Capturing the post-exertional exacerbation of fatigue following physical and cognitive challenge in patients with chronic fatigue syndrome
Introduction
Chronic fatigue syndrome (CFS) is a complex and debilitating medical condition. The international consensus diagnostic criteria for the syndrome stipulate that the fatigue is not relieved by rest; is present for at least 6 months; and co-occurs with at least four of eight additional symptoms, such as muscle or joint pain, unrefreshing sleep, and difficulties with memory or concentration [1]. CFS is also characterised by a prolonged exacerbation of symptoms induced by activity, commonly termed ‘post-exertional malaise’, or colloquially by patients as a ‘crash’ or ‘bust’. Recently devised sets of diagnostic criteria emphasise the importance of the post-exertional exacerbation of fatigue and symptoms in the diagnosis of CFS [2], [3]. The exacerbation of symptoms is commonly reported after both physical and cognitive activities [4]; however, the phenomenon is not well understood and has not been well characterised experimentally [5].
Many self-report instruments have been designed to record this symptom in patients with CFS (reviewed in [6], [7], [8]). The vast majority of the existing self-report instruments have been designed to capture the state in a retrospective, period prevalence format — e.g. “In the past week have you…?” However, to accurately characterise the ‘post-exertional malaise’ phenomenon in CFS (i.e. the change in severity and nature of this symptom following strenuous activity, and the time-course of recovery), the scale needs to be able to detect dynamic changes in fatigue over short time periods. The most common tool currently used to measure fatigue in real-time has been a simple, single variable, visual analogue scale (VAS), which allows a uni-dimensional characterisation of the severity of a symptom. Importantly, previous attempts to conceptualise the fatigue experienced by patients with CFS have highlighted the multi-dimensionality of the sensation [9], [10], [11], [12], [13], [14].
The overall aim of the studies reported here was to develop reproducible challenge protocols to investigate biological correlates of the post-exertional exacerbation of fatigue in patients with CFS. Specifically, the aims were as follows: firstly, to utilise mixed methods to design and test a brief questionnaire to characterise the fatigue state, including during exacerbation; and secondly, to develop physical and cognitive challenge protocols to allow study of the post-exertional exacerbation of fatigue; and thirdly, to assess the ecological validity on the questionnaire in real world ‘challenge’ settings.
Section snippets
Overview
Focus groups were utilised to characterise the phenomena of fatigue in patients with CFS, from which a self-report instrument — the Fatigue and Energy Scale (FES), was devised. Protocols were developed for an aerobic exercise challenge (termed here EXER), and a cognitively-demanding challenge (termed here COG) and assessed in separate case–control studies. Data from objective measures assessed in these challenge studies will be presented in supporting papers. Ecological validity of the FES was
Focus group study
Many participants indicated that the term fatigue did not adequately describe the sensation they experienced on a daily basis. Five descriptive themes emerged: feeling exhausted or tired; feeling heaviness in the limbs or whole-body; sensing fogginess in the head; feeling weakness in the muscles; and feeling drained of energy. These descriptors were nominated consistently across the groups, and no new themes emerged in the last group. Table 1 shows the word frequency analysis of the key
Discussion
CFS is characterised by a prolonged exacerbation of symptoms induced by activity, commonly termed ‘post-exertional malaise’. Accurate characterisation of this phenomenon is challenging, and it has been little studied for cognitively-demanding activities. This study aimed to characterise the post-exertional exacerbation of fatigue and to design and psychometrically analyse a new self-report instrument to measure fatigue in real-time. The instrument — the Fatigue and Energy Scale (FES) — was
Disclosures & acknowledgements
The authors declare that they have no competing interests.
AK was supported by funding from the Mason Foundation (RG114143). AL is supported by a fellowship from the National Health and Medical Research Council of Australia (No. 510246). The supporting agencies had no role in study design, data collection, analysis and interpretation, manuscript preparation and submission. The generous involvement of the participants is gratefully acknowledged.
Authors' contributions
AK, CS, UVC, AL and BB designed the study. AK and CS recruited participants and collected study data. EC contributed to the collection of data. AK, BB, UVC, CS, EC and AL analysed and interpreted the study data. AK wrote the manuscript and BB, UVC, CS and AL contributed to the manuscript. AL secured funding for the project. All authors have read and approved the final manuscript.
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