The development of hearing is a long process. It starts in the womb with the physical growth of the ear and auditory nervous system and ends when adult-like hearing abilities are achieved. It is a well-known fact that newborns can hear before and at birth, however, due to an immature nervous system and lack of experience, auditory function is incomplete. Although the growth of the ear begins with the thickening of neural tissue at approximately 20 days after pregnancy, it will take another twelve to thirteen years before all of a child’s hearing abilities are functioning at adult levels. This is why young children may cover their ears in noise and cry, it’s just too much for the maturity of the auditory system to handle.
Along the way, various hearing abilities are acquired because of a maturing nervous system and environmental learning effects. An example can be seen in how we learn to identify sound direction. Researchers tell us that gross localization can be used to assess hearing by eight months of age. However, concise localization does not reach adult ability until approximately the age of five. On the other hand, the ability to discriminate some lower tone speech sounds occurs before and at birth, but continues to improve to about the age of eight. Development occurs in increments, but eventually the auditory nervous system is complete and with hearing experience all of the auditory functions eventually become adult-like.
The hearing system matures first within the ear, then the brainstem (carries information to the brain) and with time finally the cortex. As the ear begins to develop, MRI studies show the presence of the organs of hearing and balance at about the fifth week of gestation. The outer ear and basic cortex are also visible. By the end of the 20th week of pregnancy, the structures of the inner ear nerves are completely developed to adult size. The human inner ear is very small at about half the size of a pencil eraser. By the 30th week of development, the brainstem is capable of transmitting signals to the brain. Around this same time, the cortex also becomes clearly visible. This rapid growth occurs because brain cells develop at a rate of about 250,000 cells per minute (Akerman S., 1992). That is fast!
Research suggests that an infant’s brain contains about 100 billion nerve cells. Even though at birth the hearing system is infantile, the brain can still learn speech sounds in any language or languages. Refinement of speech, language, and hearing abilities continues to occur over time. By adolescence, the connections between the right and left-brain are generally complete, signaling the final stages of maturity and hearing abilities are typically at adult levels.
As previously reported, nerve cell development is not the only important influence on emergent hearing abilities. Normal hearing creates and influences physical changes in the actual structures of the brain, while evolving critical networks that create better processing. This “neuroplasticity” continues throughout development and is life long. A good example of how experience can change function is seen in infants over their first year of life. With regular exposure to a language and its speech sounds, that particular language becomes preferred while other unknown sounds are filtered out. In fact, a neurological map is created for speech and language development that is used to guide the filtering process, including leaning and filtering out of environmental noises. However even with this filtering, separating multiple voices in a sea of background noise is difficult for young children. For these reasons, children may shy away from loud sounds, be uncomfortable in background noise, appear to be inattentive, distracted, or demonstrate fatigue. The development of the hearing system is not just about observing ear growth, but learning about sound and connecting areas within the brain to maximize network processing.
The evaluation of hearing in children usually starts with an assessment of family members for possible genetic influences. Should there be a pattern of total or partial hearing loss in direct descendants of the same or different ages, it would be best to consult a pediatrician and consider taking genetic tests to identify chromosomes that produce hearing loss. An example of a rare genetic hearing loss is seen in a family where all the boys have a hearing loss in the left ear. Another genetic effect is seen in a family where one parent has hearing loss, passing it on to half of the offspring. More common is the hearing loss created by matching recessive genes where neither parent has a hearing loss, but carries a gene for deafness. The contribution of genetic effects cannot be overlooked, as one in every eight in the general population carries one or more of 400 possible genes for hearing loss. This is the reason why nine out of ten children born with hearing loss are born to normal hearing parents.
Genetic hearing loss does not necessarily only cause nerve deafness. Some genetics may result in a missing outer ear, a closed off ear canal (atresia), or damage to the middle ear bones. In other cases, hearing loss is part of a syndrome or disease process. In rare cases, some of these can be progressive. Fortunately, the incidence of severe to profound deafness represents a small population by comparison and modern treatments for all children, regardless of the degree are better than ever before. The improvements in early identification, surgery, education and training, cochlear implants, and hearing aid technology are remarkable. Many children can eventually become mainstreamed normalizing social and educational opportunities, which have never been more available.
Newborn Hearing and Starting Education
In order to identify newborns with hearing loss and start treatment as soon as possible, the field of Audiology has spearheaded a remarkable nationwide newborn infant hearing screening program. Under this program, the goal is to screen each newborn with objective testing using Otoacoustic Emissions or Auditory Brainstem Evoked Potentials prior to discharge from the hospital or within 30 days thereafter. Should any child fail the test, they are referred for further evaluation to an Audiologist who will measure brainwave responses to sound and provide other diagnostic tests. Should hearing loss be confirmed, the Audiologist working with a team of specialists will identify the best possible treatments. The primary objective is to reduce the effects of hearing loss on speech and language development and socialization. Obtain the most appropriate educational placement and services. The best results are obtained when technology is coordinated with services.
