Chronic widespread musculoskeletal pain – A comparison of those who meet criteria for fibromyalgia and those who do not

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European Journal of Pain 12 (2008) 600–610 www.EuropeanJournalPain.com

Chronic widespread musculoskeletal pain – A comparison of those who meet criteria for fibromyalgia and those who do not Lars Co¨ster a, Sally Kendall d, Bjo¨rn Gerdle b,c,*, Chris Henriksson e, Karl G. Henriksson b,c, Ann Bengtsson a b

a Section of Rheumatology, Faculty of Health Sciences, Linko¨ping University, Linko¨ping, Sweden Section of Rehabilitation Medicine, Faculty of Health Sciences, Linko¨ping University, Linko¨ping, Sweden c Pain and Rehabilitation Centre, University Hospital, Linko¨ping, Sweden d Multidisciplinary Pain Centre, Copenhagen University Hospital, Copenhagen, Denmark e The Department of Social and Welfare Studies, Linko¨ping University, Linko¨ping, Sweden

Received 18 April 2007; received in revised form 2 October 2007; accepted 2 October 2007 Available online 19 November 2007

Abstract Fibromyalgia is currently classified as chronic widespread pain with widespread allodynia to pressure pain. There are few data describing pain characteristics, quality of life, consequences for daily living, and psychosocial status in patients who meet the classification criteria for fibromyalgia proposed by the American College of Rheumatology compared with patients with chronic widespread pain but not widespread allodynia. This study used a randomly selected sample from the general population. A postal questionnaire and a pain mannequin were sent to 9952 people. The response rate was 76.7%. The pain drawings showed that 345 people had widespread pain; that is, they noted pain in all four extremities and axially. Clinical examination, which included a manual tender point examination, was performed in 125 subjects. These people answered commonly used questionnaires on pain, quality of life, coping strategies, depression, and anxiety. Compared with chronic widespread pain without widespread allodynia, fibromyalgia was associated with more severe symptoms/ consequences for daily life and higher pain severity. Similar coping strategies were found. Chronic widespread pain without widespread allodynia to pressure pain was found in 4.5% in the population and fibromyalgia in 2.5%. Ó 2007 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. Keywords: Pain; Fibromyalgia; Chronic widespread pain; Allodynia; Epidemiology

1. Introduction Chronic pain can be categorized as localised, regional, or widespread. It may or may not be associated with different somatic disorders. The majority of cases are classified as musculoskeletal (Lindell et al., 2000). Chronic *

Corresponding author. Address: Section of Rehabilitation Medicine, Faculty of Health Sciences, Linko¨ping University, Linko¨ping, Sweden. Tel.: +46 13221574; fax: +46 13224465. E-mail address: [email protected] (B. Gerdle).

muscular pain is commonly associated with pain hypersensitivity. Fibromyalgia is chronic widespread pain (CWP) with widespread allodynia to pressure pain. Mechanisms underlying widespread allodynia include central sensitisation of pain transmission neurons and decreased function of pain inhibitory mechanisms, which is possibly maintained by tonic peripheral nociceptive input (see Mense, 2003; Vierck, 2006). Health-related quality of life in subjects with CWP has been reported to be among the lowest for medical conditions (Becker et al., 1997) as chronic pain affects daily living, the work

1090-3801/$34 Ó 2007 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ejpain.2007.10.001

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life, and social life. The prognosis is poor and chronic pain may be associated with shorter life expectancy (Andersson, 2004; MacFarlane et al., 2001). Patients encountered in a tertiary referral hospital are not necessarily representative of all patients in the referral population. It has been reported that fibromyalgia patients in referral clinics differ in pain severity and lifetime psychiatric diagnoses from fibromyalgia patients found in the community (Wolfe et al., 1995; Aaron et al., 1996). In the first report of a cross-sectional general population study (Gerdle et al., 2004), the prevalence of pain was assessed from a screening questionnaire sent to a random sample of 9952 subjects between 18 and 74 years (participation rate was 76.7%; n = 7637) in the county of ¨ stergo¨tland, Sweden. The questionnaire covered O aspects such as demographic data, diagnosis, pain today, pain during previous month, recurring pain, duration, location, frequency, intensity, pain at rest or movement-related, occupational status, working hours per week, use of health care for the pain, and disability pension. The respondents marked the location of the pain on a pain drawing (Fig. 1). The prevalence of pain of more than three months duration irrespective of intensity (chronic pain) was 53.7% (Gerdle et al., 2004). The prevalence of pain was consistently higher among women than among men and associated with age. The highest prevalence was observed in subjects 50–64 years old. In the present second study of this population sample, we analysed whether pain, other symptoms, and psycho-social consequences in subjects who met the 1990 classification criteria for fibromyalgia (Wolfe et al., 1990) proposed by the American College of Rheumatology (ACR) differ from subjects who had

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CWP but not widespread allodynia. Our main hypothesis was that widespread allodynia separates fibromyalgia from chronic widespread pain without widespread allodynia with respect to different symptoms, disability, and quality of life. This study compares different clinical characteristics and functional consequences in two subgroups of participants: (1) CWP with widespread allodynia, which is fibromyalgia according to the ACR criteria (FM1) and (2) CWP without widespread allodynia (i.e., 611 tender points (non-FM)).

