Depression and anxiety disorders have overlapping symptomatology. As defined by the DSM-IV-TR (American Psychiatric Association, 2000), major depression and generalized anxiety disorder share four diagnostic symptoms, which include sleep disturbance, difficulty concentrating, restlessness and fatigue. In the DSM-III all anxiety disorders (i.e., social phobia, panic disorder, etc.) were required to occur independently of and symptoms were to be present separate from a depressive episode. However, in the DSM-IIIR (American Psychiatric Association, 1987) this hierarchy was eliminated for all anxiety disorders except generalized anxiety disorder (Zimmerman and Chelminski, 2003). The diagnostic criteria for generalized anxiety disorder states that criteria must be reached independently and outside the presence of a depressive disorder, and this exception is likely related to the fact that both disorders have overlapping symptoms (Zimmermann and Chelminski, 2003; DSM-IV-TR, 2000).
Generalized anxiety disorder was first listed as a separate disorder in the DSM-III (American Psychiatric Association, 1980). According to Kessler et al. (1999b) , since its introduction generalized anxiety disorder has been noted to be frequently comorbid with other depressive disorders as well as other psychiatric disorders. Using the results from two population-based studies, Kessler et al. (1999b) examined the 12-month prevalence rates of individuals with generalized anxiety disorder as the primary disorder who also met criteria for major depression. In the National Comorbidity Survey (NCS) 58.1% had both disorders, and in the Midlife Development in the US Survey, 69.7% met criteria for both depression and anxiety. Interestingly, among individuals with a primary diagnosis of depression, only 17.5% in the NCS and 16.3% in the Midlife Development in the US Survey met criteria for comorbid generalized anxiety disorder and major depression.
In the Early Developmental Stages of Psychopathology Study, a prospective longitudinal study, Bittner et al. (2004) found the presence of any anxiety disorder significantly increases the risk for a subsequent major depressive disorder (OR = 2.2, 95%; CI 1.6 — 3.2). Additionally, any of the individual anxiety disorders also increase the risk for a depressive disorder. Generalized anxiety disorder had the largest risk (OR = 4.5, 95%; CI 1.9 - 10.3). Panic disorder (OR = 3.4, 95%; CI 1.2 - 9.0) and agoraphobia (OR = 3.1, 95%; CI 1.4 - 6.7) had the second and third highest risk. Social phobia and specific phobia also had an increased risk for major depression with odds ratio of 2.9 (95%; CI 1.7 — 4.8) and 1.9 (95%; CI 1.2 — 2.8) respectively. Using data from the NCS, Choy e t al. (2007) found a lifetime prevalence of major depression among individuals with specific phobia to be 40.7%, and in contrast only 14% had major depression without specific phobia. Interestingly, if more than one fear was present the risk of depression was even more elevated and ranged from OR=2.5 (95%; CI 2.0 — 3.1) for two fears to OR = 5.7 (95%; CI 4.3 — 7.6) for more than five fears. After adjusting for lifetime comorbid anxiety disorders, an odds ratio of 1.9 (95%; CI, 1.6 — 2.4) was reported as the risk for major depression if diagnosed with a specific phobia. Additionally, after adjusting for other anxiety disorders, the ECA study reported an odds ratio of 1.7 (95%; CI, 1.6 - 1.8) for depression at 12-month follow-up (Goodwin, 2002).
This frequent co-occurrence adds to speculation and debate over the possibility that generalized anxiety disorder might not be an independent disorder but an actual presentation of major depression or other comorbid psychiatric diagnoses (Kessler et al., 1999b). One central issue in this debate is the extent to which generalized anxiety disorder actually results in functional impairments and to what extent these impairments are related to comorbid depression and other psychiatric disorders (Kessler et al., 1999b).
There is evidence to support the notion that generalized anxiety and depression are two distinct disorders. First, Brown et al. (1998) identified separate latent factors of positive and negative affectivity, providing support for the argument that defines the disorders as separate and distinct in spite of the fact that they share four core symptoms. Additionally, Kessler et al. (1999b) found impairment ratings of generalized anxiety disorder alone were equal to those of major depression alone even when controlling for other DSM disorders. In conjunction with the above finding, if both disorders are present, the impairment rating is more severe than either of the two disorders alone (Kessler et al., 1999b , Zimmerman and Chelminski, 2003). These results indicate impairments caused by generalized anxiety disorder are likely to be independently significant and not the result of another disorder. Finally, and somewhat contradictory, twin studies have indicated that generalized anxiety disorder and major depression share the same gene; however, epidemiological research indicates there are different sociodemographic predictors for each disorder (Kendler et al., 1992, Skodal et al., 1994). It appears there is evidence to support two distinct disorders (Angst, 1997); however, the exact relationship between anxiety and depression remains to be more clearly delineated.
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