Concurrent inflammatory myopathy and myasthenia gravis with or without thymic pathology: A case series and literature review.

The association of myasthenia gravis (MG) and inflammatory myositis (IM) is rare and typically only one of the diseases is present. The management of the 2 diseases differs, therefore it is important to recognize the concomitant presentation. Here, we report a case series of 7 patients with co-existing MG and IM with review of the literature. We identified 7 patients with concurrent MG and IM who were followed at the Neuromuscular Disease Program at a tertiary referral center in Vancouver, British Columbia from 2004 to 2017. All 7 patients had ocular or bulbar involvement as manifestation of MG. Three patients had simultaneous onset of MG and IM, 2 of whom presented with myasthenia crisis and fulminant myositis. In the other 4 patients, MG was the initial presentation and IM occurred 3-11 years after MG. Among these 7 patients, 4 had underlying thymic pathology, including 2 with benign thymoma and 2 with stage IV thymoma; all 4 patients had antibodies to acetylcholine receptor (AChR). Of the 3 patients with no thymic pathology by imaging or histology, 2 had positive AChR antibody titer. For treatment, the thymoma was resected and chemotherapy was administered if appropriate. Additional immunosuppressive therapies including high-dose glucocorticoid, intravenous immunoglobulin (IVIG), methotrexate, mycophenolate, or cyclosporine were necessary to achieve remission. Two patients with no thymoma had refractory MG and IM, and both responded to rituximab. We also conducted a literature review on the clinical characteristics and management of this condition, and compared the previously reported cases to the patients in our series. This is one of the largest case series of MG-IM overlap with or without thymic pathology. In this cohort, the 2 disease entities can occur simultaneously, or one presents before the other. Most of the patients responded well to steroid, acetylcholinesterase inhibitor, and immunosuppressive agents. In very refractory cases, rituximab appeared to be effective, which has not been reported for the treatment of this condition before.

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