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Hearing is the perception and interpretation of biologically relevant acoustic information. The external, middle, and inner parts of the ear are responsible for sound detection and transduction into electrical signals. These signals are transmitted to the brain by the auditory nerve for central auditory processing. Hearing impairment can be associated with a variety of factors including genetics, age, trauma, drugs, and infections. In children, hearing loss is of particular clinical interest because it significantly impacts language acquisition and has lifelong implications that affect the cognitive, behavioral, and social development of a child. While its diagnosis and treatment are challenging in resource-constrained areas, childhood hearing loss remains a serious global health issue.
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Hearing loss is the most prevalent sensory impairment in children. Worldwide, its prevalence at the severe-to-profound level is 4 in every 10,000 infants, although the number of infants with hearing loss detected by universal newborn screening programs is higher in developed countries, likely reflecting increased access to early screening. Children with moderate-to-profound degrees of hearing loss can be identified by early screening and targeted for appropriate intervention.
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About 50% of newborns with hearing loss have a genetic basis for their loss, although in developed countries, this percentage rises to approximately 70%; in the remainder, the hearing loss is attributed to environmental factors (Fig. 174-1). Hearing loss, once diagnosed, is described as syndromic or nonsyndromic, depending on whether the loss occurs in isolation or is associated with concurrent clinical findings. Of genetic hearing loss, 70% is nonsyndromic and 30% is syndromic. By severity, hearing loss is graded as mild, moderate, severe, or profound, a classification based on a quantitative assessment of sound intensity measured in decibels and recorded as an audiogram (Table 174-1).
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PATHOGENESIS AND MECHANISMS OF AUDITORY PERCEPTION
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Auditory perception is a complex process whereby acoustic signals from the environment are transduced by the auditory system into electrical signals that are interpreted by the brain and allow humans to interact with the acoustic environment. Acoustic energy enters the external auditory canal and vibrates the tympanic membrane where it is converted to mechanical energy. The mechanical energy is transferred from the tympanic membrane through the ossicular chain (malleus, incus, and stapes) to the inner ear. Vibration of the stapedial footplate at the oval window displaces fluid in the cochlea, leading to hair cell stimulation and depolarization. An electrical signal is transmitted from inner ear hair cells to the dendrites of the neurons that have cell bodies in the ...