Idiopathic inflammatory myopathy: clinicopathological observations in the Indian population.

The present study attempts to investigate the pathological basis of the two clinically different forms of idiopathic inflammatory myopathy (IIM) namely, polymyositis (PM) and dermatomyositis (DM). Clinicopathological analysis of 73 cases showed that muscle fibre necrosis and regeneration were more frequent in PM than in DM, the latter being significantly so (P < 0.05). On the other hand, vasculitis was more associated with DM while perifascicular atrophy of the muscle fibres was confined to it. Vasculitis was present in eight cases. Its incidence in patients with myositis with systemic connective tissue disease (4/9) was significantly more than in other PM and DM patients (P < 0.01). An equally significant higher frequency of perimysial inflammatory infiltrate was also seen in the former as compared to the latter. Interestingly, idiopathic DM affected men as often as women and juvenile IIM affected boys more frequently than girls. A female predilection was noted in the remaining groups of IIM. These observations indicate that there may be some basic immunopathogenetic differences between polymyositis and dermatomyositis as well as between idiopathic PM/DM and that associated with systemic connective tissue diseases. Though the number of patients studied is small, the absence of female predilection in idiopathic DM and juvenile IIM may be peculiar to the IIM in the tropics.