Cough

A cough is a sudden and forceful expulsion of air from the lungs through the large breathing passages, serving as an essential protective reflex of the respiratory system. Its primary function is to clear the airways of mucus, fluids, irritants, foreign particles, and microorganisms. Although coughing is a normal physiological mechanism, frequent or persistent coughing commonly indicates an underlying disease process and is one of the most frequent symptoms prompting medical consultation.

Physiology of the cough reflex

The cough reflex is a complex, coordinated response involving sensory receptors, neural pathways, and respiratory muscles. It occurs in three sequential phases:

• Inspiratory phase, during which a deep inhalation increases lung volume
• Compressive phase, characterised by forced expiration against a closed glottis, generating high intrathoracic pressure
• Expulsive phase, marked by sudden opening of the glottis and rapid release of air, producing the characteristic cough sound

Sensory receptors capable of triggering cough are located throughout the respiratory tract, particularly in the larynx, trachea, and large bronchi. These receptors respond to mechanical and chemical stimuli and transmit signals via the vagus nerve to the cough centre in the medulla oblongata.

Biological and public health significance

Coughing plays a dual role in health and disease. From the host perspective, it helps maintain airway patency and prevents aspiration. However, from an evolutionary and epidemiological perspective, many viruses and bacteria benefit from inducing cough, as it facilitates the spread of infectious droplets to new hosts.
Simple behavioural measures, such as coughing into the elbow or towards the ground, rather than forward at breathing height, significantly reduce airborne transmission of respiratory pathogens.

Classification by duration

Clinically, cough is commonly classified according to its duration:

• Acute cough: lasting less than 3 weeks
• Subacute cough: lasting 3–8 weeks
• Chronic cough: persisting for more than 8 weeks

This classification assists in narrowing the differential diagnosis and guiding investigations.

Common causes of cough

Cough has a wide range of potential causes, reflecting its non-specific nature as a symptom.
InfectionsThe most frequent cause of acute cough is a viral upper respiratory tract infection, such as the common cold. Other infectious causes include COVID-19, acute bronchitis, pneumonia, pertussis, and tuberculosis. In individuals with a normal chest X-ray, tuberculosis is an uncommon finding. Following resolution of an infection, some individuals develop a post-infectious cough, typically dry and non-productive, which may persist for several weeks due to residual airway inflammation.
Asthma and reactive airway diseaseAsthma is a common cause of chronic cough in both adults and children. In some individuals, coughing may be the only presenting symptom, a condition known as cough-variant asthma. Related conditions include atopic cough and eosinophilic bronchitis, both characterised by airway eosinophilia and responsiveness to corticosteroid therapy.
Chronic bronchitis and smoking-related diseaseChronic bronchitis is defined by a productive cough lasting at least three months in two consecutive years. It is strongly associated with smoking and long-term exposure to airborne irritants. Tobacco smoke promotes mucus hypersecretion and impairs clearance mechanisms, making cough a compensatory response.
Gastroesophageal reflux disease (GERD)GERD is a major cause of unexplained chronic cough. Acid reflux into the oesophagus and upper airway can trigger cough either directly or via reflex mechanisms. Notably, many individuals with GERD-related cough lack classic symptoms such as heartburn.
Upper airway conditionsPostnasal drip due to allergic rhinitis or chronic sinusitis is a frequent contributor to chronic cough. Air pollution, including tobacco smoke, particulate matter, irritant gases, and indoor dampness, can also provoke coughing through airway irritation.
Foreign bodies and aspirationA sudden onset cough during eating may suggest aspiration of a foreign body. Chronic microaspiration, particularly in individuals with swallowing disorders, can also lead to persistent cough.
Drug-induced coughCertain medications can cause cough as an adverse effect. Angiotensin-converting enzyme (ACE) inhibitors, commonly prescribed for hypertension and heart failure, are well recognised causes. Beta blockers may also provoke cough in susceptible individuals.
Neurological and behavioural causesSome cases of chronic cough are attributed to sensory neuropathic disorders, termed neurogenic cough. Habit cough, which often resolves during sleep and responds to behavioural therapy, is more common in children. Coughing may also occur as a tic in conditions such as Tourette syndrome.
Other medical conditionsCough may arise from structural lung diseases including bronchiectasis, cystic fibrosis, interstitial lung disease, and sarcoidosis. It can also be associated with lung cancer, mediastinal masses, heart failure, pulmonary infarction, and aortic aneurysm. Irritation of the external auditory canal, such as from ear wax, may trigger cough via shared neural pathways.

Cough frequency in health and disease

Occasional coughing is normal. Healthy children may cough up to ten times per day without underlying illness. Healthy adults cough on average around 18 times daily, whereas individuals with respiratory disease may cough substantially more frequently.

Complications of coughing

Complications of cough can be divided into acute and chronic effects.
Acute complications include:

• Cough syncope due to transient cerebral hypoperfusion
• Insomnia
• Cough-induced vomiting
• Subconjunctival haemorrhage
• Urinary or faecal incontinence

Chronic or severe coughing may result in:

• Abdominal or pelvic hernias
• Rib fractures and costochondritis
• Fatigue
• Pelvic floor damage, including cystocele in women

These complications highlight the potential morbidity associated with persistent or violent coughing.

Diagnostic approach

Evaluation of cough depends on duration, associated symptoms, and patient age. In children, cough may be physiological or pathological. In adults with chronic cough, over 90% of cases are attributable to postnasal drip, asthma-related conditions, eosinophilic bronchitis, or GERD. A systematic approach focusing on these common causes is recommended before extensive investigations.

Treatment principles

Management of cough should primarily focus on treating the underlying cause, such as smoking cessation, asthma control, or discontinuation of causative medications. In many cases, reassurance alone is sufficient.
Pharmacological cough suppressants, including codeine and dextromethorphan, are widely used but are not recommended in children and have limited efficacy in many conditions. As coughing is a protective reflex, indiscriminate suppression may be harmful, particularly in productive coughs that clear mucus from the airways.
Other treatment strategies include:

• Anti-inflammatory therapies, such as inhaled corticosteroids
• Mucoactive agents to facilitate sputum clearance
• Bronchodilators for obstructive airway disease