ME/CFS Primer/Education Bulletins Fatigue in Myalgic Encephalomyelitis
IACFS/ME Bulletin


Fatigue in Myalgic Encephalomyelitis 

Ellen M Goudsmit1*, Bart Stouten2, Sandra Howes3

1 Health Psychologist, 23 Melbourne Road, Teddington, Middlesex, TW11 9QX, UK
2 Co-editor, ME and CFS References
3 Co-editor, ME and CFS References.

* Corresponding author.

Email: EMG:



Background: The objectives of this study were to measure fatigue in patients with well-defined Myalgic Encephalomyelitis (ME) and to assess if  there are any problems associated with the Chalder Fatigue Scale, which has been widely used to assess fatigue in patients with chronic fatigue syndrome (CFS).

Methods: Twenty-six patients were recruited from a local support group. All had been diagnosed by physicians and met research criteria for ME. They completed  the 11-item Chalder Fatigue Scale and were also asked to rate the severity of their illness. The fatigue scores were calculated using both the Likert method (0,1,2,3) and the bimodal method (0,0,1,1,). 

Results: The mean Likert score was 26.65 (SD 5.36) and the mean bimodal score was 9.81 (SD 2.04).  Fifty per cent of the patients recorded the maximum score using the bimodal method and 77% recorded the two highest scores. Moreover, there was a marked overlap between those who rated themselves as moderately or severely ill. These findings are indications of a low ceiling.

Conclusions: The findings from this study using the Chalder Fatigue Scale show that the low ceiling associated with the bimodal method means that this scoring system is not suitable for use in clinical trials.  Researchers may wish to consider alternative instruments to obtain a more accurate measure of fatigue in patients with moderate to severe ME and similar conditions.  

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Fatigue in Myalgic Encephalomyelitis  

Myalgic encephalomyelitis (ME) has been described in the medical literature since the nineteen thirties (1). It is characterised by marked fatiguability following minimal exertion, plus a delayed recovery in muscle strength after exertion ends (2, 3). Additional symptoms include myalgia,  poor concentration and memory,  giddiness or vertigo, sleep disorders, visual disturbances, mood swings, thermoregulatory abnormalities, and intolerance to alcohol.  However, it is the emphasis on muscle fatigue and the prolonged recovery period, plus the marked diurnal variability of the symptoms,  which differentiate ME from other chronic fatigue syndromes (CFS) and disorders such as depression (4).  

The aetiology of ME is unclear but some cases are associated with enteroviral infection (5-7), and abnormalities have been found in both muscle (7-8) and brain (9).  Although it has been recognised since 1956 and included in the WHO International Classification of Diseases since 1969, there has been little research into ME since the introduction of the concept of  CFS in 1988  (10-12). This effectively diverted attention away from post-viral syndromes and towards a much larger and  more heterogeneous  population, dominated by patients reporting generalised fatigue and pain (11,13).  

As the problems associated with the heterogeneity were recognised, the International Study Group advising the Centers for Disease Control and Prevention (CDC) recommended  that researchers employ stratification strategies that might reveal reliable subtypes (4, 10-11). It was hoped that the study of such groups would provide greater diagnostic clarity and, accordingly, facilitate the identification of  the  pathophysiological mechanisms underlying the onset and perpetuation of symptoms.   It is largely due to this change in focus that  there has been a resurgence of  interest  in  post-infectious fatigue syndromes and other conditions resembling ME (14-17). However, while it may seem logical to research subtypes that have already been recognised and defined, the fact that most doctors no longer diagnose ME means that there are relatively few such patients available for study. The following project was possible because two of the authors live in an area where several physicians have a special interest in the illness, and we therefore had access to a small but well-defined sample. 

The Chalder Fatigue Scale is one of the most widely used measures in research into chronic fatigue and CFS but it has not yet been evaluated in people with ME (18-22). The objectives of this study were firstly to measure fatigue in patients with well-defined ME, and  secondly, to assess if  there are any problems associated with this instrument.


The participants were recruited from a local patient support group in South London and Surrey through a notice in their newsletter and through the practice of one of the authors (EG). None were paid for their participation. All had been diagnosed by physicians and met  the criteria  below. The inclusion criteria were devised by Goudsmit  and are based on the definition of ME formulated  by Ramsay (2).  Co-morbid conditions were not excluded if they were relatively minor and clearly separable from the ME. 

