Participants
We hypothesized that other musculoskeletal pain could be associated with new onset of LBP after natural disasters. A panel study was therefore conducted with the GEJE survivors living in the severely damaged coastal areas, including Ogatsu and Oshika areas in Ishinomaki City, and Wakabayashi Ward in Sendai City, Miyagi prefecture, Japan. The surveys began three months after the GEJE and were administered every six months. The first study population included residents registered in the Residential Registry of the Ogatsu and Oshika areas and survivors living in prefabricated housing in the Wakabayashi Ward. From November 2013 to February 2014, three years after the GEJE, the residents (aged 18 years or over) who were registered in the Residential Registry of Ogatsu and Oshika areas, and the survivors who had participated in the previous survey in Wakabayashi Ward, were recruited (n = 6,396). Self-reported questionnaires and informed consent forms were mailed to these residents and a 44.6% (2,853/6,396) response rate was obtained. Among those, the participants who already had LBP were excluded (n = 663). The remaining participants were followed from November 2014 to February 2015, four years after the GEJE, and an 81.4% (1,782/2,189) follow-up rate was obtained for this period. Finally, a total of 1,782 participants were included in this study (Fig. 1). This study was approved by the institutional review board of our university (approval number: 201192) and was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.
Musculoskeletal pain
Musculoskeletal pain was assessed using self-reported questionnaires based on Comprehensive Survey of Living Conditions. The questions were: “Have you had symptoms in the last few days? If yes, please place a mark next to all your symptoms.” The examples of choices were palpitation, dizziness, diarrhea, and musculoskeletal symptoms such as low back, hand and/or foot, knee, shoulder, and neck pain [14]. The outcome of interest was new-onset LBP, which was defined as LBP absent at three years (first period), and present at four years after the GEJE (second period). The main predictor was musculoskeletal pain except for LBP at the first period which included hand and/or foot, knee, shoulder, and neck pain. Musculoskeletal pain except for LBP was categorized into three groups according to the number of painful sites (0, 1, ≥ 2).
Covariates
The following variables were included in the analysis because they were considered potential cofounding factors in previous reports [13, 15]: sex, age, body mass index (BMI), living area, smoking habits, drinking habits, comorbid conditions (hypertension, diabetes mellitus, myocardial infarction, and cerebral stroke), working status, walking time per day, living status, subjective economic conditions, psychological distress (≥ 10/24 on the Kessler Psychological Distress Scale) [16], sleep disturbance (≥ 6/24 on the Athens Insomnia Scale) [17], and social isolation (< 12/30 on the Lubben Social Network Scale) [15].
Statistical analysis
Univariate and multivariate logistic regression models were used to calculate odds ratios (OR) and 95% confidence intervals (95% CI) for new-onset LBP according to the number of musculoskeletal pain sites except for LBP in the first period. Variables included in the analysis were sex, age (< 65 or ≥ 65, years), BMI (< 18.5, 18.5 to < 25, ≥ 25, or unknown), living area (Ogatsu, Oshika, or Wakabayashi), smoking habits (nonsmoker, smoker, or unknown), drinking habits (non-drinker, < 45.6 grams of alcohol per day, ≥ 45.6 grams of alcohol per day, or unknown), comorbid conditions (absence or presence of each comorbid conditions), working status (unemployed, employed, or unknown), walking time per day (< 30 min, 30 min to < 1 h, ≥ 1 h, or unknown), living status (living in the same house as before the GEJE, prefabricated housing, new house, others, or unknown), subjective economic conditions (normal, a little bit hard, hard, very hard, or unknown), psychological distress (absence, presence or unknown), sleep disturbance (absence, presence, or unknown), and social isolation (absence, presence, or unknown). We further divided the participants into subgroups by age (< 65 or ≥ 65 years) or sex (male or female). ORs and 95% CIs for new-onset LBP were calculated in the same manner. For the stratified analysis, multiplicative interaction between musculoskeletal pain except LBP and age or sex were tested using the Wald test. In addition, the ORs and 95% CIs for new-onset LBP according to each musculoskeletal pain except LBP in the first period were evaluated. We included the same variables (Model 1) and added each musculoskeletal pain such as hand and/or foot, knee, shoulder, and neck pain as covariates (Model 2). All statistical analyses were performed using SPSS 24.0 (SPSS Japan Inc, Tokyo, Japan). A p value of < 0.05 was accepted as statistically significant.