Non-syndromic Hearing Loss

Nonsyndromic hearing loss (NSHL) includes a group of hearing loss disorders that are not associated with other signs and symptoms. On the contrary, syndromic deafness involves hearing loss that occurs with abnormalities in other parts of the body. Most forms of nonsyndromic deafness are associated with permanent hearing loss caused by damage to structures in the inner ear. It is estimated that approximately 75% of cases display autosomal recessive inheritance, 15% of cases are autosomal dominant and 2-3% cases are X-linked.  NSHL can also be due to mitochondrial inheritance. It is estimated that for autosomal recessive NSHL there are between 30-100 genes in India.

Usher Syndrome

Usher syndrome is a rare disorder that is a leading cause of deaf-blindness. It is associated with a mutation in any one of 10 candidate genes. Usher syndrome is incurable at present. In this disease, deafness is associated with a defective inner ear, and the vision loss is associated with retinitis pigmentosa (RP), a degeneration of the retinal cells. Usher syndrome has three clinical subtypes, denoted as I, II and III in decreasing order of severity. People with Usher I ( MIM: 276900) are born profoundly deaf, begin to lose their vision in the first decade of life and have problems with balance in movement. Different subtypes of Usher I can be caused by mutations in any one of several different genes that function in the development and maintenance of inner ear structures and transmit sound and motion signals to the brain eg: CDH23, MYO7A, PCDH15, USH1C, and USH1G. People with Usher II are also born deaf, but do not seem to have noticeable problems with balance; they also begin to lose their vision later. Different subtypes of Usher syndrome type II (MIM: Not available) may be caused by mutations in any of three different genes: USH2A, GPR98 and DFNB31. People with Usher syndrome III are not born deaf, but experience a gradual loss of their hearing and vision; they may or may not have balance difficulties. Mutations in only one gene, the CLRN1 gene, have been linked to Usher syndrome type III ( MIM: 276902).  In India, Usher syndrome is the second most common cause of deafness.