What are the differential diagnoses for a 65-year-old man with a 5-week history of paroxysmal cough and a known history of asthma?

Differential Diagnoses for Paroxysmal Cough in a 65-Year-Old Male with Asthma

In a 65-year-old man with known asthma presenting with 5 weeks of paroxysmal cough, the most critical differentials to systematically evaluate are: upper airway cough syndrome (UACS), gastroesophageal reflux disease (GERD), postinfectious cough, pertussis, and worsening/poorly controlled asthma itself—with these three conditions (UACS, asthma, GERD) accounting for 92-100% of chronic cough cases in nonsmokers with normal chest radiographs. 1

Primary Differential Diagnoses

Most Common Triad (92-100% of cases)

1. Upper Airway Cough Syndrome (UACS/Post-Nasal Drip)

• Most common cause of chronic cough overall 1
• Can present as "silent" without obvious nasal symptoms
• Look for: throat clearing, sensation of postnasal drainage, nasal congestion, rhinorrhea
• May coexist with asthma

2. Poorly Controlled or Cough-Variant Asthma

• Cough can be the principal or only manifestation of asthma 2
• Given his asthma history, consider inadequate control or variant presentation
• Key features: nocturnal worsening, triggered by exercise/cold air/irritants
• Requires spirometry with bronchodilator response testing 2

3. Gastroesophageal Reflux Disease (GERD)

• Third most common cause, often "silent" without heartburn 1
• Can coexist with asthma and complicate its management 2
• May present with cough alone

Critical Time-Sensitive Diagnosis

4. Pertussis (Bordetella pertussis)

• Must be considered urgently given the paroxysmal nature and 5-week duration 3, 4, 5
• Classic features to assess:
  • Paroxysmal cough (sensitivity 93.2% in adults) 5
  • Post-tussive vomiting (specificity 79.5%) 5
  • Inspiratory whooping (specificity 77.7%) 5
  • Absence of fever (sensitivity 81.8%) 5
• If whooping or post-tussive vomiting present, strongly consider pertussis 5
• Obtain nasopharyngeal culture/swab for definitive diagnosis 4
• Early macrolide treatment (within first few weeks) reduces transmission and may shorten symptoms 4

5. Postinfectious Cough

• Defined as cough lasting 3-8 weeks following acute respiratory infection 3, 4
• At 5 weeks, this remains a strong possibility
• Mechanisms: post-viral airway inflammation, bronchial hyperresponsiveness, mucus hypersecretion 4
• Important caveat: If cough persists beyond 8 weeks, consider other diagnoses 4

Secondary but Important Differentials

6. ACE Inhibitor-Induced Cough

• Critical to ask about medication history 1, 6
• Can occur anytime within the first year of use
• Resolution typically within days to 2 weeks of stopping (median 26 days) 6
• Stop ACE inhibitor regardless of temporal relationship 6

7. Nonasthmatic Eosinophilic Bronchitis (NAEB)

• Accounts for 10-30% of specialist referrals 7
• Presents with chronic cough without airflow obstruction or hyperresponsiveness
• Requires sputum eosinophil count or FeNO for diagnosis 7
• Responds to inhaled corticosteroids 7

8. COPD/Asthma-COPD Overlap (ACOS)

• Consider in 65-year-old, especially if smoking history 2
• Features: chronic bronchitis, productive cough, incomplete reversibility
• Requires spirometry with DLCO if suspected 2

9. Serious Pathology (Lower Probability with Normal CXR)

• Bronchiectasis (8 cases in one series) 8
• Lung malignancy (2 cases, but usually abnormal CXR) 8
• Interstitial lung disease 8
• Congestive heart failure (check BNP, echocardiogram) 2

Diagnostic Algorithm

Step 1: Initial Assessment

• Medication review: ACE inhibitor use? Stop if present 6
• Smoking status: Current smoker? (chronic bronchitis) 6
• Chest radiograph: Essential initial test 8, 1
• Pertussis features: Paroxysmal cough + post-tussive vomiting/whooping + absence of fever? 5

Step 2: If Pertussis Suspected

• Nasopharyngeal culture/swab immediately 4
• Start macrolide antibiotic empirically 4
• Isolate patient for 5 days from treatment start 4

Step 3: If CXR Normal and Not Pertussis

Systematically evaluate the common triad in order of prevalence:

1. Trial for UACS: First-generation antihistamine/decongestant
2. Optimize asthma control: Increase ICS dose, add LABA if not already on it 7
3. Trial for GERD: PPI therapy (even without heartburn)

Step 4: If No Response After 4-8 Weeks

• Consider NAEB: Check sputum eosinophils or FeNO 7
• Spirometry with bronchodilator response 2
• Consider bronchoprovocation testing if spirometry normal 2
• Evaluate for vocal cord dysfunction (inspiratory flow-volume loop) 2

Critical Pitfalls to Avoid

1. Don't dismiss pertussis in adults—it's underdiagnosed and highly contagious 5
2. Don't rely on cough characteristics (timing, quality, sputum) for diagnosis—they're not discriminatory 1
3. Don't assume it's just asthma without optimizing treatment and confirming diagnosis 2
4. Don't forget multiple causes can coexist—up to 62% have more than one etiology 1
5. Don't delay chest radiograph—it's recommended by all major guidelines despite low yield 8, 1
6. Don't overlook ACE inhibitors even if started >1 year ago 6

The presence of paroxysmal cough specifically warrants urgent consideration of pertussis, especially if accompanied by post-tussive vomiting or whooping, and should prompt immediate testing and empiric treatment. 5