Emergency Department management of myasthenic crisis

A 23 year old woman with a history of myasthenia gravis presented with several days of worsening generalized weakness, shortness of breath, and difficulty speaking.  She denies infectious symptoms such as fevers, chills, cough, dysuria, vomiting, or diarrhea.  Given her presentation, there is concern for myasthenic crisis.  She has had multiple similar episodes of these symptoms in the past and is currently being treated with prednisone, cyclosporine, and pyridostigmine.  She has needed IVIG and plasma exchange in the past for myasthenia exacerbations.   Her vital signs were within normal limits, but she was in mild distress from shortness of breath. 

Clinical question: 

What should be done in the ED for patients presenting with signs and symptoms of a myasthenia gravis exacerbation?

Literature:

Myasthenia gravis (MG) is an autoimmune disorder characterized by antibodies to post-synaptic acetylcholine receptors which results in fluctuating weakness.  In severe cases where weakness results in respiratory failure or the inability to swallow, the term myasthenic crisis is used.  Facial weakness, diaphragmatic and accessory muscle weakness may mask typical symptoms of respiratory distress.  Myasthenic crisis and impending respiratory failure is heralded by a forced vital capacity (FVC) of less than 1L and negative inspiratory force (NIF) less than 20 cm of water.  Blood gas measurements are poor indicators of impending respiratory failure since hypoxia and hypercarbia are late indicators of respiratory failure.  Common precipitants of myasthenic crises include infection, certain antibiotics, iodinated contrast agents, surgery, and weaning of immunosuppressants.  

Treatment in the emergency department should focus on frequent evaluation (e.g. every 2 hours) of the patient’s respiratory status with serial FVC and NIF and intubating promptly at signs of respiratory failure.  Sitting the patient upright may help temporize the patient’s dyspnea while preparing for intubation.  Even if not intubated, patients presenting with myasthenic crises will need admission to the ICU.  First line therapies include IVIG and plasmapheresis, both of which take several days to reach full clinical effect by removing acetylcholine receptor antibodies from the circulation.  High dose glucocorticoid therapy and other immunosuppresants such as azathioprine and cyclosporine can be initiated but are intended as long-term therapies and do not provide any benefit in the emergent setting.  Anticholinesterase use, such as pyridostigmine, remains controversial because of the risk of coronary artery vasospasm (resulting in MI) and arrhythmia.  A basic infectious workup, including a chest x-ray and urinalysis, should be considered.  

Take home:

Emergency department management of a patient with myasthenic crisis should focus on frequent and repeated assessment of respiratory status (including NIF, FVC) +/- intubation as necessary and disposition to an ICU.  Medical intervention helps over the longer term, but provides little benefit in the emergency department setting.

References:

1) Chaudhuri A and Behan PO.  Myasthenic crisis.  Q J Med 2009; 102:97–107.

2) Jani-Acsadi A and Lisak RP.  Myasthenic crisis: Guidelines for prevention and treatment. J Neurological Sciences 2007; 261:127–133. 

Kindly contributed by Philip Chan, PGY-2.