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We report a case of a 40-year-old woman who developed generalized muscle weakness over a period of 2 months. Physical examination revealed palpable masses in her arms and hands. Serum creatine kinase levels were elevated. Electromyography showed myopathic changes and 3 Hz repetitive nerve stimulation revealed a decremental pattern on repetitive nerve stimulation. Muscle MRI demonstrated increased signal intensity in the biceps brachii on T1-weighted images. Chest CT scan showed a mediastinal mass suggestive of thymoma. Muscle biopsy revealed giant cell polymyositis. The patient was treated with cholinesterase inhibitors and corticosteroids with improvement of strength, and subsequently underwent thymectomy followed by radiotherapy., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Polymyositis revealing myasthenia gravis and malignant thymoma: a case report. The increased frequency of thymoma associated with autoimmune diseases has been well documented in the literature. However the association of polymyositis (PM) and myasthenia gravis (MG) revealing a malignant thymoma is rare. We report a case of a gentleman whose presenting symptoms were those of polymyositis.

The aim of the study is to provide evidence that the lymphocytic infiltration of myasthenia gravis (MG) muscle do not represent a true autoimmune myositis, rather an infiltration by naive lymphocytes derived from lymphocyte-rich thymomas. Muscle biopsies from 179 patients with pure MG, 6 thymoma patients without MG and 15 patients with definite polymyositis were analyzed. In 18 patients with MG (all associated with lymphocyte-rich thymomas) and in two thymoma patients without MG, lymphocytic infiltrates were identified in muscles. By use of immunohistochemistry, we demonstrated that the lymphocytes in MG differ from those in polymyositis, being mature but in contrast to polymyositis naive CD45RA+ T lymphocytes. We suggest that the lymphocytic infiltrates in patients with MG and thymoma represent an infiltration of muscle by thymoma-derived mature but naive T cells. The finding of CD8+CD45RA+ lymphocytes in muscle may signify an underlying thymoma and should not be misdiagnosed as polymyositis.

Although most patients with thymoma present with a mediastinal mass amenable to surgical resection, some patients develop metastatic disease requiring systemic therapy. The majority of thymomas express somatostatin receptors as demonstrated by octreotide scanning, an observation which has prompted the clinical use of octreotide in patients with this disease. Many patients with thymoma exhibit autoimmune paraneoplastic syndromes, most frequently myesthenia gravis. We report here the case of a patient with metastatic thymoma who developed a profound autoimmune polymyositis and lupus-like syndrome that flared following treatment with octreotide and was associated with a clinical response to this agent. No evidence for myesthenia gravis was discovered. The severity of the myopathy necessitated mechanical ventilation for 12 weeks. The natural history of thymoma, treatment options including recent combination chemotherapy regimens, and potential mechanisms for flaring of autoimmune paraneoplastic syndromes triggered by therapy of thymoma are discussed.