Incidence of inflammatory myopathies in Victoria, Australia, and evidence of spatial clustering

Objective. To determine the incidence of idiopathic inflammatory myopathies (IIM) in Victoria, Australia, and look for evidence of space-time or spati al clustering. Methods. Cases of IIM diagnosed between 1989 and 1991 were identified by mu scle biopsy and hospital discharge diagnosis review. Diagnosis was verified by medical record review and included if Bohan and Peter criteria for defi nite or probable disease were mel. The pair-wise Euclidean distances betwee n cases' residences were computed using grid references, and temporal dista nces were calculated between biopsy dates. The Mantel test for space-time c lustering was computed. Each patient was also characterized by statistical local area (SLA) according to place of residence. For each SLA, the expecte d annual incidence of IIM was calculated, based upon its population distrib ution, and these were compared to the observed annual incidence. Confidence intervals for the true rate ratio (RR) for each SLA were calculated assumi ng a Poisson distribution, and the level of heterogeneity in the data was e xamined by calculation of a chi-squared for homogeneity. Results. Ninety-four cases met inclusion criteria for an annual incidence o f 7.4 (95% CI 6.0-9.0) per million person-years, No space-time clustering w as found (z = -0.434, p = 0.665), but there was evidence of spatial cluster ing. A total of 67 observed cases were distributed among 58 urban SLA. Four SLA had a greater than expected incidence of myositis (95% Poisson based C I excluded 1), accounting for 20 of the observed cases. Conclusion. The incidence of IIM in Australia is higher than most previous population based estimates. The finding of spatial clustering supports the hypothesis that environmental factors may be important in the pathogenesis of these diseases.