Childhood Deafness

Recessively Inherited Deafness

The most common form of genetic deafness is called "autosomal recessive". This accounts for approximately 80% of inherited deafness. Parents may wonder how their child can have a genetic form of inheritance if they can both hear. The reason is that each parent has two genes for every component of the hearing mechanism. If both parents have a single copy of a recessive gene for deafness, and one normal gene, they will be carriers of deafness. If a child inherits one deafness gene from each parent, i.e. a "double dose", they will be deaf. With recessive inheritance the chance of each subsequent child being deaf is one in four.

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Up to several percent of the normally hearing adult population in any country may be carriers of a recessive gene for deafness.

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The most common type of recessively inherited condition involves a mutation in the Gap Junction B2 gene (GJB2) which codes for the creation of a protein named "connexin 26". There are different types of connexins, which bind together in the cell wall, to form a "connexon". The connexons are conduits for communication from one cell to another. Genetic code mutations may result in an abnormally formed/shaped connexin, or depending on the mutation, may stop formation of the connexin protein at all. While it is possible, it is not common for these mutations to affect other organ systems, other than the ear.

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The GJB2 gene, speaking in evolutionary terms, seem to have been quite prone to mutations. Different populations in the world had their own specific mutations in the genetic code. The mutations causing the most severe forms of deafness were identified first- including, in Asian populations the "235DelC" mutation, and in caucasians, the "35 del G" mutation which both resulted in " instruction" to stop/prevent the formation of connexin 26 protein. 

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Dominantly Inherited Deafness

This accounts for possibly 15-20% of "genetic deafness". People with a dominant form of genetic deafness usually have a deaf parent. The chance that subsequent children will be deaf may be as high as 50%. Some dominant genes are not as "strong" and may not necessarily result in deafness or the deafness may skip a generation.

 

Sex Linked Inherited Deafness

In this condition, there usually is a clear pattern of deafness in males of the family, but minimal deafness or no deafness in the females. The condition is transmitted by females to their male children, but is not transmitted by males.

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Mitochondrial Inherited Deafness

This is a rare form of deafness transmitted by the mitochondria, which are tiny "energy organs" inside cells. This is passed to children from the mother and is thought to contribute only 1% or less to the population of deaf children. 

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Syndromic Deafness

Deafness may occur by itself or it may occur as part of a syndrome in which the deafness is associated with distinctive facial features, heart, eye, skin or other organ involvement or other medical conditions. Syndromic deafness may be inherited from the parents in a manner similar to recessive or dominant non- syndromic deafness, or may occur spontaneously or secondary to infection.

Some of the most common " syndromes" that are associated with hearing loss are

• Hemi facial microsomia/Goldenhaar Syndrome

• Waardenburg's syndome

• Congenital cytomegalo virus

• Usher's syndrome

• Pendred's syndrome

• CHARGE Association

• Neurofibromatosis type II

• Mitochondrial disorders

• Stickler's syndrome

 

Auditory Neuropathy (Auditory Dys synchrony Spectrum Disorder or ANSD or auditory synaptopathy):

This is a group of conditions which results in poor synchronisation or transmission of auditory nerve impulses from the inner ear, to, or along the audtiory nerve. Oto-acoustic emissions may still be present early on ( and are often lost later) , but the ABR test is abnormal or unreadable. An MRI scan is always necessary to asssess auditory dys synchrony spectrum disorder, because sometimes it can be mistaken for  small or absent auditory nerves- and this is important to guide treatment options.

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I have put this condition in its own section, because this is a group of conditions, which produce the same symptoms and test results.  It may arise from genetic recessive or dominant reasons, and can also arise as a result of neonatal jaundice. One of the best understood genes, within which there a variety of mutations causing ANSD is the Otoferlin gene. Mutations within this gene result in a loss of release of the chemical neurotransmitter from the auditory hair cells to the auditory nerve.

