Supplementary Material for: A Nationwide, Population-Based Prevalence Study of Genetic Muscle Disorders

Background: Previous epidemiological studies of genetic muscle disorders have relied on medical records to identify cases and may be at risk of selection biases or have focused on selective population groups. Objectives: This study aimed to determine age-standardised prevalence of genetic muscle disorders through a nationwide, epidemiological study across the lifespan using the capture-recapture method. Methods: Adults and children with a confirmed clinical or molecular diagnosis of a genetic muscle disorder, resident in New Zealand on April 1, 2015 were identified using multiple overlapping sources. Genetic muscle disorders included the muscular dystrophies, congenital myopathies, ion channel myopathies, GNE myopathy, and Pompe disease. Prevalence per 100,000 persons by age, sex, disorder, ethnicity and geographical region with 95% CIs was calculated using Poisson distribution. Direct standardisation was applied to age-standardise prevalence to the world population. Completeness of case ascertainment was determined using capture-recapture modelling. Results: Age standardised minimal point prevalence of all genetic muscle disorders was 22.3 per 100,000 (95% CI 19.5–25.6). Prevalence in Europeans of 24.4 per 100,000, (95% CI 21.1–28.3) was twice that observed in NZ’s other 3 main ethnic groups; Māori (12.6 per 100,000, 95% CI 7.8–20.5), Pasifika (11.0 per 100,000, 95% CI 5.4–23.3), and Asian (9.13 per 100,000, 95% CI 5.0–17.8). Crude prevalence of myotonic dystrophy was 3 times higher in Europeans (10.5 per 100,000, 9.4–11.8) than Māori and Pasifika (2.5 per 100,000, 95% CI 1.5–4.2 and 0.7 per 100,000, 95% CI 0.1–2.7 respectively). There were considerable regional variations in prevalence, although there was no significant association with social deprivation. The final capture-recapture model, with the least deviance, estimated the study ascertained 99.2% of diagnosed cases. Conclusions: Ethnic and regional differences in the prevalence of genetic muscle disorders need to be considered in service delivery planning, evaluation, and decision making.