Review of Myasthenia Crisis

By Damian Baalmann, M.D.
Reviewed by Sara Hocker, M.D.

Myasthenia Crisis

A 34 year-old male presents to your emergency department in moderate respiratory distress. He is conscious and as the nurses are hooking up monitors, you are able to obtain a history of recent sore throat, followed by increasing weakness in his extremities with progressive dyspnea on exertion. Several years ago, the patient underwent a thymectomy and has since been maintained on prednisone 30-mg/day and pyridostigmine 60-mg Q4H. Vitals: pulse 112, blood pressure 110/80 mmHg and respiratory rate of 26/min. His oxygen saturation is 87% on room air. What are your next steps in management?

Myasthenia Crisis: What is it?

• Myasthenia Gravis: blocking of AchRs (acetylcholine receptors) at the muscle endplate by antibodies
  • Remember, AchRs are those post-synaptic receptors found on neuromuscular junction as well as the autonomic ganglia
• Myasthenia Crisis: exacerbation of muscle weakness often resulting  in neuromuscular respiratory failure
  • Number of AChRs is reduced to ≤30% of normal
  • Pooling of secretions
  • Precipitants
    • #1 Cause is Concurrent Infection- check urine, chest X-ray, and blood cultures
    • Other stressors including surgery, pregnancy or trauma
    • Changes in the patient’s immunosuppression regimen
    • Certain drugs may exacerbate myasthenia but the evidence for this is very limited. These include aminoglycosides, erythromycin, azithromycin, beta-blockers, procainamide, quinidine and magnesium.
• Cholinergic Crisis: excessive anticholinesterase medication; aka too much acetylcholine
  • Easily confused with a Myasthenia Crisis
  • Typically not seen with doses of pyridostigmine <120-mg Q3h

Can differentiate by the presence of cholinergic symptoms including bradycardia, diarrhea, lacrimation, salivation and miosis. Bronchorea may be present in both myasthenic and cholinergic crisis.

Management

With myasthenic crisis, there are several important clinical parameters to assess to help determine whether assisted ventilation is required.

• Tachycardia, tachypnea, perspiration
• Use of accessory muscles and inability to lift the head from the bed
• Orthopnea or development of an abdominal paradoxical breathing pattern when supine
• Weak cough
• Interrupted speech (inability to perform single-breath count: count from 1 to 10 in single exhalation (roughly equal to FVC <1.0 L)
• Vital capacity < 20 mL/kg
  • Inspiratory Reserve Volume + Tidal Volume + Expiratory Reserve Volume (or AKA, the total amount of air you can exhale)
• Maximum inspiratory pressure >-30 cm H2O
  • The patient’s ability to pull air in
• Maximum expiratory pressure <40 cm H2O
• Hypoxemia and hypercarbia (late findings)

If assisted ventilation is indicated clinically, the patient denies an increase in secretions or difficulty handling their saliva, and the patient is not hypercapnic, a trial of BiPAP may prevent intubation and prolonged ventilation, reduce pulmonary complications and the length of intensive care unit and hospital stay.

If hypercapnia has already developed or there is a pooling of secretions and intubation is necessary, consider the following issues when choosing your paralytic

• Succinylcholine
  • A depolarizing neuromuscular blocker
    • meaning that it causing paralysis by flooding acetylcholine receptors and prevents repolarization
  • Because of the smaller number of receptors available in patients with MG, larger doses of succinylcholine are required to activate sufficient receptors to cause depolarization
  • How large? Probably about 2.6 times the normal dose, meaning, 3.5 to 4.0-mg/kg
  • Also, expect a longer duration because pyridostigmine inhibits pseudocholinesterase
• Rocuronium
  • A non-depolarizing neuromuscular blocke
    • Prevents acetylcholine from causing depolarization
  • Need smaller doses  of rocuronium
  • How small? Probably about 0.5 times the normal dose, meaning 0.6-mg/kg
  • Expect longer onset of action. Normal is 30-45 seconds and with MG could be up to 90-seconds for paralysis

After airway stabilization, consider some of these interventions (after consulting your friendly neurologist)

• High dose glucocorticoids (exercise caution is the patient is not intubated as this may initially worsen neuromuscular respiratory failure)
• IVIG: 400-mg/kg/day X5 days
• Plasmapheresis: 5 exchanges over 7-14 days

Revisiting the Case

Management assessment would include need for emergent intubation, elective intubation, non-invasive ventilation (caution in the setting of excessive  secretions) or supplemental oxygen. If intubating, we would be mindful of our paralytic and use more succinylcholine or less rocuronium. We would consider steroids, IVIG or plasmapheresis after consultation with neurology.

References

Seneviratne J, Mandrekar J, Wijdicks EF, Rabinstein AA. Noninvasive ventilation in myasthenic crisis. Arch Neurol. 2008 Jan;65(1):54-8.

Bird, S. (2012, December 18). Myasthenic crisis. Myasthenic crisis. Retrieved June 18, 2014, from http://www.uptodate.com/contents/myasthenic-crisis?source=machineLearning&search=myasthenic+crisis&selectedTitle=1~33&sectionRank=1&anchor=H4#H4

Flower O, Bowles C, Wijdicks E, Weingart SD, Smith WS. Emergency neurological  life support: acute non-traumatic weakness. Neurocrit Care. 2012 Sep;17 Suppl 1:S79-95.