Usher syndrome is a genetic disorder involving the loss of both sight and hearing. The hearing loss generally occurs at birth or shortly thereafter; while a progressive loss of vision due to retinitis pigmentosa (a degeneration of the retina of the eyes) begins later in life, usually before adolescence. AT first, the vision loss may be gradual and barely noticeable, but in almost all cases the result is legal blindness.
There are currently three identifiable types of Usher syndrome: Usher syndorme Type 1, Usher syndrome Type II and Usher syndrome Type III. An individual with Usher Type I has a profound hearing loss at birth, experiences balance problems due to vestibular loss, and experiences vision loss usually in early childhood. In Usher Type II, an individual has a sloping audiogram going from a moderate loss in the low frequencies to profound impairments in the higher frequecies, has no balance problems, and usually begins to lose his/her vision in the midteens. A person with Usher syndrome Type III starts with normal or near-normal hearing that progressively deteriorates, might have some balance distrubance, and experiences vision loss by midteens.
Usher syndrome occurs in approximately 1 in 25,000 and the incidence is estimated to posibly be as high as 1 in 10,000 individuals. This range is due to the difficulty in identifying persons with Usher syndrome, particulary persons with Type II Usher. Of persons iwth Usher syndrome, approximately 90% will have Type I Usher, with Type II accounting for almost 10%. At this time, Usher syndrome Type III appears to occur primarily in Finland.
While only approximately 4 in 100,000 people are diagnosed with Usher syndrome, it is estimated that 3 to 6 percent of people who have a hereditary hearing loss have the syndrome. An equal percentage of children who are hard of hearing my also have Usher syndrome.