Anesthesia Considerations 

Myasthenia Gravis 

 

 

 

Background

 

• Autoimmune disorder characterized by weakness & fatigability of skeletal muscles

• Weakness results from an antibody-mediated immunological attack directed at acetylcholine receptors (or receptor-associated proteins) in the postsynaptic membrane of the neuromuscular junction

 

 

Considerations 

 

• Bulbar/skeletal muscle weakness resulting in ↑ risk of:

  • Aspiration

  • Perioperative respiratory failure

• Potential systemic complications:

  • Thymoma & possible anterior mediastinal mass

  • Myocarditis causing cardiomyopathy, atrial fibrillation, heart block

• Altered response to NMB (neuromuscular blocking) medications:

  • Very sensitive to NdMR (nondepolarizing muscle relaxants): avoid or use 1/10 normal dose with continuous monitoring 

  • Resistant to succinylcholine (ED95 2.6X normal)

• Treatment: steroids, immunosuppressants, anticholinesterases

• Risk of perioperative myasthenic or cholinergic crises

 

 

Goals 

 

• Minimize risk of aspiration (prophylaxis, RSI)

• Minimize risk of perioperative respiratory failure (judicious NMBs & opioids) & anticipate need for post-op ventilation

• Minimize risk of myasthenic or cholinergic crisis

• Optimize neuromuscular function

 

 

Conflicts

 

• RSI vs altered response to neuromuscular blockers

• RSI vs cardiac involvement 

• RSI vs anterior mediastinal mass

• Magnesium for pre-eclampsia in pregnancy vs contraindicated due to muscle weakness

 

 

Pregnancy Considerations 

 

• 1/3 improve, 1/3 stay the same, 1/3 get worse

• Exacerbations usually in 1st trimester with improvement in 2nd & 3rd

• ~ 30% experience relapse postpartum

• ↑ abortion, preterm labor, maternal morbidity & mortality

• Cholinesterase inhibitors: minimal placental transfer but have uterotonic effects

• Monitor for fatigue/weakness during labour (consider measuring vital capacity)

• Magnesium is relatively contraindicated due to muscle weakness 

• Generally, neuraxial anaesthesia preferred (for labour & vaginal or cesarian delivery) unless severe bulbar or respiratory involvement, then consider general anesthetic for cesarian delivery:

  • Ester local anaesthetics may have prolonged half-life in patients taking cholinesterase inhibitors → increased risk of toxicity, use amide local anesthetics

  • In patients with moderate respiratory compromise, the use of BiPAP may improve the safety of neuraxial anesthesia

• Be prepared for transfer to ICU for postpartum ventilation if severe disease

• Neonatal myasthenia in 16% due to transfer of maternal IgG antibodies across the placenta, resolves in 3 to 4 weeks

 

 

Weakness in PACU 

 

• Management

  • Attend to patient: rule out airway obstruction, life-threatening hypoventilation, hypoxemia, hypercarbia, or anything requiring immediate airway management & PPV

  • Examine:

    • Vitals

    • ? aspiration, sepsis, surgical complication

    • ? myasthenic crisis (weakness improves with tensilon test)

    • ? cholinergic crisis (salivation, lacrimation, urination, diarrhea, GI symptoms, emesis, bradycardia, bronchoconstriction, bronchorrhea)

  • Review medications given in OR, PACU

  • Send ABG, electrolytes

 

 

Myasthenic Crisis vs Cholinergic Crisis 

 

• Myasthenic Crisis:

  • Weakness exacerbated by infections, electrolyte abnormalities, pregnancy, surgery, emotional stress, drugs (aminoglycosides), or interruption of immunosuppressants

  • Improvement with edrophonium (tensilon test):

    • Tensilon test: 1.5 mg increments of edrophonium to 10 mg total (should get better in about 2 minutes)

  • Consider elective intubation if vital capacity < 20cc/kg or maximum inspiratory force worse than -30 cmH2O

  • Consider PO or IV dose of pyridostigmine:

    • PO: 30-120 mg/day, onset 15-30 min, peak 2 hrs, duration 4 hrs

    • IV dose is 1/30 of PO dose

  • Alternative treatment is neostigmine 0.5-2.5 mg IV/SC q1-3 hours titrated to response (max = 10mg/24hours)

  • Neurology consult for management (plex, IVIG, steroids)

• Cholinergic Crisis:

  • Due to excessive cholinesterase inhibitors

  • Symptoms of acetylcholine excess (SLUDGE BBB): salivation, lacrimation, urination, diarrhea, GI symptoms, emesis, bradycardia, bronchorrhea, bronchospasm

  • Distinguish by giving edrophonium (tensilon test) which improves symptoms if myasthenic crisis & worsens symptoms if cholinergic crisis

  • Treatment includes endotracheal intubation, atropine & cessation of cholinesterase inhibitors until the crisis is over