Myasthenia Gravis: Disease Overview

  • Who: Bimodal distribution. Ages 20-40 (female predominance) and 50-70 (male predominance). Increased risk of onset in postpartum period; juvenile and congenital forms possible
  • What: At the NMJ, Ach receptors are blocked by antibodies = decreased muscular contraction. Associated with fluctuating weakness.
  • Home Treatments: Pyridostigmine (AchE inhibitor); +/- glucocorticoids, azathioprine, mycophenolate (immunosuppression) or rarely po (i.e. liquid) albuterol (mechanism not well understood); +/- thymectomy surgical hx

Myasthenic Crisis:

Worsening of myasthenic weakness (and here the literature differs on exact definition) that either requires or threatens to require intubation or noninvasive ventilation.

Severe bulbar weakness can produce dysphagia and aspiration that often complicates the respiratory failure.

Signs/sxs:

  • Generalized weakness
  • Ptosis often 1st sign
  • +respiratory failure
    • Generalized muscle weakness can mask signs of respiratory distress (i.e. accessory muscle use may not be apparent)
    • Can have precipitous respiratory collapse

Making the dx in the ED:

  • Is a clinical dx
  • To aid in dx, consider:
    • 1) Exam: weak cough? Difficulty holding head up? Bulbar dysfunction signs (i.e. difficulty swallowing, tolerating secretions, speaking, sticking tongue out, etc.)? Poor air movement? Tachypnea/shallow breathing? +/- accessory muscle use as above
    • 2) Single breath test: all patients should be able to count from 1 to 20 in a single breath. If not, concerning for impending respiratory failure
    • 3) Call respiratory to attempt forced vital capacity (FVC) and negative inspiratory force (NIF) testing
      • Some patients simply will not be able to do this as requires a good seal made on the mouthpiece. Also are effort-dependent and tired pts might not give their best effort so are not fully accurate
      • Should not base decision to intubate solely on values of these but can be further diagnostic tools in an otherwise mixed picture
      • NIF – measures inspiration: think of an incentive spirometer set up to a manometer
        • Concerning if <20 cm H2O
      • FVC – measures expiration: max amount of air you can forcibly exhale after a full inspiration
        • Concerning if <1L or <20-25 mL/kg
  • Eval for other dx’s in the ED; i.e. your regular respiratory compromise workup

Don’t stop there:

  • As with most exacerbations, it’s pretty cool if you figure out the cause so you can try to reverse it in the ED
  • Consider:
    • Infection (esp pna, viral respiratory illnesses)
    • Medication changes – particularly nonadherence or dose changes to pyridostigmine, tapering of immunosuppressive medications or initiation of steroid, and new meds that exacerbate MG
    • Electrolyte disturbance
    • Thyroid disease
    • Toxin/drug exposures – note, don’t give MG pts Mg in addition to a whole bunch of other meds listed here: https://emcrit.org/ibcc/myasthenia/#drugs_to_avoid_in_MG
    • Trauma
    • Childbirth (traumatic too, amiright?)
    • Surgery

Treatment:

  • Airway/Breathing:
    • Consider non-invasive measures first. Esp if only mild distress can try HFNC, and if mild-mod can try bipap.
    • Severe distress including evidence of hypoxemia, poor respiratory effort, or CO2 retention = an indication for intubation and mechanical ventilation
      • Intubation: use half dose nondepolarizing paralytic (i.e. 0.6 mg/kg roc)
        • Succ can fail to work b/c of reduced Ach receptor density on muscle
  • Treat underlying cause if identified and something can be done about it (i.e. abx for pna, giving dose of missed pyridostigmine)
  • Neuro consult (particularly to discuss):
    • IVIG
    • Plasmapheresis
    • Usually no steroids up front as can cause initial clinical deterioration but neuro may recommend them in refractory cases

TL/DR Version

  • Myasthenia gravis crisis is exacerbation of disease with concern for respiratory failure
  • Clinical dx; exam, single breath test, and PFTs of NIF and FVC all aid in dx
  • Treatment options: non-invasive ventilation, intubation, neuro consult, IVIG, plasmapheresis
  • Intubation: Do not base decision solely on NIF/FVC testing. Only use half dose of your nondepolarizing paralytic
  • Consider cause of myasthenic crisis and likewise treat if can

Further Reading:

http://www.emdocs.net/em3am-myasthenia-gravis-and-crisis/

https://www.uptodate.com/contents/myasthenic-crisis

https://www.uptodate.com/contents/clinical-manifestations-of-myasthenia-gravis

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3726100/

August 2022
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