What are USH1C-related Disorders?
USH1C-related disorders are inherited conditions caused by harmful genetic changes (mutations) in the USH1C gene. USH1C-related disorders represent a group of disorders associated with hearing loss with or without vision loss. This group of disorders does not affect intelligence or cause any other primary health problems.
Usher Syndrome Type IC
There are three types of Usher syndrome, identified as type I, type II, and type III. The different types of Usher syndrome are grouped by the severity of the disease and the age when symptoms appear. Usher syndrome type I has several associated genes, and the type associated with the USH1C gene is referred to as Usher syndrome type IC or USH1C.
USH1C is an inherited disease that causes hearing loss, balance problems, and progressive vision loss. Infants with USH1C are profoundly deaf in both ears at birth. They have severe balance problems caused by abnormalities of the inner ear (vestibular system) that can lead to delayed development. Children with USH1C sit and walk at later ages and have difficulties sensing changes in speed or direction. In childhood or by early adolescence, individuals with USH1C develop retinitis pigmentosa, an eye disease that causes night blindness and a gradual loss of peripheral or side vision. Eventually only the central vision remains, creating "tunnel vision." This central vision can also be impaired and can lead to total blindness in a small number of individuals with the disease. In some cases, individuals with Usher syndrome type 1 develop clouding of the lens of the eye (cataracts), which can further impair vision.
Some mutations in USH1C have been reported in recessive non-syndromic hearing loss and deafness (hearing loss but no vision loss), referred to as DFNB18A. Individuals with DFNB18A typically have severe to profound hearing loss at birth. Unlike other forms of hearing loss, DFNB18A does not affect movement or balance.
How common are USH1C-related Disorders?
The global incidence is unknown for both Usher syndrome type 1 and DFNB18A. In most countries, the frequency ranges from approximately 1 in 45,000 to 1 in 65,000, except in Germany, where the frequency is 1 in 90,000. Approximately 1-15% of people with Usher syndrome type I have a harmful change in USH1C. Incidence for USH1C-related disorders is higher in the French Canadian population.
DFNB18A is extremely rare, having been reported in at least two families of Indian and Chinese ethnicities.
How are USH1C-related Disorders treated?
There is no cure for USH1C-related disorders, but early treatment is important to give an affected child the best opportunity to develop communication skills. While a child is young, his or her brain is most receptive to learning language, either spoken or signed. It is also important to take advantage of the time when the child's vision is normal. Individuals with Usher syndrome type 1C generally do not respond to hearing aids, but cochlear implants may help regain some form of hearing. Sign language is a good option for communication. Specialists can introduce other tools and methods of instruction available to people with hearing loss. It is often helpful if the whole family undergoes such instruction and, as a family unit, helps the child adapt.
For those individuals that develop vision loss, visual aids and specialized instruction (for example in tactile signing) help children adapt to their limited vision. Individuals can be prone to accidental injury due to their vision loss and balance problems. Well-supervised participation in sports may help an individual with Usher syndrome type 1 compensate for balance issues, but swimming may be particularly difficult, and strategies to ensure safety are needed. Use of UV-A and UV-B blocking sunglasses is recommended, and other optical aids may increase eye comfort. Therapy with vitamin A palmitate may slow retinal degeneration for some.
What is the prognosis for an individual with an USH1C-related Disorder?
Usher syndrome type IC results in severe hearing and vision impairment, and DFNB18A results in hearing impairment only. However, neither condition affects one's lifespan or intelligence.