Paediatric polymyositis and dermatomyositis

Juvenile polymyositis and dermatomyositis are inflammatory myopathies that affect muscle. Dermatomyositis also affects skin, and can have many extramuscular manifestations. Inflammatory myopathies are uncommon in childhood, with dermatomyositis occurring more than polymyositis. For this reason, published research has concentrated on juvenile dermatomyositis. The spectrum of disease severity ranges from mild cases that can recover completely without treatment, to multisystem inflammation that can be fatal. Treatments have improved over the decades, reducing mortality from 30% before the era of steroids, to less than 1% in the present day. Juvenile cases of dermatomyositis differ from those seen in adulthood, without tendency for associated malignancy, and a far greater incidence of calcinosis. Calcinosis can be deposited as small calcinotic lumps or as sheets of calcinosis. It is very difficult to treat and causes extensive morbidity, and depending on where the calcinosis is deposited, it can cause severe disability or even death. Over the last decade, international collaborative work has concentrated on developing disease activity and assessment tools for both adult and juvenile forms of myositis. This will enable more subjective study of these rare diseases in multinational cohort studies, and enable clinical trials to investigate drug treatments. This work led to the first international double-blind placebo controlled trial of treatment in both adults and children with dermatomyositis (using rituximab as the drug). Further international collaboration has led to the development of core outcome variables, a definition of disease flare, and classification criteria.