Introduction

Low back pain (LBP) is a common musculoskeletal health problem. Approximately 80% of people experience LBP at some point during their lifetime [1]. Globally, LBP is the leading cause of years lived with a disability [2]. Therefore, LBP is a significant public health issue.

The etiology of LBP is multifactorial. Individual, physical, and psychosocial factors such as depression are associated with a risk of developing LBP [3–5]. Notably, depression is also a predictor of chronicity [4, 6]. Quality of life is lower in patients with chronic LBP and depression compared to those without depression [7]. Direct health care costs are higher in LBP patients with depression compared to those without depression [8]. Therefore, assessment of depressive symptoms in patients with LBP may be important for predicting prognosis and choosing treatment options in clinical care settings.

Somatisation, which often coexists with depression [9], is defined by Lipowski as “a tendency to experience and communicate somatic distress in response to psychosocial stress and to seek medical help for it” [10]. A systematic review pointed out that most previous studies of pain have used questionnaires on somatic complaints to assess somatisation, which may not account for the full range of the concept of “somatisation” as defined by Lipowski and recommended to use the term “multiple physical symptoms” rather than “somatisation”[11]. Nevertheless, there is some evidence that a higher somatic symptoms burden or “somatisation” is playing some role in LBP. One study reported that the comorbidity of somatic symptoms predicted the development of persistent LBP in the individuals with mild LBP [12]. Another study found that somatisation at baseline, assessed based on the number of medically unexplained symptoms, predicted treatment outcome in patients with LBP [13]. Although depression has been studied in association with LBP, currently, the role of somatic symptom burden in chronic LBP (CLBP) has not been fully explored.

Several self-reported questionnaires have been used to assess common somatic symptoms. A recent systematic review reported that the Patient Health Questionnaire-15 (PHQ-15) and the 12-item Symptom Checklist-90 somatization scale are the most suitable questionnaires for use in large-scale studies [14]. These questionnaires survey relevant symptoms, are relatively short, and have well-established psychometric properties. The Somatic Symptom Scale-8 (SSS-8) was developed as an abbreviated 8-item version of the PHQ-15 [15]. The aim of the present cross-sectional study was to examine whether somatic symptom burden assessed using the SSS-8 is associated with health-related quality of life (HRQoL) in Japanese individuals with CLBP independent of depressive symptoms. We hypothesized that a higher somatic symptom burden is associated with a lower HROoL for patients with CLBP.

Materials and methods

Results

The characteristics of the participants are shown in Table 1. The mean age of the participants was 44.5 ± 11.2 and 48% were women. The PHQ-2 score was 0 in 1,576 (51%) participants, 1 in 632 (20%), and 2 in 892 (29%). In these individuals with chronic LBP, 20.6% answered they had chronic knee pain. The mean EQ-5D score was 0.78 ± 0.18, and this score decreased as the PHQ-2 score increased. Mean SSS-8 score was 9.67 ± 6.68. As the PHQ-2 score increased, the proportion of individuals with a very high SSS-8 score increased.

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Table 1. Characteristics of the participants with chronic low back pain.

https://doi.org/10.1371/journal.pone.0193208.t001

SSS-8 total score and EQ5D score were significantly negatively correlated (Spearman correlation coefficient = -0.55, p-value<0.0001). The results of the regression models are shown in Table 2. In the crude model, SSS-8 categories were significantly associated with EQ-5D. EQ-5D scores were lower in those who had higher SSS-8 scores. With the adjustment for PHQ-2, regression coefficients for SSS-8 were attenuated greater than 10%, but the SSS-8 categories were significantly associated with EQ-5D scores independent of the PHQ-2 score. The PHQ-2 score was also independently associated with EQ-5D. Adjustment for demographic, lifestyle and other individual variables did not change the estimates essentially. In the final multiple model adjusted for the number of the comorbid conditions, the regression coefficients for SSS-8 were further attenuated but the association was still significant. Individuals with CLBP and a higher SSS-8 score had lower EQ-5D scores (regression coefficient β = 0.040 for SSS-8 high vs. very high, and β = 0.218 for non to minimal vs. very high, p-trend<0.0001).

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Table 2. Association between EQ-5D and SSS-8 from linear regression models.

https://doi.org/10.1371/journal.pone.0193208.t002

The results of the stratified analysis by sex, age and sex-age group are shown in Table 3. The results were consistent with those in the overall sample. Although the p-values for high SSS-8 in men aged ≥50 year and women aged ≥50 years were >0.05, this could be due to the small sample size in these groups. No significant interaction by age, sex or age-sex categories was found.

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Table 3. Association between EQ-5D and SSS-8 from linear regression models by sex and age.

https://doi.org/10.1371/journal.pone.0193208.t003

Discussion

We found that a higher somatic symptom burden was associated with lower EQ-5D scores independent of depressive symptoms and the number of comorbid diseases in Japanese individuals with CLBP. The results were consistent across sex and age. The difference in EQ-5D between SSS-8 high vs. very high was 0.04 and was 0.22 for SSS-8 non to minimal vs. very high among all participants. Yoshizawa et al. reported that the minimal clinically important change (MCIC) of EQ-5D in Japanese with chronic noncancer pain was 0.10 [30]. Soer et al. reported a MCIC of EQ-5D as 0.03 in patients with chronic LBP in Netherland [31]. According to these studies, the differences in EQ-5D between SSS-8 groups would be clinically meaningful. Our result suggests that somatic symptom burden is important for the quality of life of individuals with CLBP.

