Evolution of cognitive impairment in bipolar disorder: a systematic review of cross-sectional evidence

Objectives: The notion that sufferers of bipolar disorder achieve complete syndromal and functional recovery between illness episodes has been brought into question by evidence that a large proportion of patients fail to regain premorbid levels of functioning after the resolution of major affective symptoms. A growing body of evidence suggests that bipolar patients exhibit neuropsychological impairment that persists even during the euthymic state, which may be a contributory factor to poor psychosocial outcome. However, the aetiology of such impairment and its relation to progression of illness are not well understood. This review aims to consider evidence from studies investigating both the relationship between cognitive impairment and clinical outcome and studies of neurocognitive function in unaffected first-degree relatives (FDRs) of bipolar sufferers to address issues of the temporal evolution of cognitive impairment in bipolar disorder. Methods: Systematic literature review. results: The weight of evidence suggests that greater neuropsychological dysfunction in bipolar disorder is associated with a worse prior course of illness, particularly the number of manic episodes, hospitalizations and length of illness. The most consistent finding was a negative relationship between the number of manic episodes and verbal declarative memory performance. Impairment in unaffected FDRs was reported in verbal declarative memory and some facets of executive function. Conclusions: Cognitive impairment may be a trait vulnerability factor for bipolar disorder that is present before illness onset and worsens as the illness progresses. Further investigation into the causal relationship between cognitive impairment and illness course is essential.