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PEM is a defining feature of ME. The term was first recorded in 1994 but the phenomenon itself and its importance have been recognised since at least the 1950s. Sometimes the term Post-Exertional Neuro-immune Exhaustion (PENE) is also used.

PEM is an abnormal physiological response to normal activity. It presents as a significant and prolonged worsening of baseline ME symptoms, and as a reduction in the ability to function. Patients feel sicker, not just more fatigued. The impact of PEM can be reduced by careful pacing of activity.


Why is PEM important?

Recent diagnostic criteria recommend no diagnosis of ME be made unless PEM is present.**

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The presence of PEM can help distinguish ME from other conditions presenting with fatigue such as idiopathic chronic fatigue, MS, and Major Depressive Disorder.

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Commonly prescribed exercise treatments for fatigue are contraindicated in patients with PEM.

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Patients severely affected by PEM may be unable to access health care and other services unless special accommodations are made for them.

** Note that some older criteria do not require PEM and select a different patient population. This information sheet concerns ME patients who experience PEM.

Recognising PEM

PEM is an abnormal response to minimal exertion of any type. There are as yet no diagnostic tools that can differentiate reliably between PEM and other forms of exertion intolerance, and there is symptom overlap with other conditions. However, in many illnesses such as depression symptoms improve with exercise rather than worsen. Other conditions such as cardiovascular disease, overtraining syndrome, or delayed onset muscle soreness can be distinguished from PEM by paying careful attention to all three of the following questions in conjunction.

  1. Symptoms

Ask about the type, range and severity of symptoms

PEM has a wider range of symptoms than other forms of exertion intolerance (EI). While some common EI symptoms such as breathlessness or fatigue may be present, PEM also includes symptoms not usually associated with EI, such as sore throats, sleep problems, orthostatic intolerance, cognitive dysfunction, and more.

An individual patient's PEM symptom complex usually combines a worsening of baseline symptoms with additional symptoms only perceptible during PEM.

Functional ability may be significantly reduced during PEM and may leave severe patients bedbound and requiring care.

Significantly reduced stamina following exertion is a hallmark of PEM. ME studies consistently find an objective, measurable inability to reproduce a physical effort on day 2 of a repeat CPET. This finding distinguishes ME from deconditioning and some other conditions in the research setting, and it can demonstrate the presence of PEM. However, it remains to be elucidated to what degree the CPET findings reflect cause or consequence of PEM.


2. Timing

Ask about the timing of symptoms

Symptoms starting and/or peaking during or directly following exertion, and which resolve within hours, are suggestive of EI.

Symptoms starting and/or peaking after significant delay - hours to days - and which take days to weeks (or even longer!) to resolve, are suggestive of PEM.

Tolerating an activity once but not on consecutive days is suggestive of PEM.

Note that the delayed onset of PEM means many patients may not have made the connection between symptom increase and preceding trigger - and some patients may be overdoing it to such an extent they are permanently in PEM.


3. Triggers

Ask about the types, range, intensity and frequency of triggers

PEM triggers include physical, cognitive and orthostatic exertion and, sometimes, strong emotions, excessive sensory input and environmental stressors.

Individual thresholds beyond which exertion provokes PEM vary greatly, can be very low and can be crossed by basic activities of daily living such as bathing, reading or talking. Severity of symptoms is often grossly out-of-proportion to the trigger.

Also, activities tolerated individually can cumulatively provoke PEM if too many are attempted at once and/or with insufficient rest between them.

Patients may not have made the connection between their symptoms and a preceding trigger as it is counter-intuitive that such minimal and varied triggers could have such disproportionate effects.

How to SUPPORT patients & minimise harm

A medical consultation exposes patients to several PEM triggers at once. To minimise harm from PEM:

  • Consider home visits (especially for more severely affected patients).

  • Offer a quiet, low-lit room with a bed for the patient to wait; ask the patient what other accommodations would be helpful.

  • Keep appointments short and focussed on essential physical examinations - prepare and/or follow up by email.

  • Because of PEM, standard exercise advice (e.g. for pain, sleep or mood) is contraindicated in ME. Instead, support the patient's efforts to pace their energy during the consultation. Refer to our Explanation of PEM for pwME and The Art and Science of Pacing resources.

  • Document the patient's PEM for other service agencies.

The delayed onset of PEM can make the illness "invisible";
patients may appear relatively well in the clinic but suffer PEM - unseen - afterwards.

Research into the long-term effects of PEM is lacking. There is, however, some evidence that Graded Exercise Therapy not only causes PEM and significant short-term distress, but that it may also lead to long-lasting, possibly even permanent, PEM symptom exacerbation and a reduction in function and quality of life.

3 Minute Video on ME and PEM

This 3 minute animation gives some insight into what PEM is and how it impacts the lives of those living with ME.

The video was produced by Norges Myalgisk Encefalopati Forening - Rogaland, (Norwegian ME Association, Rogaland County). 

Resources


Sources

Alternative Formats for Printing

You may wish to print this 4-page A4 pdf in A5 booklet form and/or in grey scale.