Finding the cause of a chronic cough can often be difficult. It is clear from your message that you are familiar with the more common causes and that these have been checked-for and ruled out. The following is taken from an article that appeared in the Journal Lung. It addresses the issue of chronic cough historically and then provides information about some newer concepts regarding causes of such a cough. I have provided the article for you to share with your doctors.
My advice to you for optimum evaluation of your cough, thus far refractory to diagnosis and treatment is that you request consultation at one of the many Cough Clinics at academic medical institutions in the U.S. These include National Jewish Health in Denver, CO., the University of Maryland Medical Center (For more information, consultation, or patient referral, contact the University of Maryland Dyspnea and Cough Clinic at 410-328-8141. Consumers or patients can call 1-800-492-5538., The Mayo Clinic, The Cleveland Clinic and Dr. Richard Irwin at the University of Massachusetts Medical center
Good luck. The Article follows:
Lung (2011) 189:73–79
Cough Hypersensitivity Syndrome: A Distinct Clinical Entity
A. H. Morice • S. Faruqi • C. E. Wright •
R. Thompson • J. M. Bland
Chronic cough in the absence of obvious respiratory disease is one of the most common presentations to secondary care. It has been repeatedly suggested that the triad of asthma, reflux disease, and postnasal drip syndrome, or a combination of these, was the cause of the cough in the
majority of patients . The form of asthma, however, was unlike classic atopic asthma, being of late onset without airflow obstruction (cough-variant asthma) , or even without bronchial hyperresponsiveness (eosinophilic bronchitis) . The diagnosis of postnasal drip syndrome was inferred by the reduction in cough on treatment with first-generation antihistamines  rather than by any objective assessment. Finally, gastro-oesophageal reflux
was clearly associated with coughing episodes in some patients, but studies using pH criteria have found a low symptom association probability . In recent series, it has been noted that no cause for cough could be found despite extensive investigations and these patients have acquired
the label of ‘‘idiopathic cough’’ . Millqvist et al.  have described a group of patients with upper airways hypersensitivity to a wide variety of
airway irritants, ranging from ethanol to perfumes and smoke. These patients typically have a chronic cough and have been demonstrated, using objective cough challenge techniques, to have hypersensitivity of their cough reflex. Similar hypersensitivity to inhalational cough challenge
has been demonstrated in chronic cough patients . We hypothesised that patients with chronic cough represent a distinct clinical entity consisting of chronic cough with hypersensitivity. It was our experience that these patients have a characteristic symptom complex in association with
their chronic cough which enables it to be categorised as a single syndrome. We have termed this the ‘‘Cough Hypersensitivity Syndrome.’’We proceeded to test this hypothesis by constructing a questionnaire that elicits the major components of this syndrome and subsequently testing its
validity. Some of the results of these studies have been previously reported in the form of an abstract [9, 10]
Thanks for that David. I wish I lived in America so I could try those clinics you mentioned. I'm in Australia - and I've never even heard of anything like a "cough clinic" here. So from what I gather there is no cure for "cough hypersensitivity syndrome".