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Chronic cough is defined as cough of more than 2 months duration. If it continues to persist despite thorough diagnostic evaluation and appropriate therapy it is termed as unexplained cough. Chronic cough is more common in females than in males. The patients with chronic cough with normal CXR should be evaluated for the commonest causes first and it they are normal then move on to the uncommon causes of cough.

Causes of cough with normal CXR
The following are some of the cause of chronic cough with normal chest X ray.

1. Upper airway cough syndrome or post nasal drip
2. Asthma – undiagnosed
3. Non-asthmatic eosinophilic bronchitis (NAEB)
4. Gastroesophageal reflux disease (GERD)
5. Smoking related
6. Drugs like ACE inhibitors
7. Sinusitis
8. Nasal polyposis
9. Benign or malignant airway lesion
10. Obstructive sleep apnea
11. Psychogenic cough

Approach to patient
The commonest cause of unexplained cough should be evaluated first in the order of frequency. The four commonest causes of unexplained cough are UACS, Asthma, GERD and Non-asthmatic eosinophilic bronchitis. The therapy should be added one by one for each of these conditions sequentially and in additive manner, as more than one etiology may be contributing to chronic cough.
1) Advice the patient to discontinue smoking and ACE inhibitor is stopped if the patient is already taking it.
2) Empiric treatment for Upper airway cough syndrome/post nasal drip with H1 blockers
3) Pulmonary function test to rule out asthma. If asthma is strongly suspected but the PFT is normal, bronchial provocation challenge can be done.
4) Empiric treatment of GERD
5) Assess compliance with the medications.

UACS
UACS can be asymptomatic. H1 antogonists are the commonly used drugs. Other drugs like intranasal antihistamines, nasal steroids, mucolytics and leukotriene modifiers have also been used.

Asthma
Cough variant asthma can present as chronic cough. Pulmonary function testing and methacholine challenge testing can be done to diagnose asthma

GERD
Silent GERD can cause chronic cough. Even with the initiation of appropriate treatment the cough may continue for some time as it is estimated that is may take 2 to 3 months of treatment for symptoms to improve and upto 6 months for the cough to resolve.

Drugs
ACE inhibitor is the commonest drug class causing cough. Drug needs to be stopped atleast a month for symptomatic improvement

If there is no significant response, the following investigations may be done based on clinical suspicion in individual patients.

1. HRCT – Chest
2. X ray PNS and CT-Sinuses
3. Bronchoscopy – trans nasal route to examine the upper airway
4. 24 hour pH monitoring to rule out GERD
5. Endoscopic swallow evaluation
6. Echocardiogram to rule out LV dysfunction
7. Environmental assessment to rule out allergen induced cough

HRCT – Chest can diagnose conditions like interstitial lung disease, lung infections, emphysematous changes, mediastinal nodes causing external compression of bronchi, bronchiectasis and other pulmonary conditions. Bronchoscopy should be done to rule out any benign or malignant lesions in the airway that may be the cause of cough. If the cough is unexplainable with routine evaluation, the patient should be assessed by a multidisciplinary team involving pulmonologist, ENT specialist, Gastroenterologist and cardiologist

Refractory Cough
Currently no guidelines exists for the treatment of truly refractory cough. Several therapeutic options like speech therapy, nebulised local anaesthetics, gabapentin, amitriptyline, short term opiate trial, unilateral superselective vagotomy are suggested but none of them are widely used and more research is needed to test the efficacy of these modalities. Procedures like super selective vagotomy may obliterate the cough reflex and increase the risk of lung infections.