2. Methods 2.1. Subjects Subjects were drawn from the above-mentioned epidemiological study (Gerdle et al., 2004). The criterion for inclusion in our study was widespread musculoskeletal pain of more than three months duration indicated on a drawing (Fig. 1). CWP was defined as pain spinally and in all four extremities. Pain in anterior thorax, lumbar region, and abdomen were not considered as spinal pain (Fig. 1: spinal pain: one of the areas 2, 11, 7, or 15). On returning the anonymous questionnaire, 290 of the 345 respondents with CWP indicated that they were willing to be contacted and provided their name and address. All the 290 respondents were offered a clinical examination and 125 (97 women and 28 men) accepted. Respondents who declined to come for examination were contacted by telephone. Reasons for not coming included being too ill to participate, already knowing the cause of pain, not having the time, or language difficulties. The Research Ethics Committee of the University of Linko¨ping, Sweden approved the study. 2.2. Data collection Fibromyalgia tender points were palpated according to the ACR criteria (Wolfe et al., 1990) by four examiners who had trained together. Laboratory tests were performed (creatine kinase, thyroid-stimulating hormone, and C-reactive protein) to exclude other illnesses. Muscle biopsies were performed in those with increased CK level. Because no single specific instrument covered all the aspects we wished to investigate, we used several instruments (Swedish versions). The SF-36 Health Survey Short Form (SF-36) (Ware, 1988; Sullivan et al., 1995) is a frequently used instru1

Fig. 1. Pain drawing used in the postal questionnaire.

This article uses the terms ‘‘FM’’ and ‘‘fibromyalgia’’. ‘‘FM’’ refers to people with fibromyalgia in our study group, whereas ‘‘fibromyalgia’’ is used as a general term for the diagnosis.

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ment measuring global health-related quality of life (HRQL) on eight scales: physical functioning, role-physical, bodily pain, general health, vitality, social functioning, role-emotional, and mental health. The participants are also asked to use a five-point scale to rate their present general health compared to their one year previous. A higher score on the SF-36 (0–100 scale) indicates a better state of health. Reliability for each scale in this version (Cronbach’s alpha) ranges from 0.79 for roleemotional to 0.93 for bodily pain. Validity for item discrimination is reported as an average scaling success rate ranging from 94.6% to 100% in age groups younger than 72 (Sullivan et al., 1995). The generic quality of life was measured using the Quality of Life Scale (QOLS) (Flanagan, 1978; Burckhardt et al., 1989). Respondents rate satisfaction with 16 single items in 6 different domains: physical and material well-being; relations with other people; social, community, and civic activities; personal development and fulfilment; recreation; and independence. A higher score, maximum 112, indicates a better quality of life. Reliability is reported as between 0.89 and 0.92 (Cronbach’s a). Discriminant validity measured by correlation with SF-36 (Spearman’s coefficient) showed poorest correlation (0.08–0.23) with the physical function scale and highest with general health (0.64–0.72) (Liedberg et al., 2005). The Coping Strategy Questionnaire (CSQ) (Rosenstiel and Keefe, 1983; Jensen and Linton, 1993) measures how patients cope with pain and includes eight types of coping strategies: diverting attention, re-interpreting pain sensation, coping self-statements, ignoring pain sensations, praying and hoping, catastrophizing, increased behavioural activities, and pain behaviour. Each strategy is evaluated according to its frequency of use: from never (0) to always (6) with a maximum score of 36. Two additional questions concern the perception of control and possibility to minimize pain. Reliability (Cronbach’s a) is reported for the cognitive coping strategies as between 0.69 for pain behaviour to 0.84 for coping self-statements (Jensen and Linton, 1993). In Swedish patients with FM, reliability coefficients ranged from 0.63 to 0.86 (Burckhardt and Henriksson, 2001). In both studies, reliability of behavioural coping strategies was poorer. Self-efficacy was measured using the Arthritis SelfEfficacy Scale (ASES) (Lorig et al., 1989; Lomi and Nordholm, 1992; Lomi, 1992). It consists of 20 items divided into three scales: Controlling Pain (5 items); Performing Functions in Daily Living (9 items); and Controlling Other Symptoms (6 items). A higher score (0– 100) indicates a higher level of self-efficacy. Reliability coefficients (Spearman) for Swedish women with FM are reported as between 0.79 and 0.83, with strong evidence for construct validity (Lomi, 1992; Lomi et al., 1995).

