Review articleInvestigating the nature of co-occurring depression and anxiety: Comparing diagnostic and dimensional research approaches

- Despite its prevalence, co-occurring depression and anxiety is rarely studied directly.
- Previous findings on the pathophysiology of diagnostic co-occurrence are inconsistent.
- Evidence suggests that, dimensionally, depressive and anxiety symptoms interact.
- Our sample data analysis indicates such interactions in relation to diurnal cortisol.
- A dimensional approach to co-occurrence is conceptually and practically advantageous.

BackgroundAlthough approximately half of adults diagnosed with a depressive or anxiety disorder exhibit their simultaneous co-occurrence, traditional research has centered on single-target diagnoses, overlooking comorbidities within samples. In this article, we review and extend the literature that directly investigates co-occurring depression and anxiety, with the goal of shifting the focus from co-occurring diagnoses to symptom dimensions.MethodsFirst, we review studies that have directly compared psychobiological features (neural, neuroendocrine, autonomic) across depression, anxiety, and their co-occurrence, defined either categorically or dimensionally. Second, we analyze adults' diurnal cortisol secretion to examine the independent and interactive relations of continuously-assessed depressive and anxiety symptoms to neuroendocrine function.ResultsPrevious findings on the psychobiology of diagnostic co-occurrence are mixed. While nascent, evidence from dimensionally focused studies suggests that co-occurring levels of depressive and anxiety symptoms can interact with one another, as reflected in a distinct psychobiological profile for individuals with high levels of both symptom dimensions. Results of our analyses support this formulation: we found that depressive and anxiety symptom dimensions interacted consistently in their relation to the measures of diurnal cortisol.LimitationsThe illustrative sample was relatively small and included only women; future research should examine generalizability of these findings.ConclusionsA dimensional approach to investigating the psychobiology of co-occurring depression and anxiety affords both conceptual and practical advantages. Simultaneously assessing depressive and anxiety symptom dimensions can efficiently capture their unique, shared, and interactive features, thereby identifying targets for intervention across a wide range of symptom presentations.