Usher syndrome, type iiia (USH3)
- Clarin 1 (CLRN1)
Out of the three types of Usher syndrome, characterized by combined hereditary hearing and visual loss, type 3 is enriched in the Finnish population where it accounts for 40 percent of all Usher cases. Basic mechanisms causing the defects of sensory cells in the inner ear and retina wait for functional studies made possible by recent discovery of the USH3 gene. The clinical features of USH3 include postlinqual hearing impairment which usually develops by the age of eight years. The hearing loss is progressive, sensorineural and cochleal resulting in moderate to severe deafness within a few years or decades. Patients have either normal or decreased vestibular function. Retinal manifestation, retinitis pigmentosa, is diagnosed at the mean age of 17 years. The symptoms of night blindness, followed by mid peripheral visual field defects, lead to narrowed visual fields and slowly progressive tunnel vision at a mean age of 30 years. The progression of RP results in a severe visual handicap. Diagnosis of USH3 on clinical findings can be confirmed by gene tests, which also allow carrier identification.