Anti-signal Recognition Particle-positive Juvenile Polymyositis Successfully Treated with Rituximab

Abstract

To the Editor: Juvenile polymyositis (PM) is a rare form of idiopathic inflammatory myositis (IIM) in children, accounting for 2%–8% of cases1,2. In contrast to juvenile dermatomyositis (DM), the characteristic rashes are absent. Myositis-specific antibodies (MSA) can be used to further classify IIM. Anti-signal recognition particle (SRP) antibody is present almost exclusively in PM, and is usually associated with a necrotizing myopathy with a severe or fulminant course and a poor response to therapy3. A previously healthy 12-year-old girl presented with a 1-month history of increasing proximal muscle weakness, alopecia, dysphagia, and Raynaud’s phenomenon. There was no history of rash or skin changes. On examination, she had visible wasting of her shoulder muscles, manual muscle testing (MMT) of 3/5 in a proximal distribution, and a Childhood Myositis Assessment Scale (CMAS)4 score of 15/52. There were no skin findings associated with juvenile DM. Serum inflammatory markers were normal. Muscle enzymes were elevated (creatine phosphokinase 8826 U/l, lactate dehydrogenase 4305 U/l, aspartate aminotransferase 368 U/l), and electromyography demonstrated an active myopathic process. Magnetic resonance imaging (MRI) of the shoulder and hip girdles revealed symmetrically increased T2 signal in hip adductors and shoulder muscles. Pulmonary function tests demonstrated a mild chest wall restrictive pattern with normal carbon monoxide diffusion capacity. Electrocardiogram … Address correspondence to Dr. B.M. Feldman, Division of Rheumatology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada. E-mail: brian.feldman{at}sickkids.ca