The Relationship between Cognitive Functions and Clinical Features in Bipolar I Disorder Patients: A Comparison with Bipolar II Disorder

Objective: Cognitive impairment is increasingly recognized as a core feature of bipolar disorder and may persist during euthymic phases. However, direct comparisons of cognitive profiles between Bipolar I Disorder (BD-I) and Bipolar II Disorder (BD-II) remain limited, particularly in studies that include healthy control groups.Methods: In this cross-sectional study, euthymic patients with BD-I (n = 65), BD-II (n = 32), and healthy controls (n = 65) were assessed using a comprehensive neuropsychological test battery, including the Stroop Test, the WAIS-R Digit Span and Digit-Symbol Coding subtests, and the Verbal Fluency Test. Group differences were initially examined using one-way analysis of variance (ANOVA). Variables demonstrating statistically significant group effects were subsequently evaluated using Pearson correlation analyses.Results: Omnibus ANOVA revealed significant group differences for Stroop Test subtest 4 (F(2,159) = 16.00, p < .001), WAIS-R Digit-Symbol Coding (F(2,159) = 16.55, p < .001), and Verbal Fluency Test scores (F(2,159) = 14.67, p < .001). Post hoc analyses indicated that BD-I patients performed significantly worse than both BD-II patients and healthy controls on Stroop-4 and Verbal Fluency measures, whereas the difference between BD-I and BD-II patients on Digit-Symbol Coding was attenuated after adjustment. Effect size estimates indicated medium-to-large group differences, particularly for comparisons involving BD-I patients and healthy controls.Conclusions: These findings suggest that cognitive impairment persists during euthymic phases of bipolar disorder, with more pronounced deficits observed in BD-I patients, particularly in executive functioning and processing speed. Although cognitive difficulties in BD-II patients appear less extensive, they remain clinically relevant.