Non-Syndromic Genetic Deafness

Deafness is the most frequent form of sensorial deficit. In the vast majority of cases, the deafness is termed nonsyndromic or isolated and the hearing loss is the only clinical anomaly reported. In developed counties, 60-80% of cases of early-onset hearing loss are of genetic origin.

Epidemiology

Between 1 in 1,000 and 1 in 700 children were born with profound or severe hearing loss.

Clinical description

The majority of cases presenting at birth concern perceptive deafness (with a neurosensory origin associated with the inner ear) rather than conductive deafness (anomalies in the amplification of sound waves between the middle ear - tympanum and auditory ossicles - and the outer ear). Autosomal dominant forms are characterised by very early onset and bilateral hearing loss with varying degrees of severity (ranging from mild to profound). No malformations of the inner ear can be detected by CT scan. Mutations in the PDS gene are responsible for 7% of cases of childhood deafness. In these cases, the deafness is marked by early-onset, usually bilateral (but sometimes asymmetric) hearing loss with autosomal recessive transmission. This form of deafness is always associated with malformations of the inner ear that can be detected by CT scan. In rare cases, thyroid gland disease may also be present. For the autosomal dominant forms of deafness, mutations in the COCH gene result in progressive postlingual deafness associated with severe attacks of vertigo and subjective tinnitus. This form of deafness should be distinguished from Meniere's disease (see this term). Autosomal dominant mutations in the WFS1 gene cause either a form of hearing loss affecting mainly low frequency sounds or deafness associated with optic atrophy.

Etiology

To date, 86 causative genes for sensorineural deafness have been identified and localised and 37 of these genes have been cloned. Eleven of these genes have been implicated in both isolated and syndromic forms of deafness. Mutations in the gene encoding connexin 26 appear to be responsible for the majority of cases of childhood perceptive deafness.

Diagnostic methods

Hearing loss can be evaluated by audiogram and grouped as either mild (loss between 20 and 40 dB), moderate (loss between 41 and 70 dB), severe (loss between 71 and 90 dB), profound (loss between 91 and 120 dB) or as cophosis (loss beyond 120 dB).

Genetic counseling

In 85% of cases, the deafness is transmitted as an autosomal recessive trait (DFNB type). Autosomal dominant inheritance accounts for between 10 and 15% of cases (DFNA type), whereas only 1% of cases are inherited as an X-linked trait (DFN type).

Management and treatment

Management should be multidisciplinary (involving a paediatrician, general practitioner, ORL specialist, speech therapist and hearing aid specialist). Hearing aids (either implanted or external) generally provide the only management option for improving hearing loss, although surgical treatment may sometimes be proposed for some forms of conductive hearing loss.