Elements of fibromyalgia in an open population
T. Schochat and
H. Raspe1
Institute for Occupational, Social and Environmental Medicine, Medical University Ulm and
1 Institute for Social Medicine, Medical University Lübeck, Germany
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Abstract
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Objective. To examine the nosological concept of fibromyalgia in the general population.
Methods. A postal survey of rheumatic pain and non-specific bodily complaints was sent to all 3174 German female residents of Bad Säckingen, Germany, aged 35 to 74 yr. A stratified random sample of 653 subjects was further examined in a clinical survey.
Results. On the population level the point prevalence of chronic widespread pain was 13.5%. In the clinical survey, tender point count was associated not only with the extent of rheumatic pain, but also independently with the extent of bodily complaints. Subjects with no history of rheumatic pain but with non-specific bodily complaints had as many positive tender points as subjects without bodily complaints but with a history of rheumatic pain. Subjects could be identified who met the tender point criterion of the ACR without a history of widespread pain. Multivariate analyses demonstrated that some symptoms carry a risk for positive tender points (low physical mobility, pain, bodily complaints) and some for chronic widespread pain (poor health status, catastrophizing, emotional reactions, low energy level, sleep disturbances) that are independent of each other and of age.
Conclusions. The results do not only question the relevance and specificity of a history of widespread pain in diagnosing fibromyalgia, but also the concept of fibromyalgia as a distinct rheumatological disorder. The results support the concept of fibromyalgia as part of a wider spectrum of dysfunctional syndromes.
KEY WORDS: Chronic widespread pain, Concept, Nosology, Epidemiology, Disease burden.
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Introduction
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International interest in fibromyalgia started with the seminal paper by Smythe and Moldofsky in 1977 [1]. For the first time, fibromyalgiaor fibrositis as it was then calledwas defined as a clinical syndrome in terms of a symptom (widespread pain) and a sign (multiple active tender points). In their dependence on physical signs as diagnostic tools these criteria have determined the current concept of fibromyalgia. Following this publication, a number of criteria sets, all generally similar in that they were based on some combination of tender point examination and symptoms, were proposed [28]. The American College of Rheumatology (ACR) 1990 Criteria for the Classification of Fibromyalgia [7] are now the most widely used. They characterize fibromyalgia as a musculoskeletal disorder with chronic widespread pain and exaggerated tenderness as exclusive features. The criteria have not been further validated [9]. Nevertheless they are the de facto standard for diagnostic classification. They are suggested as adequate in the clinical setting [10]. The concept of the ACR 1990 criteria of fibromyalgia as a musculoskeletal disorder is supported by the Copenhagen Declaration [11, 12]. This consensus document refers to widespread pain and exaggerated tenderness as prominent features of fibromyalgia. Other criteria sets refer to a different concept of fibromyalgia as a functional disorder [2, 3, 8].
The primary objective of the study was to examine the nosological concept of fibromyalgia as a musculoskeletal disorder in the general population. The specific aim of the study was to investigate whether the occurrence of tender points in the general population is related to the same indices of poor health and other somatic complaints as widespread pain and independent of it.
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Methods
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Study design
A population-based two-stage cross-sectional screening survey was carried out between April 1993 and October 1994. The addresses of the subjects were drawn from the state registration office. Inclusion criteria were age (35 to 74 yr), gender (female), German citizenship and first residence in any of the three districts (Kernstadt, Obersäckingen, Wallbach) of Bad Säckingen, Germany. No further restriction was made. A total of 3174 persons met the inclusion criteria. The study had two phases: a postal screening and a clinical survey.
Postal screening
All subjects were mailed a questionnaire comprising several scales and a few selected additional questions. The first question of the screening questionnaire asked about general health status (How would you describe your general health status? Would you say it is very good/good/fair/poor/very poor). This question was followed by a pain mannequina drawing of a person in front and back viewwhere subjects could mark freely where they have pain today. The second page started with a region-of-interest drawing. In the back view, back (between C7 and the gluteal folds) and neck were marked and in the front view, all the joint regions the following question (pain today) referred to (16 regions of interest). Further questions asked about pain duration (For how many days do you have pain in one or more of the mentioned pain locations without interruption? and During the last 12 months, how many days did you have pain in one or more pain locations?). Pain intensity and disability due to pain were asked about by numerical rating scales (010). Functional capacity was measured with the FFbH-R [13] and bodily complaints with the BL scale [14]. The BL scale is a well-validated measure of bodily complaints [14]. The FFbH-R has been validated for use as a measure of functional capacity in musculoskeletal disorders associated with back pain [13]. The questionnaire closed with sociodemographic questions and questions about the utilization of the rehabilitation system.
