Patients with fibromyalgia were referred to the study by a consultant in rehabilitation medicine of the department of rehabilitation medicine in one of the five participating hospitals in the South of the Netherlands. They were referred to the department by a medical specialist or general practitioner. Their pain syndrome was labelled as fibromyalgia by a rheumatologist (94.6%) or a general practitioner (5.4%). In order to be able to contrast the impact of fibromyalgia on a patient's daily life situation, patients with two other non-specific pain syndromes were included. Firstly, patients with Complex Regional Pain Syndrome (CRPS) were included in the study; they were referred to the rehabilitation department by one of the anaesthesiologists of the pain clinic in the Laurentius hospital in Roermond. And secondly, patients with chronic low back pain (CLBP) were included who visited a physiatrist in a tertiary care rehabilitation centre after referral by a physiatrist of one of the five before mentioned hospitals. Inclusion criteria were: (1) one of the pain syndromes: fibromyalgia, CRPS or CLBP. (2) no other somatic disease, that could be responsible for the reported pain complaints. (3) sufficient knowledge of the Dutch language in order to be able to read and interpret the questionnaire.

The information and informed consent procedure was approved by the Medical Ethics Committee of the Rehabilitation Foundation Limburg, the Netherlands.

For all participating patients:

To answer the first research question, contributing factors for the explanation of disability in fibromyalgia were explored based on a linear regression analysis. The dependent variable in the regression model was disability as measured with the total FIQ score. Independent variables were selected based on a holistic view on pain related disability in which biomedical, psychological as well as social factors have their influence. From a biomedical perspective gender and physical functioning (subscale of the SF36) were included as independent variables. To reflect psychological functioning both fear of injury and mental health (subscale of the SF36) were included. A patient's social situation was represented by social functioning, which is assessed based on the score on this subscale of the SF36. Collinearity control included checking variable inflation factors (VIF), which had to be below 10. Extreme values, more than 3 box lengths from the upper or lower edge of the box, and outliers, with Cook's distance above 1, were discarded.

To answer the second research-question three groups of patients with different pain syndromes were compared using the following tests: (1) a χ² analysis for dichotomous variables; (2) a one-way analysis of variance (ANOVA) including a post hoc range test according Tukey for normal distributed continuous variables; (3) a Kruskal-Wallis one-way analysis of variance for non-normal distributed continuous variables (two tailed-test with significance level of p < 0.05).

To interpret the impact of the fibromyalgia syndrome on the quality of life, the total score of the SF36 of patients with fibromyalgia in this study was studied in comparison with scores of patients with fibromyalgia from other studies, quality of life scores of patients with other chronic pain conditions and scores of persons out of the general population.

Analyses were performed using SPSS software (SPSS Inc., Chicago, Ill. Version 14).

This study is performed based on data of patients entering rehabilitation departments in a chronological order. Although patients with fibromyalgia entered the study after referral by consultants in rehabilitation medicine of one of five rehabilitation departments, patients with CRPS and CLBP were included out of only one department. As a result, the number of patients in both other pain conditions (22 with CRPS and 33 with CLBP) is rather low, and group sizes are unequal. For this reason non-parametrical testing was used when population based scores were compared. The group size of patients with fibromyalgia also had implications for the regression analysis performed. Due to the number of 54 participants, the number of independent variables that could be introduced in the regression analysis was rather limited. As a result of this, only the most prominent factors out of the explanatory models for pain related disability were chosen and introduced in the current regression model. A second drawback within this study is the fact that the composition of the three groups of patients appeared to be unequal regarding the male/female ratio. In addition, although all patients had a chronic pain condition, patients with fibromyalgia and CLBP had pain for a longer period in comparison with CRPS patients. However, differences in pain duration will presumably not have influenced the results of this study. Although median pain duration of 1.5 years for patients with CRPS was significantly shorter, this time period seems however extensive enough to elicit psychological distress. On the other hand, an interfering influence of gender could be hypothesized. For this reason, in the regression analysis the model is corrected for gender. Based on the results of the regression analysis, it appeared that gender did not significantly influence the explanation of the level of disability. In the analysis for psychological distress no correction for gender was performed, merely because of the low number of participants. As a result of this, in interpreting the result of this analysis, gender related differences have to be considered. Thirdly, the inclusion of the three groups of patients with chronic pain was based on their referral to secondary or third care rehabilitation care. Patients who entered the study visited the department of a rehabilitation specialist referred by different medical specialists or general practitioners. For the patients with fibromyalgia, at the moment of inclusion in the study, no additional check, such as a check of the American College of Rheumatology guideline, was performed by the researcher or consultant in rehabilitation medicine. However, before entering the rehabilitation department, for 94.6% of the patients fibromyalgia was diagnosed by a rheumatologist, according to their professional reumatological guidelines, which includes a check of the criteria of the American college of Rheumatology. In case the analyses were repeated with patients included referred by the rheumatologist (N = 51), results didn't differ from the results found on the total population of patients with fibromyalgia. The study population represents a population of patients with fibromyalgia and CRPS normally being referred to secondary care rehabilitation services in The Netherlands. The CLBP patients however, were referred to a tertiary rehabilitation centre, which might indicate that their level of distress and fear of injury might be higher than the ones normally seen in secondary care.

