The respiratory airways are lined with nerve fibres located just beneath the surface.
These fibres are vital for keeping your airways clean, and respond to changes in temperature, dust and other particles, acid and other irritants by inducing the cough reflex.
This isn’t the only way in which a cough can be triggered, as there are receptors in the throat, nose, ears and even on the lining of the lung and diaphragm as well.
There’s considerable overlap in these complicated nerve pathways, which sometimes makes the diagnosis of the cause for a chronic cough difficult.
The approach to identifying the cause of a chronic cough is usually to:
1.) Look for the most common causes.
2.) Exclude and/or treat these causes.
3.) Do further investigations for the rarer causes (unless they’re obviously apparent in the beginning).
Generally, a chronic cough occurs as an isolated symptom that isn’t accompanied by other complaints such as fever, weight loss or coughing of blood. If any of these symptoms are present, your doctor will first direct his or her attention at specific or alternative causes.
The most common causes of a chronic cough are post-nasal drip (often called the “upper airway cough syndrome”), acid reflux, asthma, and chronic obstructive pulmonary disease (COPD).
A smoker’s cough (experienced early morning with sputum production) isn’t medically considered to be part of this chronic cough constellation. If you do have a smoker’s cough, the treatment is to quit smoking.
The cough will usually resolve within a month of giving up smoking, but may actually get worse before resolving as your airways start to clear out all the “gunk”.
Be aware that if your “normal” smoker’s cough changes, if you see blood in the sputum and/or if the colour and amount of sputum changes significantly, you should see a doctor.
The most common causes of chronic cough are listed below:
• Upper airway cough syndrome (chronic post-nasal drip). Mucus produced in the nose may either be swallowed or removed by clearing the throat during the day. At night, mucus may pass through the vocal cords into the upper airways. This results in irritation and inflammation of the airways, and a resultant cough.
The post-nasal drip may be a result of allergies, and treatment of the allergic response in the nose may resolve the problematic cough. Symptoms may be subtle and a trial of treatment is often instituted. Even if you don’t have known allergies, this is usually the starting point.
• Asthma. Asthma can develop at any age and is often missed as a diagnosis for many months. A chronic cough may be the only symptom of asthma in some people, and investigations to confirm asthma should be done if this is suspected. Standard asthma treatment with inhalers should solve the problem cough.
• Chronic obstructive pulmonary disease (COPD). Smoking-induced airways disease (COPD/emphysema) frequently result in a cough, over and above the usual “smokers cough”. The hallmark of COPD is a chronic cough and slowly progressive shortness of breath. Stopping smoking, and using specific inhaled treatment for COPD, will help reduce the cough.
• Acid reflux. Symptoms of heartburn may be an indication of acid reflux, but this symptom isn’t always present. Chemical irritation of the airways by stomach contents being breathed in (reflux) may, however, lead to a chronic cough. A special 24-hour pH monitoring test can be used to measure the acid levels in the oesophagus and to relate these to the episodes of coughing.
Treatment of the reflux with medication may be all that’s required, but more intensive treatment can be employed if no response to medication is achieved and the acid reflux is confirmed to be the cause of the cough.
• Anti-hypertensive drugs (angiotensin-converting enzyme inhibitors or ACE inhibitors). Commonly used blood-pressure medication has a particularly bad reputation for inducing a chronic cough. The cough usually starts quite soon after starting the drug, but sometimes occur some time after first commencing its use.
One always needs to consider an ACE inhibitor as a possible cause, even if you’ve been taking the drug for some time. Replacement of this class of drug with other forms of anti-hypertensives will stop the cough, usually over a few days to weeks. It’s important to switch the anti-hypertensive drug under a doctor’s supervision, as simply stopping it may result in uncontrolled blood pressure.
• Chronic inflammatory conditions such as TB or HIV-associated respiratory disease. These may also present with just a chronic cough and, in developing countries, they’re frequently the starting point of investigations. They’re given a high priority, and always have to be excluded in people with respiratory symptoms (especially in individuals living with HIV/AIDS).
In developed countries, on the other hand, these conditions may not be initially recognised as a cause of a chronic cough. If not suspected, it can go undiagnosed for several months.
• Lung cancer may induce chronic cough due to the presence of a tumour in the airway or irritation of the lung lining. In smokers who present with a chronic cough, the likelihood of lung cancer starts to increase significantly over the age of 40.
For this reason, a chest X-ray is often requested to look for possible cancer. Note that cancers aren’t always visible on a plain chest X-ray. If there’s a high suspicion or concern about a cancer, bronchoscopy (where a video camera is inserted into the airways) or a computerised tomography (CT) scan may be required to exclude cancer as a possible cause.
• Interstitial pulmonary fibrosis (IPF). This is a rare lung condition that usually occurs in older people. The cause of pulmonary fibrosis isn’t known but, essentially, the lungs begin to stiffen like an old sponge in the sun. This results in a persistent cough and shortness of breath.
On examining the lungs, a fine crackling sound can be heard – almost like the sound you hear when you pull on Velcro. The disease is confirmed by a CT scan or lung biopsy. The condition needs to be evaluated by a specialist, as treatment is available but expensive and not always easy to take.
There are many other causes (e.g. non-asthmatic eosinophilic bronchitis, cancer, presence of a foreign body) that could be considered if none of the more common causes can be identified.
Reviewed by Professor Richard van Zyl-Smit, Head of the Lung Clinical Research Unit at the University of Cape Town. MBChB, MRCP(UK), Dip HIV(Man), MMED, FCP(SA), Cert Pulm(SA), PhD.