This longitudinal study recruited patients with SSRD aged 18 to 84 years who visited the Department of Psychosomatic Medicine, Toho University Medical Center Omori Hospital between November 10, 2023, and August 21, 2024.

The exclusion criteria were: (1) schizophrenia spectrum disorders and other psychotic disorders; (2) dementia (such as Alzheimer’s, vascular, Parkinson’s disease, and Lewy body dementia); (3) suicidal ideation; (4) Japanese was not the first language; and (5) patients who could not be accurately assessed because of missing or incorrect responses.

All patients responded to self-report questionnaires at baseline and at the 6-month follow-up. Participants took approximately 15 min to respond to the questionnaire forms. Participants’ demographic characteristics such as age, sex, alcohol consumption history, smoking history, educational duration, disease duration, treatment history, and comorbid symptoms were assessed. Participants’ physical symptoms were reassessed approximately 6 months after the initial assessment, and the magnitude of change was evaluated. In addition, participants underwent evaluations of the subjective outcomes of symptoms. All participants continued to receive outpatient treatment during this period.

The primary endpoint was assessed using the Patient Global Impression of Change (PGIC) [12], whose component questions were set as the anchor questions. The secondary endpoint was assessed using the subscale pertaining to physical function from the Multidimensional Evaluation Scale for Patient Impression Change (MPIC) [13], whose component questions were set as the anchor questions, similar to the primary endpoint. Two experts unanimously diagnosed all the participants with SSRD, whose diagnostic criteria were based on the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision [1].

Changes in participant physical symptoms were assessed using the SSS-8. The SSS-8 is a scale of 8 general symptoms including “chest pain or shortness of breath” and “headaches”, which are rated along a 5-point scale ranging from “0 (not at all)” to “4 (very much)”; higher scores indicate more severe somatic symptoms. The Japanese version of the SSS-8 [14] has been validated for linguistic, psychological, and internal consistency [15]. Cronbach’s alpha for the Japanese version of the SSS-8 was found to be 0.86 [15].

The PGIC is scored using a 7-point Likert-type scale from 1 = “Very Much Improved” to 7 = “Very Much Worse”. As the purpose of this study was to identify clinically definite changes in physical symptoms, a score of 1 or 2 on the PGIC was designated as a clear improvement in physical symptoms.

The MPIC is an expansion of the PGIC [12], and includes 7 additional domains (Pain, Mood, Sleep, Physical Functioning, Cope with Pain, Manage Pain Flare-ups, and Medication Effectiveness), and each of the 8 domains can be used as an independent scale [13]. Cronbach’s alpha value was 0.89 [13, 16]. The MPIC is also scored along a 7-point Likert-type scale from 1 = “Very Much Improved” to 7 = “Very Much Worse”, and the Japanese version is evaluated in the same way [16]. In this study, a clear improvement in physical function was defined as a score of 1 or 2 on the MPIC Physical Functioning scale.

Participants also underwent initial assessment for anxiety and depressive symptoms [17] as these psychological conditions can co-exist with SSRD. The Hospital Anxiety and Depression Scale (HADS) [18] consists of anxiety and depression scales. Each scale is scored from 0 to 21, with higher scores indicating more intense symptoms. The Japanese version of the HADS was validated for reliability and validity [19] and factor structure [20] by a Japanese study. Cronbach’s alpha coefficients for the HADS-A and HADS-D were 0.81 and 0.76, respectively [21].

The demographic characteristics of the participants in the improved and non-improved groups were compared. In the analysis of the primary endpoint, given the sample size and expectations, sex, marital status, smoking and drinking habits, and medical therapy (antidepressant and/or benzodiazepine use) were compared using the chi-squared test. Medical therapy: antipsychotic use was compared using the Fisher exact test. Given the normality and variance of the data, Student’s t-test was used to compare the age and scores on each questionnaire, and the Mann–Whitney U test was used to compare educational history and disease duration.

In the analysis of the secondary endpoint, sex, marital status, drinking habit, and medical therapy (antidepressant and/or benzodiazepine use) were compared using the chi-squared test. Medical therapy (antipsychotic use) and smoking habit were compared using the Fisher exact test. Given the normality and variance of the data, Student’s t-test was used to compare the age and scores on each questionnaire, and the Mann–Whitney U test was used to compare educational history and disease duration. Normally distributed data were presented as the mean and standard deviation (SD), and non-normally distributed data as the median and interquartile range.

Thereafter, receiver operating characteristic (ROC) analyses [22] were used to calculate the MCID values of the SSS-8 for each of the primary and secondary endpoints to identify those with clear improvements in physical symptoms. The MCID cutoff point was identified by calculating the Youden index [(sensitivity + specificity) – 1] to maximize sensitivity and specificity [23].

EZR (“Easy R”) software program, version 1.54 (Saitama Medical Center, Jichi Medical University, Saitama, Japan) [24] was used for all analyses, and two-tailed p-values 0.05 were considered statistically significant.

In this study, we calculated the MCID value of the SSS-8 to develop criteria for assessing the clinical outcomes of patients with SSRD. The optimal SSS-8 MCID for the PGIC and MPIC physical function subscale was –6 points. Our results suggest that the SSS-8 possesses potential for assessing the clinical outcomes of somatic symptoms. A reduction of 6 or more points in the SSS-8 score was interpreted as a clinically meaningful change.

Most patients with somatic and related syndromes have a high degree of functional disability [25], although no abnormalities are found upon biological examination [3]. The SSS-8 is used as an adjunctive screening tool for diagnosis [1] and determining the clinical severity [8, 10]. According to a German study, the SSS-8 represents five levels of severity in 4-point increments [8]. Assessing disease severity is important for patients and clinicians, as each stage of the illness indicates different clinical symptoms and numbers of health care visits. A previous study showed that a shorter educational duration was associated with lower treatment responsiveness [26], which is consistent with the results of the present study. Additionally, studies have shown that smokers are more prone to pain [27], somatization, and psychiatric problems [28], which may explain the large number of smokers in the non-improvement group for physical function.

This study had some limitations. First, numerous patients had anxiety and depressive symptoms, making it difficult to separate the effects of the comorbidities in SSRD. Second, because of the lack of established treatment for SSRD, we could not eliminate the possibility that the lack of uniformity in the treatment approach could potentially have affected the results. Third, since we did not consider differences in disease severity, we could not determine whether the significance of a change in 6 or more points differed between participants with high and low SSS-8 scores. Fourth, the AUC values in this study indicated moderate diagnostic accuracy [29], but our interpretation of the longitudinal results was limited because the SSS-8 was developed as a screening instrument basically. Finally, although the participants exhibited characteristics of common SSRD such as a high female preponderance and longer chronic disease duration [1], the sample size was limited because of its single-center setting, and the participants’ educational level may have been higher than that of patients with general SSRD, limiting the generalizability of the results.

It would be meaningful for clinicians to visualize the clinical course of SSRD using the SSS-8 for quantitative evaluation. Since improvement in subjective symptoms is an important treatment goal for SSRD, a six-point or greater reduction in the SSS-8 scores shown in this study can be used as a clinical indicator. However, the results suggest that patients with SSRD may not perceive a clear improvement in their subjective symptoms when severity is improved by merely one level [8], reflecting excessive health consciousness among this population.

Since pharmacotherapy for SSRD has not been established, benzodiazepines may be used to mitigate physical symptoms by providing emotional relief, although there is a risk of dependence [30]. Even though the causal relationship is unknown, the persistence of physical symptoms in the non-improvement group may have promoted the use of benzodiazepines as symptomatic treatment.

This study could serve as a clinical benchmark for patients with SSRD and inform future research to establish treatment plans. Future research should consider differences in disease severity and control treatments that participants receive to rule out the possibility that differences in treatment methods may affect subjective symptoms. Additionally, larger, multi-center studies are warranted to expand the generalizability of the results and stratify participants by background factors.