Polymyositis

'An inflammatory myopathy of unknown cause' remains the least. objectionable definition of polymyositis, emphasizing our continuing ignorance of the aetiology and pathogenesis of this condition and the diagnostic need to exclude accepted causes of muscle inflammation, particUlarly those which are viral (Schwartz et al. 1978) or drug-induced (Lane & Mastaglia 1978). Usage ~f the alternative term 'dermatomyositis', though tIme-hallowed and apt when the characteristic rash is present, carries the misleading implication that skin involvement defines a separate entity (Bohan et al. 1977).