedbound versus not and the NICE severity scale).
Yet to meet any of the ME/CFS criteria, people needed to "experience substantial reductions in occupational, educational or personal activities". This was defined as scoring at or below at least two of the three following subscale SF 36 subscale cut-offs: Role physical 50 or less; Social Functioning 62.5 or less; Vitality 35 or less.
The authors could have have defined a severe group on the basis of the same SF 36 subscales but with a lower cut-off score.
It would be interesting to see results of a genuine severe category created on such a basis.
Does the S-ME/CFS group have any useful meaning?
Looking at the results for questionnaires and cytokines, there is a trend with S-ME/CFS worse than regular ME/CFS which is worse than the recovered group — even though most of the differences are not statistically significant. So the S-ME/CFS category does seem to be a bit different.
Potentially, this is simply because IOM and CCC cases are, on average, more severely affected than those with just Fukuda i.e. these differences could just be reflecting a group difference in severity.
A. Is it all about PEM?
The neatest explanation would be that the regular ME/CFS cases, which only meet only one case definition, are Fukuda cases that simply lack PEM.
There are two problems with this:
1. I would be surprised of this research group missed such an obvious difference. If it was the case, I would have expected them to simply select on a mandatory requirement for PEM to define a PEM group (Versus no PEM).
2. Also, the DSQ assessment of PEM is debatable: many patients believe it simply measures exertional intolerance. So we can't be sure that the DSQ would allow a clean separation on PEM.
If we assume that S-ME/CFS is defined by presence of PEM, does that mean that after Mono people can either develop "true" ME or a different illness that is similar in many ways? That doesn't feel right to me.
B Is it a more subtle difference?
Fukuda-but-not-IOM
The surprising thing (to me) is that so many people meet Fukuda but not IOM criteria. If it isn’t simply down to PEM, what is the other explanation?
IOM = fatigue +3 mandatory symptoms, Fukuda is fatigue + any 4 of 8 possible symptoms.
IOM and Fukuda non-fatigue symptoms
IOM: 1. Post-exertional malaise, 2. Sleep problems 3. Cognitive impairment and/or orthostatic intolerance.
Fukuda: Any four of eight symptoms. 1. Post-exertional malaise, 2. Sleep problems 3. Cognitive impairment (memory/concentration difficulties). 4 Sore throat. 5 Lymph-node pain 6. Muscle pain 7. Joint pain 8. New or different headaches.
There are two ways somebody could meet Fukuda but not IOM criteria:
— they simply don’t have all three IOM symptoms (sleep or cognitive issues, as well as PEM)
— They have all three IOM criteria but don’t meet the study’s symptom severity and frequency thresholds for them (the method section makes no mention of any thresholds for Fukuda). In other words, they are milder IOM cases.
Conclusion
As things stand, I can't see what we learn from the S-MEcfs category results.
It would be useful to have more data from the authors on the differences between the S-MEcfs group and other MEcfs cases and to see an analysis that really was based on severity. Plus seeing the results for all ME/CFS cases combined.