In order to keep the time span between screening and clinical examination as short as possible the postal screening was conducted in five waves. The screened subjects with valid answers were classified according to their answers concerning rheumatic and non-specific bodily complaints into six groups (Table 1
). The operational distinction between subjects with low and high levels of non-specific bodily complaints was made with the help of the BL scale [14] and a cut-off of 22.
Clinical survey
From each group a random sample was invited to the hospital to participate in the clinical survey (n=1022). The questionnaire of the clinical survey repeated all questions from the screening questionnaire (except questions about sociodemography and utilization of the rehabilitation system). Additionally, the German versions of several validated and internationally used questionnaires were employed in the clinical survey: the Center for Epidemiologic Studies Depression Scale (CES-D) was used to measure depressive symptomatology [1517], the Nottingham Health Profile (NHP) [1820] to survey health problems, and the Pain-related Self-Statements Scale (PRSS) to assess the subjects' cognitive coping with pain [21]. Thirty-four predefined tender points were examined by the clinical examiner (TS). These 34 points comprise all non-redundant sites suggested by Lautenschläger et al. [8], Yunus et al. [3] and the ACR 1990 Criteria for the Classification of Fibromyalgia [7]. Control points known not to be related to fibromyalgia were also examined (n=10). The examination followed the recommendation of the ACR [7]. Manual pressure was applied with the thumb. Reliability of tender point assessment by manual palpation is considered as acceptable [22]. Subjects were told to indicate if the pressure became painful. Exaggerated tenderness was considered to be present if some verbal or facial expression of pain occurred or a wince or withdrawal was observed. The level of pressure was tested periodically by having the examiner palpate the end of a dolorimeter and observing the effort required to reach the 4 kg mark. The study protocol was approved by the ethics committee of the Medical University at Lübeck.
Statistical analysis
In order to test the study hypothesis the influence of a number of pain locations and bodily complaints on mean tender point count was evaluated on the level of the clinical survey. Simple pairwise comparisons were made by t-test. Additionally, an analysis of variance applying the Scheffé adjustment for multiple comparisons was made. To test the association with tender point count further, a linear regression model was fitted to the data and the main effects of number of pain locations, non-specific bodily complaints, the interaction term of these two factors and age were evaluated.
In an analysis of the association between tender points and potential determinants, tender point count was dichotomized (at least 11 positive tender points vs less than 11 positive tender points) and univariate logistic regressions with all relevant independent variables were performed. Where possible the independent variables were categorized in accordance with the study hypothesis or at meaningful cut-off points (health status: very good or good vs fair, poor or very poor; extent of pain: three categories; pain duration: no pain, less than 3 months, at least 3 months; bodily complaints: according to the manual of the BL scale in two levels). Age, depression, active coping and functional capacity were dichotomized at their median. Due to high levels of zero scorers, catastrophizing and all areas of the Nottingham Health Profile were dichotomized at zero score vs any higher score. All variables that tested significant in the univariate analysis were further examined in a multiple logistic regression. The log likelihood ratio test was employed to compare possible models in order to find the best fitting and most parsimonious model to describe the relationship between the response variable tender point positive and the set of independent variables. The association between chronic widespread pain and potential determinants was evaluated analogously. Analysis was carried out in SAS version 6.12.
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Results
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Postal screening
A total of 2253 people replied to the postal screening questionnaire (response rate 74%). In order to utilize as much as possible of the information given and to comply with the definition of chronic pain as used in the context of fibromyalgia (for at least 3 months), we decided to combine the answers to the two questions concerning pain duration and to categorize the answers. We could identify three categories of pain duration: no pain days (30.4%), pain for less than 3 months (29.9%) and pain for at least 3 months (39.8%).
The list of 16 possible regions of pain today was not answered by 62 persons (2.8%). A total of 18.9% indicated in this question no pain at all, the remaining 81.1% of the subjects had pain in one or more regions. Pain today is most prevalent in the neck (40.8%) and in the back (48.5%). Overlapping and cumulating of complaints occurs. Of the women aged 35 to 74, 61.5% currently suffer from pain in the vertebral column, 59.7% from arthralgia and 76.1% from pain in the vertebral column or arthralgia. Looking only at subjects reporting any pain, the mode is at 2 pain locations. The median is 3, and the mean 4.1 (±2.9). If, in addition, the definition of chronic pain (for at least 3 months) is applied, 623 women with chronic rheumatic pain are found among the 2253 women. Here, the mode is at 3, the median 5 and the mean 5.5 (±3.2). A total of 466 women have rheumatic pain that has a duration of less than 3 months. Since the 16 locations in the region-of-interest drawing represent typical rheumatological sites of predilection, pain in these regions is summarized under rheumatic pain for the remainder of this article.
