The prevalence of Usher syndrome in Sweden: A nationwide epidemiological and clinical survey

The purpose of this study was to estimate the prevalence of Usher syndrome types I, II and III in Sweden and to estimate possible regional differences in the prevalence of different Usher syndrome subtypes within the country. Probands were ascertained through multiple sources and almost complete ascertainment was achieved. A total of 370 subjects with a referring diagnosis of Usher was ascertained. Of those, 77 subjects proved to have an incorrect preliminary diagnosis. Of the remaining 293 individuals, 140 were found to have Usher syndrome type I, 122 subjects were diagnosed as type II, and 27 subjects had Usher syndrome type III. The corresponding prevalence in Sweden was estimated to be 1.6/100,000 for type I, 1.4/100.000 for type II and 0.3/100.000 for type III. The prevalence of Usher I was found to be significantly higher (8.6/100.000) in the two northernmost counties. This is consistent with an earlier report made in the 1950s. Similar geographic concentrations of Usher II and III were not observed. Usher syndrome types IB, ID and IIA were found to be the most common subtypes in Sweden. The age of diagnosis in all 3 types was late and the prevalence was higher at older ages. To our knowledge this is the largest genetic/epidemiological study on patients with Usher syndrome carried out to date. It provides critical information for genetic counselling and public health planning of this important disorder.