Idiopathic inflammatory myopathy (myositis)

Introduction

Inflammatory myopathies have been recognized for many years (see Marinacci, 1965); an early review was written by Steiner (1903). The inflammatory myopathies include primary inflammatory muscle diseases and inflammatory muscle diseases in association with other autoimmune diseases (the overlap syndromes) or with malignancy. Bohan and colleagues (Bohan & Peter, 1975a,b; Bohan et al., 1977) established diagnostic criteria for myositis. They divided patients with myositis into five categories: polymyositis, dermatomyositis, polymyositis or dermatomyositis associated with malignancy, childhood polymyositis or dermatomyositis, and polymyositis or dermatomyositis associated with connective tissue disorder (overlap group). Inclusion body myositis is another inflammatory myopathy that is now regarded as a distinct entity, separate from polymyositis (Yunis & Samaha, 1971; Lotz et al., 1989). As outlined by Dalakas (1992a), the clinical and pathological features of polymyositis, dermatomyositis and inclusion body myositis remain constant whether or not these diseases are associated with malignancy or with connective tissue diseases. With the exception of inclusion body myositis, which at present is of unknown aetiology, it seems likely that these conditions have an autoimmune basis.

Clinical features

Polymyositis

Polymyositis is a disease of adults. It usually develops subacutely, presenting with proximal muscle weakness and later producing widespread weakness of the limb muscles and sometimes the bulbar muscles (Dalakas, 1992a).