To describe the phenotype of and to support further research into the etiology and appropriate therapeutic management of thymoma-associated myasthenia gravis (MG)-inflammatory myopathy (IM) overlap syndrome. 

While rare, the combination of MG and idiopathic IM has been previously documented. The prevalence of thymoma in patients with both IM and MG (70%) is significantly higher than in those with MG alone (10%). It has been postulated that thymoma-related immunopathogenic mechanisms, including dysregulation of the immune checkpoint pathway, may increase the risk of idiopathic IM and MG. 

A 47-year-old male with a history of thymoma complicated by recent tumor recurrence presented to our tertiary referral center with rapid deterioration in bulbar and appendicular motor strength and a presumed diagnosis of amyotrophic lateral sclerosis. Prednisone, pyridostigmine, and IVIG previously provided no relief. Further workup was undertaken to elucidate the true etiology of his symptoms. 

Electromyography/nerve conduction study was unremarkable. CSF studies revealed a normal protein and cell count. Creatine kinase was elevated at 2578 units/L. MRI of his cervical spine was remarkable for inflammatory changes along the myofascial planes of the posterior paraspinal muscles, concerning for myositis. Neural autoantibody testing, albeit after he had received IVIG, revealed multiple positive findings including acetylcholine receptor (AChR) blocking and modulating, smooth muscle, F-Actin, fibrillarin, and voltage-gated potassium channel antibodies. High dose methylprednisolone and plasmapheresis were initiated with marked clinical improvement. He has since been maintained on rituximab. On follow-up testing, continued positive antibodies include AChR binding, AChR blocking, and elevated striated muscle antibody. 

When presented with a patient with a history of recurrent thymoma and weakness of unclear etiology, MG-IM overlap syndrome should be considered in the differential diagnosis. Further study into the mechanism and appropriate management of this autoimmune dysregulation syndrome should be undertaken.