A 49-year-old African American male presents with progressive proximal muscle weakness, dysphagia, dysphonia and weight loss. Six months prior to his hospitalization he endorsed slowly progressive weakness primarily affecting his proximal muscle groups. Following initial evaluation by rheumatology, prednisone 60mg/day was initiated for presumed polymyositis. His clinical condition deteriorated even on high dose steroids. One week prior to hospitalization, he developed diplopia, had worsening of dysphagia, hypophonia, dyspnea with increased proximal muscle weakness and ultimately required mechanical ventilation. His presenting creatine kinase was markedly elevated at 1871 IU/L. CT chest demonstrated a mediastinal mass measuring 11cm x 3.9cm x 12cm. No other mass/tumor was detected. The patient was treated with IV steroids, plasmapheresis, IVIG and underwent thymectomy with good recovery.  Mass biopsy was consistent with thymoma. Nerve conduction study/electromyography was suggestive of a diffuse myopathic process. Myasthenia gravis evaluation demonstrated elevated acetylcholine binding, blocking and modulating antibodies.  Muscle biopsy was consistent with necrotizing inflammatory myopathy and predominant type 2 fiber atrophy.  Focal perifascicular inflammation was suggestive of dermatomyositis.  

Inflammatory myopathies are a treatable group of myopathies. Diagnosis is based on clinical and pathological features with subdivision into four types: polymyositis, dermatomyositis, inclusion body myositis and necrotizing autoimmune myositis. Typical presentation (excluding inclusion body myositis) involves symmetric proximal muscle weakness which may include neck-extensors and pharyngeal muscles. There is relative sparing of ocular muscles with all subtypes. It is rare to see an overlap syndrome of inflammatory myopathy with myasthenia gravis. If found, it is often associated with thymoma and polymyositis subtype.

We report a rare case of overlap myositis evidenced by coexistence of dermatomysotis sine dermatitis and myasthenia gravis. When treating a myopathy and clinical deterioration is appreciated with high dose steroids, it is prudent to consider this rare overlap syndrome.