Juvenile Dermatomyositis and the Inflammatory Myopathies.

The inflammatory myopathies comprise disorders of immune-mediated muscle injury. The histopathology and clinical features help distinguish them. Juvenile dermatomyositis (JDM) is the most common form of myositis in children and adolescents. Children with JDM present with proximal muscle weakness and characteristic rashes. The presentation is similar in children and adults, but JDM is a primary disorder and the adult form often is concerning for a paraneoplastic syndrome. Proximal muscle weakness occurs with dermatomyositis, polymyositis, and immune-mediated necrotizing myopathy, but the latter two conditions have no dermatologic findings or distinct tissue changes which set them apart from dermatomyositis. Inclusion body myositis, also included in the inflammatory myopathies, presents with more distal involvement, and microscopically exhibits identifiable rimmed vacuoles. We review key features of these disorders, focusing in more detail on JDM because it is more often encountered by the child neurologist.