[Sorry, this is a long post, looking at the history. I am in the process of writing an article related to this which I may post a draft of fairly soon.]
But I had thought that the term ‘multi system’ had another meaning too, that of many body systems and functions being affected.
It is another of those cases where people pick up words and use them in inconsistent ways to try to prove a point when all they are really doing is confusing people and making themselves look foolish to people familiar with the words.
Multi system-disease is used in medicine to mean a
process that has identifiable effects on the structure or workings of many systems. Which diseases that covers is pretty arbitrary. Lupus is called multi-system because you get kidney failure as well as arthritis. Diabetes has multi-system effects but because early on we knew there is one central problem of lack of insulin it isn't so often described as multi-system, but it can be. Multiple sclerosis tends not to be called multi-system, although it causes blindness and leg paralysis, because all features of MS are attributable to the same damage to central nervous cells (not even peripheral nerve cells).
So there is no agreement on exactly what deserves calling multi-system but the term is used when discussing what we know about underlying processes
we think are causing symptoms. In ME/CFS we have no idea what the processes are so talk in these terms is cheating.
How am I sure this is how it used? In the context of ME/CFS politics I think the best illustration is that if 'ME' were, maybe like 'CFS', psychosomatic, nobody would call it a multi-system disease, despite symptoms everywhere. If it were caused by viruses or autoimmune cells hidden in lots of different tissues it would.
My guess is that the use of 'complex multi-system disease' for ME/CFS (or in fact ME) started with the people who decided to re-write the Canadian criteria with the International Criteria. Fukuda had given criteria for 'CFS' that could be due to anything. The Canadian group wanted to create a consensus that included 'ME'. That made sense in the clinic since both terms pretty much described the same group of people - at least those whose lives were seriously impacted. I forget how much the Canadian criteria talk of causes but it is interesting that the group included people who were to re-convene for the ICC.
The ICC group decided to write new criteria just for 'ME'. I.e. the consensus hadn't worked. Moreover, members like Nancy Klimas were convinced that ME was a 'neuroimmune' disease. (It may be but we don't know.) The ICC criteria were written specifically to describe a neuroimmune disease - a multi-system disease (other systems were thrown in, making it more 'complex'). As a result the ICC do not really make sense and we have no idea whether they recognise anything worth recognising.
The reason why 'neuroimmune' was brought in, I presume, was because 'ME' had originally been the name of a proposed process that produced neurological signs as part of an acute illness at the Royal Free, in Iceland and maybe Incline Village. We still don't know whether there really were any neurological abnormalities in this acute illness and anyway, it is not ME/CFS as we now understand it. It was just one of many infections that can trigger ME/CFS.
So 'complex multi-system disease' entered talk about ME as a way of saying 'this cannot just be psychosomatic CFS because we know it is a specific process that affects nervous immune and other systems - we have evidence'. But we don't. The reason it isn't psychosomatic is that if you listen to people with ME/CFS over a period of years you can see that it doesn't add up. And psychological treatment doesn't work.
But the medical profession as a whole didn't buy this muddle about ME. What they were prepared to buy was the rather sensible Canadian idea that CFS=ME or ME=CFS, if you limit CFS to people with PEM, and they are the people who really need help. And ME here is Ramsay's 'chronic ME' not the acute 'neurological' ME. The psychiatrists preferred CFS/ME but didn't really want to recognise even the limited PEM based Canadian illness. ME/CFS won out, somehow, but 'complex multi-system disease' may have sounded impressive, so people wanted to justify it. They did that by saying that it meant that symptoms were produced in lots of parts of the body. But the truth is that they liked it because it implied that there was a multi system cause that was not psychogenic.
The end result is that although the 2021 NICE Guidelines are a major step forward they still carry all sorts of unhelpful baggage. Ironically, that is baggage of both sorts. There is still mention of multidisciplinary teams of rehabilitating therapists, of the sort beloved by the BPS crowd. There is also still the ghost of the confusion between acute neurological Royal Free disease and ME/CFS. ME/CFS isn't a neurological disease in any sense we understand, either. The WHO probably were as muddled as the ICC people.
I think it is important that people with ME/CFS do not get the impression that there is some dreadful process eating away at all their organs that may eventually kill them in the process. People get so ill they die, yes, but as far as we know if someone found the magic infusion these people could return to normal in an hour or so, just as people with myasthenia or Parkinson's, diabetic coma or Addison's can. In fact even more normal because as far as we know there is no permanent damage to anything in ME/CFS.
