Biomedical Engineering Reference
In-Depth Information
Chapter 7
Application of Next-Generation-Sequencing
in Hearing Loss Diagnosis
7.1
Introduction
Hearing loss is the most common birth defect and sensorineural disorder in humans.
Hearing loss results from obstructions in the transmission of the sound anywhere
between the outer ear and auditory cortex in the brain. In a normal condition, the
sound signal that is collected by the outer ear is amplifi ed by the middle ear for trans-
mission to the cochlea, which then converts this energy into electrical signals that are
ultimately transmitted to the brain through the auditory nerves (Smith et al. 1993 ).
Based on the defective anatomical structure involved, hearing loss can be classifi ed as
conductive, sensorineural, or mixed. Conductive hearing loss is a defect in conducting
sound waves through outer and middle ear due to abnormalities of outer ear, tympanic
membrane (eardrum), or ossicles of the middle ear. Sensorineural hearing loss (SNHL)
is due to a defect located anywhere from cochlea to the auditory cortex. Mixed hear-
ing loss is a combination of both conductive and sensorineural abnormalities.
Depending on the age at onset, hearing loss can be classifi ed as prelingual, present
before speech development, or postlingual, present after speech development. Severity
of the hearing loss is measured by decibels (dB), can be graded from mild (26-40 dB)
to profound (90 dB), affecting from low to high frequencies (Smith et al. 1993 ).
In this chapter, we introduce syndromic and nonsyndromic hearing loss (NSHL)
alongside the importance of hearing loss detection. Then, we focus on covering the
recent advances of hearing loss diagnosis in light of the application of next-generation-
sequencing (NGS) to this fi eld.
7.2
Hearing Loss Syndromes
One in 500 newborns is affected with bilateral permanent SNHL in developed coun-
tries; this number is increased to 2.7 per 1,000 before the age of 5 years and 3.5 per
1,000 during adolescence (Morton and Nance 2006 ). Approximately two-thirds of
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