Myasthenia gravis anesthesiology

1. Myasthenia gravisanesthesiology Jasdeep Dhaliwal Clinical Rotation

2. Overview • Pathophysiology • Autoimmune disease producing autoantibodies against nicotinic acetylcholine receptors at the NMJ • Bulbar symptoms most common • Ptosis, Diplopia, Blurred vision • Drooling, difficulty chewing, swallowing, facial weakness, dysarthria, and dysphagia • Weakness worsens with activity and fluctuates throughout the day • Muscle weakness leads to increased risk of aspiration, respiratory failure and cardiomyopathy • Does not affect smooth muscle or neuronal nicotinic acetylcholine receptors • No autonomic dysfunction • Hyperplasia of thymus seen in 65% of patients • Antibodies produced in Thymus

3. Picture!

4. Classification • Type I • Extraocular muscle only • Type IIa • Mild with sparing of respiratory muscles • Type IIb • Rapidly progressive involving respiratory muscles • Type III • Acute onset and rapid progression • Type IV • Severe form

5. Overview • Diagnosis • Edrophonium test • Short acting acetylcholinesterase inhibitor • Improvements seen in 5 minutes and last 10 minutes • Autoantibodies detected in serum • Electromyography • Classic decrement in action potential after successive nerve stimulation

6. Treatment • Acetylcholinesterase Inhibitors – first line • Pyridostigmine • Thymectomy • Improves symptoms, remission rate and clinical course • Immunosuppressants • Decreases production of autoantibodies • Plasmapheresis • Physical Therapy • Especially after crisis

7. Myasthenic Crisis vs. Cholinergic Crisis • Mysathenic Crisis • Serious complication where patients are unable to breathe adequately and possibly develop respiratory failure • Impaired swallowing and managing of secretions leading to aspiration • Monitory NIF, vital capacity, tidal volume • Cholinergic Crisis • Due to and excess of acetylcholine at the NMJ as seen in organophosphate poisoning • Fasiculations, sweating, myosis, abdominal pain, bradycardia • Flaccid paralysis and respiratory failure • Differentiate with edrophonium test

8. ANESTHETIC CONSIDERATIONS • PFT’s suggested to determine whether post-op intubation will be required • Monitor NIF, vital capacity and tidal volumes • Myasthenia gravis patients have decreased respiratory reserve so depressant drugs should be used with caution

9. ANESTHETIC CONSIDERATIONS • Myasthenia gravis patients are extremely sensitive to non-depolarizing blockers Dose-response for vecuronium in normal patients and patients with myasthenia gravis. (Reprinted from Eisenkraft JB, Book WJ, Papatestas AE: Sensitivity to vecuronium in myasthenia gravis: A dose-response study. Can J Anaesth 1990: 37: 301–306

10. ANESTHETIC CONSIDERATIONS • Cholinesterase inhibitor drugs used to treat myasthenia will influence the response to both depolarizing and non-depolarizing muscle relaxants • Although patients with myasthenia have a resistance to succinylcholine, a dose of 1.5 to 2 mg/kg will be adequate for rapid tracheal intubation. Preoperative administration of pyridostigmine, however, may prolong the duration of action of succinylcholine • Monitoring neuromuscular function with nerve stimulation is not always reliable in patients with MG

11. Notable Patients • Aristotle Onassis • Christopher Robin Milne • Brandon Cox

12. Resources • Miller’s Anesthesia, 7th ed. • Clinical Anesthesia, 6th ed. – Paul Barash • Big Blue Book