Inflammatory myopathy (polymyositis / dermatomyositis)

What it is

Polymyositis and dermatomyositis are autoimmune disorders causing muscle inflammation, that is myositis. Dermatomyositis is clinically similar to polymyositis but also carries characteristic skin lesions; polymyositis is the variant of the disease in which skin manifestations are not prominent. The muscle findings differ: polymyositis shows endomysial inflammation with CD8+ T cells and most often involves the shoulders, while dermatomyositis shows perimysial inflammation and atrophy with CD4+ T cells.

Causes and risk

The disorder may be idiopathic, or associated with connective tissue disorders or an underlying malignancy, typically lung cancer, found in 20-25% and more so in the older patient. Because an underlying cancer can cause either disease, both are called paraneoplastic syndromes. A viral infection may be the trigger, for example Coxsackie virus or HIV, and certain HLA genes are risk factors. The juvenile form affects girls more than boys and is triggered post infection; unlike the adult form it does not carry an increased risk of malignancy.

How it presents

The typical presenting symptom is gradual-onset, symmetrical, proximal muscle weakness, causing difficulty standing from a chair, climbing stairs or lifting overhead. Muscle pain, that is myalgia, may or may not be present, and is mild or absent. Polymyositis occurs without any skin features. The skin changes of dermatomyositis are Gottron papules, roughened red papules or scaly erythematous patches over the extensor surfaces of the fingers, the knuckles, the elbows and the knees; a heliotrope rash, violaceous and periorbital, on the face and eyelids; periorbital oedema; a photosensitive macular rash over the back, the shoulders and the neck; nail fold capillary dilatation; and darkening and thickening of the fingertips. Outside the muscle there may be fever, arthralgia, polyarthritis, Raynaud phenomenon, interstitial lung disease such as fibrosing alveolitis or organising pneumonia, myocardial involvement such as myocarditis, esophageal dysmotility, dysphagia and dysphonia, and respiratory muscle weakness. The juvenile form adds calcinosis, a positive Gower and a multisystem vasculitis. Two further skin signs belong to dermatomyositis, the shawl sign and the V-sign.

The exam clue is the pair itself: proximal muscle weakness with a rash means dermatomyositis, and the Gottron papules are pathognomonic. Asked to name the lesions in the picture, the three to name are the Gottron papules over the knuckles, the heliotrope rash - the purple rash on the eyelids - and the shawl sign on the back and shoulders.