An 18 year old woman presented with a year of progressive proximal limb weakness. Serum creatine kinase (CK) was elevated and electromyography suggested an irritable myopathy. Muscle biopsy revealed severe, chronic, active, necrotizing myopathy. Myositis-specific autoantibodies were initially negative; however, an immune-mediated necrotizing myopathy was suspected. She had only minimal response to variable immunomodulatory therapies over 17 years, with progression of weakness. Subsequent repeat testing confirmed positive anti-Signal Recognition Particle (SRP) autoantibodies. A thigh MRI, 17 years after symptom onset, showed extensive fatty replacement and significant muscle atrophy, suggesting a low likelihood of response to further immunosuppression. Nonetheless, motor function significantly improved after initiation of subcutaneous methotrexate (MTX). She has been stable off immunosuppressive therapy for 4.5 years. This report exemplifies that a protracted clinical course, extensive fatty replacement and atrophy on muscle MRI and normal CK levels do not preclude a late response to immunomodulatory therapy in anti-SRP myopathy.