The Fibromyalgia Impact Questionnaire (FIQ) measures the influence of pain on daily life (Burckhardt et al., 1991; Hedin et al., 1995) and contains 10 items: physical disability; overall well-being during the previous 7 days; sick-leave; impact of fibromyalgia on work; pain; fatigue, morning tiredness; stiffness; anxiety; and depression during the previous week. A higher score, maximum 100, indicates a greater impact on daily life. Reliability (Pearson’s r) for the sub-scales is reported to range from 0.56 (stiffness) to 0.99 (work missed), with 0.81 for the Total FIQ score. Discriminant validity tested with FM patients with minor or major work incapacity was good (Hedin et al., 1995). Self-rated depressive symptoms were recorded using the Beck Depression Inventory (BDI) (Beck et al., 1961). A higher score indicates more severe depression with 10–18 indicating mild to moderate depression, 19–29 moderate to severe depression, and P30 indicating severe depression (Beck et al., 1988a). BDI is an established and well-researched scale (McDowell and Newell, 1996). Anxiety symptoms were measured using the Beck Anxiety Inventory (BAI) (Beck et al., 1988b) and consists of 21 common symptoms of anxiety. The respondents used a 4-point scale to rate the influence of each symptom (0 = not at all and 3 = severely). The rating of each item is summed to a score from 0 to 63; a higher score indicates more anxiety. The instrument was constructed to avoid confounding with depression and has been shown to be a reliable and valid instrument for measuring anxiety (Steer et al., 1993). The original study (Beck et al., 1988b) reported a high level of internal consistency (Cronbach’s a 0.92) and good test–retest correlation (r = 0.75). Extending the work, Creamer et al., found a Cronbach’s a 0.91, and a poorer discriminant validity against the BDI and STAI-State suggesting that the BAI reflects state anxiety rather than depression or trait anxiety (Creamer et al., 1995). The Multidimensional Pain Inventory (MPI) was developed for patients with chronic pain (Kerns et al., 1985). The instrument has one psychosocial section (Part I) dealing with pain severity, pain-related interference in everyday life, perceived life control, affective distress, and perceived support from significant others. It also contains two behavioural sections, one dealing with patient’s perception of how significant others respond to the patient’s pain and suffering: punishing, solicitous, and distracting (Part II). Part III, which deals with a patient’s engagement in daily life activities, was not used in this study (Bergstro¨m et al., 1998). A 7-point numerical scale (0–6) anchored with ‘Never’ and ‘Very often’ is used, and the median value is calculated for each index. Reliability (Cronbach’s alpha) in the Swedish version Parts I and II is reported as 0.74–0.85 for females, similar for males, and construct validity was good for both parts and genders. Discriminant validity for

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females Pain-related interference was poorer, with a correlation with depression and catastrophizing thoughts (Bergstro¨m et al., 1998; Bergstrom et al., 1999). We also used the Pain Processing Inventory (PPI) to assess pain. This test is a self-administrated questionnaire with nine items: pain intensity during previous week (highest, lowest, average), pain intensity on motion and at rest, pain unpleasantness, pain interference in daily activities, ability to reduce pain, and the part of the day pain was felt. The patients marked their answers on a visual analogue scale (no – worst imaginable; not at all – could do nothing; no pain – pain all day; or can totally eliminate pain – cannot decrease pain at all). Four additional questions (on pain on the worst, least, and average pain experienced during last week) were taken from the work done by Wade et al. (1996). A higher score indicates more pain. The psychometric properties of PPI is less known than for the other instruments but have been summarized by Wade and Hart (2002). A General Questionnaire (GQ) recorded background data, such as age, family situation, education, employment, work tasks, pain-free periods, sickness benefits, frequency of symptoms, and satisfaction with present life situation.

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parts. X-variables with a VIP P 1.0 are most influential for the model. The PLS regression coefficients may be re-expressed as a regression model and express the influence of each X-variable on Y in each single component. In the present study, the variable of importance for explaining Y was primarily identified by a VIP value P1.0 and secondary by the regression coefficient in relation to Y. PLS has the advantage that it does not require interval-scale measurements and they are not sensitive to violations of multivariate normality (Henningsson et al., 2001). Two concepts are further used to describe the results: R2 and Q2. R2 describes the goodness of fit – the fraction of sum of squares of all the variables explained by a principal component. Q2, in turn, describes the goodness of prediction – the fraction of the total variation of the variables that can be predicted by a principal component using cross validation methods (Eriksson et al., 1999). Outliers were identified using the two powerful methods available in SIMCA-P: score plots in combination with Hotelling’s T2 (identifies strong outliers) and distance to model in X-space (identifies moderate outliers). In all statistical analysis, p 6 0.05 was regarded as significant.