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Deafness of varying severity results. Most often, the ability to understand speech is worse than expected, given a certain degree of deafness and the hearing may appear to fluctuate from day to day. Speech development is significantly impaired. Sometimes hearing aids may not be as effective as expected when used to help treat this condition, but most often cochlear implants can help restore clarity to speech, as they stimulate the auditory nerve directly. Currently, there are trials of treatment of Otoferlin gene transfer, to see whether this is a reliable way of correcting this type of deafness. Otoferlin Gene therapy successful

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Other Causes of Deafness

• Prematurity

• Difficult birth

• Very low birth weight

• Jaundice after birth.

• Infections -some of the most common occurring infections that may spread to baby in the womb include cytomegalovirus (CMV), rubella, toxoplasmosis, herpes type viruses and syphilis. CMV and Meningitis are the most common infections to cause deafness in early childhood. Both may also result in a progressive loss of hearing over several or many years. In New Zealand, so far I have yet to see a case of toxoplasmosis or syphilis related deafness ( although currently syphilis is increasing in the general community)

• Antibiotics- the most common form of deafness in this situation is due to Gentamicin or other potent antibiotics used usually in an intravenous form because of  infections during pregnancy, or very early life.

• Cancer chemotherapy- A side effect of cisplatin, is to cause a high frequency cochlear/inner ear hearing loss, which at times may be of sufficient severity to justify the need for cochlear inmplantation.

 

What can be done about my child's deafness?

Confirmation Of The Severity Of The Deafness

Deafness is confirmed using a variety of audiologic tests. Your audiologist will also discuss these with you. If your child is new born or in the first few months of life, testing is performed by measuring the electrical impulses in the hearing nerves (auditory brainstem response testing, ABR.) Small earphones are placed in or on your child's ears and clicks or short sounds are passed into the ears. Electrical impulses are measured from small electrodes/gelpads placed on the skin. This is done whilst your child is asleep and sometimes may need to be performed under sedation or a general anaesthetic. Older children who are able to turn their heads to sound or to reliably respond to sound can be tested in a sound- proof booth using room speakers or headphones. For the technically minded, for an extremely technical and detailed review of ABR testing, use this link ABR Testing technique and theory.

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Oto-acoustic emissions are tiny sounds, which are actually produced by hair cells in the inner ear. These can be measured. Absent emissions may indicate inner ear hair cell damage. Otoacoustic emission testing is usually performed at the same time as ABR or sound-proof booth testing Simple explanation of Otoacoustic emissions

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Although hearing tests are very accurate in predicting the level of hearing, it is possible that the ABR test could over- estimate the severity of hearing loss. In very rare instances, possibly because of prematurity of the auditory nerve, the absence of an ABR response does not necessarily always imply total deafness. The progress gained with a hearing aid trial is important confirmation of the severity of deafness demonstrated by other audiologic tests. 

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Other Investigations may Include

Blood Testing: This may be necessary to test blood chemicals (e.g. Thyroid hormones) or to test for abnormal genes. In New Zealand, all infants have a variety of blood tests performed at birth (the "Guthrie test")-this includes thyroid function testing.The Guthrie Card test is used mainly to detect inborn errors of metabolism, which are very rare. It can also be used retrospectively to look for CMV ( Cytomegalovirus), using PCR testing ( the same as has been used for "Covid" testing), with about 50% sensitivity and about 100% specificity ( that is to say, if a child actually has CMV, there is about a 50% chance of detecting it on the Guthrie card, and if the Gurthrie card shows CMV is present, it is almost 100% accurate).

 

Genetic Testing: Firstly, this is not always an essential test. It can however be useful to provide an explantation to the parents  ( and some degree of "closure") as to why deafness has occcurred. It can be useful to help predict the likely progression of a hearing loss, and into the near future, it may help indicate which children might be suitable for gene transfer treatment.

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At this stage, we know the most common cause of non-syndromic recessive inherited deafness is due to an abnormality in the connexin 26 gene (about 20% of all children in New Zealand and Australia with non- syndromic recessive deafness have an abnormality in this gene). This gene (when functioning)  produces a protein which enables inner ear cells to communicate with each other.Testing for abnormalities of this gene are available in New Zealand and in many other countries. The most frequent abnormalities which occur in this gene vary, country by country and sometimes region by region.

