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Occupational Medicine Advance Access originally published online on September 20, 2005
Occupational Medicine 2005 55(8):612-617; doi:10.1093/occmed/kqi142
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© The Author 2005. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Disabling musculoskeletal pain and its relation to somatization: a community-based postal survey

Keith T. Palmer1, Michael Calnan2, David Wainwright2, Jason Poole1, Claire O'Neill2, Anna Winterbottom2, Chris Watkins2 and David Coggon1

1 MRC Environmental Epidemiology Unit, Southampton General Hospital, Tremona Road, Southampton, SO16 6YD, UK
2 MRC Health Science Research Collaboration, Department of Social Medicine, Canynge Hall, Bristol, BS8 2PR, UK

Correspondence to: Keith T. Palmer, MRC Environmental Epidemiology Unit, Southampton General Hospital, Tremona Road, Southampton, SO16 6YD, UK. Tel: +23 80 777624; fax: +23 80 704021; e-mail: ktp{at}mrc.soton.ac.uk


   Abstract
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 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflict of interest
 References
 
Background Somatization (a tendency to report distress from somatic symptoms) is a little studied, but potentially important, confounder and effect modifier in occupational studies of musculoskeletal disease.

Aims To assess the role of somatization as a risk factor for disabling regional pain.

Methods A questionnaire was mailed to 4998 subjects of working age. Questions were asked about chronic and disabling pain in the past 12 months affecting the arm, low back, knee or combinations of these sites. Distress from physical symptoms was assessed using elements of the Brief Symptom Inventory and mental well-being was assessed using the short-form 36 (SF-36). Associations were examined by modified Cox regression and expressed as hazard ratios (HRs) with 95% confidence intervals (CI).

Results Among 2632 responders, 24% reported chronic pain and 25% disabling pain at one or several sites. Risk of chronic or disabling pain increased strongly according to the number of somatic symptoms reported as bothersome. For example, the HR for chronic upper limb pain in those distressed by ≥2 somatic symptoms in the past 7 days versus none was 3.9 (95% CI 2.9–5.3), and that of disabling upper limb pain was 5.8 (95% CI 4.1–8.3). Similar patterns were found for the low back and knee, and there was a gradient of increasing risk according to the number of sites with disabling pain. In comparison, associations with SF-36 mental well-being score were weaker.

Conclusion Somatizing tendency should be evaluated as a possible confounder or effect modifier in studies of occupational risk factors for musculoskeletal pain.

Keywords      Arm pain; mental health; somatizing tendency; widespread pain


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflict of interest
 References
 
Musculoskeletal pain is a major cause of disability among adults of working age in developed countries. It is associated with occupational activities that physically stress tissues, but is also importantly determined by cultural and psychosocial influences. Thus, it occurs more frequently in people who perceive their work as entailing higher demands, lower decision latitude and less support from colleagues [1–8], and also in those with poorer personal mental health [9–11].

The mechanisms that underlie such associations are unclear, with many possibilities [12]. For example, ‘stress’ and distress may promote musculoskeletal pain by extending the duration of muscle tension, or altering the processing of nociceptive stimuli to intensify pain perception, or by undermining coping or modifying physical and behavioural responses in ways that promote disability.

In the absence of a specific understanding on mechanisms, researchers have used proxy measures to gauge the impact of psychosocial factors on outcome. Mood and vitality, usually assessed by the short-form 36 (SF-36) questionnaire mental health subscale [13], are the two aspects of personal mental health that have been most studied in this context, but they may not be the only ones that matter. In particular, it is possible that experience and report of musculoskeletal pain and associated disability for everyday activities (e.g. washing, dressing and doing household chores) is linked also to a generally heightened awareness of somatic symptoms and tendency to attribute such symptoms to physical illness [14]. In support of this, two studies have found that ‘somatizing tendency’ of this sort is a risk factor for long-lived widespread pain [15,16]. However, little is known about its relation to more common, localized complaints such as low back pain and upper limb pain.

If somatizing tendency were a strong risk factor for common musculoskeletal disorders, acting independently of other aspects of personal mental health, this would have important implications for their investigation and prevention. When assessing the causal role of occupational activities, somatizing tendency would need to be considered as a potential confounder and/or effect modifier, in addition to the other established psychosocial risk factors.

As a first step in exploring this possibility, therefore, we used data from a community-based cross-sectional survey to assess the relation of musculoskeletal pain and disability to somatizing tendency, and to compare associations with those for low mood as measured by the SF-36 questionnaire.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflict of interest
 References
 
A questionnaire was mailed to 4998 subjects aged 25–64 years who were randomly selected from the age–sex registers of five general practices in north Somerset, England. The practices were chosen to include a mixture of large and small practices in urban and rural areas. Non-responders were sent a reminder postcard after 3 weeks and, if necessary, a repeat questionnaire 3 weeks later. The study received approval from the NHS South West Local Research Ethics Committee.

