Objective: Differentiation between inflammatory myositis and genetic muscle disease is important and challenging. cases are presented to highlight the clinical and laboratory features that help with distinguishing between inflammatory myositis and genetic muscle disease.

Methods: clinical features including age at onset, pattern of muscle weakness,  laboratory data including muscle biopsies, myositis-specific-autoantibodies and gene mutations are reported in eight cases.

Results: Six of patients were initially diagnosed as inflammatory myositis based on clinical manifestation and inflammation on muscle biopsy,  and received steroids therapy for a short period of time until their gene mutations were identified(1 TK2, 5LMNA). Two patients were initially diagnosed as metabolic myopathy and muscular dystrophy respectively according to slowly progressive course, and finally they were diagnosed as inflammatory myositis(1 dermatomyositis, 1 anti-SRP autoimmue myopathy), and  all respond to immunosuppressive therapy.

Conclusions: Genetic muscle disease may accompanied with inflammatory phenomenon on muscle specimen and some inflammatory myopathies may progress slowly mimic hereditary muscle disease. Patients diagnosed with inflammatory myopathies should be carefully reevaluated when fail to respond to immunosuppressive therapy. Immunohisochemical staining of MHC-I and detection of  myositis-specific-autoantibodies help a lot in the identification of inflammatory myopathies.