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I read with interest the study by Dohertyet al 1 in which cough reflex sensitivity in asthmatic subjects and patients with chronic obstructive pulmonary disease (COPD) was examined. The authors found that subjects with asthma were more sensitive to the tussive agent capsaicin than were normal subjects and, furthermore, that capsaicin sensitivity was related to the subjective assessment of symptomatic cough. They concluded that an enhanced cough reflex is an important contributor to cough in asthma.
I wish to share my own experience in this field, which has led me to a somewhat different conclusion. In our laboratory measurement of capsaicin sensitivity in over 200 healthy volunteers, as well as in a smaller group of stable asthmatic patients in whom cough was not a reported complaint, demonstrated no significant difference in cough reflex sensitivity between these two groups. Our findings are consistent with those of previous investigations2 3 and support the well documented dissociation between cough and bronchoconstriction,4 responses that are controlled by distinct neural pathways.
We have recently shown, however, that asthmatic subjects in whom cough is the sole or predominant symptom have significantly enhanced cough sensitivity compared with stable asthmatics without cough.5 I would therefore suggest that individuals with cough variant asthma comprise a distinct subgroup of asthmatics in whom the afferent airway receptors controlling cough are hypersensitive, whereas those in whom cough is not a significant feature do not differ from normal subjects in terms of cough reflex sensitivity.
Lending further support to this concept is our recent demonstration that the leukotriene receptor antagonist zafirlukast inhibits capsaicin sensitivity and symptomatic cough in subjects with cough variant asthma6 but does not affect cough reflex sensitivity in patients with stable asthma without cough7 or in healthy volunteers.8
authors' reply We were interested to read Dr Dicipinigaitis' comments about our paper. We are familiar with his contributions to the ongoing discussion about the role of sex differences in the response to inhaled capsaicin. The methodology used in his laboratory is similar to our own, although clearly differences in the dosimeter output might influence the response. Our study was not directed at this specific issue and is not appropriately powered to exclude a significant sex related difference in responsiveness in our control population. We believe our asthmatic patients to be more severe than those which he quotes in reference 5, and certainly our patients with COPD have evidence of substantial persisting pathology which we think is more likely to explain their enhanced responses. In our relatively large patient and control group combined we saw no evidence of sex differences in the degree of capsaicin response. This makes us suspect that enhanced responsiveness in our patient population is due to their underlying disease rather than to other factors. Clearly, this view cannot be extended to the important area of idiopathic cough where differences in sex may play a role.
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