In a 47-year-old woman with a 4-day dry cough, sensation of phlegm stuck in the throat, cough peaking around midday, partial relief with dextromethorphan‑guaifenesin (Robitussin DM), and two episodes of projectile vomiting after intense coughing, with normal vital signs and an essentially normal physical exam, what are the likely diagnoses and recommended next steps?

Likely Diagnosis and Next Steps

This patient most likely has post-infectious cough (subacute cough) from a viral upper respiratory infection, with post-tussive vomiting as a complication of severe coughing paroxysms; however, pertussis must be ruled out given the 4-day duration, post-tussive vomiting, and peak cough timing. 1, 2

Immediate Diagnostic Priorities

Rule Out Pertussis First

• Any cough ≥2 weeks with paroxysmal episodes and post-tussive vomiting should be considered pertussis until proven otherwise, but this patient is only at day 4, making typical viral post-infectious cough more likely 1, 2
• However, the combination of paroxysmal coughing with post-tussive vomiting warrants obtaining a nasopharyngeal aspirate or Dacron swab for Bordetella pertussis culture to definitively exclude this diagnosis 1
• If pertussis is suspected based on clinical presentation or local outbreak, start azithromycin 500 mg daily for 3-5 days immediately without waiting for culture results, as early treatment within the first 2 weeks diminishes paroxysms and prevents transmission 1, 2
• PCR testing is available but not standardized for routine clinical use 1

Assess for Red Flag Features

• Check for tachypnea (≥24 breaths/min), tachycardia (≥100 bpm), or fever (≥38°C) which would suggest pneumonia requiring chest radiography 2, 3
• The normal vital signs and physical exam make serious pathology unlikely, but remain vigilant 2
• Hemoptysis, breathlessness, or focal lung findings require immediate chest imaging 4, 3

Most Likely Diagnosis: Viral Post-Infectious Cough

Given the 4-day duration, dry cough with sensation of phlegm, midday peak timing, and post-tussive vomiting after intense coughing, this represents acute viral upper respiratory infection with post-infectious airway inflammation 1, 4, 3

Pathophysiology

• Post-viral airway inflammation causes bronchial hyperresponsiveness, mucus hypersecretion, and impaired mucociliary clearance 1, 3
• The vomiting is a mechanical consequence of severe coughing paroxysms, not necessarily indicative of pertussis at this early stage 1
• Most acute viral coughs are self-limiting and resolve within 1-3 weeks 4, 3

Treatment Algorithm

First-Line Symptomatic Management (Current Stage: Day 4)

1. Reassure the patient that this is likely self-limiting viral cough expected to resolve within 1-3 weeks 4, 3

2. Simple home remedies are first-line: honey and lemon mixture is the simplest, cheapest, and often effective treatment 4

3. The dextromethorphan + guaifenesin (Robitussin DM) she is taking is appropriate, but the dose may be subtherapeutic 4, 2

   • Maximum cough suppression occurs at dextromethorphan 60 mg, and commonly prescribed doses are often inadequate 4, 2
   • Most over-the-counter preparations contain only 10-20 mg per dose 4
   • Consider increasing to therapeutic dosing if cough severely impacts quality of life 4

4. Voluntary cough suppression through central modulation may reduce cough frequency 4

If Symptoms Persist or Worsen (Days 7-21)

At 2 weeks: If cough persists with paroxysmal features and post-tussive vomiting, strongly suspect pertussis and obtain nasopharyngeal culture while starting empiric macrolide therapy 1, 2

At 3 weeks: If still symptomatic, consider trial of inhaled ipratropium bromide as first-line pharmacologic therapy for post-infectious cough 1, 4

• Ipratropium has been shown to attenuate post-infectious cough 1

If ipratropium fails and cough adversely affects quality of life:

• Consider inhaled corticosteroids for persistent post-infectious airway inflammation 1
• For severe paroxysms, prednisone 30-40 mg daily for a short, finite period after ruling out other causes 1, 2
• Central antitussives (codeine or dextromethorphan 60 mg) only when other measures fail 1, 4

Critical Threshold: 8 Weeks

If cough persists beyond 8 weeks total duration, the diagnostic approach must fundamentally shift to systematically evaluate for 1, 3:

• Upper airway cough syndrome (chronic rhinosinusitis/postnasal drip)
• Asthma or cough-variant asthma
• Gastroesophageal reflux disease (GERD)
• Eosinophilic bronchitis

What NOT to Do

• Do not prescribe antibiotics for viral post-infectious cough unless bacterial sinusitis or pertussis is confirmed, as they provide no benefit and contribute to resistance 1, 3
• Avoid codeine or pholcodine as they have no greater efficacy than dextromethorphan but significantly worse side effect profiles 4
• Do not use subtherapeutic doses of dextromethorphan (<60 mg) and expect adequate cough suppression 4, 2
• Do not misclassify this as chronic cough at day 4, as management differs significantly 2, 3

Additional Considerations

Assess for Contributing Factors

• GERD can be triggered or worsened by vigorous coughing and may require empiric proton pump inhibitor trial if symptoms suggest reflux 1, 3
• Postnasal drip from concurrent upper airway inflammation may benefit from first-generation antihistamines, particularly for nocturnal cough 4
• Asthma or reactive airway disease should be considered if there is any wheezing, breathlessness, or response to bronchodilators 1, 3

Common Pitfalls to Avoid

• Delaying pertussis treatment while waiting for laboratory confirmation decreases effectiveness 2
• Failing to recognize that the vomiting is post-tussive (mechanical from coughing) rather than primary gastrointestinal pathology 1
• Not recognizing that dextromethorphan preparations often contain additional ingredients like acetaminophen, requiring caution with dosing 5
• Assuming all cough requires suppression—if productive cough develops, suppression may impair clearance 4