REM sleep behavior disorder (RBD) is a type of parasomnia characterized by the loss of muscle atonia during REM sleep, which leads to the enactment of dream content [1]. Such dream enactment behaviors (DEB) often manifest as violent or aggressive actions, including shouting, swearing, gesturing, punching, and kicking, which may result in physical injury to the patient or their bed partner [2]. According to a previous study, 31.4% of RBD patients had caused injury to themselves or others during sleep, but only 12.3% had received medical evaluation [3]. Idiopathic RBD (iRBD) has been identified as an early biomarker of α-synucleinopathies, such as Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy. Longitudinal studies have reported that approximately 40%–65% of patients with iRBD eventually develop a neurodegenerative disorder within 10 years [4], emphasizing the importance of early detection and clinical evaluation.

RBD has also been associated not only with neurodegenerative diseases, but also with psychiatric factors such as anxiety, depression [5,6], and the use of antidepressants [7-9]. Lam et al. [9] reported that among psychiatric outpatients aged 18–65, 5.8% exhibited RBD-like symptoms, 21.0% reported sleep-related injury, and 3.6% experienced sleep-related violence. In particular, patients taking antidepressants have been shown to exhibit increased DEB, likely due to the effects of these medications on REM sleep [5,10].

In a study using video-polysomnography (vPSG) among psychiatric inpatients, approximately 3.6% were diagnosed with RBD, and 8.77% showed REM sleep without atonia [11]. Among patients diagnosed with RBD through vPSG, 46.1% were taking antidepressants [12]. These findings indicate that RBD symptoms may be present among psychiatric outpatients and that they are often unrecognized in clinical assessments. To date, few studies in Korea have examined RBD symptoms and associated risk factors among psychiatric outpatients.

Although polysomnography (PSG) is essential for the definitive diagnosis of RBD, its use as a screening tool in routine clinical practice is often limited. The REM Sleep Behavior Disorder Screening Questionnaire (RBDSQ) is a self-report screening tool designed to systematically assess abnormal behaviors during sleep and is easily applicable in clinical settings [13]. A recent study from outside Korea also employed the RBDSQ to screen for RBD in psychiatric outpatients aged 45 to 80 years [14].

Accordingly, this study aimed to evaluate symptoms of probable RBD (pRBD) and investigate its prevalence and associated factors among psychiatric outpatients at a university hospital in Korea.

The characteristics of the patients visiting the Sleep Clinic of the Department of Psychiatry for the first time are shown in Table 1, divided into those who completed the self-report questionnaires (study participants) and those who did not (non-completers). Compared to non-completers, study participants had a significantly lower prevalence of schizophrenia spectrum disorder (p=0.001) and other mental disorders (p=0.002).

Table 2 presents a comparison of demographic and clinical characteristics of the study participants, divided into the pRBD group and the non-RBD group. A total of 356 patients were included in the study, of whom 179 (50.3%) were classified as having pRBD, and 177 (49.7%) were classified as the control group (non-RBD). Compared to the non-RBD group, the pRBD group was significantly younger (43.4±15.9 vs. 50.2±13.9 years, p=0.005), had a higher BMI (25.1±6.7 vs. 23.5±3.5 kg/m², p=0.007). In addition, the pRBD group scored significantly higher on the ISI (17.0±7.0 vs. 12.6±7.5, p<0.001), BDI (25.1±14.6 vs. 19.2±12.3, p<0.001), BAI (21.5±14.6 vs. 11.4±10.4, p<0.001), and LEC-5 (3.4±2.8 vs. 2.3±2.5, p=0.003) than the non-RBD group. The pRBD group had a higher proportion of patients at high risk for OSA according to the BQ compared with the non-RBD group (35.8% vs. 22.0%; p=0.034).

Table 3 shows psychiatric and sleep disorder diagnoses based on the presence or absence of pRBD. Among patients in the pRBD group, 20 patients (11.2%) were diagnosed with trauma- and stressor-related disorders, which was significantly higher than in the non-RBD group (4.5% [8 patients]; p=0.020). No significant differences were observed between the two groups in the other psychiatric or sleep disorder diagnoses.

Table 4 summarizes the use of psychiatric medications in the pRBD group and the non-RBD group. Overall, 52% of participants were prescribed antidepressants. Although the proportion was higher in the pRBD group than in the non-RBD group (57.0% vs. 46.9%), the difference was not statistically significant (p=0.057). However, subgroup analysis by antidepressant class revealed a significantly higher rate of SSRIs use in the pRBD group than in the non-RBD group (44.7% vs. 17.5%, p<0.001). There were no significant between-group differences in the use of other psychiatric medications.

Table 5 shows the factors associated with pRBD. In the multivariable logistic regression analysis, a total of 13 variables including age and sex were entered into the model. As a result, higher BAI scores (odds ratio [OR]=1.068, 95% confidence interval [CI]=1.019–1.120, p=0.006), sleep-related breathing disorders (OR=7.597, 95% CI=2.049–28.167, p=0.002), and the use of SSRIs (OR=6.914, 95% CI=1.909–22.110, p=0.003) were significantly associated with pRBD (Table 5). The model demonstrated good fit (χ²=63.07, df=13, p<0.001), explaining 33.8% and 45.1% of the variance according to the Cox & Snell and Nagelkerke R2 values, respectively.

