- Hearing loss is one of the most common health handicapping conditions.
- Hearing loss is generally identified as conductive, sensorineural or mixed.
- There are genetic and acquired causes.
- Children should undergo hearing screening tests before the age of at least nine months.
- Treatment includes hearing aids or cochlear implants.
How do we hear?
Hearing loss is one of the most common health handicapping conditions among children and adults. The hearing sense is the cornerstone upon which our unique human communication system is built.
This handicap therefore makes it very difficult for people to learn to speak a language, which impairs their ability to communicate naturally with other people. It also has a detrimental effect on personality and emotional development.
It is easy to take hearing for granted as sound is such a common part of everyday life. To be able to understand how sound is perceived by people and the processes involved in hearing and hearing losses, we should have a closer look at the anatomy of the ear.
Anatomy of the ear
The ear can primarily be divided into three parts, namely:
- the outer ear
- the middle ear
- the inner ear
The outer ear
The outer ear consists of two parts, namely the external ear or ear lobe (pinna) and the ear canal. It is the part of the ear that one can see.
The ear lobe is flexible and consists of cartilage and skin. It functions as a sound receiver.
Sounds are sent down the ear canal. There is a soft membrane or skin between the outer ear and the middle ear, called the eardrum or tympanic membrane.
The middle ear
Sound waves travelling down the ear canal cause the eardrum to vibrate. Just behind the eardrum are three very small bones – the smallest bones in the body.
The bones are called the hammer (malleus), the anvil (incus) and the stirrup (stapes). The hammer is attached to the eardrum and the stirrup to membrane between the middle and inner ear.
The vibrations of the eardrum are transferred to these tiny bones in the middle ear, which in turn cause fluid in the inner ear to move in wave patterns.
The function of the middle ear is called a transmitting function.
The inner ear
The cochlea is the most important component of the inner ear with regard to hearing sounds. Within the cochlea are located the auditory sensory elements known as hair cells. Each hair cell has a delicate series of hairs that stick up into the fluids of the cochlea.
As fluid waves travel through the cochlea, the delicate hairs on the sensory nerve endings bend, creating neural impulses in the eighth cranial nerve (acoustic nerve). These impulses are interpreted by the brain as sounds.
The fluids of the cochlea can also be sent into motion by vibrations carried through the skull. This is called bone conduction.
Perception of sound
The main aim of the working of the ear is the perception of sound. Let use an example: If Lisa says to George: “what is the time?” George will detect that someone is talking to him. He will discriminate speech sounds from other sounds, for example an aeroplane flying past or a car hooting. He will identify that someone is asking him a question about time and he will understand the question and react by answering: “It is three o’ clock”.
The ability to perceive what was heard (auditory perception) plays an important role in human communication.
There are many children whose hearing is within normal limits, but give the impression that they cannot hear. These children have an auditory perceptual problem. They have difficulty detecting, discriminating, identifying or understanding sounds. An inability to listen effectively can hamper communication, the development of language abilities as well as most areas of their school work.
Children with auditory perceptual (listening skills) difficulties should be helped by a speech therapist. Parents can also help their children improve their listening and language skills by means of a home programme such as the Listening and Language Home Programme.
Location of sound
A person’s brain can work out where a sound is coming from. One’s brain receives messages from both ears. The ear closest to the sound hears first. This helps your brain work out where the sound is coming from. A person who cannot hear in one ear can therefore not locate the source of the sound.
Types of hearing loss
Hearing loss is generally identified as conductive, sensorineural or mixed.
Conductive hearing loss
Conductive hearing loss occurs when there is interference of any kind in the transmission of sound from the outer ear to the inner ear. In other words, the inner ear is capable of normal functioning, but the sound vibration is not able to reach the cochlea through the outer and middle ear pathway. Fortunately most conductive hearing losses can be corrected through medical treatment or surgery.
Sensorineural hearing loss
Sensorineural hearing loss occurs when there has been damage to the hair cells of the cochlea (thus, the inner ear) or the auditory nerve fibres.
Sensorineural hearing loss may easily be overlooked during physical examination, since both the external ear canal (auditory canal) and ear drum (tympanic membrane) appear normal. Unfortunately this type of hearing loss is generally irreversible.
Mixed hearing loss
Mixed hearing loss occurs when a combination of both types of hearing loss is present.
