Seasonal affective disorder (SAD) is a type of depression with seasonal pattern. Although it involves some idiosyncratic symptoms, it can overlap with other depressive disorders such as major depressive disorder (MDD) or dysthymia. We aimed to characterize the differences in specific cognitive and clinical symptoms between SAD and depressive-related disorders.

In total, 4554 Finnish subjects from the population-based Health 2011 Survey were interviewed with the Munich version of Composite International Diagnostic Interview (M-CIDI) and filled in the Seasonal Pattern Assessment Questionnaire (SPAQ). From this sample for our analysis, we included those participants who fulfilled the criteria for SAD (n=171), MDD (n=153) or dysthymia (n=84) and their 816 psychologically healthy controls matched by age and gender. In addition to M-CIDI and SPAQ, the Beck Depression Inventory, the General Health Questionnaire, an abbreviated version of the Mini-Mental State Examination, the category verbal fluency test, and the CERAD 10-word list were used.

Subjects with dysthymia showed major deficits in both clinical and cognitive domains compared to MDD, SAD and healthy controls. Although clinical comorbidity was mild in SAD, these participants showed similar cognitive deficits to dysthymic subjects and greater impairments than MDD.

SAD subjects show a differential clinical and cognitive profile compared to other depressive-related disorders. Although less severe clinical symptoms are found in these individuals, some cognitive impairment already appears in subjects with SAD recruited from a population-based study.