What is Usher syndrome?

Risk Factors

The only known risk factor for Usher syndrome is having parents with the disorder and/or parents who carry the genes for the disorder. If both parents carry the abnormal gene, their child has a 25 percent chance of inheriting both of these abnormal genes and therefore developing Usher syndrome.

Etiology and Genetics

Ten genes have now been identified that are associated with Usher syndrome, and at least three additional genes are known to be involved, although they have not yet been characterized. All these genes specify proteins that function in the development or maintenance of specialized sensory cells in the inner ear and light-sensing cells in the retina (rods and cones). While the exact functions of some of these proteins in the processes of hearing and vision may be poorly understood, the effect of mutations in the associated genes is most often observed as the gradual loss of rods and cones in the retina and of hair cells in the inner ear.

Type I Usher syndrome can result from mutations in any of the following genes: MYO7A (located on chromosome 11 at position 11q13.5), USH1C (at position 11p14.3), CDH23 (at position 10q22.1), USH1H (at position 15q22-q23), USH1G (at position 17q25.1), or PCDH15 (at position 10q21.1). Mutations resulting in Usher syndrome type II occur in the following genes: USH2A (at position 1q41), WHRN (at position 9q32-q34), and GPR98 (at position 5q13). The gene known as USH3A (at position 3q21-q25) is the only one so far identified in which mutations leading to type III Usher syndrome occur.

All forms of Usher syndrome are inherited in an autosomal recessive fashion, which means that both copies of the particular gene must be deficient in order for the individual to be afflicted. Typically, an affected child is born to two unaffected parents, both of whom are carriers of the recessive mutant allele. The probable outcomes for children whose parents are both carriers are 75 percent unaffected and 25 percent affected. If one parent has Usher syndrome and the other is a carrier of the same type, there is a 50 percent probability that each child will be affected.

Symptoms

The main symptoms of Usher syndrome are hearing and vision loss. Some people also have balance problems due to inner ear problems.

RP begins in the early teenage years as night blindness and leads to blindness by mid-adulthood. The progression of RP limits a person’s ability to see in dim light or in the dark (night blindness). It also causes a person to lose peripheral (side) vision slowly over time. Eventually, any vision left is only in a small tunnel-shaped area. Almost everyone with RP becomes legally blind. There is no known way of predicting when or how quickly a person will lose vision.

Symptoms of type I Usher syndrome include being deaf at birth and receiving little or no benefit from hearing aids, having severe balance problems, being slow to sit without support, and rarely learning to walk before the age of eighteen months. RP begins by age ten with difficulty seeing at night, and it quickly progresses to blindness.

Symptoms of type II Usher syndrome include being born with moderate to severe hearing loss, benefitting from hearing aids, and not having balance problems. RP begins in the teenage years.

Symptoms of type III include being born with normal hearing that gets worse in the teenage years and leads to deafness by mid- to late adulthood and being born with near-normal balance.

Screening and Diagnosis

The doctor will ask about symptoms and medical history and will perform a physical exam. Hearing loss is determined with standard hearing tests. Balance problems can be detected with a test called electronystagmography (ENG). In this test, the doctor flushes the ears with warm and then cool water. This causes nystagmus, which is rapid eye movements that can help the doctor detect a balance disorder.

An eye doctor will perform an eye exam, which will likely include a visual field test to check side vision, a test to check for ability to adapt to seeing in the dark, and a test to check sensitivity to color and contrast.

If any problems are found on these tests, an electroretinography (ERG) is done. This test confirms a diagnosis of RP. It measures the electricity given off by the nerves in the retina. The test is done while wearing special contact lenses and looking at a flashing light.

Treatment and Therapy

There is no cure for Usher syndrome. The best treatment is to identify the disorder as early as possible and begin educational programs and other services right away. This helps reduce the communication and learning problems that can result from hearing and vision loss.

The specific programs and services depend on the severity of the hearing, vision, and balance problems and the person’s age and abilities. Options include hearing aids; assistive listening devices; cochlear implant, in which a small device is surgically put under the skin behind the ear to give deaf people some ability to hear; adjustment and career counseling; training to help with balance and movement; low-vision services; communications training; and skills in living independently.

Prevention and Outcomes

Currently, there is no known way to prevent Usher syndrome.

Bibliography

Cosgrove, Dominic, and Marisa Zallocchi. "Usher Protein Functions in Hair Cells and Photoreceptors." Intl. Journ. of Biochemistry & Cell Biology 46 (2014): 80–89. Print.

Millichap, J. Gordon. Neurological Syndromes: A Clinical Guide to Symptoms and Diagnosis. New York: Springer, 2013. Print.

Sutton, Amy L. Eye Care Sourcebook: Basic Consumer Health Information About Eye Care and Eye Disorders. Detroit: Omnigraphics, 2008. Print.

Williams, David. “Retinal Degeneration in Usher Syndrome.” Retinal Degenerations: Biology, Diagnostics, and Therapeutics. Ed. Joyce Tombran-Tink and Colin J. Barnstable. Totowa: Humana, 2007. Print.

Yoshimura, Hidekane, et al. "Massively Parallel DNA Sequencing Facilitates Diagnosis of Patients with Usher Syndrome Type 1." PLoS ONE 9.3 (2014): 1–7. Print.