The symptoms of hearing deficiency are so varied that no one symptom would be interpreted as indicating the possibility of a loss, but if a child exhibited a number of such symptoms he should be tested. Some of the symptoms are:
- frequent earaches and complaints of ear difficulties
- frequent colds with ear discharge
- lack of attention to casual conversation
- late to start talking
- verbal directions ignored consistently
- tendency to be withdrawn and lack of desire to be involved in social activities
- difficulty in articulating certain speech sounds
- frequent confusion as to what has been said
- constant visual scanning of the speakers’ face
- consistently turning the head to one side when paying attention to for example a teacher
- reading disability
- spelling errors
- difficulty in taking dictation
- indifference to music
- speaking very softly or too loudly
Remember, waiting for these signs is waiting until it is much too late.
The causes of hearing impairment can be divided into two main groups, namely genetic (inherited) causes and acquired causes.
Most hearing impairment is inherited. Of all types of inherited deafness only one third is present at birth. Another third starts during childhood and the other third only manifests in adulthood.
There are many ways in which people acquire hearing loss.
Causes before birth
During the first 28 days of pregnancy it is of utmost importance that the mother does not get an infection, especially German measles and cytomegalovirus.
If the mother gets German measles during the first eight weeks of pregnancy there is an 86% chance that her baby will be hearing impaired.
German measles appears to be dangerous throughout pregnancy. Any contact with a person with this illness is therefore potentially dangerous.
Other factors which can play a role in hearing impairment include Rhesus incompatibility as well as metabolic illnesses such as diabetes and thyroid problems.
Causes during and just after birth
The most common causes of hearing impairment at this stage are birth injuries, jaundice and a lack of oxygen.
Causes after birth
These causes include:
- viral and bacterial infections such as meningitis
- drug toxicity
- excessive noise exposure
- middle ear infection
- head trauma
Ear infections are painful and should be treated in time. If not, infection can cause lasting damage. Many conductive hearing losses can be eliminated or substantially improved by medical treatment.
Outer ear infection (otitis externa)
Infection can be caused by an object in the external ear canal, for example excessive ear wax. Foreign objects include peas, a piece of prestik and small insects.
Adults should not attempt to remove the object unless it can be done easily. Children should be taught that the only thing they are allowed to put into their ears is their elbow.
Rough cleaning can scratch the delicate skin in the ear canal and it may become infected.
Chlorine in swimming pools (and bacteria in dirty swimming pools) can irritate the ear canal.
If a child has earache or a discharge from his ear, he could have an outer ear infection. Ask the child to open his mouth wide, then gently pull the ear lobe back. If this is very painful, there could be an ear infection.
Middle ear infection (otitis media)
This is the most common source of conductive hearing loss. It is an inflammation and/or infection of the middle ear. Infections can be caused by a build-up of fluid in the middle ear.
Otitis media is common in children under five years, particularly those under two years. This might be because the eustachian tube connecting the throat with the ear is short. This allows bacteria and viruses to move quickly from the nose and throat to the middle ear.
Enlarged adenoids block the entrance to the eustachian tube, preventing mucus from draining into the throat. This could also cause infection.
Children prone to hayfever and those with a cleft palate are particularly at risk.
A hearing loss is an invisible problem which can go unnoticed for years. It is therefore imperative that children should undergo hearing screening tests before the age of at least nine months.
The sooner a hearing deficiency is diagnosed, the better. Nowadays even a newborn baby’s hearing can be tested directly after birth. In the United States with the exception of four states it is now compulsory to screen a newborn baby’s hearing. The hearing screening test results appear on the birth certificate.
Unfortunately the average age at which children with a hearing loss are identified is two years and four months. In some cases it can be as late as six years. This is too late and it weakens the prospects for adequate communication in the hearing world. It is because the first three years of a child’s life are the most important time for a child to acquire speech and language and thus learn to communicate with others.
How can one identify hearing loss?
Although children may exhibit symptoms of hearing loss, the only way that it can be determined for sure is through adequate testing. No whisper tests or home-made tests can do this. Testing must be performed by a qualified person such as an audiologist, using a properly functioning audiometer.
Hearing tests are performed to determine whether a person has a hearing loss, how severe the loss is, in which part of the ear the damage is, and what aid he can receive.
A person’s hearing loss is graphed on an audiogram.
A hearing aid is a small system which amplifies sound. It consists of a microphone, an amplifier and a loud speaker.
