Letters to the Editor
Reply: Experimental Affective Symptoms in Panic Disorder Patients
Dear Editor: We thank Dr Nardi for expressing his interest in our work. We are aware that the research of his team is within the same field of respiratory challenges in affective spectrum disorders, especially panic disorder (PD). Although Dr Nardi refers to results of a hyperventilation-provocation study (1) and we reported on the CO2 challenge, there is a common factor between the 2 procedures: both provoke a disturbance in respiratory homeostasis. As Nardi himself points out, the crucial issue in PD may be a problem of general instability of (cardio) respiratory control mechanisms (2,3). If this assumption is correct, other interventions resulting in disturbed respiratory homeostasis, apart from CO2 challenges, may help to reveal underlying vulnerabilities in PD, as suggested in the case of the hyperventilation-provocation paradigm.
Dr Nardi’s letter underscores the need for further research in the field of comorbidity and the complex relations of psychiatric syndromes. The Nardi and others study of hyperventilation challenge in PD and MDD showed a comparable reaction of the PD and MDP (MD with panic attacks) group to the hyperventilation challenge (1).
Patients with MDP could be considered as suffering from a less severe variant of PD than patients with MDD plus full diagnostic PD, and therefore, their comparable reaction to a hyperventilation challenge (regarding both reactive panic attacks and heart-rate acceleration) points to a high susceptibility to respiratory challenges, which they have in common with full diagnostic PD patients. This is interesting because Nardi and others’ MDP group consists of patients having MD with sporadic panic attacks, whereas in our study we included subjects with full diagnostic PD, either with or without full diagnostic MDD. The observation that a single symptom (sporadic panic attacks) suffices to alter the response to the hyperventilation challenge toward the line of response observed in subjects with the full syndrome suggests a common underlying biological susceptibility. Both studies illustrate that the presence of comorbidity modifies the outcome of a biological challenge.
Actually, the dependent variables in our study were the affective symptoms of anxiety, depression, and aggression, whereas the Nardi and others study focuses on the physiologic variable of heart rate. The results of both studies taken together call for a reappraisal of the use of respiratory challenges in affective disorders in general. As well, the scope of subjects under study could be extended to mood disorder patients and anxiety disorder patients. Also, a more comprehensive scope of dependent variables should be assessed in future studies, encompassing several biological, physiological, and psychological dimensions of negative affectivity.
References
1. Nardi AE, Valenca AM, Nascimento I, Zin WA. Hyperventilation challenge test in panic disorder and depression with panic attacks. Psychiatry Res 2001;105:57–65.
2. Perna G, Caldirola D, Bellodi L. Panic disorder: from respiration to the homeostatic brain. Acta Neuropsychiatr 2004;16:57–67.
3. Caldirola D, Bellodi L, Caumo A, Migliarese G, Perna G. Approximate entropy of respiratory patterns in panic disorder. Am J Psychiatry 2004;161:79–87.
Thea Overbeek, MD, PhD
Koen Schruers, MD, PhD
Eric Griez, MA, MD, PhD
Maastricht, the Netherlands
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