The prevalence of Usher syndrome and other retinal dystrophy-hearing impairment associations

The study was undertaken to procure population-based prevalence data on the various types of Usher syndrome and other retinal dystrophy-hearing impairment associations. The medical files on 646 patients with a panretinal pigmentary dystrophy aged 20-49 years derived from the Danish Retinitis Pigmentosa (RP) register were scrutinised. The data were supplemented by a prior investigation on hearing ability in a part of the study population. After exclusion of patients with possibly extrinsic causes of hearing impairments, 118 patients, including 89 cases of Usher syndrome were allocated to one of five clinically defined groups. We calculated the following prevalence rates: Usher syndrome type I: 1.5/100 000, Usher syndrome type II: 2.2/100 000, and Usher syndrome type III: 0.1/100 000 corresponding to a 2:3 ratio between Usher syndrome type I and II. The overall prevalence rate of Usher syndrome was estimated to 5/100 000 in the Danish population, devoid of genetic isolates. The material comprised 11 cases with retinal dystrophy, hearing impairment, and additional syndromic features. Finally, 18 subjects with various retinal dystrophy-hearing impairment associations without syndromic features were identified, corresponding to a prevalence rate of 0.8/100 000. This group had a significant overrepresentation of X-linked RP, including two persons harboring a mutation in the retinitis pigmentosa GTP-ase regulator (RPGR) gene.