3. Results 2.3. Statistics 3.1. Diagnosis according to postal survey All univariate statistics were calculated using the SPSS statistical package, version 11.5. Patients with FM were compared with patients with non-FM, and as the FM group included only few men, a comparison between FM women and FM men was not possible. In the non-FM group, women were compared with men. Women with FM were compared with women in the non-FM group. Non-parametric tests included the v2 test for comparison of categorical variables, and the Mann–Whitney test was used for comparison of differences between groups. The median was used for parametric data. For the SF-36, the mean and SD were given as these are usually used in the literature. Student’s t-test was used for parametric data with normal distribution. Partial least squares-discriminant analysis (PLS-DA) using SIMCA-P (Version 10.0, Umetrics Inc., Umea˚, Sweden) was used to regress group membership (FM or non-FM) using several other variables (X-variables) (Eriksson et al., 1999). This was done in order to multivariately determine the variables with most prominent differences between the two groups. That is, this validated the univariate statistical analyses. The VIP variable (variable influence on projection) gives information about the relevance of each X-variable and each Y-variable pooled over all dimensions. Thus the VIP parameter gives information about the relevance of each X variable, both for the X- and Y-model

A diagnosis was reported by 29% (2200 respondents) of the 7637 subjects who returned the original postal questionnaire. The majority of the reported diagnoses were related to musculoskeletal pain (93%). Fibromyalgia was reported by 111 people. A few participants (n = 6) reported diagnoses outside the musculoskeletal system, such as malignant disorders. In the CWP group, identified from the pain drawings, 50% reported diagnoses, most commonly musculoskeletal conditions such as fibromyalgia (40%), inflammatory rheumatic disorders (20%), and osteoarthritis (10%). Twenty subjects, 15 with non-FM and 5 with FM, had not sought health care for their complaints. Between the 125 participants examined and the 220 who declined participation, no significant differences were found with respect to age, civil status, education, sex distribution, sick benefits, or in answers to questions concerning pain localisation, occurrence, frequency, and intensity. 3.2. The examined subjects Of the 125 who were examined, 70 fulfilled the ACR criteria for fibromyalgia (66 women, 4 men; the FM group), and 55 (31 women, 24 men) did not fulfil the criteria (the non-FM group). The mean number of tender points in the FM group was 15 (range 11–18) and

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Table 1 Demographic data of the FM and non-FM groups FM group

Non-FM group

N = 70

N = 55

Age Mean (SD) Range

56.7 (13.0) 29–76

56.2 (12.2) 25–74

Sex Female Male

66 4

31 24

22 6 16

27 15 10 2 7 17 1 2

Employment Working Full-time Part-time Missing On sickness benefit Old age pension Seeking employment Missing

26 19 2

in the non-FM group the mean was 6 (range 0–10). Assuming that the frequency was the same in the whole CWP group as in those examined, the prevalence of CWP was 4.5% and the prevalence of fibromyalgia was 2.5% in the population. Eighteen of the FM group had not been previously diagnosed with FM. Thirteen of the FM group reported additional diagnoses: osteoarthritis, muscle overuse, stroke, whiplash associated disorders, herniated disc, and inflammatory rheumatic disorders. Twenty-eight of non-FM patients reported musculoskeletal disorders such as osteoarthritis, diabetic neuropathy, rheumatoid arthritis, polymyalgia rheumatica, or whiplash associated disorders. Laboratory tests revealed five people with raised CK levels: four in the non-FM group and one in the FM group. Muscle biopsy showed signs of myopathy in one subject and signs of denervation in one subject although this was considered to be of no clinical significance. Age distribution was the same in men and women, and the mean age was 56 for both FM and non-FM (Table 1). There were no differences in educational level between FM and non-FM groups. The highest frequency of FM and non-FM in both men and women was between 50 and 59 years. 3.3. Comparisons between the two groups In the following paragraphs, we report the results of the different instruments used (Table 2). 3.3.1. Pain The FM group scored significantly higher values for average pain severity during ‘‘last week’’ or ‘‘last four weeks’’ in the instruments in which items on pain intensity were registered (FIQ-pain: p = 0.005, MPI-pain:

p = 0.001, SF-36 body pain: p < 0.001). Experiences of least pain during the previous week were significantly lower in the non-FM group (p = 0.013), whereas worst pain showed no difference according to PPI. Pain interference was more pronounced in the FM group than in the non-FM group (PPI: p = 0.011, and MPI: p < 0.001). No significant differences were shown in the ability to control pain according to the two instruments (PPI; ASES), whereas some difference appeared in the MPI (p = 0.036). Pain at rest and pain unpleasantness were rated similarly in the two groups (PPI). The FM group also reported significantly fewer pain free periods ‘‘during last week’’ and 71.4% in the FM group reported continuous pain and no pain free periods during last week compared to 38.2% in the non-FM group. Pain-free periods every day were reported by 14.3% in the FM and 30.9% in the non-FM group.

