Usher syndrome is responsible for the majority of deaf-blindness. The word "syndrome" means that multiple symptoms occur together, in this case, deafness and blindness. It occurs in roughly 1 person in 23,000 in the United States, 1 in 28,000 in Norway and 1 in 12,500 in Germany. People with Usher syndrome represent roughly one-sixth of people with retinitis pigmentosa.

Usher syndrome is inherited in an autosomal recessive pattern. "Recessive" means both parents must contribute an appropriate gene for the syndrome to appear, and "autosomal" means the gene is not carried on one of the sex chromosomes (X or Y), but rather on one of the 22 other pairs. (See the article on human genetics for more details.)

The progressive blindness of Usher syndrome results from retinitis pigmentosa. The photoreceptor cells usually start to degenerate from the outer to the center of the retina, including the macula. The degeneration is usually first noticed as night blindness (nyctalopia); peripheral vision is gradually lost, restricting the visual field (tunnel vision), which generally progresses to complete blindness. The qualifier 'pigmentosa' reflects the fact that clumps of pigment may be visible by an ophthalmoscope in advanced stages of degeneration.

Although Usher syndrome has been classified clinically in several ways, the prevailing approach is to classify it into three clinical sub-types called Usher I, II and III in order of decreasing severity of deafness. Usher I and II are the more common forms; the fraction of people with Usher III is significant only in a few specific areas, such as Finland and Birmingham. As described below, these clinical subtypes may be further subdivided by the particular gene mutated; people with Usher I and II may have any one of six and three genes mutated, respectively, whereas only one gene has been associated with Usher III. The function of these genes is still poorly understood. The hearing impairment associated with Usher syndrome is better understood: damaged hair cells in the cochlea of the inner ear inhibit electrical impulses from reaching the brain.

Usher syndrome, also known as Hallgren syndrome, Usher-Hallgren syndrome, retinitis pigmentosa-dysacusis syndrome, or dystrophia retinae dysacusis syndrome, is an extremely rare genetic disorder caused by a mutation in any one of at least 11 genes resulting in a combination of hearing loss and visual impairment. It is a leading cause of deafblindness and is at present incurable.

Usher syndrome is classed into three subtypes according to onset and severity of symptoms. All three subtypes are caused by mutations in genes involved in the function of the inner ear and retina. These mutations are inherited in an autosomal recessive pattern.

Nonsyndromic deafness is hearing loss that is not associated with other signs and symptoms. In contrast, syndromic deafness involves hearing loss that occurs with abnormalities in other parts of the body. Genetic changes are related to the following types of nonsyndromic deafness.

- DFNA: nonsyndromic deafness, autosomal dominant

- DFNB: nonsyndromic deafness, autosomal recessive

- DFNX: nonsyndromic deafness, X-linked

- nonsyndromic deafness, mitochondrial

Each type is numbered in the order in which it was described. For example, DFNA1 was the first described autosomal dominant type of nonsyndromic deafness. Mitochondrial nonsyndromic deafness involves changes to the small amount of DNA found in mitochondria, the energy-producing centers within cells.

Most forms of nonsyndromic deafness are associated with permanent hearing loss caused by damage to structures in the inner ear. The inner ear consists of three parts: a snail-shaped structure called the cochlea that helps process sound, nerves that send information from the cochlea to the brain, and structures involved with balance. Loss of hearing caused by changes in the inner ear is called sensorineural deafness. Hearing loss that results from changes in the middle ear is called conductive hearing loss. The middle ear contains three tiny bones that help transfer sound from the eardrum to the inner ear. Some forms of nonsyndromic deafness involve changes in both the inner ear and the middle ear; this combination is called mixed hearing loss.

The severity of hearing loss varies and can change over time. It can affect one ear (unilateral) or both ears (bilateral). Degrees of hearing loss range from mild (difficulty understanding soft speech) to profound (inability to hear even very loud noises). The loss may be stable, or it may progress as a person gets older. Particular types of nonsyndromic deafness often show distinctive patterns of hearing loss. For example, the loss may be more pronounced at high, middle, or low tones.

Nonsyndromic deafness can occur at any age. Hearing loss that is present before a child learns to speak is classified as prelingual or congenital. Hearing loss that occurs after the development of speech is classified as postlingual.

Mutations in the "ACTG1", "CABP2", "CDH23", "CLDN14", "COCH", "COL11A2", "DFNA5", "ESPN", "EYA4", "GJB2", "GJB6", "KCNQ4", "MYO15A", "MYO6", "MYO7A", "OTOF", "PCDH15", "POU3F4", "SLC26A4", "STRC", "TECTA", "TMC1", "TMIE", "TMPRSS3", "USH1C", and "WFS1" genes cause nonsyndromic deafness, with weaker evidence currently implicating genes "CCDC50", "DIAPH1", "DSPP", "ESRRB", "GJB3", "GRHL2", "GRXCR1", "HGF", "LHFPL5", "LOXHD1", "LRTOMT", "MARVELD2", "MIR96", "MYH14", "MYH9", "MYO1A", "MYO3A", "OTOA", "PJVK", "POU4F3", "PRPS1", "PTPRQ", "RDX", "SERPINB6", "SIX1", "SLC17A8", "TPRN", "TRIOBP", "SLC26A5", and "WHRN".

The causes of nonsyndromic deafness can be complex. Researchers have identified more than 30 genes that, when mutated, may cause nonsyndromic deafness; however, some of these genes have not been fully characterized. Many genes related to deafness are involved in the development and function of the inner ear. Gene mutations interfere with critical steps in processing sound, resulting in hearing loss. Different mutations in the same gene can cause different types of hearing loss, and some genes are associated with both syndromic and nonsyndromic deafness. In many families, the gene(s) involved have yet to be identified.

Deafness can also result from environmental factors or a combination of genetic and environmental factors, including certain medications, peri-natal infections (infections occurring before or after birth), and exposure to loud noise over an extended period.

Types include: