Introduction
Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system that leads to demyelination and neurodegeneration [3]. Due to its ambiguous, chronic, and life-altering nature, especially in the early stages, MS induces significant levels of distress in affected individuals [3]. This condition can reduce the quality of life (QoL) of patients [5]. The World Health Organization defines QoL as an individual’s perception of their position in life within the cultural and value systems in which they live, and in relation to their goals, expectations, standards, and concerns [6]. Living with a person suffering from MS can disrupt the QoL [7], and studying its antecedent factors is of great importance. Based on the social cognitive approach, the potential antecedents of QoL in individuals with MS may include their perceptions and beliefs about their disease, which can be the constructs such as pain self-efficacy (PSE) and illness identity. The PSE refers to an individual’s belief in their capacity to manage pain symptoms, minimize its impact on daily life, and maintain functionality despite pain [9, 10]. Pain is a common symptom of MS [11], and is experienced by more than 75% of MS people [12]. Examining the effects of PSE in people with MS is important. Illness identity is important in facing chronic illnesses. There are four illness identity types: Rejection, engulfment, acceptance, and enrichment. Their relationship with psychological and physical functioning in chronic physical conditions has been examined in previous studies [22]. Illness identity is a relatively new concept in health psychology and has been studied so far in cases such as people with cardiac disease [23, 24], multisystem connective tissue disorders [23], and epilepsy [25].
The literature review indicates that the experience of MS affects individuals’ beliefs and perceptions about themselves. Understanding the extent to which PSE and each of the four illness identity types explain QoL among people with MS can help health psychologists in designing interventions aimed at improving QoL. However, there is a research gap in this area. Therefore, this study aimed to examine the relationship of PSE and illness identity with QoL among individuals with MS and to assess the moderating role of illness identity in the relationship between PSE and QoL. 

Methods
This is a correlational study with a cross-sectional design. The study population included all individuals with MS referred to Sina and Asia hospitals in Tehran, Iran, from November 16, 2023 to June 20, 2024. Using G*Power software (effect size=0.15, α=0.05, test power=0.95), the sample size was estimated at 138, but finally 166 participants were included in the study. Inclusion criteria were the MS diagnosis and age 18-60 years. Exclusion criteria were a prior psychiatric diagnosis before MS and diagnosis of comorbid chronic diseases (based on the patient’s medical records). 
For data collection, a demographic form (surveying age, gender, educational level, marital status, duration of illness, and type of MS), the Persian version of the World Health Organization Quality of Life Questionnaire- Brief version [26], the Persian version of the Pain Self-Efficacy Questionnaire [27], and the Persian version of the Illness Identity Questionnaire (IIQ) [22] were employed. 
Six participants were excluded during data collection due to outlier data, resulting in a final sample size of 160. Pearson’s correlation test and hierarchical regression analysis (with and without adjusting for age) were utilized for data analysis. Additionally, independent t-test was used to compare the mean QoL based on gender, and a one-way ANOVA was conducted to compare the mean QoL based on other demographic variables.

Results
The mean age of participants was 37.16±8.63 years, ranged 18-57 years. They included 107 females and 51 males. Additionally, 42 had a high school diploma or lower education, 82 had an associate/bachelor’s degree, and 32 had a master’s/doctoral degree. Regarding marital status, 49 were single, 97 were married, and 12 were divorced. Concerning the MS types, 63 had relapsing-remitting MS (RRMS), 67 primary progressive MS (PPMS), and 28 secondary progressive MS (SPMS). In terms of illness duration, 44 had been diagnosed for less than 2 years, 26 for 2-5 years, 43 for 5-10 years, and 45 for >10 years. Descriptive statistics for each variable, as well as the correlation coefficients are reported in Table 1.


Given the significant correlation between age and QoL, two hierarchical regressions were conducted to assess the role of independent and moderating variables in predicting QoL (with and without adjusting for the effect of age on QoL). Each hierarchical regression was conducted in three steps (or models). In the first step, PSE was entered into the equation. In the second step, the four types of illness identity were added. In the third step, four new variables (interaction of PSE with the illness identity types)were included in the model (Tables 2 and 3).




According to the results in Table 2, the third study model explained 44% of the variance in QoL before adjusting for age. However, this decreased to 38% after adjusting for age. Nevertheless, this model retained significant power in explaining the variance of QoL, even after adjusting for the effect of age.
The regression coefficients are presented in Table 3. After adjusting for the effect of age on QoL, the explained variance attributed to PSE and enrichment decreased to 26% (P=0.004) and 46% (P<0.001), respectively. However, the adjustment did not change the beta value for the engulfment (P<0.001). The rejection and acceptance domains of the IIQ did not significantly explain the variance in QoL either before or after controlling for age (P>0.05). Furthermore, among the IIQ domains, only the engulfment domain significantly moderated the effect of PSE on  QoL (P<0.001) 

Conclusion
There is a positive relationship between PSE and QoL, and PSE predicts higher QoL. The illness identity type “engulfment” has a negative relationship with QoL, while “enrichment” is positively related to QoL. Moreover, the engulfment negatively moderates the relationship between PSE and QoL. This means that the engulfment of people with MS negatively affects how they perceive themselves, reducing the impact of self-efficacy beliefs on their QoL. This, in turn, leads to decreased engagement in social and occupational activities, ultimately resulting in a decline in QoL. Therefore, it is recommended to pay attention to the illness identity (especially engulfment and enrichment) and strengthen PSE in intervention programs for patients with MS to improve their QoL.

Ethical Considerations
Compliance with ethical guidelines
Ethical approval for the study was obtained from the Ethics Committee of Iran University of Medical Sciences (Code: IR.IUMS.REC.1402.653).

Funding
This study was funded by the Iran University of Medical Sciences.

Authors contributions
Conceptualization, Methodology, Software, Original Draft Preparation: Parisa Sharifi and Mojtaba Dehghan; Validation, Investigation, Review & Editing: Parisa Sharifi , Maede Amin Roaya, Mojtaba Dehghan ; Formal Analysis, Data Curation: Parisa Sharifi; Project Administration and Funding Acquisition: Mojtaba Dehghan.

Conflicts of interest
The authors declare no conflict of interest.

Acknowledgments
The authors would like to thank the participants and the MS clinics at Asia and Sina hospitals for their cooperation in this study. 


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