3.3.2. Other symptoms On the general questionnaire, the FM group compared to the non-FM group reported significantly more sleep disturbances (p = 0.041), more restless sleep (p = 0.017), not feeling rested in the morning (p < 0.001), pronounced tiredness during the day (p < 0.005), more concentration difficulties (p < 0.001), more muscle stiffness (p < 0.001), increased pain after muscular efforts (p < 0.001). The FM group compared to the non-FM group also reported more depression, more uncomfortableness, and more sadness (p = 0.025) and were more sensitive to changes in temperature (p< 0.005). FM reported significantly more impact than non-FM on the FIQ-scales for fatigue (p < 0.001), morning tiredness (p < 0.001), stiffness (p < 0.001), anxiety (p = 0.048), and depression (p = 0.014). However, the frequency of other symptoms – feeling restless and tense, impatient and irritable, anxious, gastrointestinal symptoms, bladder irritability and palpitation of the heart – was not significantly different between the groups according to GQ. The FM group also reported more affective distress in the MPI (MPI: p = 0.007). When depression was rated using the BDI, FM had significantly higher scores (more depressed) than the non-FM group (p = 0.018): mean score 12.3 versus 8.9. In the FM group, 46% indicated no depression, 36% mild to moderate depression, 14% moderate to severe depression, and 4% severe depression; corresponding figures in the non-FM group were 64% (no depression), 27% (mild to moderate depression), 9% (moderate to severe depression), and 0% (severe depression). The FM group scored significantly higher on anxiety according to the BAI (p = 0.014) than the non-FM group; the mean total scores were 13.2 and 9.3, respectively. In several individual anxiety-related symptoms of BAI, there were significant differences between the

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Table 2 Mean ± 1 standard deviation (SD) and median values for the different instruments and scales in the fibromyalgia group (FM) and the nonfibromyalgia group (non-FM) Group

FM

Variables

Mean

SD

Median

Mean

Non-FM SD

Median

Statistics p-Value

Sex (proportion (%) women) Age (years) PPI highest pain intensity during last week PPI lowest pain intensity during last week PPI average pain intensity during last week PPI pain unpleasantness PPI pain interference PPI ability to reduce pain PPI part of the day pain was felt PPI pain intensity at rest PPI pain intensity on motion MPI, pain severity MPI pain interference MPI life control MPI affective distress MPI support MPI punishing responses MPI solicitous responses MPI distracting responses CSQ diverting attention CSQ reinterpreting pain sensations CSQ coping self-statements CSQ ignoring pain sensations CSQ praying and hoping CSQ catastrophizing CSQ increased behavioural activities CSQ pain behaviours CSQ control over pain CSQ ability to decrease pain ASES pain ASES function ASES other symptoms FIQ total FIQ – physical function scale Beck’s depression inventory Beck’s anxiety inventory Quality of life scale SF-36 physical function SF-36 role-physical SF-36 bodily pain SF-36 general health SF-36 vitality SF-36 social function SF-36 role-emotional SF-36 mental health

94.3 55.2 6.2 3.1 5.1 5.3 5.1 5.1 7.9 4.9 6.1 3.6 3.4 3.3 2.7 3.5 1.6 2.7 2.8 14.7 6.8 18.6 15.5 9.0 10.8 17.0 17.7 2.9 2.5 39.9 65.7 49.9 54.1 37.5 12.3 13.2 77.7 52.9 23.9 32.4 41.1 33.8 63.6 52.0 68.9

13.0 1.9 2.2 1.8 2.0 2.4 2.7 2.3 2.5 1.9 1.0 1.2 1.1 1.5 1.7 1.3 1.5 1.6 6.3 6.3 6.7 6.3 6.6 7.7 5.5 6.1 1.5 1.1 19.1 19.2 18.1 20.0 20.3 8.0 9.7 14.9 17.7 34.4 13.3 21.4 21.7 25.6 42.9 19.6

55.5 6.5 3.0 5.0 5.5 5.5 5.0 8.5 5.0 6.5 3.5 3.5 3.1 2.5 3.3 1.3 2.4 2.8 15.0 5.0 19.0 16.0 8.0 10.5 18.0 18.0 3.0 3.0 38.0 61.7 50.8 55.7 36.6 11.0 11.5 81.0 55.0 0.0 32.0 35.0 30.0 62.5 66.7 72.0

56.4 55.8 5.5 2.2 4.2 4.8 3.9 5.2 5.8 4.2 4.8 2.9 2.5 3.7 1.9 3.5 1.2 3.0 2.8 14.2 5.3 20.3 14.9 11.6 11.2 17.9 18.8 3.4 2.5 43.4 74.8 57.9 40.8 31.3 8.9 9.3 80.5 65.0 55.1 43.2 54.5 48.9 77.1 74.1 73.9

