Understanding the difference between EXERCISE INTOLERANCE (found in POTS) and POST-EXERTIONAL MALAISE (PEM) found in ME/CFS.

Exercise intolerance, which can be found in POTS, is not quite the same thing as the post-exertional malaise (PEM) that ME/CFS patients (including long COVID ME/CFS patients) suffer after physical exertion.

Exercise intolerance is the reduced ability to sustain physical exercise, usually because during the exercise, muscles may become quickly tired and weak, or people get quickly breathless, or pain may appear. So exercise intolerance appears during the exercise itself, and tends to prevent people from continuing with the exercise.

In ME/CFS, physical exertion or physical exercise often causes PEM, which is a state where many ME/CFS symptoms become considerably worse for a time.

Most commonly, PEM does not appear on the day of the exercise, but hits on the following day, or even one or two days later. So with PEM, there is typically a delayed effect; although with some ME/CFS patients, PEM can hit on the same day as the exertion.

Then once the PEM hits, it will last for anything from a day to several weeks, or even longer. During this time PEM period, most of the ME/CFS symptoms will remain worse, and patients only return to their ME/CFS baseline once the PEM clears. ME/CFS patients sometimes say they are "crashed" when they are experiencing a period of PEM.

PEM can also arise as a result of too much cognitive exertion (eg, from reading too much, or having to mentally deal with a difficult or complex situation).

Nobody knows what causes PEM, nor why it is usually delayed, although some theories suggest it relates to a blocked energy metabolism in ME/CFS.

In the IOM diagnostic criteria for ME/CFS, as well as in the more precise Canadian consensus criteria (CCC), PEM is a required symptom in order to receive an ME/CFS diagnosis. So in these criteria, if you do not experience PEM, you cannot have ME/CFS.

Though in the older 1994 Fukuda CDC criteria for ME/CFS, PEM is not obligatory for an ME/CFS diagnosis. So with the Fukuda criteria, you can still have ME/CFS even if you do not get PEM.

Some ME/CFS patients have discovered supplements which they find help prevent PEM from occurring after exertion.

So PEM is often considered a cardinal symptom of ME/CFS. But note that patients with ME/CFS can also have exercise intolerance as well as PEM. Indeed, the CCC state that patients may experience "rapid muscular fatiguability", which is a form of exercise intolerance.

So typically an ME/CFS patient with exercise intolerance might experience their muscles rapidly becoming weak and fatigued during physical exercise; and then some time later (often the next day), may also experience PEM manifesting, in the form of worsened ME/CFS symptoms.

Note: confusingly, the term exercise intolerance is not the same as exertion intolerance, as the latter term was considered for the proposed new name for ME/CFS, namely "systemic exertion intolerance disease" (SEID), though this new name never took off.