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

A global approach to factors responsible for functional impairment in patients with BD is necessary.

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.

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).

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.