Original Research

Clinical Factors That Predict Cognitive Function in Patients With Major Depression
Safa Elgamal, MD, PhD^; Susan Denburg, PhD^; Michael Marriott, Glenda MacQueen, MD, PhD, C
Objectives: To compare the performance of depressed patients to healthy control subjects on discrete cognitive domains derived from factor analysis and to examine the factors that may influence the performance of depressed patients on cognitive domains in a large sample. IVIethods: We compared the cognitive performance of 149 patients with major depression to 104 healthy control subjects using multivariate ANCOVA. We used principal component factor analysis to group the cognitive variables into cognitive domains. Finally, we conducted regression analysis to examine the contribution of predictor factors to the cognitive domains that were impaired in the depressed group. Results: Verbal memory and speed of processing were impaired in depressed patients, compared with healthy control subjects. Patient IQ, duration of depressive illness, and number of hospitalizations significantly contributed to the performance of patients on verbal memory and speed of processing. The severity of mood symptoms did not correlate with performance on any cognitive domain. Conclusions: Understanding the factors that predict cognitive performance of patients with depression may provide an insight into the processes by which depression leads to cognitive dysfunction. Our study showed that premorbid IQ and factors related to burden of illness are strong independent predictors of cognitive dysfunction in patients with major depression. Can J Psychiatry. 2010;55(10):653-661.

Ciinical Implications • The magnitude of cognitive deficit in depression is attributable both to patient characteristics and to illness factors. • Mood state is not associated with performance on cognitive tasks in depressed patients. • Because depression is not a homogenous disorder, the profile of cognitive impairment is variable among patient populations. Limitations • This cross-sectional study did not allow examining the magnitude of change in cognitive function over time. • We could not control for medications in a systematic manner because many chronic patients had exposure to multiple medications. • This study included patients with a relatively low intensity of mood symptoms and limited variability, which may affect the generalizability of results.

Key Words: major depression, mood disorder, cognitive dysfunction, verbal memory impairment, speed of processing, factor analysis

The Canadian Journal of Psychiatry, Vol 55, No 10, October 2010

653

Original Research

DD is associated with a range of cognitive deficits. Attention, memory, speed of processing, verbal fluency, and executive function have been examined and while some studies report impairment in these domains,'"^ others have not found differences in patients, compared with control subjects.'^'' The factors that may contribute to cognitive dysfunction in MDD have not been systematically examined in large samples. Some,*'^ but not all, studies^ report that age is negatively associated with verbal memory and executive function. Severity of mood symptoms negatively correlated with processing speed'" and executive function" in some studies, but not with verbal memory in other studies.^''^ The number of past depressive episodes predicted performance on recollection memory,''' recall memory,''* and executive dysfunction'; but not all studies report the same association.'^''* Age of onset of illness^ (Bearden et aP), duration of illness'^"'' (Reischies and Neu'' and Verdoux and Liraud^"), and number of hospitalizations'*''^ predicted poor performance on one or more cognitive domains. Medication use did not affect cognitive performance in some studies,'''•^' but other studies have reported superior performance for treated patients. "'^•^ Given the heterogeneity in study results examining the factors contributing to