Inclusion criteria:
1. Abnormal levels of fatigue and/or muscle weakness, precipitated by relatively minor levels of activity. Symptoms typically worsen during the next 24-48 hours.
2. The presence of other symptoms involving the brain and central nervous system (e.g. impaired memory and concentration, disturbed sleep patterns, balance problems and tinnitus.)
3. Periods of impaired circulation (e.g. facial pallor, feeling cold when it’s hot outside, or the reverse).
4. Marked fluctuation of symptoms, from hour to hour and day to day.
5. These symptoms must have been present during the past six months.

Exclusion criteria:
1. The presence of any condition which can  fully explain or which is likely to contribute  markedly to the degree of fatigue experienced.
2. Any person not diagnosed by a physician.

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The Fatigue Scale  devised by Chalder et al (1993) is a short, self-report  measure of the severity of  tiredness (18).  It produces a total score, with subscores  reflecting  mental fatigue (4 items), and  physical fatigue (7 items).  The 11 items are rated on a four-option continuum, from ‘better than usual’, to ‘much worse than usual’. The responses are either assigned scores from 0 to 3 (Likert method) giving a maximum of 33, or  0, 0, 1, 1 (bimodal method), giving a maximum of 11. In this study, Cronbach’s alpha was .90 using the Likert method and .83 using the bimodal method.

Other questions were included to collect demographic data and information about the duration  of the illness and  the presence of co-morbid disorders.  The patients were also asked if they experienced the symptoms listed in the inclusion criteria and to rate the severity of their illness at the time of the study. ‘Mild’ was defined as being able to lead a reasonably normal life, ‘moderate’ was defined as not being able to work, and being able to do less than 50% of what the person could do before they became ill. ‘Severe’ was defined as being able to do less than 25% of what the patient could do before they became ill, and being dependent on other people for activities such as cooking and shopping.


The data from the Fatigue scale were analyzed using both the scoring methods described above. The relationship between fatigue and other selected variables were examined using Spearman’s correlation coefficient. Alpha was set at .05 and all tests were two-tailed.


All the 26 patients who agreed to participate returned the questionnaire. The mean age of the sample was 43.77 years (SD 10.79) and 69% were female. They had been ill for an average of 155.88 months (SD 115.76), range 18-420 months, three categorized their condition as mild, 12 regarded themselves as moderately ill and the remaining 11 rated their condition as severe. Nine reported the presence of  co-morbid disorders such as  back pain and rheumatoid arthritis, or were taking a form of  medication which could theoretically have added to their fatigue.  To assess the influence of these confounding variables, the sample was divided into two groups: one with ME only and the other with ME and additional health problems (‘ME-plus’).

As Table 1 shows, the ME group was slightly older than the ME plus group but had been ill for a shorter time. Forty-seven per cent were male, compared to the  ME plus group which was made up entirely of  women.  In the ME group,  two  (12%) people categorised their illness as mild, eight (47%) as moderate and seven (41%) as severe. In the ME plus group, one (11.1%) described their illness as mild,  four (44.4%)  rated themselves as moderately  ill and a further four rated themselves as severely ill. 

The mean Likert fatigue score for the combined sample was 26.65 (SD 5.36) and the mean bimodal score was 9.81 (SD 2.04).  There were only minimal differences between the ME and the ME-plus groups in the fatigue scores, irrespective of how they were calculated.

Focusing on the individual items revealed that 86.8% of the questions making up the physical fatigue subscale received near maximal or maximum scores. The items which received the greatest number of low scores were question 3 (‘do you feel sleepy or drowsy’) and question 4 (‘do you have problems starting things’). On the mental fatigue subscale, 93% of the items received near maximal scores.

Given there were no notable differences between the two ME groups, the relationship between fatigue and other variables were calculated for the sample as a whole. The Spearman’s correlation coefficients showed no significant association between fatigue and either age or duration of illness.

Table 2 shows the mean fatigue scores and Figure 1 shows the individual fatigue scores for each of the three categories of illness severity. Four (15.4%) of the participants, all of whom rated their illness as severe, recorded the highest possible score using the Likert method and 13 (50%) recorded the maximum score using the bimodal method. Moreover, there was a marked degree of overlap between the scores from the moderately and severely ill, consistent with the view that this measure has a low ceiling.

The low ceiling is also evident in the distribution of the individual fatigue scores. As most of the scores of the moderately and severely affected patients lie close to  the maximum value, the distribution is highly skewed. This effect is more pronounced for the bimodal method than for the Likert method. Of  particular interest is the distribution of scores from the moderately affected participants.  All except one of the scores computed using the bimodal method lie in a cluster around the maximum, suggesting that this group is experiencing extreme fatigue. The Likert scores for this group, however, are not so close  to the maximum, suggesting a less disabling level of fatigue. As the questions are identical, the different interpretations appear to be artifacts  directly related to the scoring methods.

