Background: The nosological status of dysthymia has generated considerable controversy. The major issues include whether dysthymia should be classified as a form of mood or personality disorder and, if dysthymia is classified as a mood disorder, whether it is sufficiently distinct from major depression to warrant a separate category.
Methods: We conducted a family study of 97 outpatients with early-onset dysthymia, 45 outpatients with episodic major depression, and 45 normal controls, and their 882 first-degree relatives. Axis I and II disorders were assessed in relatives using direct and informant interviews and all available medical records.
Results: The rate of major depression in the relatives of early-onset dysthymic probands was significantly greater than in the relatives of normal controls and non-significantly greater than in the relatives of episodic major depressive probands. The rate of dysthymia was significantly greater in the relatives of dysthymic probands than in relatives of both major depressive probands and normal controls. Rates of most personality disorders were increased in the relatives of the dysthymic and major depressive probands compared with relatives of normal controls. In addition, the relatives of dysthymic probands had significantly higher rates of any personality disorder and any cluster B disorder than those of episodic major depressive probands, although these differences disappeared after controlling for Axis II comorbidity in the probands. Finally, dysthymic probands with and without a lifetime history of major depression did not differ on rates of psychiatric disorders in relatives.
Conclusions: There is a strong familial relationship between dysthymia and major depression. However, dysthymia is also somewhat distinct in that it aggregates specifically in the families of patients with dysthymia. Finally, dysthymia and episodic major depression both appear to have a familial association with the personality disorders, although the link appears to be somewhat stronger for dysthymia.