Verbal learning impairment in euthymic bipolar disorder: BDI v BDII

Area of Science:

• Neuroscience
• Psychiatry
• Cognitive Psychology

Background:

• Cognitive impairment is a hallmark of bipolar disorder (BD), impacting even euthymic individuals.
• Verbal Learning and Memory Tasks (VLT) often reveal the most significant cognitive deficits in BD.
• Previous comparisons between Bipolar I (BDI) and Bipolar II (BDII) disorder have yielded inconsistent findings, often attributed to small sample sizes.

Purpose of the Study:

• To compare the VLT performance between BDI and BDII out-patients in a larger sample.
• To investigate the influence of covariates such as age, education, mood scores, and age at onset on VLT performance.
• To explore the impact of current medication and illness variables on cognitive function in both BDI and BDII.

Main Methods:

• A comparative study involving 183 BDI and 96 BDII out-patients.
• Utilized an adapted version of the Rey Verbal Learning Task (RVLT) to assess verbal learning and memory.
• Controlled for demographic factors (gender, age, education), mood scores, and age at onset as covariates.

Main Results:

• BDI patients demonstrated significantly greater impairments across all five VLT outcome measures compared to BDII patients, even after controlling for covariates (Effect Sizes = 0.13–0.17).
• These cognitive impairments were largely independent of current medication status.
• Age at onset influenced performance on three VLT measures, and the number of mood elevation episodes affected one measure.

Conclusions:

• This study confirms and extends previous findings, showing a clear verbal learning deficit in BDI compared to BDII in a substantially larger cohort.
• The observed cognitive differences are unlikely to be solely explained by medication or specific illness variables.
• The findings suggest that the verbal learning impairment may represent a core phenotypic distinction between BDI and BDII subtypes.