Post-exertional malaise

One of the first definitions of ME/CFS, the Holmes Criteria published in 1988, does not use the term post-exertional malaise but describes prolonged fatigue after exercise as a symptom.[5] The term was later used in a 1991 review summarizing the symptoms of ME/CFS. Afterwards, the Canadian Consensus Criteria from 2003[6] and the International Consensus Criteria from 2011[7] used the term, as well as later definitions.

Severe symptoms triggered by a 2-day CPET in people with ME/CFS

Post-exertional malaise involves an exacerbation of symptoms, or the appearance of new symptoms, which are often severe enough to impact a person's functioning.[8] While fatigue is often prominent, it is "more than fatigue following a stressor".[4] Other symptoms that may occur during PEM include cognitive impairment, flu-like symptoms, pain, weakness, and trouble sleeping.[4][2] Though typically cast as a worsening of existing symptoms, patients may experience some symptoms exclusively during PEM.[4] Patients often describe PEM as a "crash", "relapse", or "setback".[4]

PEM is triggered by "minimal"[3] 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.[2] Sensory overload,[8] emotional distress, injury, sleep deprivation, infections, and spending too long standing or sitting up are other potential triggers.[4] The resulting symptoms are disproportionate to the triggering activity and are often debilitating, potentially rendering someone housebound or bedbound until they recover.[9][4][10][2]

The course of a crash is highly variable. Symptoms typically begin 12–48 hours after the triggering activity,[3] but may be immediate, or delayed up to 7 days.[4] PEM lasts "usually a day or longer",[9] but can span hours, days, weeks, or months.[4] The level of activity that triggers PEM, as well as the symptoms, vary from person to person, and within individuals over time.[4] Due to this variability, affected people may be unable to predict what will trigger it.[2] This variable, relapsing-remitting pattern can cause one's abilities to fluctuate from one day to the next.[1]

PEM is a hallmark symptom of ME/CFS and is common in long COVID.[11][12][13]

However, its presence can be difficult to assess because patients and doctors may be unfamiliar with it.[1][12] Hence, the WHO recommends that clinicians explicitly ask long COVID patients whether symptoms worsen with activity.[1]

The 2-day Cardiopulmonary Exercise Test (CPET) may aid in documenting PEM, showing apparent abnormalities in the body's response to exercise.[14] Still, more research on developing a diagnostic test is needed.

PEM is considered a cardinal symptom of ME/CFS by modern diagnostic criteria: the International Consensus Criteria,[2][9] the National Academy of Medicine criteria,[15][16] and NICE's definition of ME/CFS[10] all require it. The Canadian Consensus Criteria require "post exertional malaise and/or [post exertional] fatigue" instead.[17][18][19][15][20] On the other hand, the older Oxford Criteria lack any mention of PEM,[21] and the Fukuda Criteria consider it optional. Depending on the definition of ME/CFS used, PEM is present in 60 to 100% of ME/CFS patients.[4]

Studies have found that a majority of people with long COVID experience post-exertional malaise as well.[12]

There is no treatment or cure for PEM. Pacing, a management strategy in which someone plans their activities to stay within their limits, may help avoid triggering PEM.[22]

Physical therapy for people with long COVID must be modified to avoid triggering PEM in susceptible patients.[23]