1Arthritis Research Campaign (ARC) Epidemiology Unit and 2Unit of Chronic Disease Epidemiology, School of Epidemiology and Health Sciences, University of Manchester, UK.
Correspondence to:
J. McBeth, Arthritis Research Campaign Epidemiology Unit, School of Epidemiology and Health Sciences, Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK. E-mail: john.mcbeth{at}man.ac.uk
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Abstract |
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Methods. We conducted a population-based prospective survey and identified 1658 adults aged 1865 yr who were symptom-free. At baseline, detailed information was obtained on work-related mechanical and environment factors using validated instruments. Individual psychosocial features were also measured. Subjects free of chronic widespread pain at baseline were followed up at 12 and 36 months to identify those reporting the onset of new symptoms.
Results. In all, 1445 (91%) returned the questionnaire at 12 months and 978 (89%) at 36 months. Of these, 81 and 92 respectively reported new chronic widespread pain. Symptom onset was predicted by workplace factors {pushing/pulling heavy weights [relative risk (RR) = 1.8, 95% confidence interval (CI) 1.1, 3.0]; repetitive movements of the wrists (RR = 1.8, 95% CI 1.2, 2.7); kneeling (RR = 2.2, 95% CI 1.2, 4.1)} and individual factors [aspects of illness behaviour (RR = 2.9, 95% CI 1.6, 5.3); somatic symptoms (RR = 1.9 95% CI 1.1, 3.3); fatigue (RR = 1.9, 95% CI 1.2, 3.1); baseline pain symptoms (RR = 2.5, 95% CI 1.6, 3.9)]. In multivariate analysis, pushing/pulling heavy weights, repetitive wrist movements, kneeling and other pain at baseline were associated with new-onset chronic widespread pain. However, the strongest predictor was a high score on the illness behaviour scale.
Conclusion. This study provides only limited support for the hypothesis that low-level mechanical injury may be a risk factor for developing chronic widespread pain. The onset of chronic widespread pain appears to be multifactorial and is strongly predicted by individual psychosocial factors.
KEY WORDS: Occupational, Pain, Prospective, Psychosocial.
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Introduction |
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Intense debate has surrounded the theory of trauma-related fibromyalgia, which relates local injury or trauma to the onset of widespread pain symptoms [6, 7]. The retrospective reports of patients who present with symptoms appear to support this hypothesis: injuries to the neck, primarily whiplash injuries following a road traffic accident, and workplace injuries are commonly reported as precipitating events [6, 8]. Such reports are, however, subject to recall bias and the relationship between trauma and symptom onset can only be disentangled in a prospective study.
To date only one study has prospectively investigated the role of neck injuries in symptom onset [9]. In this report, fibromyalgia, classified according to the American College of Rheumatology (ACR) criteria [1], was 13 times more common in subjects who had sustained injuries to the cervical spine when compared with those with leg fractures. This finding is perhaps unsurprising considering that 10 of the 18 tender point sites required for the classification of fibromyalgia are found in the broad region around the cervical spine, and it was the difference in the number of tender points above the waist which distinguished the two groups [10]. The relationship between whiplash injuries and future widespread pain therefore remains equivocal.
The relationship between work-related mechanical injury (arising from activities involving repetitive movements, poor posture, working with heavy loads, and the workplace environment) and the onset of widespread pain symptoms is also unclear [11]. While it has been established that such factors predict the onset of regional pain syndromes, including back [12] and shoulder [13] pain, there are no prospective studies which have examined the association with new-onset chronic widespread pain. In the present study we tested the hypothesis that low-level work-related mechanical trauma would predict the onset of new chronic widespread pain. To understand the relationship more fully, we determined the relative contributions of low-level workplace mechanical trauma, posture and environment, and individual psychosocial factors to symptom onset.
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Methods |
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Study cohort
Study subjects were individuals who had responded to a cross-sectional postal survey. The study has been described in detail elsewhere [5]. Briefly, a random sample of adults aged 1865 yr were selected from a population-based primary care register, covering a mixed sociodemographic suburban area in south Manchester in the UK. A total of 1953 subjects returned the questionnaire (75% response rate), of whom 1658 were free of chronic widespread pain and were therefore eligible for the present study.
Assessment of pain status
Each subject was sent a questionnaire by post which enquired whether subjects had experienced any pain during the previous month that had persisted for at least 24 h and, if so, whether the pain had lasted for more than 3 months. Four line drawings of the body were included (front, back and sides) on which subjects were asked to indicate the site(s) of pain. These methods have been used previously to determine the location and duration of pain [14, 15] and have satisfactory intrasubject and inter-rater reliability [16, 17]. On the basis of this information, subjects were categorized by the presence or absence of chronic widespread pain. The former was defined using the definition as part of the ACR criteria for fibromyalgia [1]. To satisfy these criteria, subjects must have pain that is present both in two contralateral quadrants of the body and in the axial skeleton, and this pain must be present for at least 3 months. A total of 1658 subjects were free of chronic widespread pain and were therefore eligible for follow-up. Of those, 825 (50%) reported no pain and 833 (50%) reported some pain.
