What is the assessment and treatment plan for a patient in myasthenia crisis?

Assessment and Treatment of Myasthenia Crisis

Myasthenia crisis requires immediate respiratory support, discontinuation of cholinesterase inhibitors to rule out cholinergic crisis, and prompt initiation of plasmapheresis as the preferred immunotherapy. 1

Assessment of Myasthenia Crisis

Definition and Recognition

• Myasthenia crisis is defined as respiratory insufficiency requiring invasive or non-invasive ventilation due to respiratory muscle weakness or bulbar weakness causing upper airway collapse 1
• Occurs in approximately 15-20% of myasthenia gravis patients, typically within the first 2-3 years of disease 1, 2

Respiratory Monitoring

• Negative Inspiratory Force (NIF) measurements are essential:
  • NIF > -60 cm H₂O: Continue routine monitoring
  • NIF between -20 and -60 cm H₂O: Close monitoring needed
  • NIF less negative than -20 cm H₂O: Indicates impending respiratory failure 3
• Forced Vital Capacity (FVC) values <80% predicted with symptoms or <50% without symptoms indicate respiratory compromise 3
• Peak Cough Flow (PCF) <270 L/min indicates impaired ability to clear secretions 3

Critical Assessment Points

• Identify triggers (present in 60-70% of cases):
  • Respiratory infections (most common)
  • Medication changes (especially rapid tapering of immunosuppressants)
  • Recent surgery
  • Pregnancy
  • Initiation of corticosteroids 1, 4
• Differentiate between myasthenic crisis and cholinergic crisis:
  • Both present with extreme muscle weakness
  • Cholinergic crisis results from overdosage of cholinesterase inhibitors
  • May require edrophonium (Tensilon) test for differentiation 5

Treatment Algorithm

Immediate Management

1. Secure airway and provide ventilatory support

   • Early intubation is essential when bulbar symptoms with aspiration risk and/or respiratory insufficiency are present 2
   • Monitor for 1-2 weeks (median duration of mechanical ventilation is 12-14 days) 2

2. Differentiate myasthenic from cholinergic crisis

   • Temporarily withdraw cholinesterase inhibitors (pyridostigmine) 5
   • Consider edrophonium test if diagnosis is unclear 5
   • If cholinergic crisis is suspected, administer atropine to counteract muscarinic effects 5

3. Treat identified triggers

   • Aggressively treat respiratory infections
   • Discontinue medications that may exacerbate myasthenia:
     • β-blockers
     • IV magnesium
     • Fluoroquinolones
     • Aminoglycosides
     • Macrolides 3

Acute Immunotherapy

1. First-line treatment: Plasmapheresis/Plasma Exchange

   • Preferred over IVIG for myasthenic crisis 1
   • Typically 5-6 exchanges over 10-14 days
   • Clinical improvement can be seen within days 2

2. Alternative: Intravenous Immunoglobulin (IVIG)

   • 2 g/kg IV over 5 days 3
   • Consider when plasmapheresis is unavailable or contraindicated

Concurrent Immunosuppression

1. Corticosteroids

   • Prednisone 0.5-1.5 mg/kg orally daily 3
   • Caution: Initial steroid therapy may temporarily worsen symptoms

2. Steroid-sparing agents (initiated concurrently with corticosteroids)

   • Methotrexate: 15 mg weekly
   • Azathioprine: 2 mg/kg of ideal body weight in divided doses
   • Mycophenolate mofetil: Start at 500 mg twice daily, increase to 1000 mg twice daily 3

3. For refractory cases

   • Consider rituximab 2

Monitoring and Follow-up

During Crisis

• Regular assessment of respiratory parameters:
  • NIF measurements
  • Vital capacity
  • Arterial blood gases 3
• Monitor for complications:
  • Ventilator-associated pneumonia
  • Deep vein thrombosis
  • Delirium 2

Post-Crisis Management

• Gradual reintroduction of pyridostigmine when improvement is noted
  • Starting dose: 30 mg orally three times daily
  • Maximum: 120 mg orally four times daily as tolerated 3
• Continue immunosuppressive therapy
• Consider thymectomy if not previously performed, especially with thymoma 3
• Regular follow-up with neuromuscular specialist 3

Prognosis

• Mortality rate has dramatically improved from 75% historically to less than 5% currently 4
• Most deaths are due to complications rather than the crisis itself 2
• Approximately 30% lifetime risk of recurrence 2
• Most patients can be weaned from mechanical ventilation within one month 1

Common Pitfalls to Avoid

• Failing to recognize impending crisis before respiratory failure occurs
• Misdiagnosing cholinergic crisis as myasthenic crisis (leading to inappropriate increase in cholinesterase inhibitors)
• Delaying immunotherapy while waiting for confirmation of diagnosis
• Inadequate respiratory monitoring during initial treatment with corticosteroids
• Not addressing underlying infections or other triggers