We report a case of myositis with anti-signal recognition particle (SRP) autoantibodies that successfully responded to subcutaneous methotrexate (MTX) after 17 years of poor responses to immunomodulation.

Anti-SRP myopathy is an immune-mediated necrotizing myopathy characterized by rapidly progressive weakness that can result in severe disability. In those with younger onset, the response to immunomodulatory therapy is often incomplete or poor.

Case report

An 18 years old woman presented with 1-year of progressive proximal limb weakness. Serum creatine kinase (CK) was 4,384 units/l and electromyography suggested an irritable myopathy.  Muscle biopsy revealed a severe, chronic, active, necrotizing myopathy.  Myositis-specific autoantibodies were initially negative, however an immune-mediated necrotizing myopathy was suspected and variously treated with corticosteroids, IVIG, subcutaneous MTX that was briefly used and stopped due to severe diarrhea, azathioprine, mycophenolate, plasma exchange, and rituximab. She had only transient and suboptimal responses to immunomodulatory therapy over 17 years, with progression of weakness and disability requiring a wheelchair, and multiple hospitalizations for exacerbations including respiratory impairment.   Subsequent repeat laboratory testing confirmed the presence of anti-SRP autoantibodies.  A thigh MRI showed fatty replacement and muscle atrophy without muscle edema and CK levels were normal, suggesting that a response to further immunosuppression was unlikely. Nonetheless, subcutaneous MTX was added to the prednisone and azathioprine she was already on.  Surprisingly, motor function had significantly improved 1 year after initiation of MTX, with recovery of independent ambulation. The patient has been functionally stable off all immunosuppressive therapy for the past 18 months.

This report exemplifies that in anti-SRP myopathy, chronic damage on muscle MRI and normal serum CK levels do not preclude the possibility of a response to immunomodulatory therapy.