Larkin_2025_BMJ.Case.Rep_18_

Overlap syndromes involving myasthenia gravis, myositis and myocarditis are recognised adverse effects of immune checkpoint inhibitors (ICI). We report a similar syndrome of myasthenia gravis and myositis in a patient without prior ICI exposure. Diagnostic tests showed signs of both immune-mediated necrotising myopathy and myasthenia gravis. Symptoms included proximal muscle weakness, ophthalmoplegia, bulbar symptoms and neuromuscular respiratory failure. Initial treatment with glucocorticoids and intravenous immunoglobulin for myositis was partially effective. Significant improvement occurred with rituximab and an acetylcholinesterase inhibitor. Although imaging had not revealed a thymic mass suggestive of thymoma, the patient underwent thymectomy several weeks after recovering from the acute illness. Histopathological examination of the resected tissue confirmed the presence of thymoma. This case illustrates that myasthenia gravis and myositis can spontaneously co-present. Clinicians should consider the possibility that a seemingly atypical presentation of a condition may reflect a more typical presentation of two conditions, such as in this case. In this situation, paraneoplastic causes should be considered.