12.0 2.4 2.3 2.1 2.3 2.9 3.1 3.3 3.0 2.6 1.2 1.3 0.9 1.4 1.7 1.3 1.5 1.6 7.2 6.4 6.4 5.5 7.0 7.1 5.4 5.6 1.5 1.4 19.1 19.6 17.9 18.9 22.6 5.9 7.0 17.1 22.5 40.5 16.5 20.9 18.1 25.1 37.6 17.4

59.0 5.8 1.0 4.0 5.0 3.5 5.0 6.5 4.0 4.5 3.0 2.4 3.5 2.0 3.5 0.7 3.0 3.0 14.5 3.0 20.0 16.0 11.0 11.0 18.5 19.0 3.5 3.0 40.0 78.9 59.2 39.6 27.3 8.0 8.0 83.0 70.0 50.0 41.0 55.0 50.0 81.3 100.0 76.0

<0.001a 0.696 0.076 0.007a 0.013a 0.150 0.011a 0.917 0.001a 0.114 0.005a 0.001a <0.001a 0.036a 0.007a 0.801 0.127 0.156 0.986 0.700 0.102 0.198 0.724 0.033a 0.826 0.396 0.192 0.133 0.632 0.24 0.010a 0.008a <0.000a 0.097 0.018a 0.014a 0.188 <0.001a <0.001a <0.001a <0.001a <0.001a 0.003a 0.003a 0.179

Furthest to the right is the result of the statistical univariate evaluation (p-values). a Denotes significant group difference between the two groups.

two groups. Numbness or tingling (p = 0.009), dizziness or light-headedness (p = 0.020), trembling hands (p = 0.033), or fainting spells (p = 0.028) were rated as more severe by the FM group than non-FM group. 3.3.3. Coping There were no significant differences between the FM and non-FM except for the strategy ‘‘praying and hoping’’, which was used more often by the non-FM

(p = 0.033). In both groups, coping self-statements were the most frequently used strategy. 3.3.4. Disability According to ASES, the FM group compared with the non-FM group was significantly less confident about their ability to perform functions in daily living (p = 0.007) and control other symptoms (p = 0.007). The total FIQ score showed a significantly higher

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impact in the FM group than in the non-FM group (p < 0.001; Table 2). For the subsection in the FIQ covering physical disability, there was no significant difference between the two groups. Hence, the differences for the total FIQ score was mainly caused by differences in the severity of experienced symptoms. There was no significant difference in the employed proportions in the two groups (FM versus non-FM) (Table 1). Working women in both groups reported similar work conditions without significant differences with respect to daily heavy lifting and carrying (43%), repetitive and monotonous work tasks (24%), stress, (52%), and having responsibility as a manager or supervisor (33%). A comparison of mixed sex FM and non-FM groups showed the same result as the women-only group comparison. 3.3.5. Quality of life The two groups similarly rated generic quality of life measured with the QOLS. Health-related quality of life showed significant differences between groups. The FM group scored the SF-36 with significantly lower values on all scales except for mental health. Non-FM also rated global satisfaction with present life situation as more satisfactory than the FM in the GQ (p = 0.012) (Table 2). 3.4. Multivariate analyses We used a PLS-DA to gain further insight in what variables and groups of variables in the multivariate context showed the strongest differences between the two groups of subjects (FM versus non-FM). In a first

Table 3 The PLS-DA regression of group membership (non-FM or FM) based on all subjects Variables

VIP

Sex CSQ praying/hoping PPI ability to reduce pain PPI pain unpleasantness CSQ control over pain SF-36 role-physical PPI part of the day pain was felt Self-Efficacy Scale; Pain MPI solicitous responses SF-36 mental health MPI pain interference FIQ total SF-36 vitality SF-36 bodily pain R2/Q2

2.78 1.70 1.53 1.43 1.41 1.25 1.19 1.18 1.14 1.12 1.07 1.06 1.04 1.04 0.27/0.19

VIP are given for the variables >1.0. Variables with VIP P 1.0 are considered the most important; i.e., they showed the largest differences between the two groups and are given in bold. R2 and Q2 are also given for the model at the bottom row.

Table 4 The PLS-DA regressions of group membership (non-FM or FM) based on women Variables

VIP

SF-36 role-physical FIQ total SF-36 bodily pain PPI part of the day pain was felt MPI affective distress SF-36 vitality MPI pain intensity MPI pain interference PPI pain in motion SF-36 role-emotional CSQ praying/hoping PPI lowest pain intensity during last week FIQ – physical function scale SF-36 physical function Beck’s depression inventory SF-36 general health Beck’s anxiety inventory SF-36 vitality PPI pain interference R2/Q2

1.89 1.71 1.65 1.64 1.56 1.51 1.40 1.40 1.39 1.27 1.26 1.24 1.19 1.14 1.12 1.11 1.08 1.07 1.04 0.17/0.10

