Statistics from Altmetric.com
We read with interest the recent article by Niimi et al reporting low levels of exhaled breath condensate (EBC) pH in patients with chronic cough.1 We and others have described low EBC pH in association with airway inflammation in allergic asthma, cystic fibrosis, and chronic obstructive pulmonary disease.2–4 In these studies there is a relatively close association between inflammation and low pH which is shown by the further fall in pH during exacerbations.2 However, in non-asthmatic chronic cough, while there is a low grade inflammation present in some subjects, this is much less than would be required to invoke inflammation as the major cause of airway acidification.
It is unclear from the description of the assessment protocol how patients were allotted their individual diagnostic categories. A positive methacholine challenge test is not infrequently found in patients with reflux5 and, even in classical asthma, reflux is a common phenomenon.6 We would suggest that there has been a significant underdiagnosis of reflux disease in this cohort because of the lack of a structured history, the non-uniform application of investigations, and the failure to perform full oesophageal assessment, particularly manometry. We have shown that, when oesophageal manometry is not performed, a significant number of patients with reflux cough will be missed.7 Proton pump inhibitors at conventional doses only temporarily increase the pH of gastric reflux and do not prevent reflux per se and, unsurprisingly, only improve symptoms in a proportion of patients with reflux cough. A failure of cough to improve with proton pump inhibitors does not therefore adequately rule out reflux cough.
The simplest explanation for the low airway pH observed by Niimi et al is that a large proportion of the subjects had laryngopharyngeal reflux. This would also explain the otherwise surprising finding of a similar EBC pH across the authors’ diagnostic categories.