What is the most likely diagnosis for a patient who had an upper respiratory tract infection 5 days ago and suddenly developed loss of consciousness and difficulty breathing, now intubated?

Most Likely Diagnosis: Myasthenic Crisis

The most likely diagnosis is myasthenic crisis triggered by the upper respiratory tract infection, not pulmonary edema. 1

Clinical Reasoning

Why Myasthenic Crisis is the Primary Diagnosis

• Upper respiratory tract infections are well-established triggers for myasthenic crisis, causing acute decompensation in patients with underlying neuromuscular disease. 1

• The temporal relationship is pathognomonic: The 5-day interval between URTI onset 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

• Loss of consciousness in the context of neuromuscular weakness suggests hypercapnic respiratory failure from inadequate ventilation, not cardiogenic causes. 1 This occurs when respiratory muscles fail to maintain adequate gas exchange, leading to CO2 retention and altered mental status.

• The combination of difficulty breathing and altered consciousness requiring intubation is pathognomonic for myasthenic crisis. 1

Why Pulmonary Edema is Less Likely

• Pulmonary edema typically presents with signs of fluid overload (peripheral edema, jugular venous distension, crackles on auscultation), which are not mentioned in this clinical scenario. 2

• Cardiogenic pulmonary edema would not explain the loss of consciousness unless accompanied by severe hypotension or arrhythmia, which would be the primary presenting feature. 2

• Post-obstructive pulmonary edema (POPE) requires acute relief of chronic airway obstruction (type 2) or sudden severe obstruction (type 1), neither of which fits this presentation. 3

Critical Diagnostic Considerations

Myasthenic Crisis Can Present in Previously Undiagnosed Patients

• Respiratory infection can be the unmasking event for previously undiagnosed myasthenia gravis, with the patient having no prior known neuromuscular disease. 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 Diagnostic Workup Required

Once intubated, the patient requires:

• Bedside pulmonary function testing (negative inspiratory force, vital capacity) 1
• Acetylcholine receptor antibodies (anti-AChR, anti-MuSK) 1
• Urgent neurological consultation 1
• Continuous waveform capnography monitoring to detect changes in ventilation 1

Common Pitfalls 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

• Pneumonia would typically present with fever, productive cough, and radiographic infiltrates, not sudden loss of consciousness as the primary feature. 4

Do Not Delay Neurological Evaluation

• Do not delay neurological evaluation in patients with unexplained respiratory failure requiring intubation, especially when preceded by infection. 1

• Time is critical: Early recognition and treatment of myasthenic crisis significantly impacts outcomes. 1

Avoid Medications That Worsen Myasthenia Gravis

• Avoid aminoglycosides, fluoroquinolones, and neuromuscular blocking agents, which can precipitate or worsen crisis. 1

• Standard rapid sequence intubation medications may need modification in suspected myasthenic crisis. 1

Management Algorithm

1. Maintain mechanical ventilation with appropriate settings (avoid excessive tidal volumes that could worsen respiratory mechanics) 4

2. Obtain urgent neurology consultation for bedside assessment 1

3. Send acetylcholine receptor antibodies immediately 1

4. Perform bedside pulmonary function testing if patient cooperative enough 1

5. Consider empiric treatment with pyridostigmine or plasmapheresis/IVIG if high clinical suspicion, pending antibody results 1

6. Avoid contraindicated medications (aminoglycosides, fluoroquinolones, neuromuscular blockers) 1