According to the ACR 1990 criteria for fibromyalgia Pain is considered widespread when all of the following are present: pain in the left side of the body, pain in the right side of the body, pain above the waist, and pain below the waist. In addition, axial skeletal pain (cervical spine or anterior chest or thoracic spine or low back) must be present. [7]. From the pain drawing we could identify 597 subjects with widespread pain according to this definition (26.5%) out of the 2253 subjects with a valid questionnaire. According to the classification criteria of fibromyalgia by the ACR, widespread pain must be present for at least 3 months. If this additional requirement is applied, the number of subjects with chronic widespread pain is 304, resulting in a point prevalence of 13.5% on the population level.
Clinical survey
A total of 653 subjects agreed to further questioning and examination (participation rate 64%). The distribution of the tender point count in the clinical examination is displayed in Table 2
. At this level no statistically significant differences in tender point count could be demonstrated between group 1 and 2 (P=0.115, power=69%), 2 and 3 (P=0.303, power=31%), and between group 4 and 5 (P=0.792, power=5%), but between group 3 and 4 (P=0.006, power=79%) and between group 5 and 6 (P<0.001, power=99%) the differences were significant (pairwise comparison; two-sided t-test). In an ANOVA procedure applying the Scheffé adjustment for multiple comparisons, the results remain basically the same except the comparison between group 3 and 4, which no longer gained statistical significance.
To test the association with tender point count further, a linear regression model was fitted. In the best fitting model, both variablesnumber of pain locations and non-specific bodily complaintsare included as well as age (Table 3
). If we look at the subjects who were positive for at least 11 of the 18 tender points (n=72), we find a considerable number of subjects who fulfil this criterion of fibromyalgia according to the ACR, even in groups 1 to 4; however, no one from these four groups has a history of chronic widespread pain (Table 4
).
For evaluating the association between tender points and potential determinants all summary variables of the included questionnaires were analysed in univariate logistic regressions. Tender point count was used as an independent variable. Subjects with at least 11 positive tender points were considered as exposed. At this level of multiple univariate logistic regressions an impressive list of potential determinants for positive tender points exists (Table 5
). All variables that tested significant in the univariate analysis were further examined in a multiple logistic regression. In the final, best fitting and most parsimonious model the variables age, physical mobility, pain and bodily complaints were included (Table 6
). Compared with subjects negative for tender points, those with positive tender points are older, have limited mobility and suffer more from pain and bodily complaints.
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TABLE 5. Associations between tender points and potential determinantsunivariate logistic regressions; dependent variable: tender point according to the ACR definition (exposed: at least 11 active tender points; not exposed: less than 11 active tender points)
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TABLE 6. Associations between tender points and potential determinantsmultiple logistic regression; dependent variable: tender point according to the ACR definition (exposed: at least 11 active tender points; not exposed: less than 11 active tender points)
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A total of 96 of the 653 examined subjects (14.7%) fulfilled the criterion chronic widespread pain according to the ACR criteria (as opposed to 13.5% on the population level). In univariate logistic regressions these subjects with chronic widespread pain were compared with all other subjects (who are not necessarily free of pain and complaints) in respect to potential determinants (Table 7
). All variables that tested significant were further examined in a multiple logistic regression. The final, best fitting and most parsimonious model reveals that subjects with chronic widespread pain according to the ACR are older, rate their health status, emotional reactions, energy level and sleep worse than subjects without chronic widespread pain and focus cognitions on the aversive aspects of the pain experience (catastrophizing) (Table 8
).
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TABLE 7. Associations between chronic widespread pain and potential determinantsunivariate logistic regressions; dependent variable: chronic widespread pain according to the ACR definition: yes or no
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TABLE 8. Associations between chronic widespread pain and potential determinantsmultiple logistic regression; dependent variable: chronic widespread pain according to the ACR definition: yes or no
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Discussion
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In this population-based two-stage cross-sectional study, 26.5% of the screened subjects had widespread pain according to the ACR; 62.3% of these had chronic and 37.7% pain of less than 3 months duration, yielding a point prevalence of 13.5% with chronic widespread pain. Other epidemiological studies from different countries report a point prevalence of chronic widespread pain in adults between 10 and 17% [2327].
In the development of classification criteria for fibromyalgia two concepts can be traced [28]. The first is represented by Yunus et al. [2, 3], who required few positive tender points for diagnosis, but laid stress on a range of associated symptoms. In Germany, a similar concept was developed by Müller et al. [8, 29]. The concept of Yunus views fibromyalgia on a spectrum of dysfunctional syndromes. In this concept, functional symptoms are given the same weight as rheumatic pain and tender points. Implicit in this is the idea that pain and nociception can be substituted by signs and symptoms of other sensory systems (proprioception and enteroception) [30]. The second concept was developed by Wolfe et al. [7] and led to the ACR 1990 criteria for the classification of fibromyalgia. They concentrate on chronic widespread pain and a high number of positive tender points without requiring further symptoms. This concept of fibromyalgia as a distinct rheumatological disorder is reflected in the Copenhagen Declaration [11, 12].
