What is a myasthenic crisis?

What is Myasthenic Crisis?

Myasthenic crisis is a life-threatening complication of myasthenia gravis defined by respiratory failure requiring mechanical ventilation (invasive or non-invasive), resulting from severe weakness of respiratory muscles and/or bulbar muscles causing upper airway collapse and aspiration. 1, 2

Clinical Definition and Pathophysiology

• Myasthenic crisis represents the maximal manifestation of myasthenia gravis, characterized by rapidly progressive weakness of respiratory and bulbar muscles leading to respiratory insufficiency and aspiration 3, 4
• The crisis results from weakness of upper airway muscles causing obstruction and aspiration, weakness of respiratory muscles causing reduced tidal volumes, or both 4
• Bulbar weakness compromises airway protection, secretion clearance, and upper airway patency, while diaphragm weakness directly impairs ventilation and can lead to sudden respiratory decompensation 5

Epidemiology and Risk Factors

• Approximately 15-20% of patients with myasthenia gravis will experience at least one crisis during their lifetime 3, 2
• Most crises occur within the first 2 years of disease onset (52% of cases), and in up to 20% of patients, crisis is the first manifestation of previously undiagnosed myasthenia gravis 3, 6
• Patients at higher risk include those with severe disease, oropharyngeal weakness, muscle-specific kinase (MuSK) antibodies, thymoma, and prior history of crisis 2

Common Precipitating Factors

• Respiratory infections are the most common trigger, occurring in approximately 65% of crisis episodes 6, 4
• Other precipitants include aspiration, sepsis, surgical procedures, rapid tapering of immunosuppression, initiation of corticosteroids, and exposure to medications that worsen myasthenic weakness 4
• No specific trigger is identified in 30-40% of patients 2

Clinical Presentation

• Weakness develops within minutes to days, encompassing flaccid tetraparesis with immobility, severe dyspnea, and respiratory insufficiency 3
• Bulbar symptoms include dysphagia, dysarthria, facial weakness, and difficulty managing secretions 7, 5
• Ocular and bulbar involvement may include ptosis, extraocular movement abnormalities, and neck weakness 7
• Critical warning: Most episodes do not occur suddenly, providing a window of opportunity for prevention and early intervention 2

Duration and Prognosis

• Median duration of crisis is approximately 11-14 days of mechanical ventilation under appropriate treatment, though prolonged courses occur in about 20% of patients 3, 6
• The majority of patients can be weaned from mechanical ventilation within 1 month 2
• Current mortality rate is less than 5% in the United States, dramatically improved from the historical rate of 75% four decades ago 2, 4
• Deaths are almost never caused by the crisis itself but by comorbidities and complications such as ventilator-associated pneumonia and septicemia 3, 6
• The lifetime risk of recurrence is approximately 30% 3

Key Distinguishing Feature from Cholinergic Crisis

• Myasthenic crisis must be distinguished from cholinergic crisis (overdosage of anticholinesterase medications), as the two conditions require radically different treatment approaches 8
• Myasthenic crisis requires more intensive anticholinesterase therapy and immunotherapy, while cholinergic crisis requires prompt withdrawal of all anticholinesterase drugs 8
• Differentiation may require edrophonium chloride testing and clinical judgment, as both conditions present with extreme muscle weakness 8