@Scarecrow I agree with you that PEM may be unrecognisable to the patient at some stages (though maybe with proper questioning we would notice), but I think the answer is: do like any other disease.
That is, make the criteria for the central hallmark features (in this case, including PER, cognitive dysfunction, sleep problems, and whatever else the biomedical researchers decide helps differentiate, when observed as a set).
When someone doesn't exactly fit, doesn't have a different diagnosis, and seems to fit the diagnosis in question, either give them an 'atypical' diagnosis or follow them in clinic and treat what you can and see what emerges.
That is, make the criteria for the central hallmark features (in this case, including PER, cognitive dysfunction, sleep problems, and whatever else the biomedical researchers decide helps differentiate, when observed as a set).
When someone doesn't exactly fit, doesn't have a different diagnosis, and seems to fit the diagnosis in question, either give them an 'atypical' diagnosis or follow them in clinic and treat what you can and see what emerges.