Neuropsychological functioning in bipolar disorder and schizophrenia

Abstract

Background: Some patients with bipolar disorder (BD) demonstrate neuropsychological deficits even when stable. However, it remains unclear whether these differ qualitatively from those seen in schizophrenia (SZ).

Methods: We compared the nature and severity of cognitive deficits shown by 106 patients with SZ and 66 patients with BD to 316 healthy adults (NC). All participants completed a cognitive battery with 19 individual measures. After adjusting their test performance for age, sex, race, education, and estimated premorbid IQ, we derived regression-based T-scores for each measure and the six cognitive domains.

Results: Both patient groups performed significantly worse than NCs on most (BD) or all (SZ) cognitive tests and domains. The resulting effect sizes ranged from .37 to 1.32 (mean=.97) across tests for SZ patients and from .23 to .87 (mean=.59) for BD patients. The Pearson correlation of these effect sizes was .71 (p<.001).

Conclusions: Patients with bipolar disorder suffer from cognitive deficits that are milder but qualitatively similar to those of patients with schizophrenia. These findings support the notion that schizophrenia and bipolar disorder show greater phenotypic similarity in terms of the nature than severity of their neuropsychological deficits.

Figure 1. Mean effect sizes for six cognitive domains based on demographically-adjusted T-scores produced by patients with schizophrenia (SZ) and bipolar I disorder (BD) compared to healthy adults

MANOVA planned contrasts confirmed that each patient group differed significantly from healthy controls on every cognitive domain (p < 0.01). Bonferroni-corrected post hoc comparisons showed the SZ and BD groups differed significantly (p < 0.5) on all domains except Divided and sustained attention.

Figure 2. Effect sizes for 19 individual test scores based on demographically-adjusted T-scores produced by patients with schizophrenia (SZ) and bipolar I disorder (BD) compared to healthy adults

MANOVA planned contrasts confirmed that each patient group performed significantly (p < 0.01) worse than healthy adults on every measure except CPT Hit RT, on which the BD group did not differ significantly from healthy controls. Bonferroni-corrected post hoc comparisons showed the SZ and BD patient groups differed significantly (p < 0.05) on 11 of 19 measures. Note: GP-dom and GP-ndom = Grooved Pegboard dominant and non-dominant hands; Trails = Trail Making Test; BTA = Brief Test of Attention; CPT-HRT, CPT-RTse, and CPT-d’ = Continuous Performance Test hit reaction time, hit RT standard error, and discrimination; WCST-cat and WCST-pe = Wisconsin Card Sorting Test categories and perseverative errors; WF-letter and WF-cat = Word Fluency letters (S & P) and categories (animals & supermarket items); DFT = Design Fluency Test; HVLT-lrn, HVLT-del, HVLT-rec and BVMT-lrn, BVMT-del, BVMT-rec = Hopkins Verbal Learning Test and Brief Visuospatial Memory test learning, delayed recall, and recognition discrimination, respectively.