Successful Recovery from Life-Threatening Ventilatory Failure with Efgartigimod in Seronegative Myasthenia Gravis: Two Case Reports

Background: In Japan, efgartigimod is approved for treating generalized myasthenia gravis in patients who are anti-acetylcholine receptor antibody-positive, anti-muscle-specific kinase antibody-positive, or seronegative. Efgartigimod efficacy has been reported in seropositive patients with myasthenic crisis; however, evidence in seronegative patients with myasthenic crisis remains limited. We report two patients with seronegative generalized myasthenia gravis presenting with myasthenic crisis as the initial manifestation, successfully treated with efgartigimod. Case presentation: Case 1 involved an 83-year-old man admitted with acute dyspnea requiring mechanical ventilation for respiratory failure initially attributed to acute exacerbation of heart failure with pleural effusion. Due to poor respiratory recovery, he was referred to neurology on hospital day 14. Clinical features including ventilatory impairment and fatigable limb weakness, together with electrophysiological findings, supported myasthenia gravis diagnosis. Following oral immunosuppressants and intravenous efgartigimod, he was weaned from mechanical ventilation on hospital day 42. Case 2 involved a 75-year-old man with progressive dyspnea and signs of right heart failure requiring mechanical ventilation for respiratory failure. He was referred to neurology on hospital day 2. Electrophysiological testing under sedation suggested myasthenia gravis. After methylprednisolone pulse therapy, subcutaneous efgartigimod was initiated, achieving ventilator liberation on hospital day 40. Conclusion: Efgartigimod was effective in treating myasthenic crisis requiring mechanical ventilation in seronegative generalized myasthenia gravis, regardless of autoantibody status. These cases suggest efgartigimod may serve as an alternative or adjunctive therapeutic option for severe myasthenia gravis exacerbations.