Reply: Experimental Affective Symptoms in Panic Disorder Patients

Abstract

Dear Editor: In a recent issue of The Canadian Journal of Psychiatry, Overbeek and colleagues assessed depression and aggression symptoms beyond anxiety symptoms in panic disorder (PD) patients (1). They administered a gas mixture of 35% CO2 and 65% O2 to 25 patients with PD, 9 of whom also met the criteria for a comorbid major depressive disorder (MDD). It is suggested that, in addition to anxiety, CO2 challenge induces depressive and aggressive symptoms, specifically in PD patients with comorbid depression Multiple lines of evidence link biological, especially respiratory, findings to anxiety disorders (2,3) and mood disorders (4). Strategies that involve the use of respiratory challenge tests have been especially fruitful in generating hypotheses about PD (2,5) and about groups without PD but with a high susceptibility to respiratory-induced panic attacks (3,4). The strength of Overbeek and colleagues' study (1) is the inclusion of comorbid PD and MDD as an independent variable. In our trial with a hyperventilation test in PD, MDD, and MDD with panic attacks, we differentiated PD and MDD patients with panic attacks from MDD patients and normal control subjects (3). Our PD patients and control subjects reported similar symptoms, and we observed a similar heart rate, suggesting that PD patients perceive these symptoms to be more aversive, perhaps resulting in a greater likelihood of panic attack. The issues related to the concept of comorbidity between mood and anxiety disorders are complex and not fully defined (3). What is of concern from the biological point of view is the real possibility of a biological comorbidity that alters the pathogenesis of some syndromes so as to affect symptoms, diagnosis, and treatment. Investigations into the possible neurobiological abnormalities that might subserve the abnormal subjective response to these provocative agents are clearly worthwhile. One interesting observation from our data was the lower heart-rate response to the hyperventilating challenge test in the MDD group (3). Perhaps this differential response could also be detected in a CO2 challenge test. In our trial, all groups had an increase in heart rate, but the MDD patients had a significantly lower increase (3). We speculated that, as the anxiety baseline levels were lower in the MDD group before the test, they tended to be less anxious and less responsive to the possible anxiety-inducing symptoms that could occur during the session, because they had never had a panic attack. The patients in the panic groups PD and MDD with panic attacks had a higher level of anticipatory anxiety, and perhaps they might be included in a subgroup of PD patients with a hyperactive response to respiratory tests. Recent studies investigating the complexity of respiratory physiology have revealed consistent irregularities in respiratory pattern, suggesting that these abnormalities may be a vulnerability factor to panic attacks (5). The source of the high irregularity observed together with unpleasant respiratory sensations in PD patients is still unclear, and different underlying mechanisms might be hypothesized. Panic attacks could be the expression of primal emotion arising from an abnormal modulation of the respiratory or homeostatic functions (5), and provocative tests observing other symptoms besides anxiety symptoms can bring more substantial data to comorbidity research. Funding and Support This research was supported by the Brazilian Council for Scientific and Technological Development (CNPq), Grant 300500/93-9.