The questionnaire inquired, among other things, about: pain lasting a day or longer in the past 12 months in the upper limb, the low back (both illustrated on a mannequin) and the knee; the duration of such pains; the difficulty they caused in sleeping, getting dressed and doing everyday chores; employment status; depressive symptoms and tendency to report somatic symptoms. These last two features were assessed using the mental health section of the SF-36 [13] and elements of the somatic subscale of the Brief Symptom Inventory (BSI) [17], which is a self-reported measure of distress and psychopathology comprising items on bothersome nausea, faintness, dizziness, weakness, numbness in the body, chest pain and breathing difficulties during the past 7 days.

After inspection of simple contingency tables, two new response variables were created for (i) ‘disabling pain’—defined as pain that made it difficult or impossible to sleep and get dressed and do daily chores, and (ii) ‘chronic pain’—pain that had been present for >6 months during the past 12 months. In addition, subjects were categorized (i) in approximate thirds of the distribution of SF-36 mental health score, and (ii) according to the number of BSI somatic symptoms (out of 6) reported as bothering them ‘extremely, quite a bit, or moderately’ in the past 7 days. To facilitate direct comparison with percentiles of the SF-36 score, we also calculated a BSI score (out of 24) by summing responses to the six somatic questions on a scale of 0–4 (4 = extremely distressed) and by classifying subjects to thirds of its distribution.

All analyses were carried out using STATA software (version 7). The association of regional pain with levels of the two principal measures of psychological well-being were examined by modified Cox regression [18] and expressed as mutually adjusted hazard ratios (HRs) with associated 95% confidence intervals (CI). The baseline in these analyses was subjects free from pain during the past 12 months at the site or sites considered.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflict of interest
 References
 
Usable responses were received from 2632 of those mailed (response rate 53%), including 1963 (74.6%) who were in current employment. The non-responders included 73 subjects who had died or moved away (1.5%) and 149 subjects (3%) who provided an incomplete response. The response rate was broadly similar across practices, but in comparison with the age–sex distribution of the practice-registered patients and that of the local health authority area (North Somerset Primary Care Trust population of 100 000) women were somewhat over-represented (55% versus 50 and 51%, respectively), as were older subjects (47% aged >50 years versus 37–38%).

The respondents included 590 subjects who reported chronic pain and 589 who reported disabling pain during the past 12 months at one or several of the sites considered (Table 1). The prevalence of chronic pain varied by site from 7% for the knee to 14% for the upper limb; while the commonest site for disabling pain was the low back (19%).


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  		Table 1. Prevalence of pain in upper limb, low back and knee during the past 12 months, according to duration, severity and overlap

 
Table 2 shows the relation of somatization and SF-36 mental health scores to upper limb pain of various durations, and also to disabling upper limb pain. Risks tended to increase according to the number of somatic symptoms reported as bothersome, the association becoming stronger with increasing duration of pain and being especially strong for disabling pain. For example, the HR for chronic pain was 3.9 (95% CI 2.9–5.3) and that for disabling arm pain 5.8 (95% CI 4.1–8.3) in those reporting ≥2 somatic symptoms as extremely, quite, or moderately distressing in the past 7 days as compared with those untroubled by somatic symptoms over this period. The association with the SF-36 score, although apparent for disabling and chronic pain, was weaker than that for somatization. Thus, for example, being in the highest versus the lowest band for the SF-36 score was associated with a HR of 1.6 for chronic pain and 2.2 for disabling pain.


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  		Table 2. Relationship of somatization and mental health scores to the prevalence of upper limb pain during the past 12 months

 
For the low back, a similar set of associations was evident—the risks of chronic low back pain and disabling low back pain were increased in those reporting ≥2 bothersome somatic symptoms versus none, and greater in this band than in the worst versus the best band of SF-36 score (Table 3).


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  		Table 3. Relationship of somatization and mental health scores to the prevalence of low back pain during the past 12 months

 
At the knee this pattern was even clearer (Table 4). There was little or no association between the SF-36 score and chronic or disabling knee pain, but there was a strong association of these outcomes with the number of bothersome somatic symptoms reported (HRs for ≥2 versus no symptoms are 3.9 and 5.6, respectively).


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  		Table 4. Relationship of somatization and mental health scores to the prevalence knee pain during the past 12 months

 
The foregoing analyses were based on all subjects, including the unemployed. However, when adjustment was made for employment status, associations with the top tier by somatization and SF-36 scores were little changed. For example, the HR for disabling pain in those with ≥2 bothersome somatic symptoms versus none decreased marginally from 5.8 to 5.4 for the upper limb, from 2.3 to 2.2 for the low back and from 5.6 to 5.0 for the knee.

