Dermatomyositis & Polymyositis (Idiopathic Inflammatory Myopathies)

Symmetric proximal muscle weakness with a violaceous periorbital rash - the heliotrope rash - and scaly papules over the knuckles - Gottron's papules - is dermatomyositis, one of the idiopathic inflammatory myopathies. These are autoimmune muscle diseases with symmetric proximal weakness and a raised creatine kinase; dermatomyositis is distinguished by its skin signs and is a complement-mediated, interferon-driven microangiopathy, whereas polymyositis has the myopathy without the skin signs and is a diagnosis of exclusion. I would confirm the diagnosis with myositis-specific antibodies, which also define subtypes and predict complications, together with EMG, MRI showing muscle oedema, and muscle biopsy. The two things I must not miss are the systemic associations. First, adult dermatomyositis is strongly paraneoplastic, so I must perform an age- and risk-appropriate search for an occult malignancy, because the myositis can be the presenting sign of a cancer. Second, interstitial lung disease is a major and sometimes rapidly progressive complication, particularly with the anti-synthetase syndrome and the anti-MDA5 antibody, so I must assess the lungs. Management is medical and multidisciplinary - corticosteroids with steroid-sparing immunosuppression such as methotrexate or azathioprine, and IVIG, which is efficacious in refractory dermatomyositis. From an orthopaedic standpoint I would address weakness-related disability, joint contractures, and the calcinosis seen especially in juvenile dermatomyositis, excising deposits that ulcerate, become infected or restrict joints.