Thymoma-associated myasthenia gravis coexisting with myotonic dystrophy: a case report

Background: Myotonic dystrophy (dystrophia myotonica [DM]) is an autosomal-dominant inheritance, and myasthenia gravis (MG) is an autoimmune disease characterized by weakness of skeletal muscles. Cases of both DM and MG are extremely rare and distinguishing DM and MG symptoms is challenging. Case presentation: We herein report a 49-year-old woman presenting with subacute dyspnea and muscle weakness. She had previously been diagnosed with DM 24 years earlier. Computed tomography (CT) revealed an anterior mediastinal 32-mm solid mass that was suspected of being thymoma. The clinical features and neurological examination findings confirmed the diagnosis of thymoma-associated MG coexisting with DM. Intensive treatment for MG, including surgery, resulted in an improvement in some of her neurological symptoms. Conclusions: The symptoms of DM usually progress slowly, so the sudden exacerbation of symptoms indicates the involvement of other factors. It is important to be aware of these associations, as an early diagnosis with proper treatment will result in a better outcome.