What are the clinical features of myasthenic crisis?

Clinical Features of Myasthenic Crisis

Myasthenic crisis is characterized by respiratory failure requiring mechanical ventilation due to severe weakness of respiratory and bulbar muscles in patients with myasthenia gravis. 1, 2

Primary Clinical Features

• Respiratory muscle weakness is the defining feature of myasthenic crisis, leading to respiratory insufficiency requiring invasive or non-invasive ventilation 1, 2
• Bulbar muscle weakness manifests as dysphagia (difficulty swallowing), dysarthria (slurred speech), and facial weakness, which can lead to upper airway collapse 3, 4
• Fatigable or fluctuating muscle weakness that typically affects proximal muscles more than distal muscles, worsening with continued activity and improving with rest 3, 5
• Ptosis (drooping eyelids) and diplopia (double vision) from extraocular muscle weakness are common preceding symptoms 3, 5

Respiratory Assessment Findings

• Decreased negative inspiratory force (NIF) and vital capacity (VC) are objective measures of respiratory compromise 6, 4
• Forced vital capacity (FVC) <20 mL/kg indicates need for mechanical ventilation 6
• Maximum inspiratory pressure <30 cm H₂O and maximum expiratory pressure <40 cm H₂O are associated with need for ventilation 6
• Rising end-tidal CO₂ or arterial CO₂ strongly predicts need for mechanical ventilation 6

Bulbar Symptoms

• Dysarthria (slurred speech) indicates bulbar involvement 3, 5
• Dysphagia (difficulty swallowing) increases risk of aspiration 3, 6
• Facial muscle weakness causes reduced facial expressions 3, 5
• Neck weakness may lead to difficulty holding up the head (head drop) 3, 5

Associated Findings

• Autonomic dysfunction may present as dry mouth, urinary retention, constipation, and orthostatic hypotension 6
• Generalized limb weakness typically affects proximal muscles more than distal muscles 3, 5
• Concurrent myositis may be present, especially in immune checkpoint inhibitor-related cases 6
• Cardiac involvement may occur, particularly in immune-related cases 6

Progression Pattern

• Myasthenic crisis often develops over hours to days, not suddenly 1, 2
• Most crises occur within the first 2-3 years of myasthenia gravis diagnosis 1, 2
• In 15-20% of cases, myasthenic crisis is the first manifestation of previously undiagnosed myasthenia gravis 2, 7
• Median duration of crisis is about 2 weeks under appropriate treatment 2

Common Precipitating Factors

• Respiratory infections are the most common trigger 1, 8
• Other triggers include:
  • Aspiration 8
  • Sepsis 8
  • Surgical procedures 8
  • Rapid tapering of immunosuppressive medications 8
  • Beginning treatment with corticosteroids 8
  • Medications that worsen myasthenia (beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, macrolide antibiotics) 6, 4
  • Pregnancy and childbirth 9, 8
  • No specific trigger is identified in 30-40% of cases 1

Risk Factors for Developing Myasthenic Crisis

• History of previous myasthenic crisis 1
• Severe myasthenia gravis 1
• Oropharyngeal weakness 1
• Muscle-specific kinase (MuSK) antibodies 1
• Thymoma 1

Clinical Pitfalls to Avoid

• Failure to recognize early respiratory compromise - Hypoxia and hypercapnia may develop late in respiratory failure 6
• Missing bulbar dysfunction - A strong predictor of need for intubation 6
• Overlooking concurrent myositis or myocarditis - Particularly in immune checkpoint inhibitor-related cases 6
• Continuing medications that worsen myasthenia - Beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolide antibiotics should be avoided 6, 4