What pulmonary complications can occur during a myasthenic crisis?

Pulmonary Complications of Myasthenic Crisis

The primary pulmonary complication of myasthenic crisis is respiratory failure requiring mechanical ventilation, which occurs in 15-20% of patients with myasthenia gravis during their lifetime, with secondary complications including pneumonia, atelectasis, and aspiration. 1, 2, 3

Primary Respiratory Complications

Respiratory Muscle Failure

• Respiratory failure is the defining feature of myasthenic crisis, characterized by progressive weakness of the diaphragm and intercostal muscles leading to inadequate ventilation and the need for mechanical ventilation. 1, 2, 3
• The "20/30/40 rule" identifies patients at imminent risk: vital capacity <20 ml/kg, maximum inspiratory pressure <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O. 1
• Respiratory decompensation can develop within minutes to days, progressing to flaccid tetraparesis with immobility and severe dyspnea. 3

Bulbar Muscle Weakness and Aspiration

• Bulbar muscle weakness leads to dysphagia, dysarthria, and inability to protect the airway, creating high risk for aspiration of secretions and gastric contents. 1, 4, 3
• Globus events (difficulty swallowing) can be life-threatening due to rapidly exhausting coughing and swallowing attempts. 3
• Early intubation is essential to secure the airway before aspiration occurs. 2, 3

Secondary Pulmonary Complications

Pneumonia

• Pneumonia develops in approximately 46% of mechanically ventilated myasthenic patients, though this rate varies by institution and aggressiveness of respiratory care. 5
• Respiratory infections are the precipitating factor in 60-70% of myasthenic crises. 4
• Community-acquired pneumonia can be both a trigger and a complication of myasthenic crisis. 6

Atelectasis

• Atelectasis occurs in approximately 46% of mechanically ventilated myasthenic patients when aggressive respiratory interventions are employed. 5
• The incidence is significantly lower (46%) compared to historical series (91%) when aggressive respiratory therapy including frequent suctioning, chest physiotherapy, and bronchodilator treatments is used. 5

Critical Monitoring Parameters

Pulmonary Function Assessment

• Forced vital capacity (FVC) and negative inspiratory force (NIF) must be measured every 4-6 hours during the initial admission phase to detect deterioration before overt respiratory failure. 4
• The single-breath count test (counting aloud at 2 numbers/second while exhaling) provides a rapid bedside screen, with counting to ≥25 correlating with normal respiratory muscle function. 1, 4

Important Pitfall: Delayed Recognition

• Pulse oximetry and arterial blood gases are NOT reliable early indicators of impending respiratory failure and may remain normal until failure is imminent. 1, 4
• End-tidal CO₂ monitoring is an optional early warning tool for respiratory decompensation. 1, 4
• Do not wait for hypoxemia or hypercapnia before escalating care, as respiratory failure can progress rapidly once these abnormalities appear. 4

Impact on Clinical Course

Duration and Outcomes

• Median duration of mechanical ventilation is 12-14 days under sufficient treatment, though prolonged courses are common due to complications. 3
• Approximately 20% of patients remain mechanically ventilated after 1 month. 3
• Mortality ranges from 2-5% to over 16%, with lethal outcomes almost never caused by the crisis itself but by comorbidities and complications. 3

Effect of Aggressive Respiratory Management

• Aggressive respiratory treatment including frequent suctioning, chest physiotherapy, bronchodilators, and intermittent positive-pressure breathing significantly reduces the incidence of atelectasis and pneumonia, leading to shorter mechanical ventilation duration and ICU stays. 5
• The respiratory intervention index (measuring aggressiveness of therapy) correlates inversely with lung injury scores. 5

Management Implications

Immediate Interventions

• Early intubation and mechanical ventilation is the most important step in managing myasthenic crisis to prevent aspiration and respiratory arrest. 2, 3
• Orotracheal intubation is preferred, with placement of small-bore duodenal feeding tubes to decrease aspiration risk. 2
• ICU-level monitoring with continuous pulse oximetry and cardiac telemetry is mandatory for all patients with generalized weakness or bulbar symptoms. 4

Medication Precautions

• Immediately discontinue medications that worsen neuromuscular transmission: β-blockers, IV magnesium (absolutely contraindicated), fluoroquinolones, aminoglycosides, and macrolide antibiotics. 1, 7, 4
• Pyridostigmine should be withheld in intubated patients. 1, 4