Polymyositis and dermatomyositis are systemic connective tissue diseases which are characterised by acute and chronic inflammation of striated muscle. In dermatomyositis there is an accompanying dermatitis. The limb girdle or proximal muscles are most severely affected but their bulk is preserved beyond that expected from their weakness: this is an important sign distinguishing this condition from a limb girdle dystrophy.
Myositis is usually the initial presentation. Some patients present with dermatitis and later develop muscle weakness; rarely, only dermatitis occurs.
The aetiology is unknown but there is an association with HLA-B8 and HLA-DR3.
Dermatomyositis in males over the age of 60 years may be suggestive of an underlying systemic malignancy.