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This exploratory study found that patients with ME recorded comparatively high scores on the Chalder Fatigue Scale. To compare the fatigue scores with other samples, we searched for all studies on CFS that had used the 11-item Chalder Fatigue Scale.  The mean values reported in the identified articles  ranged from 17 (19) and 28.24  (20) using the Likert method (n=25) and from 6.89 (21) to 10.6  (22) using the Bimodal method (n=12).  Thus the fatigue levels found in our ME sample are similar to the higher values documented in the patients with CFS.

Although our sample was small and the findings should be interpreted with caution, the results are consistent with the anecdotal reports of profound, disabling fatigue as well as the raised scores on measures of  physical and functional impairment in the literature (3, 23).

Like previous reports on patients with CFS, our scores also revealed a low ceiling effect (24, 25).    Using the bimodal method, 50% of the participants recorded the highest score and 77% recording the two highest scores. This may present a problem in clinical trials, since those with a maximum score at baseline will not be able to record an increase in fatigue during or following treatment. There was also evidence of a ceiling effect when studying the Likert scores from the severely ill participants. Aside from the difficulties in obtaining an accurate measure of exacerbations, the clustering of scores also suggest that this measure may not be sensitive enough to reflect minor improvements.

A more detailed analysis revealed that 86.8% of the items on the physical fatigue subscale received near maximal scores using both Likert and bimodal methods. The questions which received the highest scores were item 5 (‘do you lack energy’) and item 7 (‘do you feel weak’).  The latter is particularly noteworthy as muscle fatiguability is a cardinal symptom of ME. The two questions which attracted the lowest scores and which were responsible for most of the variance  were ‘do you feel sleepy or drowsy’ and ‘do you have problems starting things’. These findings are similar to those of Morriss et al (1998) whose subjects recorded maximal scores on six of the eight physical fatigue items, although in their study, most of the variance was accounted for by the items relating to mental fatigue (24).  

Comparing the means of the patients with ME with those documented in people with other fatigue syndromes supports the view that this subgroup cannot be differentiated from patients with CFS using this particular instrument  (20, 26-28). However, while the mean  for ME was similar to  the scores from  patients who fulfilled the 1994 criteria for CFS, it was higher than those from people with chronic fatigue and infectious mononucleosis (29, 30).  

A study by Jason et al (2003) also failed to differentiate individuals with ME and CFS using the Chalder Fatigue Scale (15). However, they found that a greater percentage of  the patients with ME reported post-exertional malaise, problems with concentration and memory, lymph node pain, early morning stiffness  and dizziness than the group with CFS.  

The nature of the neurological and neuropsychiatric symptoms associated with ME highlights the need to include these as variables in clinical trials and to address them  in rehabilitation programmes. The current  tendency to focus on fatigue and emotional distress may not give an accurate view of the effect of treatments in this subgroup, and interventions which do not recognise the diversity of  symptoms are less likely to meet these patients’ needs (22, 31-32).    

The main limitation of this study was the small sample size. However, ME is a difficult disease to study. It is much less common than CFS as currently defined  - 1 per 1000 versus 10-26 per 1000 in the UK, respectively (13, 33), and there are therefore fewer patients available for research. In addition, there are currently no international consensus criteria for ME, and there has been little discussion in countries outside the US about the potential value of studying subgroups. This has limited  both interest and access to funding.  Another issue is that many of the participants were recruited from support groups, and although they closely resembled samples described elsewhere, the results may not be  applicable to patients in the community (3, 5, 9, 33).

Despite the diagnostic and practical challenges associated with the study of ME,  it is important that we do not use these as reasons to stop the research. Indeed, based on our findings, we would encourage further studies into  fatigue syndromes using  the stratification strategies recommended by the CDC (4).  Knowledge of the similarities and differences is essential not only to increase our understanding of the different diagnostic subgroups, but also a pre-requisite to improve patient care. 

In summary, the patients with ME were found to have fatigue scores comparable to those documented in people with CFS.  The ceiling effect associated with the Chalder Fatigue Scale means that the bimodal method is not appropriate for use in clinical trials, and that more accurate data may be obtained using a different measure.  

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Authors’ contributions

EMG designed the study, collected the data and drafted the manuscript. BS performed the statistical analysis and helped to draft the manuscript. SH participated in the design and helped to draft the manuscript. All authors read and approved the final manuscript.



The authors wish to thank Professor Trudie Chalder and all the individuals who participated in this study.



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