This study investigated the prevalence and associated factors of pRBD among first-visit outpatients at a psychiatric sleep clinic. Among psychiatric outpatients who presented with sleep-related complaints, 74.1% completed the RBDSQ, and 50.3% of these were classified as having pRBD. Factors significantly associated with pRBD were higher BAI scores, a diagnosis of sleep-related breathing disorder, and the use of SSRIs.

The prevalence of RBD has been reported as 1.06% in a largescale study of the general population aged 35 to 75 years [27] and 2.01% among individuals aged 60 and older in a Korean community-based sample [28]. In a psychiatric outpatient clinic-based study of individuals aged 45 and older, the prevalence of pRBD was reported to be 15.9%. Subgroup analyses revealed considerable variation in pRBD prevalence across primary psychiatric diagnoses, ranging from 6.7% to 26.4% [14]. In our study, among patients who visited the Sleep Clinic of the Department of Psychiatry, the prevalence of pRBD was 50.3%, notably higher than that reported in previous studies. Several factors may account for this discrepancy.

First, only patients who voluntarily completed the RBDSQ were included, leading to potential selection bias. Patients who completed the RBDSQ in our study showed a different diagnostic distribution compared with those who did not. Specifically, completers (study participants) had significantly lower proportions of schizophrenia spectrum disorders and other mental disorders. Since previous research has suggested a lower risk of pRBD in schizophrenia spectrum disorders (odds ratio=0.573) [14], their absence from our sample may have contributed to the elevated prevalence. Moreover, the category of other mental disorders in non-completers primarily included patients with mild or subthreshold symptoms or those who visited mainly for routine psychiatric consultation. Such patients may have been less motivated to report their symptoms accurately in the self-report questionnaire. In addition, because this study targeted psychiatric outpatients who primarily presented with sleep-related complaints, even minor sleep problems may have been overreported.

Second, among those who did complete the questionnaires, the pRBD group exhibited more severe psychiatric symptoms, including significantly higher ISI, BDI, and BAI scores, compared with the non-RBD group. This suggests that severity, rather than diagnostic category alone, may have contributed to an elevated prevalence.

Third, pharmacological effects may have influenced RBDSQ responses. SSRIs are known to induce vivid dreams [29] and sleep-related behaviors such as sleep talking [30]. These sleep-related side effects could have been reported as positive responses on the RBDSQ, which may have led to an overestimation of the prevalence of pRBD in our sample.

Fourth, undiagnosed OSA may also have contributed to false positives on the RBDSQ, as severe OSA can mimic RBD-like behaviors. In our study, considering the higher BMI and BQ scores of participants, the possibility of unrecognized OSA increased risk of false-positive findings, thereby inflating the prevalence estimates.

Taken together, these factors indicate that the prevalence observed in this study is likely an overestimation and should be interpreted cautiously. The findings may not be fully generalizable to the broader population of psychiatric outpatients.

In this study, the use of SSRIs was identified as a significant factor associated with RBD symptoms, consistent with findings from numerous previous studies. According to a recent meta-analysis, antidepressants—particularly SSRIs—are among the major risk factors for RBD, with SSRI users exhibiting approximately a 2.4-fold increased risk of developing RBD compared to non-users [8]. Antidepressants interfere with brainstem regulation of REM sleep by disrupting normal muscle atonia during this stage, thereby enabling DEB [31]. While such symptoms may subside after discontinuation of the medication [32,33], in some cases they may represent an early manifestation of iRBD, warranting longitudinal monitoring [34]. Further studies should systematically examine RBD symptoms after antidepressant withdrawal, as their course and underlying mechanisms have not yet been fully clarified.

When comparing the pRBD group to the control group, no diagnostic differences were found for diagnosis of mood disorder or anxiety disorder; however, a significant association was confirmed with higher BAI scores. This suggests that the severity of symptom is more relevant to RBD symptoms than diagnostic category. Unlike the prevalence analysis, where inflated rates may have reflected more severe cases, here the finding indicates that anxiety severity itself may be independently associated with RBD symptoms. In this study, diagnoses were categorized into groups such as mood disorder and anxiety disorder, and the severity of the disorder was not considered. Therefore, the significantly higher BAI scores in the pRBD group suggest that patients with more severe psychiatric disorders may be included in the pRBD group. Previous studies have reported positive correlations between RBDSQ scores and both anxiety (r=0.43, p<0.05) and depression (r=0.56, p<0.05) [35]. In a PSG-confirmed RBD cohort, early-onset psychiatric disorders were common, with depressive disorder found in 49% and anxiety disorder in 43% [36]. Given that depression is generally more prevalent than anxiety disorders, this suggests a high prevalence of anxiety disorders in the RBD patient group. In line with these reports, the significantly higher BAI scores in our pRBD group suggest that anxiety severity plays a critical role. Further research is needed to examine the association between RBD and psychiatric severity, with a particular focus on anxiety.

