Thymoma-associated myasthenia gravis coexisting with myotoni
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 dystrophia myotonica and MG are extremely rare and distinguishing dystrophia myotonica and myasthenia gravis symptoms is challenging.

A 49-year-old woman presented with subacute dyspnea and muscle weakness. She had previously been diagnosed with dystrophia myotonica 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 myasthenia gravis coexisting with dystrophia myotonica. Intensive treatment for myasthenia gravis, including surgery, resulted in an improvement in some of her neurological symptoms.

The symptoms of dystrophia myotonica 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.