There are three general categories of Usher syndrome. Type 1 and type 2 are the most common forms of Usher syndrome in the United States. These 2 types account for about 95% of all cases of the syndrome. A total of nine genes that cause Usher syndrome have been identified.
Children with type 1 Usher syndrome are born profoundly deaf and have severe balance difficulties. Many of these children do not benefit from hearing aids and may be candidates for cochlear implants.
Because of the balance problems associated with type 1 Usher syndrome, children with this disorder are slow to sit without support. They typically are not able to walk until 18-24 months old.
These children usually begin to develop vision problems in early childhood, almost always by age 10. The vision loss is caused by retinitis pigmentosa (RP), a degenerative condition of the retina. Vision problems most often begin with difficulty seeing at night, but tend to progress to total blindness.
The associated genes are MY07A, USH1C, CDH23, PCDH15, and SANS.
Those with type 2 Usher syndrome are born with moderate to severe hearing loss and normal balance. Most of these individuals can benefit from hearing aids and can communicate orally. Vision problems in those with type 2 Usher syndrome tend to progress more slowly than those in type 1, with the onset of RP often not apparent until adolescence or shortly after. Type 2 does not result in total blindness. Hearing loss usually remains stable.
The associated genes are USH2A, VLGR1, and WHRN.
Children with type 3 Usher syndrome have typical hearing at birth. Although most children with the disorder have normal to near-normal balance, some may develop balance problems later on. Hearing and sight worsen over time, but the rate at which they decline can vary from person to person, even within the same family. A person with type 3 Usher syndrome may develop hearing loss by the teens, and he or she will usually require hearing aids by mid- to late adulthood. Night blindness usually begins sometime during puberty. Blind spots appear by the late teens to early adulthood, and, by mid-adulthood, the person is usually legally blind.
The associated gene is USH3A.
Sources: Cleveland Clinic; Genetic and Rare Diseases Information Center; National Institute on Deafness and Other Communication Disorders