Chronic Cough (Over 8 Weeks): Common Causes and Which Tests to Expect

Across the literature the same picture keeps appearing: in most non-smoking adults with a normal chest X-ray, one or several of a few 'classic' causes are responsible for the cough. The CHEST panel checked in a systematic review whether the older duration-based algorithms still hold up, and confirmed this list.

What does this mean for the reader? First, the list is short and familiar to primary care doctors, so a well-taken history often does lead to the diagnosis. Second, causes frequently overlap (hence 'combinations' in the summary), so improvement after treating one of them does not always mean the problem is over. Third, the authors themselves stress the low quality of the evidence: the algorithms are a sensible, practical framework, not something proven in large, well-designed trials.

It used to be assumed that behind every chronic cough sits one specific disease and that the cough will vanish once it is treated. The 2020 ERS guidelines shift that emphasis. In their view, many people with chronic cough have so-called cough hypersensitivity: excessive sensitivity of the sensory nerves of the airways (vagal afferents), so that the cough is triggered by stimuli that cause no reaction in others, such as cold air, perfume, smoke or cleaning products. This matters in practice: the cough has a physical explanation and is not a matter of 'nerves' or weak willpower.

The guidelines also describe treatable traits. Cough-variant asthma and eosinophilic bronchitis respond to anti-inflammatory treatment. According to the authors, non-acid reflux should be treated with agents that improve motility rather than with drugs that reduce acidity. For some patients with refractory cough, treatment that acts on nerve hypersensitivity is considered: low-dose morphine (effective in some patients, used in specialist centres), gabapentin and pregabalin (limited by side effects), and in the future drugs that block the P2X3 receptor. Cough suppression therapy is also an effective option when delivered by experienced therapists.

Cough hypersensitivity explains well what is seen in clinical practice. A post-infectious cough can last for weeks because the nerves in the airways stay 'irritated' after the infection is over. Coughing can also create a vicious circle: cough irritates the airways, and the irritation increases the cough. This does not mean the cause need not be sought. It means that after the trigger is removed the cough does not always clear completely, and that it takes patience and continued investigation.

The conclusion from PROCOUGH is both reassuring and honest. Reassuring, because guideline-based treatment cut cough frequency by more than half. Honest, because most patients, despite improvement, still did not feel well, and the study covers people treated in a specialist clinic with a cough averaging 7 years, that is, extremely difficult cases. These numbers cannot be transferred to everyone who has coughed for two months: in many people the cough resolves completely once a simple cause is found and treated.

Two factors are worth checking right at the start, because they are the cheapest to exclude and may be enough to solve the problem: drugs from the angiotensin-converting enzyme (ACE) inhibitor group, and smoking. ACE inhibitors (in Poland substances such as ramipril, enalapril or perindopril are used) are widely prescribed for high blood pressure and heart failure, and a dry, tickling cough is among the best-known adverse effects of this class. It can appear not only in the first days but also after months of use, which is why patients rarely link it to a tablet they have taken for a long time.

Two things deserve attention. First, almost one third of cases resolved without a new drug, probably by addressing treatable traits such as smoking and ACE inhibitors (the authors cautiously call this 'likely'). Second, in another, more recent analysis of 199 patients on ACE inhibitors attending a chest clinic with cough, as many as 76.9% had cough of pulmonary origin and only 23.1% had no identified lung disease. Longer-lasting cough, accompanying breathlessness and higher CRP more often pointed to lung disease rather than the drug alone. That is a single-centre, retrospective study and the authors themselves describe it as hypothesis-generating, but it illustrates an important point: taking an ACE inhibitor does not exclude another cause of cough, so one should not assume in advance that 'it must be the tablet'.

Gastro-oesophageal reflux is one of the most often named causes of chronic cough and at the same time one of the most misleading. Cough can occur even when there is no heartburn at all, and heartburn in a person who coughs does not prove that reflux causes the cough. In practice a 'therapeutic trial' with acid-suppressing drugs is therefore often used, but its efficacy is considerably more modest than once assumed.

This does not mean that reflux never causes cough or that acid-suppressing drugs help nobody. It means that as a routine, 'blind' trial in everyone who coughs the evidence is weak, and some people with reflux cough through a non-acid mechanism, which proton pump inhibitors do not address (hence the mention of prokinetic drugs in the ERS guidelines). Prolonging an ineffective course of a heartburn drug also has a price: the effects of long-term use are covered in our article on proton pump inhibitors. A sensible approach is to check whether there are other typical reflux symptoms, run a short, pre-planned trial of treatment, and end it or change strategy if nothing improves.

The standard first step is the history and physical examination, followed by a chest X-ray, usually ordered for every adult with cough lasting over 8 weeks, and spirometry. The X-ray looks for focal lesions, infiltrates, tumours, an enlarged heart and fluid around the lung. Spirometry checks whether the airways are narrowed (obstruction) and whether the narrowing eases after a bronchodilator, which suggests asthma. We describe these tests in detail in our entries on chest X-ray and on spirometry.

What follows from this? A normal X-ray is good news, but it does not 'close' the subject: in this group with difficult-to-treat cough, CT found something that changed further management in about 1 in 7 people with a normal X-ray. That is why a doctor who has not found the cause may order a CT, especially when the cough is persistent, there are red flags, or there is a smoking history. On the other hand, X-ray is not there to 'find the cause of cough' in general: the classic causes (UACS, asthma, reflux) cannot be seen on it. Its main job is to exclude serious, visible changes.

Equally misleading is the belief that a cough is 'just after an infection'. Post-infectious cough is a real cause of subacute cough (3-8 weeks), but once it passes 8 weeks other causes must be sought, including asthma, which can begin after a viral infection. Likewise, the belief that a doctor 'must see' the cough on an X-ray leads to false reassurance after a normal chest film, as discussed above. There is no magic tablet, only methodical elimination of causes.

Chronic cough is one of those symptoms where patience and method deliver more than guessing. Studies agree that there are usually several causes and that they overlap, and that hypersensitivity of the airway nerves can keep the cough going even after the original trigger has gone. It therefore pays most to start with the simple things (medicines, smoking, nose and sinuses, reflux symptoms, spirometry and X-ray), to set short, clear trials of treatment and judge their effect, rather than endlessly extending further syrups and antibiotics. At the same time it is worth remembering the red flags: chronic cough is also a symptom that in a small number of cases heralds serious lung disease, and then speed of diagnosis matters most.