27PNon-caseating granulomatous myopathy associated with thymoma and atypical presentation of myasthenia gravis

Abstract

Non-caseating granulomatous myositis is a rare inflammatory myopathy that is most associated with sarcoidosis, and to a lesser degree with other inflammatory disorders including inclusion body myositis and myasthenia gravis (MG). When associated with MG, a thymoma is also typically present. We report a case of non-caseating granulomatous myositis associated with thymoma and an atypical presentation of acetylcholine receptor (AChR) positive MG. A 64-year-old woman undergoing workup for pneumonia was found to have a large anterior mediastinal mass, which was diagnosed as an AB type thymoma and resected in October 2023. She was also found to have AChR antibody positivity but lacked a clinical myasthenic syndrome at the time. In November 2024 she presented with a 2-month worsening of painless limb muscle weakness associated with severe edema, more pronounced in the arms than legs. Weakness progressed to the point where she was no longer able to feed herself with a utensil and could not walk as long as previously. Prior baseline was 2 hours daily at the gym. She denied any dysarthria, dysphagia, diplopia, dyspnea, numbness or tingling, bowel or bladder dysfunction. Serology was consistent with myopathy and included CK elevated to greater than 6,000. Electrodiagnostic studies revealed irritable myopathy, and muscle biopsy confirmed the presence of non-caseating granulomas. No evidence of sarcoidosis was found on imaging or serology. She was found again to have positive AChR antibodies but lacked typical oculopharyngeal symptoms of MG. However, she did have severe upper limb weakness more profound in the distal muscle groups, as well as positive repetitive stimulation testing (performed at the ADM muscle with stimulation of the ulnar nerve); these findings were most suggestive of an atypical MG presentation. She was treated with IVIG and oral prednisone and had significant improvement at one month follow up. We report a severe case of non-caseating granulomatous myositis associated with recent thymectomy and an atypical presentation of AChR positive myasthenia gravis. We suggest that granulomatous myositis should be suspected in subacute onset of upper limb distal predominant weakness associated with edema and rhabdomyolysis in the setting of history of thymoma and MG.