Myasthenia gravis

Keyword

Myasthenia gravis

Definition

Nicotinic acetylcholine receptors antibodies로 인한 skeletal muscle neuromuscular junctions의 Autoimmune disorder

Signs & Symptoms

• 활동 시 심해지고 쉬면 호전되는 muscle weakness
• Affected individuals may have thymic hyperplasia and tumors (-> intubation 어려울 수 있음)
• Cranial nerve and bulbar involvement are common (pharyngeal, laryngeal muscle weakness-> aspiration risk ↑)

Osserman Classification System for Myasthenia Gravis

• Class I: Ocular myasthenia
• Class IIA: Mild generalized myasthenia with slow progression: no crises, responsive to drugs
• Class IIB: Moderately severe generalized myasthenia: severe skeletal and bulbar involvement but no crises; drug response less than satisfactory
• Class III: Acute fulminating myasthenia: rapid progression of severe symptoms, with respiratory crises and poor drug response
• Class IV: Late severe myasthenia, same as class III but progression over 2 years from class I to II

Treatment

Patients are usually treated with thymectomy, anticholinesterase(severe case), and immunosuppressant medications (corticosteroids, azathioprine, cyclosprine)

Preoperative evaluation

Patients taking azathioprine -> CBC and LFTs (drug-induced bone marrow suppression, liver dysfunction)

Patients treated with steroids need glucose measurement, Steroids는 지속 필요

Anticholinesterase 지속 필요

Management of anesthesia

Anticholinesterase tx. -> muscle relaxant dosage reassessment 필요

succinylcholine resistance, prolonged response, antagonize the effect of nondepolarizing muscle relaxant 때문에 anticholinergic drugs -> withheld for 6 hours preoperatively, 환자의 sensitivity 바뀔 수 있어 수술 후 medication은 재조정 필요함

Corticosteroid tx. -> succinylcholine 에 영향 없으나 vecuronium(steroidal muscle relaxant)에 resistance

• Induction : small dose succinylcholine 사용가능하며 보통 Myasthenia gravis 환자에서 non-depolarizing agent에 대한 sensitivity 증가하므로 initial dose ½ or ⅔ 를 사용하고 nerve stimulator monitoring 권고
• Tracheal intubation은 awaken으로 이루어지거나 muscle relaxant 없이 (volatile agent 의 relaxant효과만으로) 하기도 함
• Maintenance : Intravenous anesthetics, volatile agent(with / without N₂O) - muscle relaxant 사용 줄일 수 있어 좋음. short acting, small doses non depolarizing muscle relaxant로 intraoperative relaxation 유지 가능
• Postoperative : postoperative ventilation 중요!! 특히 6년 이상 MG병력, COPD 동반 시

significant bulbar weakness, vital capacity < 40mL/kg -> pyridostigmine 750mg/daily

Rapid recovery of neuromuscular function in patients receiving rocuronium when sugammadex was used for reversal.

Reference

Miller’s Anesthesia, 8th edition, Chapter 38: Preoperative Evaluation p.1129

Miller’s Anesthesia, 8th edition, Chapter 39: Anesthetic Implications of Concurrent Diseases p.1200-1201

Stoelting’s anesthesia and co-existing disease, Chapter 21. Skin and musculoskeletal diseases p.448-451