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

Abstract

Background: Although 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.

Methods: First, 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.

Results: Previous 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.

Limitations: The illustrative sample was relatively small and included only women; future research should examine generalizability of these findings.

Conclusions: A 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.

Keywords: Anxiety; Comorbidity; Cortisol; Depression; Symptom dimension.

Fig. 1

Examples of diverse psychobiological profiles that may be observed in individuals with two co-occurring disorders on the basis of the presumed features of each disorder alone. Note. Color denotation visually represents the profile of functioning observed in a given disorder (i.e., a disorder ‘feature’), compared to the profile of functioning observed in a control group, for particular psychobiological measures of interest (e.g., cortisol awakening response [CAR] and afternoon decline in cortisol). (A) Different colors denoted for Disorder A and Disorder B indicate unique psychobiological features (e.g., a flattened CAR versus a steeper afternoon decline). The table illustrates the varied possible profiles of functioning that may be observed when the two disorders co-occur. (B) The same color denoted for Disorder A, Disorder B, and the two co-occurring disorders presents an alternative model of a shared profile of functioning for the measure of interest (e.g., a flattened CAR and steeper afternoon decline across all groups). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Research approach for investigating the unique, shared, and interactive relations of two symptom dimensions to psychobiological functioning. Note. (A) Specific recommendations for sample selection with respect to levels of both depressive and anxiety symptoms are provided in Beuke et al. (2003) and Ingram and Hamilton (1999) and illustrated in Oehlberg et al. (2012). (B) Graphs on the left illustrate a unique positive relation of scores on Dimension A, and a unique negative relation of scores on Dimension B, to levels of functioning along the psychobiological measure of interest. In the top left graph, parallel lines indicate that scores on the two dimensions do not interact in their relation to the psychobiological measure. In the bottom left graph, non-parallel lines indicate a significant interaction between scores on the two dimensions in their relation to the psychobiological measure. Graphs on the right illustrate a shared positive relation of scores on Dimension A and scores on Dimension B to levels of functioning along the psychobiological measure. In the top right graph, parallel lines indicate that scores on the two dimensions do not interact in their relation to the psychobiological measure. In the bottom right graph, non-parallel lines indicate a significant interaction between scores on the two dimensions in their relation to the psychobiological measure.

Fig. 3

Interactive effects of depressive symptoms and anxious arousal on the cortisol awakening response (CAR), peak (post-awakening) cortisol, and afternoon decline in cortisol. Note. (A) Higher BAI scores associated with a steeper CAR, β=0.26, p=.042; qualified by an interactive effect of CES-D and BAI scores on the CAR, β= −0.19, p=.043. (B) Higher BAI scores associated with higher peak cortisol, β=0.12, p=.039; qualified by an interactive effect of CES-D and BAI scores on peak cortisol, β= −0.14, p=.001. (C) Higher CES-D scores associated with a flattened afternoon decline in cortisol, β=0.01, p=.043, and higher BAI scores associated with a marginally steeper afternoon decline in cortisol, β= −0.01, p=.063; qualified by an interactive effect of CES-D and BAI scores on the afternoon decline in cortisol, β=0.01, p=.005.