Post-exertional malaise

Post-exertional malaise (PEM), sometimes referred to as post-exertional symptom exacerbation (PESE)^[1] or post-exertional neuroimmune exhaustion (PENE),^[2] is a worsening of existing symptoms and/or appearance of new symptoms that occurs after minimal exertion.^[3] It is the hallmark symptom of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and common in other infection-associated chronic conditions (IACCs) such as long COVID and fibromyalgia.^[4][1] PEM is often severe enough to be disabling, and is triggered by ordinary activities that healthy people tolerate. Typically, it begins 12–48 hours after the activity that triggers it, and lasts for days, but this is highly variable and may persist much longer.^[5][6][7] Management of PEM is symptom-based, and patients are recommended to pace their activities to avoid triggering PEM.

Melvin Ramsay, the infectious diseases specialist who coined the term "myalgic encephalomyelitis", insisted that:

Muscle fatigability whereby, even after a minor degree of physical effort, three, four or five days or longer elapse before full muscle power is restored is unique and constitutes the sheet anchor of diagnosis. Without it I would be unwilling to diagnose a patient as suffering from ME.^[8]

Although Ramsay's focus on physical triggers of PEM is now understood to be too restrictive (see below), modern diagnostic criteria for ME/CFS all require PEM for diagnosis. This includes the Canadian Consensus Criteria from 2003^[9], the International Consensus Criteria from 2011^[3], and the Institute of Medicine criteria from 2015,^[7] as well as later definitions.

The term "post-exertional malaise" itself was coined in a 1991 review summarizing the symptoms of ME/CFS.^[10] However, the 2021 NICE committee stated they consider the term PEM outdated, as it may give the impression of just a "vague discomfort", and argued that the term post-exertional symptom exacerbation better captures the symptom. Nonetheless, they decided to continue using PEM as it is the more familiar term.^[11]: 49

PEM is triggered by "minimal"^[6] physical or mental activities that were previously tolerated, and that healthy people tolerate.^[5] Which activities trigger PEM depends on illness severity.^[12]

• For someone with so-called "mild" ME, PEM can be triggered by a full day of work.^[12]
• For someone with so-called "moderate" ME, it can be triggered by attending a social event, grocery shopping, or cooking a full meal.^[12]
• For someone with so-called "severe" ME, it can be triggered by taking a shower, or cooking a simple meal.^[12]
• For someone with so-called "very severe" ME, it can be triggered by sitting up, saying a few words, or turning over in bed.^[12]

In addition to these physical activity triggers, sensory overload,^[13] emotional distress, injury, sleep deprivation, and infections are other potential triggers.^[7] The resulting symptoms are disproportionate to the triggering activity and are often debilitating, potentially rendering someone who is able to leave the house at baseline housebound, or someone who is able to leave the bed at baseline bedbound.^{[3][7][14][5]}

Symptoms typically begin 12–48 hours after the triggering activity,^[6] but may be immediate, or delayed up to 7 days.^[7] PEM lasts "usually a day or longer",^[3] but can span hours, days, weeks, or months.^[7][15] In some cases, PEM can be permanent, and thus result in a permanent decrease of baseline capacity.^[15][16]

Symptoms that may occur during PEM include cognitive impairment, flu-like symptoms, pain, weakness, dizziness, fatigue, and trouble sleeping.^[7][5] In more severe cases, visual disturbances, stroke-like symptoms, and episodes of temporary paralysis have been recorded. Though typically cast as a worsening of existing symptoms, patients may experience some symptoms exclusively during PEM.^[7] Patients often describe PEM as a "crash", "relapse", or "setback".^[7]

The experience of PEM has been described as follows:

Some days can be absolutely horrendous […] when I wake up, it will feel like I'm being pushed into that like gravity is kind of wrapped it up and I'm being kind of sucked, pulled into the bed aches and pains. Because of my fine motor control, I struggle to do up buttons, zip up shirts […] hold a knife. Legs are heavy, it's almost like dragging feet across the floor, and every small […] effort and heart rate [spikes].^[17]

“A few months ago, I went to a therapy pool which was heaven getting in, but once I got home I just about made it up the stairs and it was like a switch went off in my whole body and I face planted on the landing […] couldn't move anything in and my body felt like it shut down […] And I just had to lie there for half an hour […] I couldn't cope with anything. I couldn't speak. I couldn't look at anything. I was just faced down eyes shut for half an hour, and it is like a total sensory overload”^[17]

When I do any activity that goes beyond what I can do—I literally collapse—my body is in major pain, it hurts to lay in bed, it hurts to think, I can’t hardly talk—I can't find the words, I feel my insides are at war.^[18]

The level of activity that triggers PEM, as well as the symptoms, vary within individuals over time.^[7] Due to this variability, affected people may be unable to predict what will trigger it.^[5] 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.^[19][20][21]

Its presence can be difficult to assess because patients and doctors may be unfamiliar with it.^[1][20] 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.^[22] The hand-grip test has also shown promise as a diagnostic tool.^[23] 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,^[5][3] the National Academy of Medicine criteria,^[24][5] and NICE's definition of ME/CFS^[14] all require it. The Canadian Consensus Criteria require "post exertional malaise and/or [post exertional] fatigue" instead.^{[25][26][27][24][18]} On the other hand, the older Oxford Criteria lack any mention of PEM,^[28] 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.^[7]

A majority of people with long COVID experience post-exertional malaise as well.^[20]

Numerous biological findings have been associated with PEM.^[29]

For example, extensive evidence from 2-day cardiopulmonary exercise testing (CPET) shows significant reductions in oxygen consumption and workload at the ventilatory anaerobic threshold during PEM.^{[30][31][32][33]} As a result of these findings, PEM has been described as a state of "bioenergetic failure".^[31]

Evidence also shows skeletal muscle tissue damage and intramuscular infiltration of immune cells in PEM.^[34]

There is no treatment or cure for PEM. Pacing, a management strategy in which someone plans their activities to stay within their limits, is essential to avoid triggering PEM.^[35]

Graded exercise therapy is counter indicated in the presence of PEM,^[36] and physical therapy for people with ME and long COVID must be modified to avoid triggering PEM in susceptible patients.^[1]