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A "positive" test result may be very helpful to clarify the cause of deafness, but a negative result does not mean that the deafness is not genetic-it may simply be that another abnormal gene may be involved. Defining a genetic anomaly may not only give a person or family "closure" but in some instances it may help identify those in whom the hearing would be expected to remain stable, and those in whom the hearing may eventually deteriorate.

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We anticipate many more tests will become available in the next few years. To have the genetic test done, a blood test is performed on your child. The DNA may also be stored and re assessed in the future, without the need for another blood test. My current practice is to offer an approximately 240 gene test , through Auckland Healthcare/ Te Whatu Ora- Te Toka Tumai at Invitae Laboratory in San Francisco, and less commonly a more expensive "Otoscope" panel at University of Iowa. These usually take 4-12 weeks to come back with a result- however very often, these results are inconclusive!

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MRI Scan: This gives useful information about the anatomy of the inner ear, the brain and auditory nerves. It can be done without a general anaesthetic if your child is under 3 1/2 months of age (which is called a 'feed N wrap' technique) but if your child is older, until the age of 7 or so, they will require a general anaesthetic for the scan-- simply to keep them still enough for the 40 minutes required to do the scan. No x rays are involved with this type of scan.

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CT Scan:Generally I don't recommend a CT scan unless the hearing loss is a mechanical " conductive" loss. An MRI is preferred.

Opthalmology Doctor (Eye Specialist) Referral: This is advisable in all newly diagnosed hearing impaired children to ensure there is no eye abnormality. Up to 60% of all children with hearing impairment will have visual difficulties or eye abnormalities.

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ERG testing: (Electro retinography) This is another electrical type test similar to the ABR. A flash of light is made in front of your child's eyes, and the brain waves which results from this are measured. This is useful in particular to exclude Retinitis Pigmentosa which can occur with Usher's Syndrome. This test is performed at the recommendation of the eye specialist, but is not used nearly as often as we used to do in the past. Genetic testing will also compliment any assessment of the eyes, as it may identify the possible risk of deaf blindness in later years.

Hearing Testing For Other Family Members: This may be very useful and important to develop a better picture about the pattern of inheritance in the family- and may clarify the likelihood of deafness in further children, and the likelihood of deafness occurring later in your child's children.

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Genetic Evaluation and Counselling

Genetic Counselling

An individual or family may wish to see a geneticist for a variety of reasons. In the case of hearing loss, common questions are how genetic factors could cause hearing loss, whether there are any associated medical complications, whether the hearing loss might progress, and what the chances are that another family member might be born with or develop hearing loss.

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Evaluation

The genetic evaluation is usually done by a team made up of a geneticist, (a physician specially trained to recognize signs of genetic conditions) and a genetic counsellor. The medical history (including records from the Audiologist and Otolaryngologist) and family history are reviewed and a physical examination is performed. The medical history may identify a medical cause of hearing loss such as a serious illness, an injury, or an infection the mother had in pregnancy, or it may point out an important symptom of a genetic condition, such as kidney infections or difficulty seeing in the dark. The physical examination may reveal subtle characteristics that can be related to specific genetic conditions. These characteristics by themselves may not be abnormalities, and they may appear to be unrelated to hearing loss. For example, small pits in front of the ears could indicate "branchio-oto-renal syndrome", which also involves the kidneys; a white forelock could be part of "Waardenburg's syndrome"; night blindness or tunnel vision could indicate "Usher syndrome", or severe near sightedness could be a sign of "Stickler syndrome".

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Counselling

Even when the cause of the hearing loss remains unknown, the family can be given estimates of the chance it will recur in another relative. These estimated risks are based on studies of many families with similar family histories. For example, if the family is a hearing couple who has had one child who is severely or profoundly deaf, the chance that the next child would be deaf is about 25%.

