A persistent cough is a cough that continues beyond the expected duration of an acute respiratory infection. The causes are numerous and can involve the upper airway, lungs, oesophagus, heart or medicines. Common causes include asthma, chronic obstructive pulmonary disease, post-infectious airway irritation, upper-airway cough syndrome associated with nasal or sinus disease, gastro-oesophageal reflux and smoking. The nature of the cough, whether it produces mucus, its timing, associated wheezing or breathlessness, smoking history, medication exposure and duration all help determine the likely cause. A persistent cough should not automatically be assumed to be a continuing infection, particularly when it lasts for weeks or is accompanied by warning signs.
Asthma is a common cause of persistent cough and can sometimes present predominantly with coughing rather than obvious wheezing. Asthma involves variable airway narrowing and inflammation, often producing cough, wheeze, chest tightness and breathlessness. Symptoms may be triggered by exercise, cold air, allergens, respiratory infections or other irritants and can be worse at night. Cough-variant asthma is a recognised presentation in which chronic cough is the dominant symptom. Spirometry and, when necessary, additional airway-hyperresponsiveness testing can help establish the diagnosis.
COPD is another important cause, particularly in people with a substantial history of cigarette smoking or other chronic inhalational exposure. Chronic bronchitis, one clinical component of COPD, involves persistent cough and sputum production associated with chronic airway inflammation and mucus hypersecretion. Emphysema, another major COPD phenotype, destroys alveolar structures and reduces elastic recoil, although cough may be less prominent than in chronic bronchitis. COPD can produce progressive exertional breathlessness, wheezing, recurrent respiratory infections and chronic mucus production. Smoking cessation and appropriate medical management are central to reducing further lung injury.
Upper-airway disorders can produce a persistent cough through irritation of the nose, throat and larynx. Allergic rhinitis, sinus inflammation and mucus draining from the upper airway can stimulate cough receptors. The sensation of mucus at the back of the throat is often described as post-nasal drip, although modern clinical terminology may group several upper-airway mechanisms under upper-airway cough syndrome. Persistent nasal congestion, sneezing, throat clearing and seasonal symptoms can provide clues. Treating the underlying nasal or sinus condition may improve the cough when this mechanism is responsible.
Gastro-oesophageal reflux can produce chronic cough because refluxed gastric contents can irritate the oesophagus and, in some circumstances, the laryngeal or airway region. Reflux may cause classic heartburn and regurgitation, but some people have cough or throat symptoms without obvious heartburn. However, reflux should not automatically be diagnosed as the cause of every unexplained cough because cough itself can have many causes and reflux may coexist with other respiratory disease. The relationship becomes more suggestive when coughing is associated with meals, lying down, nocturnal symptoms, sour taste or other reflux features.
Smoking and environmental exposure are major considerations. Cigarette smoke irritates the airways, promotes inflammation and impairs normal airway defence mechanisms, making chronic cough common among smokers. Second-hand smoke, dust, fumes, workplace chemicals and air pollution can also irritate the respiratory tract. Persistent exposure can worsen existing asthma or COPD and may contribute to chronic airway symptoms. Removing the exposure is therefore both diagnostic and therapeutic: if symptoms improve after exposure stops, that provides useful evidence, although persistent cough still requires assessment when it does not resolve.
Medicines can sometimes cause persistent cough. Angiotensin-converting enzyme inhibitors, commonly called ACE inhibitors and used to treat conditions such as hypertension and heart disease, are a classic medication-related cause of a persistent dry cough. The cough may begin weeks or months after treatment starts, so a person may not immediately associate it with the medicine. Other medication effects and pulmonary conditions also need consideration. A prescribed medicine should not be stopped without professional advice; a clinician can determine whether changing treatment is appropriate and whether another cause needs investigation.
Persistent cough can occasionally indicate more serious disease, including bronchiectasis, tuberculosis or lung cancer, particularly when accompanied by haemoptysis, unexplained weight loss, persistent chest pain, recurrent infections, significant breathlessness or other systemic symptoms. Coughing up blood should be assessed because although it can result from a severe cough or common respiratory infection, it can also indicate pulmonary embolism or lung cancer. Investigation may involve examination, chest X-ray, spirometry, blood tests, sputum analysis or CT imaging depending on the clinical presentation. The appropriate response to a persistent cough is therefore to identify its cause rather than repeatedly suppress the symptom without evaluation.
