Most Likely Diagnosis: Myasthenia Gravis
The most likely diagnosis is myasthenia gravis (Option C), specifically myasthenic crisis triggered by the recent upper respiratory tract infection. 1
Clinical Reasoning
Why Myasthenia Gravis is the Answer
Upper respiratory tract infections are well-established triggers for myasthenic crisis, causing acute decompensation in patients with underlying neuromuscular disease. 1
The temporal relationship between URTI onset (5 days ago) and severe symptom development fits the typical pattern of infection-triggered myasthenic crisis, where respiratory muscle weakness progresses rapidly to respiratory failure requiring mechanical ventilation. 1
The combination of difficulty breathing and loss of consciousness requiring intubation is pathognomonic for myasthenic crisis. 1 The loss of consciousness suggests hypercapnic respiratory failure from inadequate ventilation due to neuromuscular weakness. 1
Myasthenic crisis can present acutely in previously undiagnosed patients, with respiratory infection being the unmasking event. 1 This patient may have had subclinical myasthenia gravis that decompensated with the URTI.
Why the Other Options Are Less Likely
COPD (Option A):
- COPD exacerbations typically occur in patients with known chronic lung disease and progress over days with worsening dyspnea, increased sputum production, and cough. 2
- Sudden loss of consciousness is not a typical presenting feature of COPD exacerbation. 2
Pulmonary Edema (Option B):
- Pulmonary edema would typically present with signs of fluid overload, orthopnea, paroxysmal nocturnal dyspnea, and radiographic evidence of bilateral infiltrates. 3
- The sudden onset following URTI without cardiac history makes this less likely. 3
Drug Overdose (Option D):
- No history of drug ingestion is provided in the clinical scenario.
- Drug overdose would not explain the temporal relationship with the preceding URTI. 1
Critical Diagnostic Pitfall to Avoid
Do not assume all respiratory failure post-URTI is pneumonia—the absence of fever, purulent secretions, or infiltrates on chest radiograph should prompt consideration of neuromuscular causes. 1 Infections, particularly respiratory infections, are among the most common precipitants of myasthenic crisis, accounting for a significant proportion of cases requiring ICU admission. 1
Immediate Next Steps
Once intubated, assess for underlying neuromuscular disease including bedside pulmonary function testing, acetylcholine receptor antibodies, and urgent neurological consultation. 1
Avoid medications that can worsen myasthenia gravis, including aminoglycosides, fluoroquinolones, and neuromuscular blocking agents, which can precipitate or worsen crisis. 1
Do not delay neurological evaluation in patients with unexplained respiratory failure requiring intubation, especially when preceded by infection. 1