In this study, it appeared that the level of psychological distress of patients with fibromyalgia exceeds the scores for patients with CRPS and CLBP. The median score of 159 and 152 for consecutively patients with CRPS and CLBP are in agreement with published mean score of 146 (SD 49) for male and 150 (SD 44) for female patients with chronic pain who visited Dutch pain clinics N = 2458 12. In contrast, patients with fibromyalgia score higher. The finding that scores for somatisation, depression and anxiety are higher in patients with fibromyalgia as compared to scores for patients with other pain conditions have been reported before. Most studies addressing psychological distress in patients with fibromyalgia compared "fibromyalgia scores" with scores of patients with rheumatoid arthritis and reported somatisation rates for patients with fibromyalgia that exceed those found in patients with rheumatoid arthritis 181920. In addition, the level of anxiety in patients with fibromyalgia appeared to be higher as compared to the level of anxiety in patients with rheumatoid arthritis 202122. Lifetime depression rates in fibromyalgia ranged from 20% to even 86% indicating a high prevalence in comparison with other medical conditions 2324. Raphael et al, reported that in women with fibromyalgia the risk of lifetime anxiety disorders and in particular obsessive compulsive disorder, appeared to be approximately 5-fold higher as compared to the general population 25.

The fact that depression and anxiety appeared to be high in patients with fibromyalgia could support the hypothesis of van den Houdenhoven that in patients being deprived of "overactivity" as their favourite coping strategy, anxiety and depression can occur and can have a substantial influence on the level of disability. The fact that in the regression-analysis, mental health contributed significantly in the explanation of a patient's disability level confirms the influence of psychological wellbeing on the functioning of patients with fibromyalgia. It is of great importance to identify factors that are associated with disability in patients who persist in performing activities, as this group may be distinguished from patients who use avoidance strategies to cope. The idea of van Houdenhoven regarding disability in chronic pain patients with a premorbid overactive lifestyle matches the ideas explained in the self-discrepancy model of Higgins 26. In view of the explanation of disability in fibromyalgia, the selfdiscrepancy model is introduced in the discussion section of this article as a suggestion for further research. Higgin's self discrepancy theory postulates that each person has three basic domains of selves; the actual selve (e.g. describes what attributes an individual believes they actually possess), the ideal selve (the characteristics that an individual would ideally like to possess in the future) and the ought selve (the attributes that an individual believes they ought to or should possess). Individuals are motivated to work towards a condition where the actual self matches the ideal self or ought self. In this, people strive to keep the discrepancies between the actual-ideal and the actual-ought selves as small as possible, as these give rise to negative psychological situations, that are associated with specific emotions 27. This could be the explanation of persistent behaviour as reported by van Houdenhoven and Hasenbring. According to Higgins, discrepancies between actual-ideal self gives rise to dejection-related emotions (e.g. disappointment, frustration, depression), while a discrepancy between the actual-ought self may lead to agitation related emotions (e.g. fear, guilt, self-contempt). This theory postulates that discrepancies between selves (actual self vs. ideal self and actual self vs. ought self) gives rise to specific negative emotions, which will finally lead to disability. According to the self-discrepancy theory, the greater the magnitude and accessibility of a particular type of self-discrepancy, the higher the intensity of the associated discomfort when that particular self-discrepancy is activated. The concept of self-discrepancy has been applied to a number of clinical disorders, such as body dysmorphic disorder 28, depression and anxiety 29. Davies was the first to apply the self-discrepancy theory and its concept of self and identity to a group of chronic pain patients 30. She found in 89 patients with different types of specific and nonspecific pain that self-discrepancies are significant predictors for depression, anxiety and pain-related disability. In our study based on the presented data, no conclusions can be drawn on the role of the concept of selfdiscrepancies in fibromyalgia. However, the fact that psychological factors as depression and general anxiety were high in patients with fibromyalgia together with the fact that mental health was most associated with disability could support the idea of the self discrepancy theory. The finding of Natvig et al that patients with fibromyalgia had a higher leisure time physical activity level as compared to other females without pain 31 could confirm the hypothesis on the disabling role of persistent behaviour in fibromyalgia. Further research is warranted.

It seems that patients with fibromyalgia experience a lower quality of life as compared to the general population. In comparison with patients with rheumatoid arthritis, especially mental health and social functioning of patients with fibromyalgia seem to be more affected. Quality of life of patients with fibromyalgia was associated with their fibromyalgia related disability level. The total impact on quality of life of fibromyalgia, as in CLBP, seem considerable. However, in comparing data of the different studies, it is important to consider that: Firstly, no statistical testing has been performed to confirm differences in scores on quality of life of the different patients groups and secondly data within Table 4, are gathered from both patients who searched for help in a rheumatologic or rehabilitation department together with persons who filled in a questionnaire based on a survey or an advertisement and were not seeking medical care. As a result of these different recruitment procedures, differences between populations regarding the level of quality of life may have occurred. Therefore, in the interpretation of these results on quality of life, the risk of selection bias has to be considered. Nevertheless, based on this overview, it can be concluded that the quality of life of patients with fibromyalgia seem to be influenced by their pain problem.

If it can be confirmed that in a population of patients with fibromyalgia, especially patients with persistent behaviour seem to be present, these patients will not benefit from the current approach in rehabilitation medicine focusing on enhancing the level of physical activity. It may be speculated that patients who show persistent behaviour might benefit from learning how they can reduce for example their self discrepancies and associated negative emotions and fine tune their activities during the day. Distinguishing groups of activity related behaviour seems therefore an important research topic. This could lead to selecting specific treatments in the future for different patients with chronic pain, and especially in patients with fibromyalgia.

This study is based on a cross sectional design and hypotheses on disabling factors for fibromyalgia are given. To study contributing factors in chronification of the pain related syndromes and their impact on daily life a prospective cohort study seems more appropriate as compared to the current cross sectional design. However, we believe that the preliminary results of the current study are of value for further research. As a result of this, further research is warranted.