A brief, comprehensive measure of post-exertional malaise, 2025, Jason and Chee

[poetinsf]

Defining post-exertional malaise "as an abnormal response to minimal amounts of physical and/or cognitive exertion, with symptom severity and duration out of proportion to the initial trigger" may not be enough. Fatigue and other symptom response to exertion can be *predictably* out of proportion compared to healthy people even when not in PEM. At least that is my experience when I was in deeper end of ME/CFS. If I walked 1 km, for example, I could predict that I would spend a few more hours lying down the next day. But that was not PEM even though fatigue was out of proportion to the exertion; it was an expected fatigue from the exercise given my condition at the time. PEM was when I spent 3 days in bed because I walked the same distance 5% faster.

This could be what led some people to think that other conditions like COPD also feature PEM. They compare the outcome to healthy people and conclude that it is PEM because fatigue is out of proportion (i.e., exercise intolerance). But it would not be PEM if you define PEM as the response that is out of whack with what is expected for a given condition and the amount exertion. This is why I prefer to separate post-exercise fatigue (expected for the condition) from post-exertional sickness (much worse than expected).

I guess it might help reduce misdiagnosis if a good way could be found to isolate fatiguability from PEM?

Right on the money. Except that DSQ-2 as it stands does not do that because it considers the outcome by itself. They should define PEM as a large differential between the expected outcome given the condition and the actual outcome, rather than the outcome by itself.

edit: added "exercise intolerance"

 

[poetinsf]

A confusing point over and above the fact that more rapid fatiguability is different to PEM, is that this more rapid fatiguability of itself is likely to be a symptom of ME?

I think it's fair to consider rapid fatiguability a symptom of ME/CFS. But I don't think we need to conflate it with PEM. PEM is a rather distinct and abrupt phenomenon of bottom falling out when you do just a tad bit more than you should whereas rapid fatiguability is more linear.

Further when in PEM more rapid fatiguability becomes even more rapid, more pronounced, which in turn means that the circumstances which contribute to triggering PEM will also trigger PEM more readily.

Insofar as the rapid fatiguability is a symptom of ME/CFS, it would be no surprise that it gets worse in PEM which is worsening of symptoms?

 

[Mij]

Yesterday I walked to the store at around 12:30, I don't normally go out walking any distance during that time of day and I could barely walk, my gait was jittery and I felt as though my legs would give out on me after 10 minutes. This was not PEM.

Yet the evenings I have a smooth gait and can power-walk for one hour and feel good afterwards as long as I stay within my limits, and don't have anything planned the next day.

I don't understand why time of day makes such a big difference in my case.

 

[poetinsf]

I don't understand why time of day makes such a big difference in my case.

Maybe it's similar to seasonality? There could be more of whatever makes your ME/CFS worse in the afternoon. Or, less of whatever makes your ME/CFS better.

 

[Mij]

Possibly. I also don't feel the need to lie flat/down in the evening like I do during the day. I can be upright all evening without feeling unwell.

 

[Verity]

Yesterday I walked to the store at around 12:30, I don't normally go out walking any distance during that time of day and I could barely walk, my gait was jittery and I felt as though my legs would give out on me after 10 minutes. This was not PEM.

Yet the evenings I have a smooth gait and can power-walk for one hour and feel good afterwards as long as I stay within my limits, and don't have anything planned the next day.

I don't understand why time of day makes such a big difference in my case.

I have a similar experience! I often pace around at night and have the sense that I’d get in trouble if I did it during the day.

 

[ME/CFS Science Blog]

Thanks for the useful analysis, especially the comments by Trish.

I'm also a bit confused by the scoring system. What does it do or indicate? Assuming a higher score means a stronger indication of PEM, then there are a couple of issues.

Having more triggers would be a stronger indication of PEM. If people say that mold, cold and chemicals trigger PEM, is that a stronger indication that they have PEM than those who don't? I would say the opposite, it's almost an indication that people might be misinterpreting PEM as simply 'symptom worsening'.​

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A longer symptom duration also gets a higher score, and 12% said that their PEM lasted more than 2 years. In my view, that isn't an indication of having PEM. Such a long duration makes it less likely to be PEM and more ambiguous if it isn't simply a deterioration of the illness regardless of exertion levels. With PEM there is usually a clear relationship to exertion levels; you get worse when you do too much, and not otherwise. If patients get worse for 2 years continuously, then it could still be PEM but is much less clear. So responses like these should probably also results in a lower instead of a higher score.​

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You also get a higher score if pacing is less effective, which is also curious. If the worsening is related to exertion (doing too much), then doing less should be expected to be helpful. So if pacing doesn't help at all, there is again more ambiguity if the symptom worsening is indeed PEM and related to too high exertion levels. If one were to give PEM scores based on the effectiveness of pacing (wouldn't recommend), then it would make more sense to give a higher score if pacing is useful, rather than if it is ineffective.​

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Given all these issues, I'm kind of hoping that I misunderstood what the score is all about.