Comorbidity and mixed anxiety-depressive disorders: is there epidemiologic evidence?

Recent epidemiologic studies (i.e., studies conducted since 1980) have consistently demonstrated, on the basis of standardized diagnostic assessments, that there is a substantial overlap between different types of anxiety and depressive disorders. The current literature, however, discusses this issue primarily within the concept of comorbidity and there are some controversies about the existence of a separate disorder of mixed anxiety-depression (MAD). MAD can be defined by the presence of mixed symptoms of depression and anxiety that are below the diagnostic threshold for either one of these diagnoses. Since MAD has not been included in any of the current official classification systems, its prevalence, risk factors, course, and outcome have not been studied specifically in any of the recent epidemiologic studies even though MAD is thought to be very important, especially in primary care settings. This paper reviews recent epidemiologic studies and presents data from the Munich Follow-Up Study, which has found a prevalence of about 1% for MAD as defined by the ICD-10. Despite the lack of clear diagnostic criteria for MAD, there are some indications that: (1) this disorder might be frequent in primary care settings, and (2) patients with MAD frequently demonstrate subjective suffering, show impairment in personal and occupational functioning, and have high health service utilization rates. Current empirical evidence is still insufficient for deciding a suitable classificatory solution for this problem.