REM sleep behavior disorder (RBD) is a parasomnia characterized by loss of REM sleep-related muscle atonia and consequent dream enactment during sleep.¹ RBD patients are at risk of harming themselves or others during dream enactment.² RBD is known to be associated with other neurodegenerative diseases such as Parkinson's disease (PD) and Lewy body dementia.³ Additionally, psychiatric symptoms, such as anxiety and alexithymia, are known to be associated with RBD.⁴⁵ Therefore, RBD treatment should focus not only on symptom resolution but also on the evaluation and treatment of comorbid disorders.

Unlike anxiety and alexithymia, no consensus has been reached on the relationship between depression and RBD. Some studies have claimed that depression is not associated with RBD,⁶ but one study revealed a tendency for RBD to be associated with depression.⁵ Old age and neurodegenerative disorders are risk factors for both RBD and depression⁷⁸⁹ and may support the hypothesis that RBD is associated with depression.

In support of this idea, a recent case control study showed that patients with idiopathic RBD were more likely to report comorbid depression than were controls.¹⁰ However, this recent study was limited in that only histories of physician-diagnosed depression were assessed, and it ignored the possibility that some depressed RBD patients had not been diagnosed. To our knowledge, no reported study has addressed factors associated with depression in RBD patients.

In this study on RBD patients, we examined the relationship between depression (and associated factors) and RBD.

The RBD patients in this study were 66 males (70.2%) and 28 females (29.8%). The number of patients with a history of neurological disorders was 10 (10.6%), including five (5.3%) patients with Parkinson's disease or Parkinsonism and five (5.3%) patients with conditions including cerebral stroke and dementia. The number of patients with a history of psychiatric disorders was 18 (19.1%), including six (6.4%) patients with mood disorder, five (5.3%) with anxiety disorder, five (5.3%) with sleep disorder, and two (2.1%) patients with other conditions such as alcohol dependence and somatoform disorder. As shown in Table 1, comparisons between male and female patients in terms of clinical characteristics showed no significant differences. Also, BDI and ESS scores did not differ significantly between males and females. Table 2 presents the nocturnal polysomnographic characteristics of study patients and comparisons between male and female patients. The N2 and N3 sleep percentages were higher in female than in male patients (49.6±7.9% vs. 44.8±10.7%; p=0.036; 9.6±7.3% vs. 5.3±6.8%; p=0.007, respectively, all t-tests). Sleep latency and REM latency in female RBD patients were longer than those in males (22.9±25.8 min vs. 10.3±10.9 min; p=0.018; 140.3± 64.6 min vs. 108.0±65.9 min; p=0.031, respectively, all t-tests). Other PSG parameters did not differ significantly between males and females.

The mean BDI score of all patients was 12.4±10.3. Of them, 42 (44.7%) patients were depressed (BDI >11). Depressed RBD patients were less able to recall enacted dreams than were non-depressed ones (61.9% vs. 86.5%; p=0.008, χ² test) (Figure 1). As shown in Table 3, depressed patients had a higher respiratory disturbance index (17.8±19.9 vs. 9.5±10.2; p=0.017, t-test) and were more likely to have a history of psychiatric disorders compared with non-depressed RBD ones (31.0% vs. 9.6%; p=0.016, χ² test). Table 4 shows no significant difference in the PSG study between depressed and non-depressed patients.

As shown in Table 5, after controlling for the respiratory disturbance index and history of psychiatric disorders, the ability to recall enacted dreams was negatively associated with BDI scores higher than 11 (odds ratio=0.323, p=0.041, logistic regression analysis).

We aimed to explore the relationship between depression (and associated factors) and RBD and found that 44.7% of all RBD patients met the criteria for depression (BDI score >11). Compared with the prevalence of depression in the general population (21.6%),¹⁶ the higher incidence of depression found in this study may be clinically significant. Our finding is also higher than the 28.8% value from a previous study on RBD patients.¹⁰ That previous study included only patients who had been diagnosed with depression by physicians, thereby excluding non-diagnosed depressed patients. The enrollment of a higher proportion of female RBD patients in the present study might have affected the prevalence of depression in RBD patients, because females are more likely to be depressed than males¹⁷ and Our results showed no significant difference between males and females but tendencies toward gender differences in BDI scores and in the percentage of patients with depression were observed (p-value is 0.06 and 0.07 respectively). The data support the hypothesis that patients with RBD are more likely to be depressed.

It is notable that depressed RBD patients were less likely to recall enacted dreams than were non-depressed patients. Emotional dysregulation is known to trigger failure to recall enacted dreams, because dreams contain emotions.¹⁸ This study is partially supported by previous studies in which the frequencies of dream recall in alexithymic or depressive subjects were lower than those in non-alexithymic or non-depressed subjects.¹⁸¹⁹ Taken together, the data show the possibility that RBD patients suffering from emotional dysregulation may become depressed, rendering the recall of enacted dreams less likely.

Neurodegeneration may also explain our present findings. Depression is common in patients with neurodegenerative diseases such as Parkinson's disease,²⁰ and RBD is known to be associated with such neurodegenerative disorders.³ Depressed RBD patients may have possibility that more neurodegenerative changes than do non-depressed patients, and it could make them less attentive and more memory-impaired. Because of these impairments, depressed RBD patients may become less likely to recall enacted dreams.

A previous study showed that enacted dreams in RBD patients had more content with aggression, such as stranger's attacks, than other forms of parasomnia. These aggressive features in the enacted dreams could be explained by cognitive dysfunction, particularly frontal dysfunction.²¹ Thus, enacted dreams in RBD may have clinical importance in predicting the possible onset of neurodegeneration.

This study included a higher proportion of female RBD patients than did a previous study, in which females accounted for 18% of the RBD patients.⁷ The previous study reported that clinical histories or questionnaire responses alone were less sensitive than PSG for detecting RBD in females.²² In the present study, we used both history and nocturnal PSG diagnostically to avoid underdiagnosing RBD. The inclusion of nocturnal PSG may explain why the proportion of female RBD patients in this study was higher than that in the previous study.²² In PSG findings, the percentages of N2 and N3 sleep and REM latencies differed significantly between male and female RBD patients. These differences were similar to those reported in a previous study on gender differences in PSG.²³ Theses finding were also similar to a recent study on gender differences in China based on polysomnographic findings.²⁴

This study has several limitations. First, it was cross-sectional and retrospective. We were not able to identify any causal relationship between enacted dream recall and depression in RBD. Second, we assessed depression using only the BDI. More reliable results might have been obtained if depression had been diagnosed using the Diagnostic and Statistical Manual for Mental Disorders, fifth edition (DSM-5)²⁵ or the Structured Clinical Interview for DSM-IV-TR Axis I Disorders (SCID).²⁶ Third, enacted dream recall was assessed using a single question. More reliable data might have been obtained by examining detailed features of enacted dream recall, such as frequency, number of dreams and dream contents. Fourth, there could be other confounding factors that were not assessed in this study, such as medication use. The study sample has may include patients with secondary RBD, such as that due to antidepressant use or Parkinsonism. Finally, associations between enacted dream recall and cognitive disturbance were not studied.

In summary, we found that about half of all RBD patients met our depression criteria. These depressed RBD patients were less likely to recall enacted dreams than were non-depressed ones. It may be important to carefully examine mood in RBD patients to detect depressive symptoms in a timely manner.