In our study, in predicting the number of tender points, rheumatic pain can be substituted by a history of multiple bodily complaints. This conclusion, derived from groupwise comparisons, is partly based on comparisons with low statistical power. However, looking at the means of tender points, a pattern evolves. At each level of bodily complaints, increasing rheumatic pain is associated with an increasing number of positive tender points. At each level of rheumatic pain, a higher level of bodily complaints is associated with a higher number of positive tender points. In a linear regression model the number of tender points is associated with the number of pain locations andindependentlywith the extent of bodily complaints. Thus, the concept of fibromyalgia as a distinct rheumatological disorder has to be questioned. Instead, the association of positive tender points with both rheumatic pain and bodily complaints independent of the presence of pain supports the concept of fibromyalgia by Müller and by Yunus. This conclusion is further supported by two study groups from the UK and the USA [3133]. Both groups came to the conclusion that tender points are a measure of general distress independent of pain. Thus, fibromyalgia seems to be part of a wider spectrum of dysfunctional syndromes, with irritable bowel and chronic fatigue syndromes as possible members of the family. Wessely et al. [34] followed the same line of thinking. They used the term functional somatic syndromes' for disorders such as irritable bowel syndrome, fibromyalgia and chronic fatigue syndrome. They concluded that substantial overlaps between these syndromes exist. A significant overlap between fibromyalgia and chronic fatigue syndrome [35, 36] and irritable bowel syndrome [37, 38], but not between fibromyalgia and inflammatory bowel syndrome [39], could be demonstrated in clinical studies.
In the present study, we identified subjects who meet the tender point criterion of the ACR (11 or more positive tender points out of 18 examined), but without a history of widespread pain. This result is indicative of a low specificity of positive tender points. This conclusion is consistent with surveys in the UK [31] and in Norway [40]. In both populations subjects with high tender points but no chronic widespread pain were observed rather frequently.
A principal limitation of our study is its generalizability. We studied a small population. However, our results that could be compared with other studies (e.g. prevalence of chronic widespread pain) are in accordance with the literature. The restriction to women also remains a limitation of this study. It is generally agreed in the literature that the prevalence of fibromyalgia in the general population is about six to seven times higher in women than in men. Women have more than twice as many tender points as men [6]. In order to increase the economy of the study we therefore focused on women. A study of the same size but with both sexes would find considerably fewer subjects with tender points. The focus on subjects aged 35 to 74 further increased the economy of the study. Finally, it is possible that those who did not respond and those who did not participate in this study might be different, a hazard to any observational study. We previously reported analyses on a potential differential non-response or non-participation bias [41]. In summary, it can be stated that a bias was unlikely to occur in respect to the variables that are important for testing the study hypothesis.
Among the strengths of the study design is the way subjects were selected into the second stage. Based on the information from the postal screening on the number of pain regions and bodily complaints, six mutually exclusive groups covering the whole range of the population were formed. From each group, a random sample was invited to participate in the clinical examination. In this weighted, stratified sample of all women aged 35 to 74 yr we were able to analyse bodily complaints and tender point count at different levels of pain. Since the main interest of the study is a comparison within the cohort, associations were not inferred back to the population. No adjustment was therefore made for the sampling weights.
In summary, the results of the study question the relevance, specificity and sensitivity of a history of widespread pain in diagnosing fibromyalgia and the concept of fibromyalgia as a distinct rheumatological condition. Instead, they support the concept of fibromyalgia as part of a wider spectrum of dysfunctional syndromes. The results suggest that the use of the combination of chronic widespread pain and a certain number of positive tender points as symptoms defining the syndrome fibromyalgia is arbitrary.
A better decision regarding the components and cut-offs could be reached if we knew the prognostic value of the diagnostic components. It is therefore recommended to study the relevant components prospectively. There is mounting evidence that tender points function as a sedimentation rate for distress [3133]. Recent prospective studies demonstrate that chronic widespread pain is associated with features of the process of somatization, including psychological distress [42], and with an increased risk of death, mainly from cancer [43]. It has been suggested that non-biological factors such as coping strategies and socio-economic variables should be studied [44]. In addition to measurements of social well-being, endpoints for studying prognosis should include functional disability and work disability, since these are the areas in which fibromyalgia patients are likely to become affected.
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Acknowledgments
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This publication is based on a research project which received financial funding from the ministry of education, science, research and technology (BMBF) under the funding sign 01 EF 9494/9. The authors are responsible for the content.
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Notes
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Correspondence to: T. Schochat, Medical University Ulm, Institute for Occupational, Social and Environmental Medicine, Frauensteige 10, 89075 Ulm, Germany. E-mail: thomas.schochat{at}medizin.uni-ulm.de 
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Submitted 28 February 2002;
Accepted 15 November 2002