F008 Prevalence of Juvenile-onset and pediatric huntington disease and their availability and ability to participate in trials: a dutch population and enroll-HD observational study

<h3>Background</h3> Juvenile-onset Huntington Disease (JHD) represents 1–5% of HD patients, with onset before the age of 21. Pediatric HD (PHD) relates to a proportion of JHD patients that is still under the age of 18 years. Up to now, the JHD and PHD populations have been excluded from interventional trials. <h3>Objective</h3> Describe the prevalence and incidence of JHD and PHD in the Netherlands and to explore their availability and ability to participate in interventional trials. <h3>Methods</h3> The point prevalence of PHD and JHD patients in the Netherlands and 5-year incidences over the past 20 years were analyzed. In addition, by combining data from our cohort with ENROLL-HD, we explored (1) proportions of JHD patients being diagnosed at pediatric versus adult age and their diagnostic delay and (2) disease stage in JHD and adult-onset HD patients at the time of diagnosis. <h3>Results</h3> The proportional prevalence of JHD and PHD relative to the total manifest HD population in January 2024 was between 0.84 and 1.25% and 0.09 and 0.14% respectively. The mean incidence of JHD patients being diagnosed was between 0.85 and 1.28 per 1000 HD patient years and of PHD patients being diagnosed was 0.14 per 1.000.000 under-aged person years. JHD patients were in 55% of cases clinically diagnosed on adult age, with a median diagnostic delay of 4 years. The vast majority of JHD patients was functionally compromised at clinical diagnosis and adolescent-onset JHD patients were significantly less independent at diagnosis when compared to the adult-onset group. <h3>Conclusions</h3> The estimated prevalence of JHD and PHD in the Netherlands is lower than previously suggested. Due to the diagnostic delay more than half of JHD cases are not eligible for clinical trials in the PHD population. Furthermore, functional disease stage markers show that the ability of JHD patients for participation in therapeutical trials is diminished, while the CAP100 score does not seem appropriate in this particular group.