Genetics of Nonsyndromic Recessively Inherited Moderate to Severe and Progressive Deafness in Humans

Deafness is a sensory impairment which results in a partial or total loss in reception of sound. The intensity of sound can be measured in decibels (dB). It is usual to assess hearing thresholds at frequencies of 0.25, 0.5, 1, 2, 4 and 8 KHz. Sounds of each frequency are presented at different intensities to a subject and the response is recorded graphically as an audiogram. A loss in hearing is indicated if the threshold for perception of sound for any frequency is elevated by 10 dB or greater as compared to the defined standard value for each frequency. Like visual foveae, organisms also have acoustic foveae, in which a certain frequency occupies greater space and is resolved more than other frequencies. In humans, the frequencies of 2 to 4 KHz are finely resolved. The frequencies of 0.5-2 KHz are the most important for hearing conversations. Therefore, individuals can have usable hearing if deafness does not impair these frequencies to a profound degree. A hearing loss of >91 dB constitutes profound deafness while those between 41 to 90 dB are defined as moderate (41-55 dB), moderate to severe (56-70 dB) or severe hearing loss (71-90 dB), respectively. Progressive deafness involves a gradual loss in the ability to hear over time.