In order to facilitate these goals, the Individual Disabilities Education Act (IDEA) and the Early Intervention Programs for Toddlers and Infants provide assistance to help children with hearing loss. IDEA is a law that provides guidelines for children with disabilities in the schools. Under IDEA and its Early Intervention Program, children with hearing loss from birth to 2 years of age are eligible for services. Additionally, all children to young adults, up to the age of 21, may also be eligible for special education services and assistance. Pediatric Audiologists recommend that the local school district be contacted immediately following a diagnosis of hearing loss because educational Audiologist and others specialists can be very helpful in guiding development. Some programs even have infant classes to assist parents. A child born with hearing loss in these modern times has opportunities to succeed not like any other time in our history.
Checking for Hearing Loss at Home
Parents should be aware that 20% to 30% of children develop hearing loss after leaving the hospital. For this and other reasons, monitoring sound awareness and the development of speech and language should continue long after leaving the hospital. Monitoring requires knowing a few simple milestones that should occur in all children. All infants should startle to loud sounds. By approximately 3 months of age, infants will listen to a familiar voice respond to sound in the room and easily startle. In most cases, by eight to nine months of age an infant will turn their head in the direction of a parent’s voice. For most infants approaching six months of age, speech starts as cooing, babbling, and playing with their voice. Infants can typically respond to their name by 12 months of age or sooner and may say “no”, “ma ma”, or “da da” in addition to one to 10 other words. Parents should be aware that the development of speech and language is a continuous process and that children develop at different rates. As children get older, they should have a vocabulary of 20 words by 18 months of age and 50 words, including partial pronunciations, when they are 24 months old. Most 2 year olds should also be saying a few two-word sentences such as “baby cry” or “baby go”. Parents should keep in mind that speech pronunciation is also developing at this same time and that word clarity or poor articulation is common. A good rule of thumb for parents to remember, because of the time spent observing their child, is to follow their own intuition and if hearing loss is suspected consult a professional. If any child fails to respond to sound at any age or does not speak by 9 months of age, consultations with a Pediatrician, ENT physician, and Audiologist (My Baby Has Hearing Loss) are indicated.
Middle Ear Infections
After birth, the most frequent cause of hearing loss is the common middle ear infection (otitis media). These infections occur in 5 out of 6 children by the age of three and are more frequent in boys than girls. Otitis media occurs primarily because of the lack of proper drainage and function of the middle ear release valve in the back of the throat called the Eustachian Tube. This tube opens to clear the ears of pressure by swallowing or yawning, but in children, the angle of the tube is not optimal for drainage and may not function properly. Should a cold cause swelling of the Eustachian tube or milk find its way into the tube, infection can occur. Some well-known risk factors include the absence of breastfeeding, exposure to tobacco smoke, presence of siblings, and daycare attendance. In many cases, there are no outward signs (no pain, no crying, no pulling at the ears), but there is a clear decrease in the response to sound. Remarkably, the newest research has shown that approximately 80% of all pediatric ear infections resolve without treatment within three months. This new information has resulted in a reduction in the use of antibiotics for ear infections, unless medically necessary and the bacterial resistance patterns are known. Do not be surprised if the physician decides that antibiotics are not appropriate at the first visit and chooses to treat your child with other medications that resolve the symptoms and reduce swelling of the membranes. This is considered more appropriate as overuse of antibiotics has resulted in bacterial resistance. In the future, there will be a vaccine to reduce the occurrence of middle ear infection in children. Fortunately, most children outgrow middle ear infections by approximately the age of eight. This is the time when the angle of the Eustachian tubes has changed to increase drainage and reduce accessibility to the middle ear by foreign substances and bacteria.
In children who have reoccurring infections the hearing loss is typically temporary. Should a child have three infections in six months or four in a twelve-month period, this is considered chronic otitis media. In over a million cases a year of chronic childhood ear infection, small ventilation tubes are placed in the eardrums by Ear, Nose, and Throat surgeons. This relieves the pressure and allows the ear to heal. This is important because controlling frequent ear infections prevents lapses in normal hearing and therefore, development. Lapses or reduced hearing in young children, in addition to being a medical problem, can create difficulty learning phonetics and developing normal speech and language. In some cases, this has a domino effect, creating difficulty learning how to read and may be associated with auditory processing problems such as hearing speech in background noise, such as that encountered in a classroom or restaurant. Treating chronic middle ear disease is important as the consequences can result in significant developmental and educational delays.
