What are the causes of a cough in adults?

Causes of Cough in Adults

The causes of cough in adults depend critically on duration: acute cough (<3 weeks) is predominantly viral respiratory infections, subacute cough (3-8 weeks) is mainly postinfectious, and chronic cough (>8 weeks) is most commonly caused by upper airway cough syndrome (UACS), asthma, gastroesophageal reflux disease (GERD), and nonasthmatic eosinophilic bronchitis—either alone or in combination. 1

Acute Cough (<3 Weeks)

Viral upper respiratory tract infections account for the vast majority of acute cough cases 1, 2:

• The common cold is the most frequent cause 2
• Acute bronchitis is viral in >90% of cases 3
• Critical pitfall: Green or colored sputum does NOT indicate bacterial infection and should not trigger antibiotic prescription 3

Other acute causes to screen for include 1:

• Exacerbations of underlying asthma or COPD 1
• Pneumonia (look for fever >39°C, tachycardia >100 bpm, respiratory rate >24/min, focal consolidation) 3
• Pulmonary embolism (consider in high-risk patients with sudden onset) 1
• Red flags requiring immediate evaluation: hemoptysis, severe dyspnea, chest pain, high fever, or signs of respiratory distress 1

Subacute Cough (3-8 Weeks)

Postinfectious cough is the leading cause, representing 48.4% of cases 1:

• Results from persistent airway inflammation and cough reflex hypersensitivity following viral infection 2
• Bordetella pertussis should be included in the differential, with 10% of chronic cough cases showing positive nasal swabs 1

Exacerbations of underlying conditions 1:

• UACS/postnasal drip syndrome (33.2%) 1
• Asthma (15.8%) 1
• COPD in smokers 1

Chronic Cough (>8 Weeks)

The "Big Four" causes account for approximately 90% of chronic cough cases 1, 4, 5:

1. Upper Airway Cough Syndrome (UACS/Postnasal Drip)

• Most common single cause in specialist cough clinics 1
• Results from rhinosinus conditions causing mucus accumulation in the posterior pharynx 6
• Morning predominance is characteristic due to overnight mucus accumulation 6
• Associated with frequent throat clearing and sensation of postnasal drip 6

2. Asthma (Including Cough-Variant Asthma)

• Second most common cause, present in 15-36% of chronic cough cases 1
• May present as isolated cough without wheezing or dyspnea 1
• Timing clue: Often wakes patients at night or early morning due to circadian airway responsiveness 6
• Worsened by exercise or cold air exposure 6

3. Gastroesophageal Reflux Disease (GERD)

• Prevalence ranges from 5-41% as a cause of chronic cough 1
• Critical insight: GERD can be "silent" from a GI standpoint in up to 75% of cough cases—absence of heartburn does NOT rule it out 1, 6, 7
• Timing pattern: Cough that occurs when falling asleep but stops once asleep is highly characteristic of GERD due to lower esophageal sphincter closure during established sleep 7
• Three mechanisms: laryngeal irritation, microaspiration/macroaspiration, and esophageal-bronchial reflex 1

4. Nonasthmatic Eosinophilic Bronchitis (NAEB)

• Accounts for 5.4% of subacute and a portion of chronic cough 1
• Diagnosed by sputum eosinophilia without airflow obstruction 1

Additional Important Causes

Medication-induced 1, 6, 7:

• ACE inhibitors are a common and often overlooked cause 1, 6
• Must be discontinued to assess if responsible for cough 1, 7
• Sitagliptin should also be considered 1

Smoking-related 1:

• One of the most common causes of persistent cough 6
• Prevalence increases in dose-related manner 6
• Smokers at risk of developing COPD 1
• Paradox: Smoking cessation initially increases cough reflex sensitivity before improvement 6

Occupational/environmental exposures 1:

• Workplace sensitizers (hot acidic conditions, hot chili peppers, chemical exposures) 1
• Bedroom irritants (dust, allergens, dry air) can trigger morning cough 6

Serious conditions requiring exclusion 1:

• Lung cancer: Fourth most common presenting feature, especially in smokers with finger clubbing, pleural effusion, or lobar collapse 1
• Bronchiectasis: Prevalence ~4% in specialist cough clinics; can present as "dry" bronchiectasis without sputum 1
• Tuberculosis: Must be considered in endemic areas or high-risk populations even with normal chest radiographs 1

Diagnostic Approach Algorithm

Step 1: Duration classification 1:

• <3 weeks = acute
• 3-8 weeks = subacute

• 8 weeks = chronic

Step 2: Screen for red flags immediately 1:

• Hemoptysis, fever, weight loss, dyspnea, chest pain, recurrent pneumonia 5
• If present, obtain chest radiograph and consider advanced imaging 5

Step 3: Medication review 1:

• Discontinue ACE inhibitors or sitagliptin if present 1, 7

Step 4: Chest radiograph 1:

• Mandatory for all chronic cough and acute cough with atypical features 1
• Rules out malignancy, infection, parenchymal disease 1

Step 5: Systematic evaluation/empiric treatment for the Big Four 1, 4:

• Evaluate or empirically treat UACS, asthma, and GERD in stepwise fashion 4, 8
• Multiple causes coexist in up to 25% of patients—combination treatments often necessary 9, 8

Step 6: Follow-up 1:

• Reassess within 4-6 weeks using validated cough severity tools 1
• If refractory after optimal treatment, refer to specialist cough clinic 1

Key Clinical Pearls

Demographics matter 6, 7:

• Women, particularly middle-aged, have higher prevalence and more sensitive cough reflex 6, 7

Timing provides diagnostic clues 6, 7:

• Morning predominance suggests UACS (mucus accumulation overnight) 6
• Cough when falling asleep that stops once asleep is highly specific for GERD 7
• Night/early morning awakening suggests asthma 6

Common diagnostic pitfalls to avoid 1, 6, 3:

• Assuming absence of heartburn rules out GERD (75% of GERD-related cough is silent) 1, 6
• Using sputum color to guide antibiotic decisions (green sputum does NOT indicate bacterial infection in acute cough) 3
• Failing to consider multiple simultaneous causes 9, 8
• Inadequate treatment duration for GERD (may require several months for response) 7