Cognitive functions in depression and anxiety disorders : findings from a population-based study

This doctorial thesis examines cognitive functions in depression and anxiety disorders in a population-based sample that includes mostly untreated persons. It is well established that depression is associated with cognitive impairments. However, in spite of the fact that most of the depressed persons are untreated, almost all available evidence in this field is based on in-and outpatient samples. Also, little attention has been paid to cognitive functioning in anxiety disorders. The thesis includes four empirical studies that were based on data from the PART study, an ongoing population-based study of mental health in Stockholm, Sweden. The specific objectives of Study I were to examine whether there is an association between depression and cognitive abilities including episodic memory, verbal ability, psychomotor speed (TMT-A), and executive function (TMT-B) as well as to examine whether potential cognitive deficits vary as a function of DSM-IV defined depression diagnoses. Similarly, in Study II we aimed to examine the relationship between anxiety disorders and cognitive functioning in the same abilities as in Study I and whether the observed cognitive impairments varied as a function of anxiety diagnosis. In Study III, the major objective was to study cognitive functioning in recovery from depression by following up a sample of depressed persons three years later. Study IV, finally, aimed to investigate premorbid markers of depression with a specific focus on low episodic memory performance. This was accomplished by prospective examination of a cohort of depression-free persons three years after the baseline examination, at which a group of these persons received a depression diagnosis. Results from Study I indicated that depressive disorder was associated with cognitive dysfunction. Depression-related deficits were observed in tests tapping episodic memory and executive function. Further, we found that persons diagnosed with Major Depressive Disorder and Mixed Anxiety Depressive Disorder showed significant deficits in episodic memory functioning, whereas Dysthymia was associated with impaired executive function. Minor Depressive Disorder was not found to be associated with cognitive dysfunction. The pattern of results in Study II was comparable to the observations in Study I. We found anxiety-related impairments in episodic memory that remained even after controlling for comorbid depression. Specifically, Panic Syndrome (PD), Social Phobia (SP), and Obsessive Compulsive Disorder (OCD) were associated with episodic memory dysfunction, whereas Specific Phobia was not. In addition, we observed executive dysfunction in anxiety and then specifically in persons affected by PD and OCD. However, these deficits were non-significant after controlling for alcohol abuse/dependence, suggesting that excessive alcohol use may explain parts of these findings. Results from Study III demonstrated that the recovered persons suffered a continuous cognitive dysfunction. Results from Study IV suggested that low episodic memory performance, as measured by the sum of free and cued recall, was a significant risk factor for developing depression three years later independently of demographic, clinical and socioeconomic factors. Taken together, the findings from this thesis extend the picture of cognitive dysfunction in depression and anxiety disorders by including untreated persons sampled from the population. Further, the present findings suggest that episodic memory impairments persist beyond the recovery from depression and that low cognitive performance is present already three years before depression diagnoses. The overall conclusion that can be drawn from the thesis is that depression in particular, but also anxiety, are serious conditions that affect cognitive functioning indicating that these disorders are associated with brain dysfunction. This, in turn, may have a large negative impact, not only for the working and social lives of the persons affected by depression and anxiety, but also for society as a whole.