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MYASTHENIA GRAVIS ANESTHESIOLOGY Jasdeep Dhaliwal Clinical Rotation
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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
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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
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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
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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
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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
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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
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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
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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
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Notable Patients Aristotle Onassis Christopher Robin Milne Brandon Cox
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Resources Miller’s Anesthesia, 7 th ed. Clinical Anesthesia, 6 th ed. – Paul Barash Big Blue Book
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