Usher Syndrome
What Is Usher Syndrome?
Usher syndrome is an inherited condition that causes 1) a serious hearing loss that is usually present at birth or shortly thereafter and 2) progressive vision loss caused by retinitis pigmentosa (RP). RP is a group of inherited diseases that cause night-blindness and peripheral (side) vision loss through the progressive degeneration of the retina, the light-sensitive tissue at the back of the eye that is crucial for vision.
Researchers have described three types of Usher syndrome-type I, type II and type III.
- Individuals with Usher syndrome type I are nearly or completely deaf and experience problems with balance from a young age. They usually begin to exhibit signs of RP in early adolescence.
- Individuals with Usher syndrome type II experience moderate to severe hearing impairment, have normal balance, and experience symptoms of RP later in adolescence.
- Individuals with Usher syndrome type III are born with normal hearing but develop RP and then progressive hearing loss.
How is Usher syndrome inherited?
The Usher syndrome types are inherited as an autosomal recessive trait. This means that an affected person receives one abnormal gene from each of his or her parents. A person who inherits a gene from only one parent will be a carrier, but will not develop the disease.
A person with Usher syndrome must pass on one disease gene to each of his or her children. However, unless the person has children with another carrier of Usher genes, the individual's children are not at risk for developing the disease. Currently we cannot reasonably test everyone for carrier status, but this may change in the years ahead.
How is Usher syndrome diagnosed?
Since individuals with Usher syndrome have both hearing and visual symptoms, we perform testing of both systems. This testing includes:
- visual function tests: visual fields and electroretinogram (ERG)
- a retinal examination
- hearing tests
- balance tests for all patients age ten years and older
Although some of the genes that cause Usher syndrome have been identified, the diagnosis is still based on ocular and clinical testing.
Is genetic testing for Usher syndrome available?
At this time, genetic testing for Usher syndrome is done only as part of research projects. This is due to many factors. Usher syndrome is not caused by only one gene. So far, 10 Usher genes have been mapped: 7 for type I, 3 for type II, and 1 for type 3. There are still more genes to find. A few of these genes have been sequenced and described. These are MYO7A, harmonin, CDH23, PCDH15, all causing type I. The usherin gene causes type II disease.
Finding the genes is a very important advance in the fight against Usher syndrome. Further study is required to characterise these genes, and determine how the mutated genes cause Usher syndrome. Additional genes that cause Usher syndrome also need to be identified. Several researchers throughout the world are working on Usher syndrome. Findings from this research may one day allow treatments for Usher syndrome to be developed.
