Pharmacotherapy of Cough

Introduction & cough reflex

• Cough — a protective respiratory reflex that expels secretions and foreign particles from the air passages; it is a host-defence mechanism and can be initiated voluntarily as well as reflexly.
• Trigger — stimulation of mechano- or chemoreceptors in the throat & respiratory passages, or stretch receptors in the lungs. When the mucociliary escalator (beating cilia + mucus layer) is impaired (e.g. smoking), cough becomes the compensatory clearance mechanism.
• Reflex pathway — afferent arm (airway cough receptors → vagus) → cough centre in the medulla (nucleus tractus solitarius + cortex for voluntary control) → efferent arm (respiratory muscles, larynx, pelvic sphincters). Central antitussives raise the threshold of this medullary centre.
• Afferent receptors — three types — rapidly adapting receptors (RARs) (mechanosensitive), slowly adapting receptors (SARs) (Hering–Breuer inflation reflex), and C-fibre nociceptors (sensitive to bradykinin, capsaicin, PGE₂; express TRPV1 & TRPA1 channels). Larynx/large airways are mechanosensitive; peripheral airways chemosensitive.
• Useful vs useless cough — a productive (useful) cough drains the airway — suppression is undesirable & may be harmful (esp. in bacterial lung infection); a dry, nonproductive (useless) cough serves no purpose and should be suppressed, especially when tiring, sleep-disturbing or hazardous.
• Governing principle — treat the cause, not the cough — antitussives give only symptomatic relief and do not alter the cause. Persistent excessive cough can cause syncope, urinary incontinence, vomiting, chest pain, rib fractures and impaired quality of life.
• Classification by duration — acute (< 3 weeks; commonest cause viral URTI), subacute (3–8 weeks; usually post-infectious), chronic (> 8 weeks). Chronic cough in adult non-smokers — four common causes: UACS (upper airway cough syndrome / postnasal drip), asthma (incl. cough-variant), NAEB (non-asthmatic eosinophilic bronchitis) & GERD.