I wonder if perhaps the people who are questioning his ME diagnosis have never experienced ME of extreme severity where even walking to the toilet (or, in some cases, rolling over in bed or moving your limbs) depletes your muscle power -
immediately. In a very severe patient like the author, I don't think the body ever has enough energy generation capacity to be able to get through exertion without immediate problems. It may be that
delayed PEM simply indicates a higher level of health/functionality of those who are still able to participate in research studies and go to doctors' offices. It's interesting that the ICC says PENE is the cardinal symptom while Ramsay said
Muscle fatiguability, whereby, even after a minor degree of physical effort, three, four or five days, or longer, elapse before full muscle power is restored and constitutes the sheet anchor of diagnosis. Without it I would be unwilling to diagnose a patient as suffering from ME, but it is most important to stress the fact that cases of ME or mild or even moderate severity may have normal muscle power in a remission. In such cases, tests for muscle power should be repeated after exercise.
Notice how he doesn't say anything about delayed onset of symptoms. He obviously saw a lot of severe hospitalised cases during the Royal Free outbreak. I wonder if the ICC is more geared toward the mild patient who can do stuff like 2-day CPET or the moderate patient who can still potter around a bit without collapsing to the ground from minor muscle work or go to a doctor's office but then crash for days afterwards.
These days I'm moderate and the ICC describes me well (though Ramsay's definition describes me better) but there was a time when I was severe, bordering on very severe, and the PENE description was no longer relevant to me. To give you an example, a couple of years ago I was bedridden and only left the bed to go to the toilet and eat meals. Living in an apartment at the time meant that these trips only required a few steps each way. I was very depleted by each trip and it took me hours to "recover" from them. Then one day I went to answer the door and the cat ran out, ended up two floors up and refused to come down. Like a moron instead of getting the elevator I decided for some inexplicable reason (severe patient with POTS standing upright = brain hypoperfusion = reasoning skills and judgment in the toilet) to walk up the two flights of stairs to get him. What could possibly go wrong?
Anyway, I made it back to bed somehow but man, I thought I was going to die for sure. Angina, severe shortness of breath, pain in muscles, tachycardia worse than usual, extreme muscle weakness, sore throat etc. etc. This went on for several days where I couldn't catch my breath etc. So yeah, it was immediate and it lasted for days. The trips to the toilet and the dining table caused immediate symptoms but simply weren't depleting enough to cause symptoms lasting days.
It may be that the author's trip to the toilet isn't strenuous enough to trigger the true post-exertional amplification lasting days or weeks or months for him though he might have real PEM too but simply isn't in a position to trigger it because he collapses long before he's able to damage himself with a more significant overexertion. It could be the same underlying process though.
I wonder if the folks with delayed PEM who think immediate symptoms from exertion indicate "exercise intolerance" would experience immediate PEM too if they challenged their bodies with something more substantial like trying to run. Not that I would recommend doing this in a million years; I'm just making a comparison between this patient (who might be hitting the anaerobic wall from walking to the toilet since to him that is the equivalent or running a marathon) and a moderate patient who might need to run on a treadmill for a few minutes before they hit the wall and collapse with acidosis.
Re: exercise intolerance, I often compare my situation of prolonged recovery from exercise, autonomic symptoms, weird flu-like inflammatory symptoms etc. during PEM to someone like my father who has heart disease, mild COPD, is on like a dozen meds and has classic exercise intolerance with breathlessness and fatigue on exertion but after a good nap or a night's sleep he's as good as new. I don't think there anything more than a superficial similarity between ME/CFS reaction to exercise and and exercise intolerance in other diseases. I think if this guy had another disease that was causing such severe exertional intolerance that leaves him bedridden for years it would have declared itself by now on conventional medical tests.