We also examined the relation of somatization and mental health scores to the number of sites where disabling pain was reported (Table 5). A gradient of increasing risk was found with both scales, but this was stronger for the somatization scale, and especially in relation to report of disabling pain at multiple sites. Thus, the HR for disabling pain at two or three sites was increased nearly 6-fold in those with ≥2 as compared with no bothersome somatic symptoms in the past 7 days; but much less (2.3-fold) in those in the worst versus the best third of the SF-36 scale.


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  		Table 5. Relationship of somatization and mental health scores to the prevalence of disabling pain, according to the number of sites involved

 
When the BSI score was categorized in thirds, rather than according to the number of distressing somatic symptoms reported, it still showed stronger associations with chronic and disabling pain than did the SF-36 score categorized using the same percentiles. Thus, for example, the HR in the highest versus the lowest band was 4.7 (95% CI 3.1–7.0) for chronic arm pain, 12.1 (95% CI 6.1–24.1) for disabling arm pain, 4.3 (95% CI 2.5–7.5) for chronic knee pain and 20.5 (95% CI 6.3–66.6) for disabling knee pain.

Finally, we examined the association with disabling pain separately for each of the component items of the BSI somatization subscale, in case the pattern was unduly influenced by a particular symptom (Table 6). Five of the six items (all but pains in the heart or chest) were significantly associated with report of disabling regional pain, with HRs ranging from 1.9 (faintness or dizziness) to 5.0 (feeling weak in parts of the body).


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  		Table 6. Relationship of disabling pain to component questions of the somatization subscale of the BSI

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflict of interest
 References
 
Our findings suggest a clear, strong and consistent association between reports of multiple bothersome somatic symptoms (nausea, faintness, dizziness, weakness, numbness in the body, chest pain and breathing difficulties) and reports of long-lived and disabling pain in the upper limb, low back, knee and at combinations of these sites. These associations were observed after adjustment for the SF-36 mental health score, were stronger and more consistent than those by the band of the SF-36 score, existed after adjustment for age, sex and social class and were evident for all of the component questions used to measure somatization. They existed independently of employment status, although unemployment itself was modestly associated with report of disabling pain.

The response to the questionnaire was incomplete (53%), and it is possible, given the focus of study, that respondents over-represented those with musculoskeletal pain. However, our estimates of association by level of somatization would only be biased if those associations differed by response status. We have no reason to expect this and the consistent gradients by level of somatization for every site and the even stronger associations with long-lived and disabling illness argue against this explanation.

Although there have been rather few studies of the relation between somatization and disabling pain, our findings are consistent with those in a British cohort of workers, among whom somatic distress predicted incident forearm pain after adjusting for physical work demands [19], a Danish industrial cohort in which it predicted incident neck and shoulder pain after painstaking allowance for ergonomic exposures [20], a population survey in which an association existed with chronic widespread pain [21], a follow-up study of patients with acute and subchronic low back pain, in whom somatic distress predicted a delayed recovery [15], and a cross-sectional survey of vocational drivers with low back pain [22].

Assuming the association to be genuine, a pressing concern is to understand the direction of causation. One possibility is that subjects who have a low threshold for being bothered by somatic symptoms more readily report distressing regional and widespread pain, or more readily experience them. One definition of somatization, in keeping with this hypothesis, is ‘the predisposition to amplify physiological sensations or to misclassify symptoms of emotional arousal’ [23]. Distinguishing the report from the experience of pain is clearly not straightforward, but reporting at a different threshold need not be a conscious deliberate act, and beliefs could even be mediated through or underpinned by physiological alterations [24]. Nonetheless, the trait would predispose to the outcome, rather than the reverse. A second, different (but not mutually exclusive) possibility is that the experience of chronic pain exerts a sensitizing effect that heightens bodily awareness of physiological events [14]—that is, the experience of living with chronic disabling pain leads to somatization.

Our data are cross-sectional and do not enable us to distinguish between these possibilities. Prospective study is needed to determine whether a high somatization score predicts the development of disabling pain in those initially pain free or whether a low somatization score favours recovery during follow-up. A longitudinal study would be useful also in assessing how stable somatizing tendency is, and how amenable to therapeutic interventions [25–27]; and in determining whether distress is more predictive of outcome than occupational physical demands. We intend making this our next focus of inquiry.

These baseline observations suggest, however, that somatization is an important (hitherto neglected) risk factor to evaluate in patients with chronic and severe pain, relevant both in workers and the unemployed. In occupational studies it should be assessed for its potential to confound or modify the relatively weaker associations that are typically found with physical and psychosocial risk factors in the workplace.


   Conflict of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest
References
 
None declared.


   Acknowledgements
 
This project was funded by the MRC Health Science Research Collaboration, based in the Department of Social Medicine, University of Bristol. We thank the 5 general practices from Avon that allowed us to approach their patients.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest
 References
 

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