Summary

Idiopathic inflammatory myopathies are a heterogeneous group of systemic diseases characterised by variable phenotypes of chronic progressive muscle weakness. Myositis-specific antibodies (MSAs) include antibodies to cytoplasmic signal recognition particle (SRP) and various tRNA synthetases as well as the nuclear helicase protein Mi-2. These antibodies are typically found only in a fraction of true myositis cases and they tend to be mutually exclusive. Few cases of coexistence of two MSAs in the same patient have been reported and these cases all involve an antisynthetase antibody coexisting with either anti-SRP or anti-Mi-2 antibody. Peculiar clinical manifestations may be associated with different combinations of MSAs but the rarity of these cases makes their characterisation difficult. We report the first ever case of anti-SRP and anti-Mi-2 copositive polymyositis in a 19-year-old boy who presented with a week history of profound muscle weakness that attained its peak within 24 hours of onset.