Post-exertional malaise

Post-exertional malaise involves fatigue, and is of such a severity that it impacts a person's functioning.[3] Other symptoms that may be experienced during PEM include cognitive impairment, flu-like symptoms, pain, weakness or trouble sleeping.[3][1] While typically cast as a worsening of existing symptoms, patients may experience some symptoms exclusively during PEM.[3] Patients often describe PEM as "crash," "relapse," or "setback."[3]

PEM is triggered by "minimal"[2] physical or mental activities that were previously tolerated, and that healthy people tolerate, like attending a social event, grocery shopping, or even taking a shower.[1] Emotional distress, injury, sleep deprivation, infections, and spending too long standing or sitting up are also cited as potential triggers.[3] The resulting symptoms are disproportionate to the triggering activity and distinct from typical fatigue.[3][4] PEM is often debilitating,[5] and may render someone housebound or bedbound until they recover.[1] The course of a crash is highly variable. The onset of symptoms typically occurs 12–48 hours after the triggering activity,[2] but may be immediate, or delayed up to 7 days.[3] PEM lasts "usually a day or longer,"[5] but can span hours, days, weeks, or months.[3] The level of activity that triggers PEM, as well as the symptoms, vary from person to person, and within individuals over time.[3] Due to this variability, people with CFS may be unable to predict what will trigger PEM.[1]

PEM is a common symptom of ME/CFS, occurring in 60 to 100% of patients, depending on how the condition is defined.[3] PEM is also a common symptom of long COVID, where long COVID symptoms may get worse after exertion, in a similar manner as ME/CFS.[6]

PEM is considered a cardinal symptom by a number of the different diagnostic criteria for ME/CFS: the International Consensus Criteria,[1][5][7] the National Academy of Medicine criteria,[8][9] and NICE's definition of CFS[4] all require it. The Canadian Consensus Criteria require "post exertional malaise and/or [post exertional] fatigue" instead.[10][11][12][8][13] On the other hand, the older Oxford Criteria lack any mention of PEM,[14] and the Fukuda Criteria consider it optional. A 2-day Cardiopulmonary Exercise Test (CPET) has emerged as a helpful tool in documenting PEM, showing apparent abnormalities in the way the body responds to exercise.[15] Still, there needs to be more attention and urgency placed on firmly establishing a diagnostic test.

There is no treatment or cure for ME/CFS or PEM. Pacing, a management strategy in which someone plans their activities to stay within their limits, may help people avoid triggering PEM.[16]