Study population
We investigated 130 Japanese women with menstrual pain (aged 20–45 years) who had not become pregnant in the past 3 months. The new edition of ICD-11 defined chronic pain as pain that lasts or recurs for longer than 3 months [
10]. In addition, a system review of chronic pelvic pain by the World Health Organization included menstrual pain [
11]. Therefore, we considered repeated menstrual pain over three menstrual cycles as broad-sense chronic pain. Participating women were randomly selected from among women registered with a web research panel. Respondents completed online self-administered questionnaires. Information collected included demographic characteristics, pain-related items, and psychosocial factors regarding patients with chronic pain. None of the respondents were in menses when they completed the questionnaires. Respondents had to complete each questionnaire before proceeding to the next questionnaire (i.e., there were no missing data). When we examined the association between respondents’ characteristics and severity of menstrual pain, we excluded 10 women with hormone therapy because hormone therapy may affect dysmenorrhea.
Development of the Japanese version of the IEQ-chr
As shown in the Additional file
1, the original Injustice Experience Questionnaire (IEQ) is a tool used to assess perceived injustice among injured individuals [
3]. The IEQ-chronic (IEQ-chr) is a modified version of the instrument for individuals with chronic symptoms; we used the Japanese version of this tool to assess perceived injustice among women with menstrual pain [
6]. IEQ-chr items are similar to those in the original IEQ, although the word “injury” was changed to “health status” [
6]. For example, “this scale was designed to assess how your injury has affected your life” (IEQ) became “this scale was designed to assess how your health status has affected your life” (IEQ-chr) [
3,
6]. Similar modifications were made in the Japanese version.
First, we confirmed the validity and reliability of the IEQ-chr-J, as the Japanese version has not previously been validated. We used confirmatory factor analysis (CFA) to evaluate structural validity. Variation and covariation among the 12 items were assessed using fit indices for a one-factor structure model, based on ≥9 points of modification indices produced by the statistical package. We calculated several fit indices: chi square (χ
2), chi square divided by degree of freedom (χ
2/df), the root mean square error of approximation (RMSEA) with 90% confidence intervals (CI), the standardized root mean square residual (SRMR), comparative fit index (CFI), and Tucker-Lewis index (TLI). A RMSEA value < 0.08 suggests a good fit, 0.08–0.10 a moderate fit, and > 0.10 a poor fit [
12]. SRMR values < 0.09 indicate a good fit [
12], and CFI and TLI values close to 0.95 are considered to indicate a relatively good fit [
13].
The concurrent validity of the IEQ-chr-J was calculated using Pearson’s correlation coefficients for the Pain Catastrophizing Scale (PCS), Hospital Anxiety and Depression Scale (HADS) anxiety subscale, HADS depression subscale, a numerical rating scale (NRS) for maximum/average menstrual pain, the Brief Pain Inventory (BPI) pain interference domain, and the IEQ-chr-J. The English version of IEQ-chr was significantly correlated with catastrophizing, depression, pain intensity, and pain disability [
6]. Therefore, we used PCS, HADS, NRS, and BPI pain interference domain to evaluate concurrent validity. The PCS comprises 13 items on Likert-type scales from 0 to 4, which are converted into 0–52 points to measure exaggerated pain [
14]. The PCS was previously used to examine the concurrent validity of the IEQ and IEQ-chr, and was highly correlated with IEQ, IEQ-J, and IEQ-chr scores [
6,
14,
15]. Each HADS subscale includes 7 items with responses on a 4-point Likert scale from 0 to 3, which convert into scores up to 21 for anxiety/depressive symptoms in the past week [
16].
The internal consistency of the IEQ-chr-J was investigated with the Cronbach’s alpha coefficient and test-retest reliability. In total, 112 of 130 respondents repeated the questionnaires after a 1-week interval; those answers were compared with the first round answers using intra-class correlation coefficients (ICC). The sample size for ICC analysis was determined based on the following assumptions. The null hypothesis (H0) was that the ICC would be 0.70, and the alternative hypothesis (H1) was that the ICC would be 0.90. With a power of 0.80, the minimum sample size required was 19 women. In the present study, our test-retest reliability sample (112 participants) satisfied the required sample size.
Association between perceived injustice and severity of dysmenorrhea
We performed multivariable regression analysis to examine the association between perceived injustice and severity of menstrual pain. We calculated variance inflation factor (VIF) to examine multicollinearity in regression analysis. Hair et al. suggested that VIF < 10 was indicative of inconsequential collinearity [
17]. The IEQ-chr-J was considered the independent variable, and we evaluated severity of dysmenorrhea by pain intensity and interference. We used log-transformed scores for the IEQ-chr-J, HADS anxiety, and HADS depression, as HADS anxiety and HADS depression scores were not normally distributed. We adjusted models for log (HADS anxiety), log (HADS depression), age, body mass index (kg/m
2, categorized in quartiles), existence of irregular menstrual period (self-reported menstrual cycle from 25 to 35 days; yes or no), and endometriosis diagnosed by physician (yes or no). The 10 excluded women with hormone therapy might have included women with menstrual pain induced by endometriosis, because those with menstrual pain tend to take hormonal therapy; therefore, we re-ran models including these 10 women as a sensitivity analysis.
Respondents described their maximum/average menstrual pain intensity in the past 3 months using a NRS from 0 (no pain) to 10 (worst pain imaginable). An NRS is recommended when pain intensity is evaluated in clinical trials [
18]. If respondents usually took analgesics for menstrual pain, they answered by assuming their pain intensity without taking analgesics. Pain interference was measured by the BPI pain interference subscale [
19]. The BPI assesses pain interference in seven daily activities on a scale from 0 (does not interfere) to 10 (interferes completely): general activity, mood, walking, work, relationships with others, sleep, and enjoyment of life. Mean values of the scores for these seven activities were used to evaluate pain interference severity [
20].
The significance level for the statistical hypothesis testing was set at p < 0.05. CFA was performed using IBM SPSS Amos version 25 (IBM Corp., New York, USA), and the sample size for ICC analysis was determined using PASS software version 13 (NCSS, Utah, USA). Other statistical analyses were performed using SAS version 9.4 (SAS Institute Inc., North Carolina, USA).