What parenteral medication can be given in a myasthenia gravis crisis with acute muscle weakness?

Parenteral Medication for Acute Muscle Weakness in Myasthenia Gravis Crisis

For patients with suspected myasthenia gravis crisis with acute muscle weakness, intravenous pyridostigmine is the recommended parenteral medication for immediate administration, with neostigmine intramuscular as an alternative when IV access is unavailable. 1, 2

Initial Assessment of Acute Muscle Weakness

• Rapidly assess for respiratory compromise by measuring negative inspiratory force and vital capacity, as respiratory failure is the defining feature of myasthenic crisis 1, 3
• Evaluate for bulbar symptoms (dysphagia, dysarthria, facial weakness) which may indicate impending respiratory compromise 1, 3
• Check for pattern of weakness: typically fluctuating, exercise-dependent weakness of proximal extremities or bulbar muscle groups 1, 3

Parenteral Medication Options

First-Line Parenteral Treatment:

• Intravenous pyridostigmine: When oral administration is not possible, IV pyridostigmine can be administered 1, 2
  • Dosing: 1 mg IV pyridostigmine corresponds to 30 mg oral pyridostigmine 1
  • In crisis situations, pyridostigmine may be discontinued if intubation is required 1

Alternative Parenteral Option:

• Intramuscular neostigmine: 0.75 mg IM neostigmine corresponds to 30 mg oral pyridostigmine 1
  • Can be used when IV access is unavailable 1, 2

Diagnostic Testing Option:

• Edrophonium (Tensilon) test: Can be used diagnostically but not for maintenance therapy 4
  • Adult dosing: 2 mg IV initially, followed by 8 mg if no reaction occurs after 45 seconds 4
  • For children: 1 mg IV (under 75 lbs) or 2 mg IV (over 75 lbs), with potential titration up to 5-10 mg 4
  • Caution: Not recommended for maintenance therapy due to brief duration of action 4

Management Algorithm for Myasthenic Crisis

Step 1: Immediate Interventions

• Hold all acetylcholinesterase inhibitors if patient is in respiratory failure 5
• Secure airway if respiratory compromise is present; consider noninvasive positive-pressure ventilation initially 5, 6
• If intubation becomes necessary, avoid depolarizing paralytics or use reduced doses of non-depolarizing agents 5

Step 2: Rapid-Acting Immunomodulatory Therapy

• Initiate IVIG 2 g/kg IV over 5 days (0.4 g/kg/day) or plasmapheresis for 5 days 1, 2
• For life-threatening symptoms, plasma exchange may be the preferred option 1

Step 3: Corticosteroid Therapy

• Administer high-dose corticosteroids (methylprednisolone 1-2 mg/kg/day) 1
• For severe symptoms, consider pulse corticosteroid dosing (methylprednisolone 1 g/day for 5 days) 1

Important Precautions

• Avoid medications that can worsen myasthenia gravis 1, 7:

  • β-blockers
  • IV magnesium
  • Fluoroquinolones
  • Aminoglycosides
  • Macrolides
  • Anticholinergic medications (including hyoscyamine) 7

• Monitor closely for 1, 2:

  • Respiratory function with frequent pulmonary assessments
  • Cardiac involvement (check ECG and troponin)
  • Concurrent myositis (check CPK, aldolase)

Special Considerations

• Differentiate between myasthenic crisis (undertreated) and cholinergic crisis (overtreated) 8, 9
• In cases of diagnostic uncertainty, edrophonium testing can help differentiate, but should be used cautiously in crisis 4, 10
• Neurological consultation is essential for management of myasthenic crisis 1, 2
• The mortality rate of myasthenic crisis has improved significantly to approximately 4-8% with modern intensive care 6