The results from our clinical trial provide support that a structured CBT protocol for panic disorder is similarly efficacious for individuals with and without common comorbid anxiety and unipolar mood disorders. Furthermore, successful implementation of the protocol was associated with reduction in symptoms (although not necessarily diagnoses) of anxiety and depressive disorders not specifically addressed in the treatment protocol. Rates of comorbid GAD declined significantly from pre- to posttreatment, and although rates of MDD, specific phobia, and social phobia declined, these changes were not significant. Not surprisingly, treatment responders showed greater reductions on measures of anxiety and depression across treatment, as compared to nonresponders.
Another important finding was that individuals with comorbid depression (with or without comorbid anxiety) showed similar improvement across treatment on a measure of panic disorder and agoraphobia severity (PDSS-IE), as compared to participants with only comorbid anxiety or no comorbidity at all. These data contradict findings from other studies suggesting that the presence of comorbid depression is associated with worse outcome (Brown et al. 1995
; Reif et al. 2000
; Steketee et al. 2001
), although perhaps the smaller sample sizes in these studies may have been related to the differential findings. Laberge et al. (1993
) found that depression was not associated with worse outcome, despite depressed patients’ tendency to minimize gains. Similarly, McLean et al. (1998
) found no adverse effects of comorbid depression when treating panic disorder. The results from our investigation provide support that comorbid depression is not associated with worse outcome; however, this rather restricted sample due to exclusionary criteria (e.g., bipolar mood disorders) and non-assessment of Axis II disorders make it difficult to draw strong conclusions. Rates of comorbid MDD also decreased, but this decrease did not reach statistical significance. This may suggest that comorbid depressive disorders are somewhat less “responsive” to CBT for panic disorder, as compared to anxiety disorders such as GAD, or may be an artifact of a somewhat smaller rate of comorbid MDD. Similarly, rates of comorbid social phobia and specific phobia showed improvement that also did not reach statistical significance. Further exploration of the relationship of depression and specific types of anxiety disorders to panic disorder treatment is warranted.
One issue that has proven difficult to reconcile is whether the presence of comorbidity is associated with more severe symptoms of the principal disorder (Berkson 1946
). The few studies examining this aspect of comorbidity have yielded conflicting results. Brown et al. (1995
) found that individuals with panic disorder and at least one additional diagnosis were rated as having more severe panic symptoms than those without any additional diagnoses. In addition, comorbid individuals evidenced higher ASI and WSAS scores. However, Tsao et al. (1998
) found no such difference in their sample of panic disorder patients with and without comorbidity (which may have been due to relatively small sample size), although, in a follow-up investigation, Tsao and colleagues (2002
) reported that patients with comorbidity reported significantly higher ASI scores than those without comorbidity. Our study demonstrated that COM individuals were rated as more severe on all measures of panic disorder, and number of comorbid diagnoses was significantly associated with measures of panic disorder and agoraphobia severity. Given the variety of measures used, and the fact that the data were taken from both patients and independent evaluators, this finding appears robust, at least with respect to the relationship between anxiety and depression comorbidity and panic disorder. Even more encouraging is that, despite having more severe panic disorder symptoms at pretreatment, individuals with comorbid anxiety and/or depression were equally likely to respond to treatment as those without such comorbid diagnoses, at least on categorical measures of treatment response.
There are several possible mechanisms by which CBT for panic disorder may be having an impact on comorbid conditions, including 1) many of the skills taught in CBT for panic (e.g., monitoring, cognitive restructuring, and exposure) are useful in coping with other emotional disorders and 2) alleviating panic symptoms as a “cause” for other symptoms of anxiety and depression will necessarily reduce the prevalence and intensity of “secondary” comorbid disorders. A potentially more interesting hypothesis is that CBT may be targeting a broader problem experiencing and regulating emotions (see Barlow et al. 2004
; Campbell-Sills and Barlow 2007
). CBT, then, may serve to reduce the intensity of all emotional experiences, rather than just panic symptoms.
There are several limitations of the current study worth noting. First, due to the nature of the study, only patients with a principal diagnosis of panic disorder were included. In addition, we did not include an assessment of Axis II disorders, which limits generalizability of the findings to comorbid diagnoses outside of anxiety and mood disorders. Study inclusionary criteria may have also restricted the applicability of the findings to a more diverse patient population. Replication of these findings is needed in more diagnostically and ethnically diverse community and clinical samples. Additionally, there was no control condition in the current study (i.e., all patients received the same treatment), so there can be no guarantee that treatment effects are specifically related to CBT. Finally, the nature of the study design precludes follow-up analyses to explore the relationship of CBT for panic disorder to comorbidity over time.
Nonetheless, findings from the current study add to the knowledge of panic disorder and comorbid anxiety and depression in a number of important ways. These data confirm findings that comorbidity does not necessarily impede treatment for panic disorder, although highlight the possibility of differential relationships between panic disorder and specific comorbid anxiety and depressive disorders. Further research exploring these differential relationships is clearly warranted, particularly with more generous inclusionary criteria, and the relationship of comorbidity and treatment outcome for other disorders.