Characteristics
This syndrome is characterized by hearing loss and a gradual visual impairment. The hearing loss is associated with a defective inner ear, whereas the vision loss is associated with retinitis pigmentosa (RP), a degeneration of the retinal cells. Usually, the rod cells of the retina are affected first, leading to early night blindness and the gradual loss of peripheral vision. In other cases, there is early degeneration of the cone cells in the macula, leading to a loss of central acuity. In some cases, the foveal vision is spared, leading to "doughnut vision"; central and peripheral vision are intact, but there is an annulus around the central region in which vision is impaired.
Usher syndrome has three clinical subtypes, denoted as I, II and III. People with Usher I are born profoundly deaf, and begin to lose their vision in the first decade of life. They also exhibit balance difficulties and learn to walk slowly as children, due to problems in their vestibular system. People with Usher II are not born deaf, but do have hearing loss. They do not seem to have noticeable problems with balance; they also begin to lose their vision later (in the second decade of life) and may preserve some vision even into middle age. People with Usher syndrome III are not born deaf, but experience a gradual loss of their hearing and vision; they may or may not have balance difficulties.
Usher syndrome is a variable condition; the degree of severity is not tightly linked to whether it is Usher 1, 2 or 3. For example, someone with Type 3 may be unaffected in childhood but go on to develop a profound hearing loss and a very significant loss of sight by early to mid-adulthood. Similarly, someone with Type 1, who is therefore profoundly deaf from birth, may keep good central vision until the sixth decade of life, or even beyond. People with Type 3, who have useful hearing with a hearing aid, can experience a wide range of severity of the RP. Some may maintain good reading vision into their sixties, while others cannot see to read while still in their forties.
Usher syndrome I and II are associated with a mutation in any one of six or three different genes, respectively, whereas only one mutation has been linked with Usher III. Since Usher syndrome is inherited in an autosomal recessive pattern, both males and females are equally likely to inherit Usher syndrome. Consanguinity of the parents is a risk factor. Children of parents who both are carriers of the same mutation have a one fourth chance of inheriting the condition and children of such parents who are unaffected have a two thirds chance of being carriers. Children of parents where only one parent is a carrier have a no chance of having the disease but have a one half chance of being a carrier. First recognized in the 19th century, Usher syndrome was the first condition to demonstrate that phenotypes could be inherited in tandem; deafness and blindness are inherited together, but not separately. Animal models of this human disease (such as knockout mice and zebrafish) have been developed recently to study the effects of these gene mutations and to test potential cures for Usher syndrome.
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