Advances in the development of hearing aids over the last few years have brought a new dimension to the aid available to help the hearing impaired person to hear and more importantly to decipher speech despite background noise.
Hearing aids differ in size – from the older behind-the-ear type to a very small, almost unnoticeable hearing aid which fits in the ear canal.
A key to success in helping hearing impaired people adjust to the hearing world (socially as well as professionally) is the early examination of hearing loss, accurate diagnosis and the fitting of high technology hearing aids which meet the unique needs of the hearing impaired person.
Unfortunately some people wait as long as ten years before they address the problem. If a hearing loss is left untreated for a long time, the person’s ability to hear speech will gradually decrease up to the point where she begins to isolate herself by avoiding situations where communication is necessary – especially in a crowd. This could lead to depression and stress-related illnesses.
Not all deaf children benefit from conventional hearing aids. Fortunately there is a solution for some profoundly deaf children or those with total sensori-neural hearing loss who are not able to gain sufficient auditory information to acquire verbal communication – a multi-channel cochlear implant.
Many people are deaf because their inner ear is damaged. Most of them still have some remaining nerve fibres in the ear. These nerve fibres may be capable of sending impulses (sound) to the brain if they are stimulated electronically.
The cochlear implant can do just that. During cochlear implantation an electrical apparatus is surgically implanted into the bone behind the ear. It consists of a microphone (which receives sounds), a speech processor (which selects usable sounds) and a coil (which decodes and sends electric impulses to the electrodes). These components are worn externally.
The device which is implanted surgically into the bone behind the ear is a receiver which picks up the signals transmitted by the coil and in turn relays them to an electrode array which is introduced into the cochlea itself.
The electrical impulses are transmitted via 22 implanted electrodes in the cochlear to the audiotory nerve. The transmitting coil and the internal antenna of the receiver each have a magnet and these hold the coil in place over the antenna across the intact intervening skin. The impulses are then interpreted by the brain as sound and a sensation of hearing is experienced.
This system allows the deaf person who has received a cochlear implant to be able to identify normal, daily environmental sounds for example the doorbell, a dog barking, a knock on the door, car engines and hooters, a telephone ringing and background music.
Most children can recognise the rhythm and intonation of speech patterns and can identify some words. Their auditory skills also improve. It can also help the child to recognise speech even without lip-reading in some cases. This is not always the case, as individual children differ widely in the benefit that they receive from the cochlear implant, but the prognosis for normal speech and language development increases dramatically the younger the child is at implantation, which makes early diagnosis of an impairment so critical.
The cochlear implant has its limits. The apparatus does not normalise hearing and the sound that is generated by the implant differs from normal hearing, although this difference is decreasing rapidly as technology is advancing making implants a realistic option for an ever-increasing number of patients.
It is essential that a person who has received a cochlear implant receive intensive hearing and communication training from professionals who are experienced in working with the deaf. The child needs help in order to interpret the sounds he or she hears and, if possible, to use the sound to develop spoken language.
The Carel du Toit Centre at Tygerberg Hospital offers this service. For more information regarding cochlear implants, contact them at (021) 938 5312 or the Cochlear Implant Unit at (021) 938 9494/938 5080.
There are strict selection criteria for candidates of cochlear implants. The selection criteria are:
- Profound or total sensori-neural hearing loss in both ears
- Little or no benefit from hearing aids
- The family must be motivated and supportive and show realistic expectations
- For adults the criteria is also that hearing has been lost after acquisition of a spoken language
At the Cochlear Implant Programme at Tygerberg Hospital, a multi-disciplinary team is involved in the selection process. The team includes an ear, nose and throat specialist, audiologist, speech therapist, teacher, guidance therapist, occupational therapist, clinical psychologist and a social worker.
Since 1986 the Cochlear Implant Unit has made it possible for profoundly deaf people to experience the wonder of sound.
In September 1988 the first two deaf children received the Nucleus 22 Cochlear Implant at Tygerberg Hospital. In April 1997 a child received the latest technology - Nucleus 24 Cochlear Implant.
To date more than 35 000 children and adults worldwide have received a cochlear implant – a new lease on life for most of them.
Written by Liesel van Niekerk, speech and hearing teacher and author of Listening and Language Home Programme
Reviewed by Dr D. Wagenfeld, M.B.Ch.B, M.Med, F.C.S.