VIP are given for the variables >1.0. Variables with VIP P 1.0 are considered the most important; i.e., they showed the largest differences between the two groups and are given in bold. R2 and Q2 are also given for the model at the bottom row.

analysis using all subjects (both men and women), a significant model was obtained that explained 27% of the variation in group membership (R2 = 0.27, Q2 = 0.19) (Table 3). The most important variables (VIP P 1.0) were sex, three scales of the PPI, two of the scales of CSQ, four of the SF36 scales, ASES-pain, two of the MPI scales, and FIQ total. Due to the large differences in the proportion of men and women and the prominent influence of sex in the model (Table 3), only women were included in a subsequent PLS-DA (R2 = 0.17, Q2 = 0.10) (Table 4). A somewhat different pattern was found: seven of the SF36 scales were important, together with FIQ, pain intensity variables (PPI and MPI), one of the scales of CSQ, as well as BDI and BAI. To summarize, both the univariate and multivariate analyses showed that the two groups of subjects differed significantly and prominently with respect to pain, coping, self-efficacy, disability, and quality of life (Tables 3 and 4).

4. Discussion 4.1. Major results Generally confirming our hypothesis, the main results were that CWP with widespread allodynia to pressure pain (FM group) was associated with more severe symptoms – higher pain intensity, higher pain severity, fewer

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pain-free periods, and more pronounced pain-related interference in everyday life – and consequences for daily life compared with CWP without widespread allodynia (i.e., non-FM). The population prevalence for CWP without widespread allodynia was estimated as 4.5% and for fibromyalgia 2.5%. This study replicates previous estimates for FM in the population and extends previous work showing differences in pain experience between those fulfilling the ACR criteria and those with chronic widespread pain and fewer tender points. 4.2. Tender points Our study compared people with the same spread of chronic pain but a significant difference in the number of tender points. The number of TPs is probably not only a measure of decrease in pressure pain thresholds at several predefined locations but also a sign of other factors that can give rise to increased pain perception. Such factors can be expectations, fear, previous experiences, environmental factors, and psychological distress (Wolfe et al., 1995; Clauw and Crofford, 2003; Petzke et al., 2003). Petzke et al. (2003) showed that pressure stimuli have to be applied randomly in order to measure pressure pain threshold that is not influenced by psychological factors. The method used in the present study was the one recommended in the 1990 ACR classification criteria and can be regarded as an easy to perform screening method for the combination of reduced pressure pain thresholds and psychological distress. The ACR method is still clinically valuable in the clinical setting in order to identify CWP with especially severe symptoms that give rise to pronounced disability. Our results agree with earlier studies that conclude that the number of tender points is a clinically useful measure (Granges and Littlejohn, 1993), and a study of occupationally active homecare personnel in which the tender point score correlated with pain intensities, other symptoms and disability (Lundberg and Gerdle, 2002). The pathological pain amplification in fibromyalgia has a biological component with longstanding or permanent central and peripheral sensitization and/or pain disinhibition (Mense, 2003; Henriksson, 2003; Bennett, 2004; Vierck, 2006). We do not know whether the same patho-physiological changes are present in non-FM and although the use of standardized quantitative sensory testing would have been desirable, it was beyond the scope of this study. It is known that pressure pain thresholds can be decreased also in people with CWP without widespread allodynia (Carli et al., 2002; Laursen et al., 2005). 4.3. Pain and other symptoms There are few published studies on CWP in a randomly selected sample of the population using question-

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naires combined with clinical examination (Prescott et al., 1993; Croft et al., 1994; Wolfe et al., 1995; White et al., 1999b; Lindell et al., 2000; Bergman, 2005). Our results confirm the study by White et al. (1999a); they found that FM cases reported more severe pain and fatigue and more symptoms compared with other chronic pain patients. The authors concluded that fibromyalgia appeared to have distinctive features compared with the symptoms expressed by other chronic pain patients. Both in acute and in chronic pain, the activity in pain-transmitting neurons reaches into the cortex where sensory-motor information and emotional responses are perceived and interpreted. Pain – in accordance with the IASP definition of pain – has both psychological and biological components. Chronic pain is a stressor which bi-directionally interacts with different systems in the body, which in turn might explain increased prevalence of symptoms in severe pain conditions such as CWP and fibromyalgia. The spread of pain was the same in both groups as defined in the inclusion criteria. However, pain reported by FM was also more intensive and persistent. Pain in FM more than in non-FM was also described as continuous and related to movement. Although we have not investigated mechanisms of pain discrimination that might underpin the differences between FM and non-FM, measures of wind-up (Staud et al., 2003) and the regulation of diffuse noxious inhibitory control (Kosek and Hansson, 1997; Mense, 2003) might be interesting. However, mean values of pain intensity in both groups were at moderate levels and the variation in pain intensity was large within both groups. Depression and anxiety influence the ability to manage in daily life, and in women both were important variables distinguishing FM from non-FM. They may be one of the reasons for greater disability in FM. However, in 54% of the FM group their answers on BDI indicated depressive symptoms compared to 36% in the non-FM group. White et al. (2002) also found in fibromyalgia more psychological distress and physical symptoms compared to pain controls. Sleep difficulties, not being rested in the morning, tiredness, and difficulties in concentrating were more pronounced in FM group than in non-FM. Moldofsky (2001) concludes, ‘‘although insomnia and other sleep complaints are very common in fibromyalgia and related conditions, no sleep abnormalities are specific for fibromyalgia’’. There were no significant differences between FM and non-FM with respect to gastro-intestinal symptoms and bladder irritability. These symptoms are often considered to be more frequent in FM than other pain conditions although our findings indicate that the degree of spread of allodynia is not coupled to the occurrence of visceral symptoms. Only one instrument (FIQ) has been designed for fibromyalgia and is regarded as the most efficient

