Postinfectious Cough

Postinfectious cough is a cough that persists for 3 to 8 weeks after an acute respiratory tract infection (subacute cough), with a normal chest X-ray and no other cause found. It affects up to 25 to 50% of patients after a respiratory tract infection, and is commoner in smokers and in patients with pre-existing airway disease.

• The mechanism is airway inflammation and epithelial damage with increased cough reflex sensitivity, mucus hypersecretion and transient bronchial hyperresponsiveness. Postnasal drip and gastro oesophageal reflux may add to it.
• Typical triggers are influenza, common cold viruses, respiratory syncytial virus, parainfluenza, mycoplasma, chlamydia, pertussis and COVID-19. Pertussis must always be considered, because cough from this infection can last 6 to 12 weeks.
• Risk factors: smoking, asthma, atopy, chronic rhinosinusitis, gastro oesophageal reflux disease, female sex and age above 50.
• The cough is self-limiting in most patients, usually resolving by 8 weeks. Cough beyond 8 weeks is chronic cough and needs a full assessment for other causes.

• A dry or minimally productive cough that follows an acute respiratory infection with initial improvement of other symptoms (fever, malaise, nasal symptoms), and that persists for more than 3 weeks. The cough is often worse at night or on waking, triggered by cold air, talking, laughing, exercise and strong smells, and typically gradually improves.
• A tickle or irritation in the throat, hoarseness, postnasal drip and throat clearing may be present.
• The chest examination is normal or shows a transient wheeze. There are no systemic features such as fever, weight loss or night sweats.
• Differential diagnosis: pertussis, cough variant asthma, upper airway cough syndrome, gastro oesophageal reflux, ACE inhibitor cough, chronic bronchitis, bronchiectasis, tuberculosis, pneumonia not resolved, lung cancer, heart failure, sarcoidosis, interstitial lung disease, foreign body, and habit cough.
• Red flags: haemoptysis, weight loss, night sweats, fever, progressive breathlessness, chest pain, hoarseness lasting more than 3 weeks in a smoker, a neck mass, clubbing, abnormal chest signs, a history of tuberculosis exposure, or immunosuppression.
• Complications: cough syncope, rib fracture, urinary incontinence, sleep disturbance, hoarseness, social embarrassment, and anxiety.

• Clinical diagnosis by exclusion. Take a history of the preceding illness, the timing, smoking, medicines (ACE inhibitors), exposures, reflux or nasal symptoms, and pertussis or tuberculosis contacts.
• Examine the chest, nose, throat and oropharynx. Record oxygen saturation, and check for clubbing and lymph nodes.
• A chest X-ray is advised for all patients with a cough lasting 3 weeks or more, to exclude pneumonia, tuberculosis, cancer and other structural disease. A normal X-ray with a preceding infection supports the diagnosis.
• Spirometry with bronchodilator reversibility (an increase in forced expiratory volume in 1 second of 12% and 200 mL or more) or peak flow variability above 20% when asthma is suspected. Fractional exhaled nitric oxide above 40 ppb supports eosinophilic airway inflammation.
• Pertussis serology (anti-pertussis toxin IgG) or polymerase chain reaction when the cough is paroxysmal, with whoop or post-tussive vomiting, or when there is a contact.
• Sputum for acid fast bacilli or a molecular test for tuberculosis when the cough lasts more than 2 weeks in people at risk, with weight loss or night sweats, and in high prevalence settings. HIV test as indicated.
• Full blood count, with eosinophil count, when asthma or parasitic disease is suspected.
• If cough persists beyond 8 weeks, arrange a full assessment for chronic cough, covering the upper airway, asthma, reflux and medicines.

• Reassure the patient that the cough is common, harmless in most cases, and gradually resolves. Advise on avoiding smoking and irritants, and keeping hydrated.
• Self-help: honey and lemon in warm water (not for infants under 1 year), a lozenge, sipping water, and a humidifier for dry air. Avoid repeated forceful throat clearing.
• Antibiotics are not indicated for postinfectious cough, unless pertussis or another bacterial infection is suspected.
• Treat the dominant mechanism where evident, with a short course for each:
  • Upper airway cough syndrome (postnasal drip or nasal symptoms): fluticasone propionate nasal spray 50 micrograms per spray, 2 sprays into each nostril once daily (200 micrograms per day), reduced to 1 spray per nostril after 2 to 4 weeks. Use saline irrigation twice daily. First generation antihistamine such as chlorphenamine 4 mg orally every 4 to 6 hours (maximum 24 mg per day) at night for 1 to 2 weeks, or a combination with a decongestant for a short period only.
  • Airway hyperresponsiveness or wheeze: inhaled bronchodilator salbutamol 100 micrograms, 2 puffs via a spacer 4 to 6 hourly as needed for 1 to 2 weeks. Review the response.
  • Persistent or severe cough with eosinophilic airway inflammation: inhaled beclometasone 200 to 400 micrograms twice daily, or budesonide 200 to 400 micrograms twice daily, for 2 to 4 weeks. For severe, disabling paroxysmal cough, prednisolone 30 to 40 mg orally once daily for 5 to 7 days may be given after excluding infection (evidence limited).
  • Reflux symptoms: omeprazole 20 to 40 mg once daily before breakfast for 8 weeks, with diet and weight advice.
  • Ipratropium bromide inhaler 20 to 40 micrograms (1 to 2 puffs) 3 to 4 times a day may help the cough after a viral infection, from small studies.
• Cough suppressants (short term, for sleep disrupting dry cough): dextromethorphan 10 to 20 mg orally every 4 to 6 hours (maximum 120 mg per day) at night for up to 1 to 2 weeks, or codeine linctus 15 to 30 mg orally every 6 hours (maximum 120 mg per day) for a few days, if the patient is an adult and not driving. Avoid codeine in children under 12 years, in pregnancy and breastfeeding, and in respiratory disease with sputum retention.
• Pertussis, if suspected or confirmed within 3 weeks: azithromycin 500 mg orally once on day 1, then 250 mg daily for 4 days, to reduce transmission. After 3 weeks, treatment does not alter the course, and only supportive therapy is used.
• Special groups:
  • Children: avoid cough suppressants and over-the-counter cough medicines under 6 years. Use honey for those older than 1 year, treat nasal symptoms, and investigate a cough lasting more than 4 weeks (chest X-ray, asthma assessment, foreign body, tuberculosis contact, and protracted bacterial bronchitis, treated with amoxicillin clavulanate 22.5 mg/kg of the amoxicillin component orally 12 hourly for 2 weeks).
  • Pregnancy: use honey, saline, fluticasone nasal spray, and, if needed, salbutamol. Avoid codeine and dextromethorphan unless advised.
  • Elderly: investigate more readily for cancer, heart failure and drug induced cough.
  • Smokers: strongly advise cessation. Offer nicotine replacement (patch 21 mg per 24 hours for those smoking more than 10 cigarettes daily, for 6 to 8 weeks, then step down) and varenicline 0.5 mg daily for 3 days, then 0.5 mg twice daily for 4 days, then 1 mg twice daily for 12 weeks.
  • ACE inhibitor users: stop and replace with an angiotensin receptor blocker.
• Follow up: review at 3 to 4 weeks. If the cough persists beyond 8 weeks, or if red flags develop, refer to a respiratory physician and perform a full chronic cough assessment.