In order to examine the validity of diagnostic categories of depression, Li-Shiun Chen, M.D., and colleagues from The Johns Hopkins University charted the course of illness in 1856 patients with depression who had been part of the 1980-1983 Baltimore Epidemiologic Catchment Area (ECA) survey. To determine the heterogeneity of depression over time, investigators analyzed data from a 1993-1996 follow-up study. The subjects had received diagnoses of major depressive disorder, depressive syndrome only, dysthymia only or comorbid depression in 1981. The researchers also included 1,613 control subjects.
Researchers assessed the change in lifetime diagnosis status by cross-tabulating diagnoses from 1981 and 1982 with the follow-up assessments. Of the initial 59 subjects with major depressive disorder in 1981, 10 (16.9%) went on to develop comorbid depression. Of 136 subjects diagnosed with depressive syndrome in 1981, 13 (9.6%) developed major depressive disorder, 11 (8.1%) developed comorbid major depression and seven (5.1%) developed dysthymia. Of the initial 37 subjects with dysthymia only in 1981, seven (18.9%) developed comorbid major depression. Among the 1,613 controls, 38 (2.4%) developed major depressive disorder, 118 (7.3%) developed depressive syndrome, 31 (1.9%) developed dysthymia and 14 (0.9%) developed comorbid depression.
The researchers then looked at risk factors, including gender, family history of depressive disorder and stressful life event one year before first depressive episode. The odds ratios for female gender and family history were strongest for comorbid depression (5.22 and 8.32, respectively). For stressful life event the odds ratio was highest for depressive syndrome (3.92).
A multivariate analysis also showed gender to be the highest risk factor for comorbid depression (12.20 odds ratio), but it was lowest for major depressive disorder (1.33 odds ratio). Chen et al. noted, “Dysthymia was the only category that was not significantly associated with any of the examined risk factors.”
The researchers concluded that depression categories were genetically homogeneous, but environmentally heterogeneous. Symptom profiles for all categories differed only in severity. “Since…depressive syndrome is associated with similar etiologic profiles to major depressive disorder and a substantial rate of impairment, we highlight the importance of depressive syndrome in genetic and clinical research. In contrast, the dysthymia category was similar to depressive syndrome regarding symptoms but not risk profiles…Its value as a distinct nosologic entity appears to be questionable.