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