Neuropsychological performance in bipolar I, bipolar II and unipolar depression patients: a longitudinal, naturalistic study

Background: It has been suggested that cognitive deficits existed in mood disorders. Nevertheless, whether neuropsychological profiles differ three main subtypes of mood disorder (Bipolar I, Bipolar II and UP) remain understudied because most current studies include either mixed samples of bipolar I and bipolar II patients or mixed samples of different states of the illness. The main aim of the present study is to determine whether, or to some extent, specific cognitive domains could differentiate the main subtypes of mood disorders in the depressed and clinically remitted status.

Method: Three groups of bipolar I (n=92), bipolar II (n=131) and unipolar depression (UP) patients (n=293) were tested with a battery of neuropsychological tests both at baseline (during a depressive episode) and after 6 weeks of treatment, contrasting with 202 healthy controls on cognitive performance. The cognitive domains include processing speed, attention, memory, verbal fluency and executive function.

Results: At the acute depressive state, the three patient groups (bipolar I, bipolar II and UP) showed cognitive dysfunction in processing speed, memory, verbal fluency and executive function but not in attention compared with controls. Post comparisons revealed that bipolar I depressed patients performed significantly worse in verbal fluency and executive function than bipolar II and UP depressed patients. No difference was found between bipolar II and UP depressed patients except for the visual memory. After 6 weeks of treatment, clinically remitted bipolar I and bipolar II patients only displayed cognitive impairment in processing speed and visual memory. Remitted UP patients showed cognitive impairment in executive function in addition to processing speed and visual memory. The three remitted patient groups scored similarly in processing speed and visual memory.

Limitation: Clinically remitted patients were just recovered from a major depressive episode after 6 weeks of treatment and in relatively unstable state.

Conclusion: Bipolar I, bipolar II and UP patients have a similar pattern of cognitive impairment during the state of acute depressive episode, but bipolar I patients experience greater impairment than bipolar II and UP patients. In clinical remission, both bipolar and UP patients show cognitive deficits in processing speed and visual memory, and executive dysfunction might be a status-maker for bipolar disorder, but a trait-marker for UP.