The incidence of juvenile dermatomyositis: results from a nation-wide study.

There have been no previous attempts to estimate the incidence of juvenile dermatomyositis (JDM) in the UK and Ireland. Consultant paediatricians from these countries were asked to report all cases of JDM which were diagnosed in 1992 and 1993 to the British Paediatric Surveillance Unit. One-hundred-and-twenty-one reports were received, of which 48 were confirmed cases of JDM and three were confirmed cases of polymyositis. The estimated incidence of JDM was 1.9 per million children aged under 16 yr (95% confidence interval 1.4-2.6). The median age at onset was 6.8 yr and the median delay in diagnosis was 4 months. Girls were affected much more often than boys (ratio 5.0:1). In the cases with an onset in 1992 there appeared to be a cluster of cases with an onset in April and May.