The inflammatory muscle diseases polymyositis and dermatomyositis are characterised by an inflammation of striated muscle. As with other conditions in the spectrum of connective tissue disease, they can occur on their own or in association with, for example, SLE. They are rare, with only 5–7 new cases per million per year and affect women more frequently than men. In general they can affect any age, although dermatomyositis has a peak of incidence in childhood and in the elderly; in the latter it may be related to underlying malignancy. Inclusion body myositis has a different clinical, histopathological and prognostic profile (see below).

Polymyositis is characterised by insidious muscle weakness and pain that develops over 3–6 months, but acute catastrophic presentations can also occur. Large proximal muscle groups are involved preferentially and patients have difficulty climbing stairs, rising from sitting or raising their arms above their heads. Shortness of breath may also occur as a result of diaphragmatic involvement, aspiration pneumonia (dysphagia caused by upper oesophagus involvement) or interstitial fibrosis.

Dermatomyositis refers to polymyositis in the presence of a characteristic rash that frequently precedes the myopathy. Papules develop over the metacarpophalangeal and proximal interphalangeal joints (Gottron’s papules) with areas of linear erythema running the length of the fingers’ dorsal surface; the cuticles become ragged and dilated nail fold capillary loops become visible. Further cutaneous stigmata include a classically heliotrope rash across the eyelids and a macular eruption in a ‘V’ formation at the base of the neck and over the shoulders and back (the ‘shawl’ sign).

More unusual manifestations of disease include myocardial involvement, pericardial effusions and arthropathy.

Secondary post-inflammatory fibrosis and scarring lead to longterm weakness and an inability to reach pre-morbid levels of activity. This is particularly problematic in children in whom development of calcinosis also contributes to the formation of flexion contractures. Scarring following inflammatory lung disease leads to pulmonary fibrosis and an irreversible reduction in exercise tolerance.