Myasthenic Crisis

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MG Pathophysiology & Symptoms

• Autoimmune neuromuscular junction disease
• Antibodies to muscle fiber ACh receptors -> decrease function and # of ACh receptors available for neurotransmission -> decreased muscle stimulation -> weakness 
• Weakness worse with exertion and at the end of the day 
• Other symptoms: Ptosis, diplopia, weakness of proximal muscle groups, neck extensors and facial/bulbar muscles

 

MG Crisis (MC) Causes 

• MC = MG patient in respiratory distress 
• 15-20% of MG patients will undergo crisis requiring emergency intervention 
• Any form of physical or emotional stress can trigger MC
• Infection most common cause
• Other precipitants: surgery, pregnancy, childbirth, tapering steroids, medications (LOTS of them), conditions that change drug tolerance 

 
Drugs to avoid in MG Patients: 

• Fluoroquinolones, Ketolides, Botulinum toxin, D-Penicillamine

 
Drugs that should be used with caution: 

• Aminoglycosides, metronidazole, macrolides
• Beta-blockers, dihydropyridine calcium channel blockers (ex. amplodipine), quinidine, quinine 
• Corticosteroids & ACTH
• Interferons, Mg, narcotic analgesics (ex. morphine), phenothiazines, sedatives & hypnotics 
• Depolarizing & non-depolarizing neuromuscular blocking agents (succinylcholine-like, curare-like – need to use at lower doses) 

 

Evaluation

• Most come to the ED with  known MG diagnosis  
• Assess respiratory status – look for accessory muscle use, retractions, abnormal lung sounds, ability to clear secretions
• NOTE: Accessory muscle use can be blunted or absent in MC because of muscle fatigue 
• Focused review of systems: Neuro symptoms (ex. weakness/diplopia) that improve with rest; recent infections, toxins, drug exposures, trauma, stress
• Pay attention to current medication list (and any recently added meds) 
• Cholinergic crisis is rare but similar in presentation. Distinguish by assessing for autonomic and cholinergic symptoms like bradycardia, diarrhea, emesis, miosis, urination; etc. 

 
Focused 30-second Neuro Exam

• Test: Extraocular muscles, pupil reactivity, sensation in each extremity, Babinski & patellar reflexes, strength against resistance (arms, legs & head) 
• Observe for weakness, ptosis, grossly increased secretions, signs of respiratory failure

 

Airway Management

• Indications for Intubation: 

- NIF score less than -20 
- Vital capacity less than 10-20 mL/kg
- Single-breath test (unable to count past 20) 
- Unable to clear secretions

• Don’t just rely on thresholds – assess overall respiratory status (respiratory rate, work of breathing, oxygenation, phonation) 
• For borderline patients, consider noninvasive PPV like BiPap - shown to reduce need for ventilator support and time in the ICU 
• Depolarizing agents can last 2-4 times longer in MG patients. Succinylcholine can have unpredictable response so try to avoid
• Consider nondepolarizing agents like vecuronium or rocuronium, but at lower doses (still need to be cautious!). Start at one-third to one-half the standard dose. 
• Can also try awake intubation with propofol, fentanyl or ketamine 
• Avoid pyridostigmine during a crisis as it increases secretions and make airway management difficult! 

 
 
Management After Stabilization

• Look for underlying cause
• Infectious work-up: CXR, CBC, urinalysis, blood culture
• Other tests: BMP, tox screen, CK levels, EKG, beta-hcg 
• ABG useful for vent management, but not great as diagnostic tool – CO2 level and O2 sat usually change much later in myasthenic crisis 
• Discontinue meds that could cause exacerbation, replace electrolyte imbalances, treat any underlying infection (double-check to make sure antibiotic is safe in MG patients)  
• Call Neurology 
• Admit to ICU, where PLEX or IVIG will be initiated 

 
 
Sources:
Bird, S.J. & Levine, J.M (2019). Myasthenic crisis. In A.F. Eicheler & G. Finlay (Ed.), UpToDate.Retrieved February 27, 2019, from https://www.uptodate.com/contents/myasthenic-crisis
 
Howard, J.F. (2009). Myasthenia Gravis: A manual for the health care provider. Myasthenia Gravis Foundation of America, Inc.
 
Jowkar, A. (2010).  Neuromuscular topics for Neuroscience ICU RNs. Presentation, University of North Carolina at Chapel Hill.
 
Roper, J., Fleming M.E., Long. B. et al. (2017). Myasthenia Gravis and Crisis: Evaluation and Management in the Emergency Department. The Journal of Emergency Medicine, 53(5): 843-853. http://dx.doi.org/10.1016/j.jemermed.2017.06.099
 
Tintinalli, J. E., Stapczynski, J. S., Ma, O. J. et al. (2016). Tintinalli's emergency medicine: A comprehensive study guide(Eighth edition.). New York: McGraw-Hill Education.