The Hearing Loss Team
When a hearing loss is encountered, consultation with the Audiologist, Ear, Nose, and Throat physician, Pediatrician, Speech/Language Pathologist, Psychologist, and others will determine the best possible options. These may include a discussion on available medical treatment, hearing aids, cochlear implants, speech and language therapy, educational opportunities, communication skills, family counseling, and referral to a specialist, such as a geneticist. The team will be monitoring the development of speech and language and auditory function, but there is much more. Knowing how to deal with hearing loss is not about learning how to talk and understand words, but it is about the family and how they are helped to work together with their family member. Finding solutions that help everyone to adjust to hearing impairment takes time and commitment, but it is far from impossible. Specialists in habilitation and hearing counseling can be very helpful in this regard. The family has a major influence on how their child, brother or sister develops and succeeds with hearing loss. Support from family members is very important to development, but also to self-esteem. The team, which includes the family members, should be very sensitive to how the hearing-impaired child or adolescence perceives their hearing loss, themselves, and others. It also includes how other family member feel.
Fitting Infants and Children with Amplification
Fitting amplification to infants and children with hearing loss is not an easy or simple exercise. Not only does it require a plethora of scientific knowledge, it also requires specialized training in communication skills, child and family counseling, and an in-depth understanding of the milestones in speech, language, hearing, physical, social, and educational development. Among other duties, the Pediatric Audiologist must also carefully observe changes in behavior or ability and monitor the function of hearing aids or cochlear implants. This includes watching for medical complications such as ear infections and progressive hearing changes.
In infants, hearing loss should be identified as soon as possible, but no later than the first six months of life. Hearing aids should be fitted as soon as possible or in profound hearing loss, a cochlear implant scheduled absolutely by no later than the end of the first year of life (cochlearamericas.com, advancedbionics.com/us, and www.medel.com/us). This represents ideal timing for speech, language, and hearing development and should not be delayed unless medically necessary.
In the case of hearing aids, special measurements for infants and toddlers called real ear to coupler difference (RECD), should be performed to verify the prescription, set the controls for sound comfort, and ensure proper audibility for the most natural sound experience. This is the most appropriate treatment as auditory processing and development of the hearing system depends upon the sound experience. Furthermore, the hearing technology that is most appropriate for infants should not over control the sound environment with special noise reduction circuits or directional microphones. Using hearing aids that over process the environment and reduce natures signals may not help the brain to achieve a complete and correct brain map. It is best that the developing brain learns to adapt to and use sound fully, resulting in a more enriched hearing experience and a better tolerance and acceptance of noise.
In school-aged children with hearing loss, the use of special accessories that work with hearing aids can improve hearing in the classroom and in a variety of other critical listening situations. These devices provide direct access to any sound product with a plug in jack, IPods included. With a simple transmitter and receiver, typically by FM radio signals, sound can be sent directly to a hearing aid or cochlear implant. A boot is a plastic snap-on connector that attaches to a bottom of a behind-the-ear hearing aid or the spine of the cochlear implant. The boot bridges the electronics between the hearing aid and any wireless receiver that picks up audio signals, presenting them directly to the amplifier. A teleloop, on the other hand, is a wire loop that hangs around the neck and sends the sound signal to the t-coils (magnetic coils) inside the hearing aids. These new technologies can produce either FM (preferred), infrared (not recommended), or Bluetooth signals to fit every application. For example, in some movie theaters, churches, synagogues, and auditoriums special transmitters have been installed to broadcast the sound signals from film or on-stage microphones and in some movie theaters there are devices that provide closed captions right at the viewer’s seat. Special telephone and television connectors are also available for home use. In fact, any device that produces sound and has an earphone jack can be connected to amplification. Accessorizing your child’s hearing experience with devices that connect them to all the household and educational devices can make a striking difference in how children view communication and the world around them.
The development of the hearing system is a complex process requiring physical growth and years of training from sounds in our environment. The process of developing speech and language and other life skills is primarily dependent upon the hearing and vision experience, especially during the first few critical years of life. Hearing loss should be identified within six months after birth and immediate treatment should be initiated. All children with profound hearing loss should be fitted with cochlear implants because normalized interaction and communication over a lifetime is available and untreated deaf children under perform in society.
When amplification or a cochlear implant is needed, a medical evaluation followed by a visit to the Audiologist will help to determine the most appropriate care. The Federal Government mandates education at the public school by the age of three, but many programs also have infant classes for children with disabilities. If a child is identified with a hearing loss, inquire regarding the services offered in early education at your community school and fill out the necessary paperwork. The general wisdom is that the sooner the treatment and education start, the better the opportunity for eventually mainstreaming children into the classroom and daily life.
Bellis, TJ. (2003). Assessment and Management of Central Auditory Processing Disorders, 2nd Edition, Canada, Delmar Learning. (Statistical Information)
Ackerman, S., (1992). Discovery of the Brain, in Bellis TJ, Assessment of Management of Central Auditory Disorders, Washington D.C., National Academy Press.
Muller, G. III, & Hall, J. III (1998). Audiologist’s Desk Reference. San Diego, Singular Publishing Company.