In our study, higher BMI and a high risk for OSA as determined by the BQ was significantly associated with pRBD in univariate analysis. The multivariable logistic regression analysis demonstrated that a diagnosis of sleep-related breathing disorder was independently associated with pRBD. This association may be attributable to a higher likelihood of comorbid sleep apnea or alterations in REM sleep architecture. Consistent with this, a previous study reported that 107 out of 120 RBD patients (89.2%) were diagnosed with OSA, demonstrating a high prevalence of OSA in RBD patients [37]. Given the high prevalence of obesity and the potential influence of psychiatric medications on metabolism in psychiatric populations, this finding is noteworthy. Further studies are needed to clarify the relationship between RBD and factors related to weight and metabolism.

In this study, both the number of traumatic events (as measured by the LEC-5) and the diagnosis of trauma- and stressor-related disorders were higher in the pRBD group; however, these associations were not statistically significant in multivariable analysis. Prior studies have reported RBD-like symptoms—particularly DEB—among patients with post-traumatic stress disorder, and some have proposed the concept of a distinct parasomnia subtype, termed trauma-associated sleep disorder, to describe this phenomenon [38]. These findings indicate that careful assessment of trauma history and related diagnoses is warranted when evaluating pRBD symptoms in psychiatric outpatients.

The observed pRBD group was younger than the non-RBD group, in contrast to prior studies that identified old age and male sex as risk factors for RBD. To date, most RBD research has focused on middle-aged and older populations, whereas data on younger individuals are limited to studies of early-onset RBD, defined as diagnosis before the age of 50. A previous study reported that early-onset RBD patients are more likely to be female, to have psychiatric comorbidities, and to be prescribed antidepressants [39]. Notably, these characteristics parallel those observed in our pRBD group, which was younger in age, exhibited psychiatric comorbidities including insomnia, depression, anxiety, and frequently reported SSRI use. Given the scarcity of research on RBD in younger individuals, prospective longitudinal studies are needed to clarify the distinct clinical characteristics and symptom trajectories of early-onset RBD patients.

This study has several limitations. First, the possibility of selection bias must be considered. We retrospectively analyzed medical records of psychiatric outpatients whose chief complaint was sleep-related problems and who completed the RBDSQ; only 74.1% of first-visit patients completed the RBDSQ, and their diagnostic distribution differed from that of non-completers. Schizophrenia spectrum disorders were absent among study participants; given previous reports that this group is associated with a lower likelihood of pRBD [14], the prevalence observed here may have been overestimated. Moreover, the non-completer group contained a higher proportion of “other mental disorders,” which likely included mild or subthreshold cases; such patients may have been less motivated to report symptoms accurately, leading to additional bias. Finally, because the study population consisted of psychiatric outpatients with sleep-related complaints, overreporting of minor sleep symptoms is possible. Second, pRBD was assessed solely using the RBDSQ without PSG confirmation. Although the RBDSQ has been standardized in various studies and is widely used as a reliable screening tool for RBD, its specificity is limited. One study reported a sensitivity of 0.96 and a specificity of 0.56 [13], implying that a substantial number of false positives may occur. A meta-analysis reported its accuracy in general population [40], but high false-positive rates have been reported in migraine patients (58.9%) [41] and in OSA, where a higher cutoff (6.5) has been suggested with frequent false positives [42]. Given the higher BMI and BQ scores in our sample, undiagnosed OSA may have contributed to misclassification. Moreover, other sleep-related behavioral disorders (e.g., vivid dreams, sleep talking, sleepwalking, sleep-related eating disorder) can mimic RBD and require PSG for differentiation. Third, this was a single-center retrospective chart review conducted at a specialized psychiatric sleep clinic; therefore, the findings may not be generalizable to other clinical settings or the community population, and causal inferences cannot be made. Fourth, because the RBDSQ relies on self-report, recall or reporting bias may have affected responses, particularly in psychiatric patients with variable insight. Fifth, psychiatric diagnoses were grouped into broad categories such as mood disorder and anxiety disorder, which may not adequately reflect the heterogeneity of subtypes or severity levels. Finally, medication-related variables such as dosage, treatment duration, and adherence were not assessed; these should be considered in future research to more accurately estimate pharmacological effects on RBD-like symptoms.

To our knowledge, this is the first study to investigate RBD in psychiatric outpatients in Korea. Although the prevalence of pRBD observed in this study is likely to have been overestimated due to selection bias and the specific characteristics of our sample, the findings nonetheless provide important clinical implications. Our findings indicate that pRBD symptoms are relatively common among outpatients at a psychiatric sleep clinic and are independently associated with anxiety, SSRI use, and sleep-related breathing disorders. These results suggest that psychiatric patients with these risk factors should be prioritized for screening, particularly before initiating antidepressant therapy, and that careful differentiation from OSA is clinically important. Because RBD may also serve as an early biomarker of neurodegenerative diseases, psychiatric patients reporting such symptoms should be considered for regular clinical monitoring and further diagnostic evaluation, including PSG.