The quote titling this post - "illness course subtypes can account for significant differences in the experiences of patients with CFS [Chronic Fatigue Syndrome] and ME [Myalgic Encephalomlyelitis]" - comes from the findings reported by Jamie Stoothoff and colleagues  (open-access available here).
Including one Leonard Jason on the authorship list, someone who has quite a pronounced peer-reviewed research history when it comes to ME/CFS (see here and see here for examples), researchers looked at over 500 people from various research cohorts who were "at least 18 years of age with a current diagnosis of CFS or ME." Participants from one of the cohorts, we are told, included those "who self-identified as having a CFS or ME diagnosis" so one perhaps has to be little careful when interpreting results; especially given prior 'mix-ups' between notions of 'chronic fatigue' and a diagnosis of CFS (see here)...
Participants completed a few assessment instruments including the Medical Outcomes Study 36-item short-form health survey (SF-36) and the DePaul Symptom Questionnaire (DSQ), providing a snapshot in relation to "current physical and mental functioning" and "CFS-related symptomatology." Researchers also calculated "energy quotients" which, although sounding like something a bit 'new age' actually involved asking participants to "rate their available and expended energy yesterday and last week" based on items included in the DSQ. The main aim was to explore the 'feasibility' of "classifying differing experiences of CFS and ME symptomology."
Results: a few 'generalisations' were derived from the collected results, which translated into 'illness course sub-types' such as: "Fluctuating (59.7%), with 15.9% Constantly Getting Worse, 14.1% Persisting, 8.5% Relapsing and Remitting, and 1.9% Constantly Getting Better." Authors also reported that "no significant relationship was found between illness course groups and gender, race, ethnicity, marital status, education, or work status."
Outside of other pretty obvious results - "The Constantly Getting Worse group generally reported higher levels of symptom severity and frequency when compared to the other groups, while the Constantly Improving group reported lower levels of symptom severity and frequency compared to the other groups" - authors also reported findings in relation to those energy quotients previously mentioned. So: "the Constantly Getting Worse group reported overextending their personal energy envelopes to significantly higher degree compared to the average patient" with the need for a lot more research in this area.
I know some people might be shrugging their shoulders about these results with 'so what' sentiments but I'm not one of them. The reason: quite a lot of things in relation to ME/CFS have been taken 'as read' down the years. Be that something like discussions on what recovery looks like in relation to ME/CFS (see here) through to questions about whether ME/CFS in the UK for example, is the same as ME/CFS in other parts of the world (see here), [previous] research seems to have 'skipped' quite a few important steps along the way; only now being corrected. The ideas derived from this latest research on how ME/CFS is not necessarily a 'static condition' in terms of symptom profiles (albeit based on a snapshot) adds to that basic research on the label(s).
With the requirement for quite a bit more replicative work needed, I also wondered whether this research from Stoothoff et al might overlap with other, more biologically-orientated results too. So, those talking about typical/atypical ME/CFS presentation impacting on immune system markers (see here) or even the effects of duration/staging of the condition(s) when it comes to biochemistry (see here) might represent important inclusions in further drafts of work on illness subtypes?
Oh, and whilst on the topic of ME/CFS, the recent findings by Nilsson and colleagues  inform us that use of a monoaminergic stabiliser - (-)-OSU6162 - previously thought to show "promise for treating a variety of neuropsychiatric disorders such as Huntington’s disease, schizophrenia, and mental fatigue following stroke or head trauma" probably isn't going to be great for ME/CFS. Yet more evidence for the idea that mental fatigue, accompanying physical fatigue and PEM as a characteristic feature of ME/CFS, is probably not just some sort of stand-alone 'psychiatric' manifestation...
 Stoothoff J. et al. Subtyping Patients with Myalgic Encephalomyelitis (ME) and Chronic Fatigue Syndrome (CFS) By Course of Illness. J Biosens Biomark Diagn. 2017;2(1).
 Nilsson MKL. et al. A randomised controlled trial of the monoaminergic stabiliser (-)-OSU6162 in treatment of myalgic encephalomyelitis/chronic fatigue syndrome. Acta Neuropsychiatr. 2017 Dec 7:1-10.