Follow-up
Those subjects who were free of chronic widespread pain at baseline were mailed an identical questionnaire at 12 months, and those free of chronic widespread at 12 months were mailed a further follow-up questionnaire at 36 months. The methods used for categorizing pain were the same as those used for the baseline survey. The observer categorizing pain status was blind to the subjects baseline pain status and all other information included in the questionnaire. New chronic widespread pain at follow-up was defined using the ACR definition, as discussed above.
Work-related risk factors
To determine work-related mechanical, posture and environment exposures, we retrospectively assessed each subjects work history at 36 months. Subjects were asked to list the jobs they had held at the time of the baseline survey along with any subsequent changes in employment, and to include the start and, if appropriate, end dates for all jobs listed. On the basis of job history, work-related exposures were based on the subjects current job at the time of the baseline and 12-month surveys. For each job, subjects were asked specific questions relating to various aspects of the job. The questions about physical activities covered lifting or carrying weights with one or both hands and pushing or pulling weights of 25 kg or more. Working postures assessed were typing, standing or sitting for 30 min or more without a break and kneeling and repetitive movements of the arms or wrists. Aspects of the workplace environment were based upon the demandcontrolsupport model [18] and covered three domains: job demands (whether the job was boring, monotonous or repetitive, too hectic or fast, and whether it caused stress or worry); job control (the extent to which subjects could decide how to carry out their work and whether they learned new things); and social support (the degree to which subjects were satisfied with the support received from supervisors or colleagues). Persons who have high demands, low control and low social support are considered to be at risk of adverse health outcomes. For all measures, subjects were asked to indicate their workplace exposure as never, occasionally or half or most of the time.
Individual risk factors
The baseline and 12-month questionnaires also included sections to assess aspects of physical complaints, illness attitudes and behaviours, levels of psychological distress and fatigue, and other regional pain symptoms which are known to predict the onset of chronic widespread pain. These were (i) the Somatic Symptom Checklist [19], (ii) the Health Anxiety and Illness Behaviour subscales of the Illness Attitude Scales (IAS) [20], (iii) the General Health Questionnaire (GHQ) [21] and (iv) the Fatigue Questionnaire [22] (Appendix 1). Subjects free of chronic widespread pain may nevertheless report some pain at baseline. Because these subjects may be more likely to report new chronic widespread pain at follow-up, we also examined reports of any pain at baseline. Specifically, we examined reports of low back, shoulder, knee and forearm pain.
Statistical analysis
Those subjects who were free of chronic widespread pain and who provided complete data at baseline and follow-up were included in the analysis. We examined the associations between (i) exposures at baseline and new-onset chronic widespread pain at 12 months and (ii) exposures at 12 months and new-onset chronic widespread pain at 36 months.
To examine the relationship between workplace mechanical factors and new-onset chronic widespread pain, the referent group was categorized as those who reported never being exposed. In the same way, those who did not report potentially harmful work-related postures or workplace environment exposures were categorized as the referent group.
The distribution of the baseline psychosocial scale scores was not Gaussian. Thus, subjects scoring zero on the GHQ, Fatigue and Somatic Symptom scales were classified as the referent group and the remaining subjects were dichotomized to produce two approximately equal-sized groups. For the subscales of the IAS, the subjects were divided into three equal-sized groups on the basis of the distribution of their scale scores.
Generalized estimating equations [23], which are used to examine repeated-measures data and which adjust for intrasubject correlation, were employed to examine the association between scoring in the middle and highest categories and having new chronic widespread pain, compared with scoring in the lowest category. Domain-specific models examined the relative contribution of predictors with outcome for work-related mechanical load and posture, workplace environment, individual psychosocial factors and baseline pain status. Those risk factors associated with new chronic widespread pain (i.e. those factors with a statistically significant risk, a risk 1.5 or more or a risk of 0.67 or less) were entered into a multiple logistic regression model to examine the relative contributions of exposures to symptom onset. All analyses were adjusted for age and gender. Results are expressed as relative risks (RR) with 95% confidence intervals (CI). All analyses were conducted using the Stata statistical software (StataCorp, 2001).