Somatisation is defined by Lipowski as “a tendency to experience and communicate somatic distress in response to psychosocial stress and to seek medical help for it” [10]. Somatisation, depression, and anxiety are frequent mental disorders in primary health care, and often overlap in patients [9]. Depression has been studied extensively, and it is established that depression is a predictor of the chronicity of LBP [4, 6]. Somatisation has also been studied for LBP and other pain research. Nonetheless, the assessment of “somatisation” is problematic. Crombez et al. noted in their systematic review that most previous studies of pain have used questionnaires on somatic complaints to assess somatisation, which may not account for the full range of the concept of “somatisation” as defined by Lipowski, such as whether symptoms were not explained by pathological findings and whether the individual was attributing the symptom to physical illness and seeking medical help for it. The authors recommended using the term“multiple physical symptoms” rather than “somatisation”[11]. The authors also warn that somatisation scores could be elevated artificially by the somatic symptom that is the primary complaint, such as LBP.

Nevertheless, there is some evidence that somatic symptoms burden or “somatisation” is associated with HRQoL. Somatisation, depression, and anxiety can coexist, but each of them may have an independent role in HRQoL. Lowe et al. reported that depression, anxiety, and somatisation were each independently associated with Short-Form General Health Survey (SF-20) scores in primary clinic patients, although the effect size of each was only small to moderate [9]. In this study, somatisation was defined as a PHQ-15 score of ≥ 15. The authors discussed that using this high threshold would reflect somatisation and not just somatic symptom severity. A review of nine population-based studies found that total somatic symptom scores were associated with healthcare use and predicted health status independent of depression, anxiety, and a number of general medical illnesses [32]. We adjusted for the counts of chronic conditions as in this review study to try to consider the comorbid disease and to reflect how much individuals were bothered by somatic symptoms, which is one nature of somatisation. Our study results were consistent with these previous studies, showing that somatic symptom burden is associated with HRQoL independent of depression and the number of comorbid diseases in a group of individuals with CLBP.

Previous studies suggest that somatisation has a role in the outcomes of LBP [6]. Matsudaira et al. reported that the comorbidity of somatic symptoms, as assessed using the brief job stress questionnaire (BJSQ), was associated with the development of persistent LBP in urban Japanese workers with mild LBP [12]. In patients with LBP that were treated by chiropractors, somatisation at baseline that was assessed using the Four Dimensional Symptom Questionnaire was associated with pain intensity, functional status, and perceived recovery [33]. In hospital patients with LBP, baseline somatisation based on the number of symptoms for which physicians could find no clear cause assessed using the Screening for Somatoform Disorders Questionnaire, was correlated with HRQoL (MOS 36-Item Short-Form Health Survey (SF-36) scores) at follow-up, and was inversely associated with ≥ 50% reduction in pain one-year after surgical or conservative treatment [13]. These studies suggest the possibility that somatic symptom burden would lead to a worse LBP outcome and result in a lower HRQoL. However, these studies do not always adjust for depression. Depression is a risk factor for the onset and chronicity of LBP [4, 6], and somatisation often coexists with depression. Therefore, it might be difficult to determine whether somatic symptom burden predicts LBP outcomes independent of depression.

There is another possible explanation for our study results. Individuals with LBP often suffer from multisite pain or other musculoskeletal disorders [34–36], which could result in higher somatic symptom assessment scores. Previous studies show that as the number of pain sites increase, functional ability or HRQoL decreased [35, 36]. Although we did not assess pain in another body site other than LBP or knee pain, 43% of the participants answered that they were bothered at least somewhat by “pain in arms, legs, or joints” in the SSS-8. In addition, studies have showed that chronic conditions such as fibromyalgia, chronic LBP, irritable bowel syndrome, temporomandibular joint disorder, interstitial cystitis, chronic fatigue syndrome, and headache are linked by central sensitization and called functional somatic syndromes (FSS) [37–39]. The FSS conditions may overlap in one patient [39]; therefore, it is possible that individuals with more disabling chronic LBP suffered from other musculoskeletal disorders and/or FSS symptoms that decreased HRQoL in these individuals.

A strength of the present study is its large sample size with information on relevant covariables. The participants were not recruited in clinical settings; therefore, the possibility of selection bias due to seeking treatment should be low. The independent variable and the outcome were assessed using the validated tools, which would reduce the risk of classification bias. However, there are a few limitations of the present investigation. First, depression was assessed using only two questions; thus, misclassification is possible, which would be non-differential. Second, measures of anxiety were not available. Anxiety also often overlaps with somatisation and is associated with HRQoL [9]; thus, residual depression and anxiety may have confounded the results. This could have contributed to the overestimation, but the magnitude is unknown. Third, we did not collect the information on the pain in other sites than LBP and knee pain. Finally, the participants of the present study were recruited online and thus our results are not necessarily representative of the Japanese population.

In conclusion, somatic symptom burden might be an important factor for HRQoL in individuals with CLBP independent of depressive symptoms and the number of chronic conditions.

Supporting information