This study in 30 rheumatic diseases shows that severe fatigue is a widespread and highly prevalent problem across rheumatic diseases. Overall, one out of every two patients was severely fatigued. Severe fatigue was least common in patients with osteoarthritis (35 %) and most common in patients with (comorbid) fibromyalgia (around 80 %). Patients’ odds of being severely fatigued were higher when having fibromyalgia, multiple rheumatic diseases without fibromyalgia, a shorter disease duration, a younger age, and less years of education; also language was related to fatigue.
Thorough research into the prevalence of severe fatigue in rheumatic diseases would demand random sampling, certification of the rheumatic disease by a medical specialist, and identifying chronic fatigue using classification criteria next to self-report scores as has been done in chronic fatigue syndrome . This type of prevalence studies have not been done yet. Previous studies examining fatigue across rheumatic diseases all had their limitations. One study examined three rheumatic diseases and, like most other studies [5–7, 10], used a sample of patients recruited in one rheumatology clinic . Another used a sample of the general population in which the presence of a rheumatic disease was determined by self-reported diagnosis and did not present a cutoff for fatigue or prevalence per rheumatic disease . Overall measures to assess fatigue varied across rheumatic diseases and in definitions of fatigue severity [5–7, 9, 10] which impeded insight into the prevalence of severe fatigue in distinct rheumatic diseases. A strength of our study is that a uniform measure and cutoff were used to estimate the prevalence in distinct rheumatic diseases. In spite of the diversity of assessment and sampling methods in all studies thus far, our prevalence estimates are in agreement with previous studies examining the prevalence of severe fatigue [7, 13, 34] and lower than studies examining prevalence of less severe levels of fatigue in rheumatic diseases [6, 9, 35].
Although having (comorbid) fibromyalgia increased patients’ odds of being severely fatigued, this study also clearly showed that severe fatigue is by no means exclusive to patients with (comorbid) fibromyalgia; around 50 % of patients with other rheumatic diseases are also severely fatigued. Indeed, fatigue has been recognized in recent years as a core symptom and outcome measure not only in fibromyalgia but also in rheumatoid arthritis, psoriatic arthritis, and ankylosing spondylitis [2, 5, 9, 15]. Patient-focused group discussions indicated that fatigue is overwhelming and different from normal tiredness, that it permeates every sphere of life, and that the possibilities of self-management are variable and professional support is rare [2, 36]. The current study showed high prevalence of severe fatigue across 30 rheumatic diseases and different cultural backgrounds. This suggests that fatigue should be considered a core symptom and outcome measure in clinical trials and clinical practice for all rheumatic diseases, without exception. Moreover, the finding emphasizes that the development and evaluation of adequate management and treatment of fatigue in rheumatic diseases is of utmost importance.
The pathology of fatigue is largely unknown. The high prevalence of fatigue in rheumatic diseases suggests that the inflammatory process is a precipitating and possible maintaining factor of fatigue. This interpretation is somewhat supported by the observation of a lower prevalence of fatigue among patients with osteoarthritis, but it is contradicted by the high prevalence being observed in fibromyalgia. Moreover, associations of clinical and laboratory variables with fatigue are mostly low or absent [1, 2]. With current knowledge, “acute” fatigue as a result of a disease flare-up is probably best targeted by pharmacological interventions, while behavioral means such as life-style adjustment, cognitive behavioral therapy, (graded) physical exercise training, and sleep hygiene interventions should be considered in the treatment of chronic fatigue [1, 37, 38].
The study being international increased the probability that the findings are generalizable to different countries and cultures, but it also indicated some differences in the prevalence of severe fatigue between people with a different language. The presence of severe fatigue varied substantially across languages with French-speaking patients most often reporting severe fatigue and Dutch-speaking patients least often. Similar findings have been reported for functioning and wellbeing, as measured with the RAND(SF)-36, both between countries and between language regions, with Dutch speakers having more favorable scores than people with another language and French speakers scoring relatively poor [39, 40]. These differences do not seem to be due to measurement invariance [39, 41] but have been attributed to differences in access to and quality of national health systems , differences in social and economic opportunities within countries , and culture .
The current study has some limitations. It was based on self-reported diagnoses of rheumatic diseases without certification by a medical specialist, which may have led to the incorrect reporting of rheumatic diseases. Moreover, the recruitment through the Internet may have led to a lower representation of the older patient population and patients with a low social economic status, and it surely led to an overrepresentation of some patient groups (e.g., those with larger or more active patient associations). Furthermore, partly due to the overrepresentation of some patient groups such as fibromyalgia which are mostly female, men are underrepresented in this study. Finally, predominantly Western European countries participated in this study. Thus, our results might not be a fair reflection of the prevalence of severe fatigue in older, male patients and patients from other than Western European countries.
The use of a generic questionnaire to measure fatigue enabled us to measure the severity of fatigue in multiple diseases, and using a unidimensional measure may perhaps have reduced confounding by other states such as depressed mood that may differ among diseases. In rheumatoid arthritis, fatigue has been defined as multidimensional in nature, including physical, emotional, and cognitive aspects of fatigue and the daily impact of living with fatigue , a definition which can be useful in clinical assessment. As our goal was to give an overview of the prevalence of severe fatigue, not its impact, across a broad range of rheumatic diseases, we chose the Vitality scale of the RAND(SF)-36 which also had the advantage that the results are directly comparable to the level of fatigue in the general population.
A strength of the study is that a stringent uniform cutoff score was used, which has been shown sensitive to identify patients with chronic fatigue syndrome in the general population . However, to assure that this cutoff indeed measures a level of severe fatigue comparable to chronic fatigue syndrome, future studies should examine if it is sensitive to identify severe fatigue using classification criteria other than self-report only  in patients with rheumatic diseases.
This study is the first to provide an overview of the presence of severe fatigue across 30 rheumatic diseases using a large international dataset, a uniform way of recruitment, a uniform measure to assess fatigue, and a verified [31, 32] cutoff score for severe fatigue. It showed that more than 50 % of all patients with a rheumatic disease are severely fatigued. Severe fatigue can have detrimental effects for the patient, the near environment, and society at large. A better understanding of fatigue is crucial. In rheumatology, unraveling the underlying mechanisms of fatigue and developing optimal treatments should be top priorities in research and clinical practice.