Follow-up study of seasonal affective disorder in Switzerland.

Of 39 diagnosed Seasonal Affective Disorder (SAD) patients who were interviewed 2-5 years after participation in a light therapy trial, 10 continued to have recurrent major depressive episodes in winter, and 17 manifested sub-syndromal SAD (2 patients also had recurrent brief depression, seasonal type). 8 patients had recovered, and 4 had shifted in symptomatology. Thus, over a number of years, the clinical diagnosis changed for the better in 64% of the patients, suggesting that SAD is not a prodromal form of a more chronic major affective disorder, and that light therapy (and perhaps also light-oriented behaviour) reduced the incidence and depth of subsequent depressive episodes. Further evidence for this was the large reduction in use of conventional antidepressant drugs (from 17 to 1) during the follow-up period. Diagnosis of SAD was stable and reliable.