Usher syndrome is the most frequent cause of deaf-blindness in humans. It’s a genetic disease that results when there are mutations (defects) in genes that are important for the function of both photoreceptors in the retina and hair cells in the cochlea (inner ear). These mutations usually lead to either a deficiency or defective version of a protein that’s critical for the health and function of the retina and cochlea. More than 400,000 people are affected by this disorder worldwide. Currently there is no cure for Usher syndrome.
Usher syndrome impacts three major senses in the body:
Children with Usher syndrome are born with or develop hearing loss. It’s estimated that upward of 10 percent of people with congenital bilateral, sensorineural hearing loss have Usher syndrome.
Vision loss in Usher syndrome is caused by a progressive vision disorder known as retinitis pigmentosa (RP). RP causes the light-sensing cells in the retina to gradually deteriorate, initially resulting in night blindness, followed by a narrowing of the visual field, commonly known as tunnel vision.
Balance is achieved and maintained through input from your eyes, the vestibular organs in the inner ear and the sensory systems of the body, such as the skin, muscles and joints. Thus, people with Usher syndrome suffer from severe balance issues due to vestibular dysfunction.
There are three clinical types: Type 1, Type 2 and Type 3, which are distinguished by the severity and age when the signs and symptoms appear. There are at least eleven different genetic types of Usher syndrome, as determined by the genes that are involved. There are six different genes that cause Usher Type 1, three that cause Usher Type 2, and two that cause Usher Type 3. One cannot determine the genetic type by clinical testing; DNA testing is the only reliable way of determining the true genetic type.
- Type 1 is comprised of 5 different subtypes; 1B, 1C, 1D, 1F and 1G depending on the specific gene mutation.
- Children diagnosed with Usher syndrome type 1 are profoundly deaf at birth and have a dysfunctional vestibular system.
- Due to the vestibular dysfunction, a child with Usher syndrome type 1 will usually take longer to sit up and develop walking at a later stage than a typically developing child.
- Gradual vision loss in the child occurs due to Retinitis Pigmentosa (RP). The severity and onset of RP varies between individuals but commonly develops before the age of 10. Vision problems are initially characterised by night blindness or tunnel vision but this also varies between individuals.
- Usher syndrome type 2 can be grouped into 3 subtypes, 2A, 2C and 2D.
- In Usher syndrome type 2, the severity of hearing loss from birth can range from very mild to severe. Those with Usher syndrome type 2 do not have a dysfunctional vestibular system.
- Vision loss due to the RP does not usually develop until late adolescence or in some cases until the late twenties.
- Usher syndrome type 3 is very rare compared to the other Usher syndrome types and only one subtype has been discovered so far, with the majority of people with USH3 living in Finland.
- A person with Usher syndrome type 3 is born with normal hearing and close to normal balance, however hearing gradually deteriorates with age.
- A measurable hearing loss usually occurs by puberty and vision starts to deteriorate during the teenage years and may progress during life.
All forms of Usher syndrome are inherited from not one but both parents. When both parents are asymptomatic carriers of the Usher syndrome gene, they have a 25% chance with each pregnancy of producing a child with Usher syndrome.
Estimates are that 1 in 10 persons carriers some form of recessive gene for Usher syndrome, but because there are so many different genetic versions of Usher syndrome there is only a small chance of a specific carrier mating with an identical carrier.