Usher syndrome is the most common condition that involves both hearing and vision problems. A syndrome is a disease or disorder that has more than one feature or symptom. The major symptoms of Usher syndrome are hearing impairment and retinitis pigmentosa, an eye disorder that causes a person's vision to worsen over time. Some people with Usher syndrome also have balance problems. There are three general types of Usher syndrome. Although the syndrome was first described by Albrecht Von Graefe in 1858, it was named for Charles Usher, a British eye doctor, who believed that the condition was inherited or passed from parents to their children.
Who is affected?
Approximately 3–6% of all deaf children and perhaps another 3–6% of hard-of-hearing children have Usher syndrome. In developed countries such as the United States, about 4 babies in every 100,000 births have Usher syndrome.
Usher syndrome is inherited or passed from parents to their children through genes. Genes are located in every cell of the body, except for red blood cells, which don't have a nucleus. Genes contain instructions that tell cells what to do. Some genes specify traits such as hair color. Other genes are involved in the development of body parts, such as the ear. Still others determine how parts of the body work. Each person inherits two copies of each gene, one from each parent.
Sometimes genes are altered or mutated. Mutated genes may cause cells to act differently than expected. Genes for Usher syndrome are autosomal recessive, a term meaning that 1) Usher genes are located on chromosomes other than the sex chromosomes, and 2) both parents must contribute the mutated gene to the child before the disorder is seen. Usually, parents are unaware that they have an Usher gene because they would need two of the mutated genes in order to show signs of Usher syndrome. A number of different genes have been found to cause the various types of Usher syndrome.
Types of Usher syndrome
The three types of Usher syndrome are Usher syndrome type 1 (USH1), Usher syndrome type 2 (USH2), and Usher syndrome type 3 (USH3). USH1 and USH2 are the most common types. Together, they account for approximately 90–95% of all cases of children who have Usher syndrome.
People with USH1 are profoundly deaf from birth and have severe balance problems. Many of these individuals obtain little or no benefit from hearing aids. Most use sign language as their primary means of communication. Because of the balance problems, children with USH1 are slow to sit without support and rarely learn to walk before they are 18 mo old. These children usually begin to develop vision problems by the time they are ten. Visual problems most often begin with difficulty seeing at night, but tend to progress rapidly until the individual is completely blind.
Individuals with USH2 are born with moderate to severe hearing impairment and normal balance. Although the severity of hearing impairment varies, most of these children perform well in regular classrooms and can benefit from hearing aids. These children most commonly use speech to communicate. The visual problems in USH2 tend to progress more slowly than the visual problems in USH1. USH2 is characterized by blind spots that begin to appear shortly after the teenage years. When an individual's vision deteriorates to blindness, his or her ability to speechread is lost.
Children born with USH3 have normal hearing and normal to near-normal balance. Hearing worsens over time. However, the rate at which hearing and sight are lost can vary between affected individuals, even within the same family. Children develop noticeable hearing problems by their teenage years and usually become deaf by mid- to late adulthood. Night blindness usually begins sometime during puberty. Blind spots appear by the late teenage years to early adulthood. By mid-adulthood, the individual is usually blind.
Hearing loss and retinitis pigmentosa are rarely found in combination. Therefore, most people who have retinitis pigmentosa and hearing loss probably have Usher syndrome. Special tests such as electronystagmography (ENG) to detect balance problems and electroretinography (ERG) to detect retinitis pigmentosa help doctors to detect Usher syndrome early. Early diagnosis is important in order to begin special educational training programs to help the individual manage the combined hearing and vision difficulties.
Presently, there is no cure for Usher syndrome. The best treatment involves early identification in order to begin educational programs. The exact nature of these educational programs will depend on the severity of the hearing and vision impairments as well as the age and abilities of the individual. Typically, individuals will benefit from adjustment and career counseling; access to technology such as hearing aids, assistive listening devices, or cochlear implants; orientation and mobility training; and communication services and independent-living training that may include braille instruction, low-vision services, or auditory training.
Researchers are currently trying to locate the genes that cause the syndrome and identify the function of those genes. This research will lead to improved genetic counseling and early diagnosis, and may eventually expand treatment options.
Scientists are also developing mice that have the same characteristics as humans who have the various types of Usher syndrome. Mouse models will make it easier to determine the function of the various genes involved in Usher syndrome. Research is also being conducted to improve the early identification of children with the syndrome. Treatment strategies such as the use of cochlear implants for hearing impairment and intervention strategies to alleviate retinitis pigmentosa are also being examined.
Currently, 12 loci have been found to cause Usher syndrome. A locus is a small segment of chromosome on which one or more genes are housed. For seven of the 12 loci, genes and the proteins that they encode have been identified. The genes that cause Usher syndrome are MY07A, USH1C, CDH23, PCDH15, and SANS, which cause USH1; USH2A, which causes USH2; and USH3A, which causes USH3. The resulting proteins that the genes encode help cells in the retina, the part of the eye that receives images of objects, and the cochlea to function.
In Apr 2003, NIDCD researchers, along with their research collaborators from universities in New York, NY, and Tel Aviv, Israel, pinpointed a mutation, named R245X, of the PCDH15 gene that accounts for a large percentage of USH1 cases in today's Ashkenazi Jewish population. (The term Ashkenazi describes Jewish people who originate from eastern Europe.) Because of this finding, researchers conclude that Ashkenazi Jewish infants with bilateral, profound hearing loss who lack another known mutation that causes hearing loss should be screened for the R245X mutation. If a child's USH1 is discovered early on, before she loses the ability to see, then that child is more likely to benefit from the full spectrum of intervention strategies that are available to help her communicate and participate in life's activities.