The present study assessed telomere length and cognitive function in euthymic BD-I and BD-II patients and whether there were correlations between them. Additionally, we investigated the relationships between clinical characteristics, telomere length and cognitive function. In our study, individuals with euthymic BD-I and BD-II showed shorter telomere length relative to HCs, but there was no significant difference between patients with BD-I and BD-II. Compared to HCs, poor performance was detected in the cognition domain of the speed of processing, reasoning and problem solving and attention/vigilance in BD-I patients, and the speed of processing, social cognition and attention/vigilance showed decreased performance in BD-II patients. We investigated the correlations with telomere length, cognitive function and some clinical characteristics. No correlation was found between telomere length and cognitive function. However, the function of reasoning and problem solving was negatively correlated with DI in BD-I, and visual learning performance was negatively correlated with DUI in BD-II.
Our study suggested that shorter telomere length was observed in BD patients. The relationship between telomere length and bipolar disorder has been controversial in previous studies. Some have reported significantly reduced telomere length in BD patients compared to unrelated healthy controls [15, 20, 21]. In contrast, individual studies failed to find an association between BD and telomere length [14, 22]. However, a meta-analysis that enrolled 578 BD patients and 551 HCs across 10 studies showed that telomere length in BD patients was significantly shorter than in healthy controls [23]. Moreover, the telomere length in the unaffected relatives of BD patients, who are at high risk of mood disorder, was shorter than that in healthy participants [13]. The overall results demonstrated that there may be a connection between shorter telomere length and the risk of developing BD. However, investigating the difference in telomere length in BD subtypes has been limited thus far. A recent study showed significant telomere shortening in patients with BD-I but not in BD-II patients [24], which is inconsistent with our findings and an earlier result on BD-II [25]. The discrepancy in these results might be attributed to the unstable methods of measuring telomere length and the different medical treatments. More studies designed to consider the potential influencing factors could elucidate the relationship between TL and bipolar disorder.
Previous studies have shown that telomere length is associated with age-related cognitive dysfunction [26] and dementia [27]. A large sample investigation suggested that longer TL was correlated with better cognitive performance. TL attrition was inversely associated with global cognition and speed processing, visual-spatial function and memory in healthy adults [28, 29]. Compared with patients with nonpsychotic disorders, BD patients are at great risk for developing dementia later in life [30, 31]. In BD patients and their first-degree relatives, telomere length explained a small but significant difference in delayed recall [13]. However, our results failed to find an association between TL and cognitive function in BD patients. The sensitivity and dimension of the measurement tool of cognitive function might partially contribute to the distinct outcome. Moreover, although no relationship between them was shown in our small young adult samples, whether the potential cognitive deficit in later life in BD patients was related to shortened telomere length is worth discussing. Further study should focus on a large sample and include a range of ages of individuals to clarify the relationship between TL and cognitive function in BD patients.
Our study showed that the duration of the illness and the untreated illness were connected with cognitive function. Performance in visual learning was negatively correlated with DUI in BD-II, and prolonged illness duration was negatively correlated with impairment of reasoning and problem solving in BD-I. Long DUI was more likely to have adverse clinical outcomes in BD, such as a higher number of individuals with suicide attempts and a longer duration of illness [32]. DUI was one of the strongest predictors of patients’ levels of functioning outcome [33]. A meta-analysis showed that there was a small association between longer DUI and reduced performance in planning/problem-solving ability in psychosis [34]. Consistent with our findings, cross-sectional data from a bipolar cohort showed that a longer illness duration was a predictor of cognitive function in BD-I [35]. Our study reflects the combined influences of neurodevelopmental abnormalities on cognitive function in BD patients.
Several study limitations are noteworthy in our study. First, the participants in the present study were young adults, and the relationship between TL and cognition suggested that it was age-related [26]. In our study, no significant correlation between them was found in young adults, and whether the relationship was prominent in elderly BD patients is worth further investigation. Second, the relationship between TL and BD is complex, including the influence of psychotropic medication [36, 37]. Participants in our study were euthymic BD patients, and in medical treatment, the inclusion of first-degree relatives of BD will help disentangle the hereditary of TL in BD. Third, the sample size of the study was limited, which may have skewed the results to some extent. More participants should be enrolled in further studies.