Polymyositis without Beneficial Response to Steroid Therapy: Should Miyoshi Myopathy be a Differential Diagnosis?

BACKGROUND Miyoshi myopathy (MM) is an autosomal-recessive muscle disorder caused by mutations in the DYSF gene. Clinical features and histopathological changes in dysferlinopathies may mimic inflammatory myopathies and a high degree of clinical suspicion is required to guide the genetic investigation. CASE REPORT We report the case of a 16-year-old male who presented with severe bilateral calf pain and elevated CK levels (15 000 IU/l) who was on prolonged steroid therapy prompted by the clinical suspicion of inflammatory myopathy. Three years into his illness, he was referred for neuromuscular evaluation presenting with untreatable muscle pain and progressive weakness. The diagnosis of "refractory polymyositis" was revisited. Targeted exome sequencing revealed homozygous pathogenic mutations in the DYSF gene, confirming a diagnosis of Miyoshi myopathy. CONCLUSIONS Our case illustrates that severe muscle pain may be the initial feature of Miyoshi myopathy and should be considered in the differential diagnosis of inflammatory myopathies. Although the described patient reported partial clinical improvement in muscle pain, steroid treatment is not an effective therapy for dysferlinopathy patients and it did not prevent disease progression. In addition, we confirm the utility of next-generation sequencing approaches to myopathies, particularly in complex or unusual cases when muscle biopsy is not available.