Decades of molecular and cellular investigations have advanced all fields of hearing research. A multitude of genes for syndromic and nonsyndromic forms of hearing impairment including autosomal dominant and recessive, X-linked, and mitochondrial modes of transmission have been located in specific regions of the human genome. In addition, clinically relevant genes essential for normal auditory development and/or function are being identified and cloned at a rapid pace.
Other cochlear-specific genes have been isolated from enriched membranous labyrinth cDNA libraries. New technologies, including the development of detailed maps via high throughput screening platforms, RNAi, ChIP-seq, combined with knock in and knock out technologies, have facilitated identification of new genes and their subsequent cloning and expression. Scientists continue to populate comprehensive molecular maps that are critically necessary to understand the complex regulatory mechanisms underlying normal hearing.
Animal models such as the mouse continue to be instrumental in mapping and cloning many deafness genes. Because of the utility of the mouse for such studies, additional mouse models of deafness are being created through mutagenesis and screening programs as well as targeted mutation of deafness genes found in humans. In addition, numerous animal and non-animal models are being used to study the function of the proteins encoded by deafness genes and to test therapeutic approaches and screen potential therapeutic drugs. These advances offer researchers many opportunities to study the characteristics of deafness, hereditary factors involved in hearing loss, and genes that are critical for the development and maintenance of the human ear. Great strides are being made in the study of properties of auditory sensory cells and of characteristics of the response of the inner ear to sound.
Research demonstrates that infants who are born deaf or hard-of-hearing have a better chance of learning language if the hearing loss is found immediately after they are born and if they learn a spoken or signed language as early as possible. Given this information, the NIDCD has placed a high priority on understanding the causes, possible treatments, and progression of hearing loss during early childhood. Approximately two to three in every 1,000 children in the United States are born with severe to profound deafness. In 1989, less than five percent of newborns received hearing screening prior to leaving the hospital, and most children were not identified to have a hearing impairment until two to three years of age. That delay during a critical period for language development led to lifelong difficulties in language acquisition and the need for costly special education in schools for the deaf. The implementation of universal newborn hearing screening, a joint effort by the NIDCD, the Health Resources and Services Administration, and the Centers for Disease Control and Prevention, has dramatically improved the identification of infants with hearing loss early in life and accelerated the initiation of services for these children. Today, more than 95 percent of children are screened for hearing loss shortly after birth. The NIDCD continues to examine the outcomes of children identified through newborn hearing screening.
Scientific advances have also been translated into cochlear implants and hearing aids. Advances are being made in developing brainstem implants and brain-computer interfaces. Research has verified that despite considerable variability in the performance of children who have received cochlear implants, most demonstrate marked improvements in speech perception and production. Cochlear implants positively influence children's receptive and expressive language skills. The earlier the implantation and the longer children use their implants, the greater their language ability.
To achieve the most benefit from their implants, however, children generally need extensive oral-auditory training following implantation and also benefit from periodic audiological assessments. Cochlear implants have benefited children who are congenitally deaf as well as those who are postlingually deaf. Adults can also benefit from cochlear implants when their hearing ability has greatly decreased. The vast majority of adult implant recipients derive substantial benefit in conjunction with speechreading, and many are able to communicate by telephone. Dedication to research on cochlear implants and other neural prostheses will improve the quality of life of individuals with hearing loss as well as our understanding of the auditory system.
New insights have been gained concerning the encoding of complex signals transmitted from the auditory nerve to the brain. The relationship between the neural codes for sound intensity, frequency, duration and temporal characteristics of auditory signals and the perception of the stimulus variables has been further clarified. Valuable progress has been made in understanding the structure and function of efferent feedback pathways to the inner and middle ear. There is now good evidence that this system may aid in the detection of signals in noisy environments and serve to protect the ear from acoustic injury.
Gains have been made about the ways in which the brain creates maps of auditory space and how the maps interact with visual space. This research may have implications in treatment of children who acquire hearing loss in infancy or early childhood. Further, psychoacoustic and electrophysiologic studies of infants and children are providing important new insights into the development of functional hearing.
In the aging auditory system, discoveries have been made demonstrating changes in the regulation of fluid composition and autoregulation of cochlear blood flow which may underlie some of the biologic effects of aging on auditory function. Improved behavioral and electrophysiological techniques for measuring auditory function are providing more accurate assessments of the peripheral and central components of age-related hearing loss.
Recent development of animal models for bacterial and viral infections hold promise for new diagnostic and therapeutic approaches to sensorineural hearing loss caused by infections. Antiviral drugs may find rapid application in the treatment for these conditions with the advent of suitable animal models in which to test efficacy. In addition, models allow a greater understanding of why and to what degree infants and children are susceptible to ototoxic drugs used in the treatment of infections.
Otitis media continues to be a significant focus of research because of its prevalence and cost to society. Important risk factors have been identified. Studies of the eustachian tubes have provided new information on tubal mechanics, surfactant-like (fluid) substances and middle ear pressure regulation. The role of bacterial biofilms in chronic otitis media is a new and promising area of investigation. State-of-the-art molecular, genetic, and genomic techniques are being used to identify genes that may predispose an individual to chronic otitis media. These techniques are also being used to define the specific molecular changes that allow viral and bacterial infection of the middle ear as well as the host/pathogen interactions that facilitate the disease process.
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