It's possible that I have a separate immune condition in addition to ME/CFS, although it would seem that the odds of having an extremely rare immune dysfunction (SigM) on top of ME/CFS are so small as to be practically non-existent. It's more likely that whatever is causing my immune problems in ME/CFS is also at the root of the SigM. But nobody really knows. We know far too little about immune dysfunction in ME/CFS.
I am always interested in knowing how researchers decide whether a medical condition is comorbid to an illness, or whether that condition is considered part of the main illness.
For example, in ME/CFS a significant percentage of patients have
generalized anxiety disorder (GAD). However, if you look at the CCC diagnostic criteria for ME/CFS, GAD is not considered a symptom of ME/CFS (although the CCC do say that anxiety may arise after stress). However, GAD is known to be a common comorbidity of ME/CFS. And GAD you can have just on its own, without having ME/CFS.
Compare this to how, in the CCC, the condition of
emotional lability, aka pseudo bulbar affect (= unstable, rapidly changing or exaggerated emotions like bursting into tears over the smallest thing)
is considered to be a symptom of ME/CFS, even though I think it is relatively rare in ME/CFS, judging by the lack of discussion about this symptom on this forum.
So why is emotional lability considered part of ME/CFS, whereas generalized anxiety disorder is not, even though the latter is much more common in ME/CFS patients?
The only answer I can see here is that generally you do not have emotional lability as a condition on its own; it tends only to appear within neurological disease or stroke. Whereas GAD you can have on its own.
So this could be the answer regarding whether you define a condition as a comorbidity to your main disease, or one of the symptoms of the main disease. If you can have the condition/symptom on its own without any other disease, then it perhaps researchers will consider that to be comorbidity of the main disease; whereas if a condition/symptom does not occur on its own, then this condition will be considered as a symptom of the main disease.
Another example is IBS: many ME/CFS patients have IBS, but IBS is not considered part of ME/CFS: the CCC considers that IBS is a common comorbidity of ME/CFS.
Does it make any difference whether a condition is comorbid to ME/CFS, or actually part of ME/CFS? Possibly. Perhaps if it is a comorbid condition, it will not necessarily improve or be cured if you improve or cure your ME/CFS. Whereas if a condition is a part of ME/CFS, presumably it will be ameliorated or disappear alongside all the other ME/CFS symptoms if you happen to improve or cure your ME/CFS.
So if a condition is comorbid to ME/CFS, it probably needs to be addressed and treated independently to your ME/CFS treatments.
In the case of IgM deficiency, I see that this can occur on its own (albeit rarely), so perhaps it might be best seen as a comorbid condition to ME/CFS.
IgM deficiency may of course share some common causal factors with your ME/CFS.
Regarding such common causal factors: in my case, the virus I caught triggered ME/CFS, generalized anxiety disorder, depression with lots of anhedonia, and some leg weakness which I think may be mild polymyositis. So this virus appeared to be a common causal factor of these four conditions; but these four are considered separate medical conditions. And indeed, I find I have to treat them separately, as I find medications that work for one will not necessarily help another.
