Prevalence of cognitive impairment in major depression and bipolar disorder

The current study examines prevalence of cognitive impairment in four mood disorder samples, using four definitions of impairment. The impact of premorbid IQ on prevalence was examined, and the influence of treatment response.

Samples were: (i) 58 inpatients in a current severe depressive episode (unipolar or bipolar), (ii) 69 unmedicated outpatients in a mild to moderate depressive episode (unipolar or bipolar), (iii) 56 outpatients with bipolar disorder, in a depressive episode, and (iv) 63 outpatients with bipolar disorder, currently euthymic. Cognitive assessment was conducted after treatment in Studies 1 (6 weeks of antidepressant treatment commenced on admission) and 2 (16-week course of cognitive behaviour therapy or schema therapy), allowing the impact of treatment response to be assessed. All mood disorder samples were compared with healthy control groups.

The prevalence of cognitive impairment was highest for the inpatient depression sample (Study 1), and lowest for the outpatient depression sample (Study 2). Substantial variability in rates was observed depending on the definition of impairment used. Correcting cognitive performance for premorbid IQ had a significant impact on the prevalence of cognitive impairment in the inpatient depression sample. There was minimal evidence that treatment response impacted on prevalence of cognitive impairment, except in the domain of psychomotor speed in inpatients.

As interventions aiming to improve cognitive outcomes in mood disorders receive increasing research focus, the issue of setting a cut-off level of cognitive impairment for screening purposes becomes a priority. This analysis demonstrates important differences in samples likely to be recruited depending on the definition of cognitive impairment and begins to examine the importance of premorbid IQ in determining who is impaired.