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Results |
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Of the 978 subjects who completed both follow-ups, 675 (69%) reported being employed at baseline and 572 (85%) provided job information. At 36 months, 613 (63%) reported being in employment and 525 (86%) provided job information. A total of 81 subjects reported new-onset chronic widespread pain at 12 months, of whom 66 responded at 36 months and were therefore able to provide a retrospective employment history. Of these, 45 (68%) reported being in employment and 40 (89%) provided job information. Of the 92 subjects with chronic widespread pain at 36 months, 61 (66%) were employed, of whom 55 (90%) provided job information.
Workplace mechanical and posture risk factors
A number of work-related risk factors predicted the onset of chronic widespread pain (Table 1). With regard to the manual handling activities that were examined, subjects who reported pushing or pulling heavy weights with one or two hands had an increased risk of developing symptoms during the follow-up period. A small number of work-related postures were also associated with symptom onset. Thus, subjects who reported repetitive movement of the wrists were almost twice as likely to report new symptoms at follow-up. However, the highest risk was conferred by kneeling, which was associated with a more than doubling of risk.
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Discussion |
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Although there are no other studies of which we are aware which have prospectively examined the association between work-related trauma and the onset of chronic widespread pain, one recent study has examined retrospective reports of physical trauma in patients with fibromyalgia [24]. Persons with fibromyalgia were significantly more likely to report physical trauma (fracture, surgery or injury at work) when compared with age- and sex-matched controls without fibromyalgia. The predictive value of work-related factors in the onset of regional pain syndromes such as low back pain [11] and shoulder pain [12] has been demonstrated.
In the current study, kneeling and pushing or pulling heavy weights were important risk factors for new-onset chronic widespread pain. As this study was conducted amongst the general population, the range of occupational groups was wide and it was not possible to identify what activities were performed in specific occupations. We did, however, have information on social class. Factors such as kneeling, pushing or pulling weights and lifting or carrying weights tended to be more commonly reported in those performing manual or unskilled occupations (social classes IIIMV). Adjusting for social class only slightly attenuated the results and made no difference to the final model.
Symptom onset was also independently predicted by repetitive wrist movements. It may be hypothesized that the increased association with repetitive wrist movements was due to keyboard work in white-collar workers. However, repetitive wrist movements were more commonly reported in those from lower rather than higher social classes. Furthermore, in the univariate analysis, typing was not found to predict symptom onset at follow-up.
We have previously shown similar associations with forearm pain [13], which has a similar multifactorial aetiology. In particular, we reported that new-onset forearm pain was predicted by repetitive arm movements and dissatisfaction with support from colleagues, while the strongest predictor was a high score on the illness behaviour scale. However, forearm pain did not occur in isolation and we hypothesized that it may be part of a wider pain syndrome. This hypothesis is supported by the present findings, in which, of the regional pain syndromes examined, forearm pain was one of the strongest predictors of new-onset chronic widespread pain.
When examining the present results there are a number of methodological issues that need to be considered. First, we assessed pain state at three time points: baseline, 12 and 36 months. We made no attempt to identify what had happened in the intervening months and it is likely that we missed new cases. Population studies indicate that the majority of persons who develop chronic widespread pain will have symptoms that persist over many years [25, 26]. It therefore seems unlikely that we would have missed many new-onset cases that had resolved in the intervening months. Nevertheless, our interest lay not in reporting the prevalence of new pain onset over the study period but in those factors that predicted pain onset. That we may have missed cases does not affect the internal comparisons between predictors and outcome in our study. However, it is likely that we may have underestimated the strength of the relationships.
A further issue in the present study is that subjects were asked to report job exposures over the preceding 36 months retrospectively. However, the degree to which the data are subject to recall bias seems minimal. At the time of providing job information, the majority of subjects (95%) reported being in the same employment they had had when completing the baseline questionnaire 36 months previously. Although exposures may have changed within jobs over time, this seems unlikely to be the case for the majority of subjects. Nevertheless, it could be argued that those subjects who had chronic widespread pain at that time may have been more likely to recall adverse work-related exposures, such as lifting heavy weights and working postures, when compared with those who did not have chronic widespread pain. However, it has been reported recently that retrospective recall of work-related mechanical and environment exposures amongst subjects with low back, neck or shoulder pain after a period of 25 yr was satisfactory [27]. That study demonstrated that, although there was a degree of differential misclassification, a comparison of the risk estimates using data collected at the time of the original survey and that recalled 25 yr later indicated that the influence was limited. It therefore seems unlikely that differential recall of work exposures 3 yr previously in subjects with new-onset pain would explain our findings. In addition, we have found that only specific risk factors are associated with symptom onset. If subjects with chronic widespread pain were over-reporting workplace exposures, we would expect this over-reporting to be generalized across exposures.
