The auditory system contains the inner ear that is responsible for transforming the mechanical energy of the sound waves into electrical stimuli, which is translated in the brain. The auditory system is composed of a cochlea, which is a fluid-filled tube coiled in a spiral shape around the modiolus.
Hearing impairment has a wide spectrum of etiologies originating from both environmental and genetic factors. Genetic factors of hearing defects poses greater challenges. A hearing disability that appears early in childhood can have a major consequence on language acquisition. Hearing loss is categorized based on the frequency loss and the severity of hearing thresholds. When hearing loss is the only apparent abnormality, it is referred to as non-syndromic hearing loss (NSHL). In other cases, hearing loss occurs along with a variety of other malformations and thus is designated as syndromic hearing loss (SHL). A panoply of genes have been discovered to be involved in hearing loss.
To date, more than 100 genes have been linked to various degrees of hearing impairment. Hearing loss can be passed on to each generation by autosomal dominant, autosomal recessive, X-
linked, Y-linked, and mitochondrial modes of inheritance.
Causes of hearing loss
Similar to other sensory loss, hearing impairment has a wide spectrum of etiologies originating from both environmental and genetic factors. Prolonged exposure to high intensity sound poses a high risk for auditory function and can lower hearing thresholds. Acoustic trauma, as a result of a sudden loud noise, can lead to temporary and/or permanent hearing impairment. Among environmental factors, different viral infections, as well as neonatal anoxia and hyperbilirubinemia, can also cause permanent hearing defects. Long term augmentation of ototoxic drugs such as aminoglycoside and gentamicin antibiotics has an adverse effect on the auditory system and accounts for hearing defects. Unlike the genetic factors dictated by hereditary information, some of the environmental factors can be reduced and avoided by raising awareness for appropriate protection.
Genetic insults contributing to hearing defects poses greater challenges. The clinical heterogeneity of hearing loss is characterized by common classifications based on several parameters such as onset, severity and the presence of additional clinical manifestations other than deafness. Hearing loss that occurs prior to speech acquisition is termed prelingual deafness, either congenital or appearing after birth. A hearing disability that appears early in childhood can have a major consequence on language acquisition. Age-related hearing loss (ARHL) affects the elderly population with high prevalence and its appearance and progression is influenced by both genetic and environmental factors. About 60% of the population over the age of 65 suffers from different degrees of hearing loss, with a decline in sensitivity to sound, accompanied with reduced speech perception. Hearing loss is also categorized based on the frequency loss and the severity of hearing thresholds. High tone loss refers to reduced sensitivity of high frequency acoustic stimulus, as opposed to low tone loss for the low frequencies. The terms profound, mild and moderate describe the descending order of different severity levels of hearing impairments. When hearing loss is the only apparent abnormality, it is referred to as non-syndromic hearing loss (NSHL). In other cases hearing loss occurs along with a variety of other malformations and thus is designated as syndromic hearing loss (SHL).