Myotonic dystrophy type 1 coexisting with myasthenia gravis and thymoma.

Myotonic dystrophy type 1 (DM1) is an autosomal-dominant multisystemic disorder that may rarely be associated with benign and malignant neoplasms. Cases of both thymoma and myasthenia gravis in association with DM1 are extremely rare. A literature review revealed only three prior reports. We present a 51-year-old man with a family history of DM1 and fluctuating diplopia and ptosis, who was found to have acetylcholine receptor-binding antibodies, thymoma, and a clinical presentation compatible with ocular myasthenia gravis as well as positive genetic testing for DM1. Needle electromyographic (EMG) study demonstrated diffuse runs of myotonic discharges in multiple muscles, consistent with the diagnosis of DM1. Single-fiber EMG showed both increased jitter and blocking. Due to somatic instability, which has been shown previously in DM1, the myotonin protein kinase (DMPK) gene appears to act as a tumor suppressor. Therefore, abnormal CTG repeat expansions in the gene could lead to the development of thymoma and myasthenia gravis.