The effect of subthreshold depressive symptoms on cognitive functions and peripheral biomarkers in bipolar disorder

AbstractObjective A global approach to factors responsible for functional impairment in patients with BD is necessary.Method Ninety-three euthymic patients with BD [49 patients with SD (subthreshold depression) and 44 patients without SD] and 48 healthy controls were invited for evaluation of demographic, clinical, and cognitive characteristics. To define SD, the lower limit was HDRS ≥4 points and the upper limit was HDRS <9 points. Stroop test, California verbal learning test, digit-span test, controlled word association test, and clock drawing test were performed. Serum BDNF levels were measured. Additionally in the BD group; blood drug (lithium, valproic acid), leukocyte, C-reactive protein (CRP), TSH, and vitamin B12 levels were measured.Results We found no difference between serum BDNF levels of BD (n = 93) and controls. The cognitive performances of the BD group were worse than the control group (p < 0.001). Attention, working memory, and stroop performance of patients with SD were worse than patients without SD (p < 0.05). Verbal fluency, stroop test, and planning performance decreased as serum CRP level increased in patients with BD (p < 0.05).Conclusions Although the patient group with SD was in euthymia, their cognitive performance was worse than the group without SD. Poor cognitive performance in BD was associated with serum CRP levels.KEY POINTSPatients with SD are not in complete well-being, even in the euthymic period.SD symptoms in bipolar disorder cause cognitive impairment.Serum BDNF level is not affected in the euthymic period in bipolar disorder.High serum CRP levels are associated with poor cognitive performance.Keywords: Bipolar disorderDepressionCognitionBDNFC-reactive protein AcknowledgementsWe thank Professor Sut Necdet (Trakya University, School of Medicine, Department of Biostatistics) for the statistical analysis of our study.Ethical approvalReceived ethical approval from the Trakya University Faculty of Medicine Ethics Committee (TÜTF-BAEK 2020/234).Author contributionsSogut Kubra and Gorgulu Yasemin acquired funding, designed the study, collected the data, analyzed the data, and wrote the manuscript. Serum levels of BDNF, CRP, and leukocytes were measured, in the laboratory, by Palabiyik Orkide. All authors contributed to and have approved the final manuscript.Disclosure statementNo potential conflict of interest was reported by the author(s).Data availability statementThe data that support the findings of this study are available from the corresponding author and G.Y., upon reasonable request.Correction StatementThis article has been republished with minor changes. These changes do not impact the academic content of the article.Additional informationFundingThe project was supported financially by the Trakya University Scientific Research Project Committee (grant number: 2020/137). This work was not an industry-supported study. The authors have indicated no financial conflicts of interest.Notes on contributorsKubra SogutKubra Sogut is a medical doctor and specialist in psychiatry. Published article in psychotic episodes after bariatric surgery.Yasemin GorguluYasemin Gorgulu is a professor of psychiatry. She has scientific researches and published articles mostly in the field of biological psychiatry and affective disorders.Orkide PalabiyikOrkide Palabiyik, physiologist, assistant professor. Published researches on peripheral biomarkers.