Particularly in the case of hearing loss, people may have quite different attitudes about deafness in their family. For example, some hearing parents might be concerned about having another child who is deaf, while others may feel that the hearing loss would not pose a problem, but would want to know if any other medical problems might be involved. Similarly, deaf parents may feel comfortable about their own abilities, but would prefer not to have a deaf child, whereas other deaf parents may be more concerned about the challenges of raising a hearing child.

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For more detailed contact information see NZ Genetic services contacts.

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Hearing Aids

Once deafness has been confirmed it is recommended that hearing aids are fitted by the age of 6 months. The timing of intervention is critical to achieving language development in line with your child's peers. It can take time to have the hearing aids adjusted perfectly. Your child needs strong and persistent encouragement  and support to use the hearing aids as much as possible. Your child's ability to benefit from the hearing aids will be monitored by your audiologist.

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Cochlear Implants

A cochlear implant is an electronic device that is used to transmit sound directly into the inner ear and hearing nerves (Cochlear implant information)  . It has an internal part that is buried under the skin of the scalp and is practically invisible. There is an outer portion rather like a hearing aid which transmits sound through to the inner device. The cochlear implant is of benefit for and recommended for all children with severe /profound hearing loss who are unable to get enough amplification from the most powerful hearing aids. These are provided free of charge for eligible NZ residents.

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Cochlear implant assessment is performed by a multidisciplinary team which includes surgeons, audiologists and habilitationists, based at Hearing House in Auckland Hearing House - Northern Cochlear Implant Programme, St Georges Hospital in Christchurch and JAG Legal Building in Wellington Southern Cochlear Implant Programme NZ

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Maximising language development

It is critical to continue to talk with your baby and youjng child in a normal way. In fact, the more you talk (or/and sign) with your child, the better their language development will be. Read as many books to your child as you can, with them on your knee, if possible. 

Despite the diagnosis of "deafness", often there is some residual hearing, and it is possible to amplify sound with hearing aids sufficiently for your child to be able to hear sufficiently for normal speech and language development to occur. If there is insufficient amplification, (most commonly for high pitch sounds which are required for clarity) then cochlear implants are recommended.

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For those who chose sign communication for their child, early intervention with a specialised therapist is vital Deaf Aotearoa.

 NZ Advisors on Deaf and Hearing Impaired Children- Ministry of Education The advisors are wonderful resource people, funded through the Ministry of Education to assist you with therapy and intervention options. In smaller centres, they may be also able to help with hearing therapy. Advisers on Deaf Children are available through Special Education Service Learning Support

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Other important people intimately involved with the development of speech and language are Speech-Language Therapists, Teachers of the Deaf and Hearing Re-Habilitation therapists.

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Teachers of the Deaf generally are under "Ko Taku Reo"  hearing provision schools Ko Taku Reo Schooling in New Zealand 

Speech-Language Therapists are based at most primary schools but the availability of speech and language therapy for deaf children is currently very limited in Auckland. There are some private speech and language therapists available in the community.

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Hearing House, 251 Campbell Road, Auckland (ph:579-2333) is a free service set up by the New Zealand Cochlear Implant Trust partly at the instigation of hearing parents with deaf children. It provides oral habilitation and excellent support for families and children with moderate to profound deafness. Children are taught to maximise their residual hearing in order to understand) language and to speak. A visit to Hearing House is strongly recommended for all parents of children with newly diagnosed deafness.

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Further Resources

Auckland Parents of Deaf Children. I highly recommend checking out and joining this supportive group of parents who have been through what you might be going through now. You don't actually have to live in Auckland!

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• NZ/ Aotearoa Parents of Deaf Children

• Deaf Children Australia

• Hearing House Auckland This is where the audiology and habilitation for the Northern Cochlear Implant programme is based. 251 Campbell Road, Auckland, ph: 09 579-2333.

• John Tracey Clinic- Los Angeles  This has international correspondence courses for pre-school hearing impaired children and their parents. 

• UK National Deaf Children's Society

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