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instrument for discriminating and assessing the influence of fibromyalgia (Pagano et al., 2004; Bennett, 2005). The total sum FIQ did show a pronounced difference between FM and non-FM groups, but more pronounced differences in the multivariate context were seen, such as for SF-36 role-physical (Table 4). When both sexes were included, additional variables were more important than FIQ (Table 3). 4.4. Quality of life Quality of life is reported to be low in fibromyalgia (Picavet and Hoeymans, 2004). No difference between groups was found in generic quality of life (QOLS). By contrast, generally significant differences in HRQL (i.e., SF-36) between non-FM and FM were found. Perhaps, this result means that the generic quality of life depends on other factors – e.g., family situation and economic and material circumstances – that are expected to be similar. The finding that generally there was a lower HRQL in the FM group agrees with more pronounced symptoms, higher impact on daily life, and depression found using the other instruments. However, both FM and non-FM groups reported a lower health-related quality of life compared with Swedish population data (Nordlund et al., 2005). In a Dutch population study (Picavet and Hoeymans, 2004), those with fibromyalgia had lower values than other musculoskeletal diagnoses, except in the role-emotional dimension. Laursen et al. (2005) reported a low HRQL in fibromyalgia patients compared to other patients with local or regional pain conditions. People with fibromyalgia drawn from health care settings have been shown to have even lower levels on the SF-36 scales compared to the present results (Henriksson et al., 2004). 4.5. Epidemiology of CWP We estimated the prevalence of fibromyalgia in the population as 2.5%, assuming that the frequency was the same in the whole CWP group as in those examined. McBeth (2005) has reviewed on the epidemiology of fibromyalgia and CWP: Prevalence figures were retrieved from population based studies and the diagnosis of fibromyalgia was determined using the 1990 ACR criteria. Of six studies, the prevalence in five varied between 0.66% and 3.3%; thus our estimated prevalence figure for fibromyalgia appears to be reasonable. In one study (only women), the prevalence was 10.5%. The prevalence of CWP retrieved from eight studies ranged from 4.7% to 13.5% although the prevalence of CWP is influenced by the definition of widespread pain (McBeth, 2005). In our study, the prevalence (4.5%) was similar to what Hunt et al. (1999) reported (4.7%) using the Manchester definition of widespread pain. In our study, fibromyalgia was diagnosed in 56% of the

CWP group. Even though our prevalence figures are approximately in agreement with other studies the relatively low participation rate might indicate a reduced accuracy for the prevalence in this part of Sweden. The gender difference between FM and non-FM may reflect a bias in the current criteria, since the tender point construct development is derived largely from female patients (Wolfe et al., 1990; Okifuji et al., 1997) and healthy women have lower pressure pain thresholds than healthy men (Chesterton et al., 2003). 4.6. Representativity and limitations The high response rate in present study, derived from a large, non-hospital-based community population, indicates satisfactory representativity. One limitation of the study is the number (57%) of the participants invited who did not come for a physical examination (see methods) and thus not included in the study. However, there were no significant differences in age, civil status, education, sex distribution, sickness benefits, and in answers to pain questions between those reporting CWP who came for examination and those who declined. This strengthens the assumption that the examined people were representative of the whole group of CWP. However, it can be assumed that the frequency of fibromyalgia is somewhat higher among the people examined as in our experience people with fibromyalgia usually have high compliance in studies.

5. Conclusion Earlier studies (Granges and Littlejohn, 1993; Lundberg and Gerdle, 2002) reported that the number of tender points correlates with pain and other symptoms and disability. Our cross-sectional study investigated only subjects with chronic widespread pain. We found that a high number of tender points in CWP was associated with more clinical pain and negative consequences associated with pain, other symptoms, disability, and health-related quality of life, than in CWP with fewer tender points.

Acknowledgements We thank the Swedish Rheumatism Association and the University Hospital of Linko¨ping for their grants. The authors also would like to thank Ms. Ylva Billing for administrative assistance.

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