Over the follow-up period we were unable to trace a total of 294 subjects who were no longer resident at the address held by their general practice. There were no significant differences in the baseline risk factor scores of those who did and did not participate. Nevertheless, to have any impact on the reported findings, one would have to hypothesize a different relationship between exposure and outcome in those who did not participate when compared with those who did. Although this remains a possibility, it seems unlikely.
As in any study, there may be residual confounding due to other unmeasured risk factors, for example anthropometric measures or individual levels of physical fitness. Few studies of chronic widespread pain have examined such factors. However, Makela and Heliövaara [28] found no relationship between body mass index and fibromyalgia. Other interesting relationships may exist between lifestyle (smoking, alcohol, diet) and leisure activities. Future studies will be required to examine such relationships and their relative predictive effects.
We have used the term new-onset chronic widespread pain to classify those who developed chronic widespread pain at follow-up. We did not necessarily identify those subjects with a first-ever episode, but those with a new onset after a symptom-free period. However, there may have been some degree of misclassification. It is likely that such misclassification would be random with respect to baseline exposures and would be likely to attenuate the estimates towards the null.
Intuitively, it seems likely that subjects with widespread (but not chronic) pain or those with chronic (but not widespread) pain would be more likely to develop new-onset chronic widespread pain at follow-up. However, our data suggest that this is not necessarily true. On further examination, by stratifying the results by pain status prior to symptom onset, we found that effects tended to be stronger in subjects free from previous pain when compared with those who reported other previous pain (data not shown). In the final multivariate model we have adjusted for previous pain status.
A final issue to consider is that we have not examined the modifying role of compensatory claims in symptom reporting. It has been proposed that legal action plays an important part in trauma-related pain and that any examination of the relationship between work-related factors and new pain onset must address the role of litigation [6]. We do not have information on compensatory claims and cannot address this question directly. However, in our study population, of those persons with new chronic widespread pain, 74% reported consulting their health-care provider with symptoms while only 2% reported any time off sick from work. Because the number of subjects who reported sickness leave was small, it seems unlikely that compensation would have been a motivating factor in their pain reporting. The issue of compensatory claims is likely to be less of a problem in the UK than in the USA. In a previous study of low back pain we found that, of those persons with new-onset low back pain, only 2% reported litigation with respect to their pain (unpublished data).
Previous authors have hypothesized that the role of injury in the onset of widespread pain syndromes is less important than that of the persons attitudes and beliefs about health [6]. The present findings lend some support to this hypothesis. While work-related factors did predict new pain onset, the strongest predictor was a high score on the illness behaviour scale, which is characterized by frequent visits to health-care practitioners for symptoms that affect everyday activities. Importantly, the majority of persons who reported exposure to adverse working conditions did not develop chronic widespread pain.
In conclusion, we have found only limited evidence to support the hypothesis that work-related low-level trauma is associated with the onset of chronic widespread pain. A number of factors precipitate symptom onset, including having other regional pain symptoms, reporting other somatic symptoms and, particularly, aspects of illness behaviour. However, we have not examined the effect of major trauma in the onset of chronic widespread pain and this will require further prospective studies.
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Appendix 1 |
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The Illness Attitude Scales (IAS). The nine IAS [20] assess attitudes and concerns about illness and health. Each scale includes three items, each scored from 0 to 4, providing a total score between 0 and 12. Individual scales assess worry about health, concern about pain, health habits, hypochondriacal beliefs, thanatophobia (fear of death), disease phobia, bodily preoccupation, treatment experience and effect of symptoms. A recent study [30] based on a principal components analysis demonstrated that the IAS measure two dimensions reflecting health anxiety and illness behaviour. The health anxiety subscale consists of 11 items (such as Are you worried that you may get a serious illness in the future?) and has a total score between 0 and 44 with a general population mean score of 9.1 (S.D. 6.9). The illness behaviour subscale consists of six items (such as Do your bodily symptoms stop you from working?) and has a total score between 0 and 24 with a general population mean score of 4.7 (S.D. 4.2).
The General Health Questionnaire. The 12-item version of the GHQ [12] was included as a measure of psychological distress. Each item has four possible responses, but for scoring these were dichotomized at the midpoint. Scores for individual items are summed to give a total score ranging between 0 and 12, high scores indicating higher levels of psychological distress.
The Fatigue Questionnaire [22] is an 11-item instrument developed for use in population studies to measure physical and mental aspects of fatigue. Each item has four response options: two options correspond to a score of 1 being assigned and two responses correspond to a score of 0. The scores for each item are summed to provide a total score between 0 and 11, high scores corresponding to high levels of fatigue.
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Acknowledgments |
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Conflict of interest
The authors have declared no conflicts of interest.
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References |
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