Elemental EM: Myasthenia Gravis

This is the first post in the Elemental EM series, a rapid review of core emergency medicine topics. The goal is to present high yield facts as a foundation for practice and fundamentals for board review. Please enjoy the following bullet point summary on myasthenia gravis.

Author: Courtney Cassella, MD (@Corablacas, EM Resident Physician, Icahn SoM at Mount Sinai) // Edited by: Alex Koyfman, MD (@EMHighAK, EM Attending Physician, UT Southwestern Medical Center / Parkland Memorial Hospital) and Brit Long, MD (@long_brit)

Epidemiology/Pathophysiology

• 0.15-0.7% of the population, Most often 3rd to 6th decade
• Autoimmune: Acetylcholine receptor antibodies
  • Causes receptor degradation, dysfunction, and blockade
  • Impairs function at neuromuscular junction (NMJ) → decreased muscle strength
• Dysfunction of thymus gland or immune response to infectious antigens → abnormal thymus → often thymoma present

Clinical Presentation

• General weakness
  • Proximal muscles
  • Neck extensors
  • Facial or bulbar muscles: Ptosis, diplopia, dysphagia, dysarthria, dysphonia
• Worsens: as the day progresses, with prolonged muscle use, hot temperatures, stress, infection
• No deficit in sensory, reflex, or cerebellar function
• Myasthenic crisis: Respiratory failure
  • Precipitants: infection, surgery, rapid tapering of immunosuppressants, pain, menstruation, pregnancy, sleep deprivation, medications
• Evaluation for impending respiratory failure
  • Inspiratory function
    • Vital capacity <1L (<20-25 mL/kg)
    • Negative inspiratory force <20 cm H2O
    • Accessory muscle use
  • Expiratory function
    • Positive expiratory force <40 cmH2O
    • Weak cough
    • Difficulty counting to 20 in a single breath
  • Upper airway (bulbar) weakness: dysphagia, nasal regurgitation, nasal quality of speech, staccato speech, jaw weakness, bifacial paresis, tongue weakness

Differential Diagnosis

Diagnosis

• Serologic testing – acetylcholine receptor antibodies
• Ice pack test – ptosis improves with ice pack application
• Edrophonium or neostigmine (Acetylcholinesterase inhibitor) → improves muscle strength
  • Limited use in testing as may worsen weakness in other NMJ disorders or cholinergic crisis leading to respiratory failure.
  • Caution: cardiac disease → may cause bradycardia, AV block, atrial fibrillation, cardiac arrest
  • Differentiates myasthenic crisis versus cholinergic crisis
  • 
• Electromyographic testing → postsynaptic NMJ dysfunction

Treatment

• Neurology consult

• Management of respiratory failure
  • Induction with smaller doses: Etomidate, fentanyl, propofol
  • Avoid use of depolarizing or nondepolarizing paralytic agents
    • If paralytics are necessary → half-dose
  • Immunosuppression: steroids, azathioprine, mycophenolate, plasma exchange, IVIG
  • Thymectomy

Resources/References

• Handel DA, Gaines S. Chronic Neurologic Disorders. In: Tintinalli JE, Stapczynski J, Ma O, Yealy DM, Meckler GD, Cline DM. eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8e New York, NY: McGraw-Hill; 2016. http://eresources.library.mssm.edu:2744/content.aspx?bookid=1658&sectionid=109437149. Accessed March 20, 2017.
• Patti, L. Edited by Robertson, J. and Koyfman, A. Undifferentiated Weakness: ED Focused Approach and Management. http://www.emdocs.net/undifferentiated-weakness-ed-focused-approach-and-management/
• Nickson, C. Myasthenia Gravis. LITFL. https://lifeinthefastlane.com/ccc/myasthenia-gravis/
• Wendell, LC and Levine JM. Myasthenic Crisis. Neurohospitalist. 2011 Jan; 1(1): 16-22. PMID: