How Often are Pediatric Patients with Clinically Amyopathic Dermatomyositis Truly Amyopathic

Background Pediatric patients can present with skin manifestations of dermatomyositiswithout overt weakness (clinically amyopathic juvenile dermatomyositis[JDM]), but it is unclear how often this happens and how often they have subclinical muscle inflammation. Objective Our goal was to determine the frequency of clinically amyopathic JDM and the frequency with which a thorough evaluation uncovers subclinical myositisat a single institution. Methods A retrospective review was performed of 46 patients diagnosed with JDM at Children's Hospital of Wisconsin. Results Of 46 patients presenting with skin findings consistent with dermatomyositis, 10 patients (21.7%) did not have evidence of muscle involvement on history or exam, and these tended to be the younger patients. Of these 10, only 2 (4% of all the JDM patients) were truly amyopathic upon further evaluation (all five muscle enzymes [aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, creatine kinase and aldolase], magnetic resonance imaging [MRI], muscle biopsy). In our series, muscle biopsywas not helpful in identifying subclinical myositis. In contrast, MRI did uncover subclinical muscle disease. Conclusion These data suggest that truly amyopathic JDM is rare and that a thorough workup that includes all five muscle enzymes and MRI may uncover occult myositis.