Your Genes and Hearing Loss
One of the most common birth defects is hearing loss or deafness (congenital), which can affect as many as three of every 1,000 babies born. Inherited genetic defects play an important role in congenital hearing loss, contributing to about 60 percent of deafness occurring in infants. Although exact data is not available, it is likely that genetics plays an important role in hearing loss in the elderly. Inherited genetic defects are just one factor that can lead to hearing loss and deafness, both of which may occur at any stage of a person’s lifespan. Other factors may include: medical problems, environmental exposure, trauma, and medications.
The most common and useful distinction in hearing impairment is syndromic versus non-syndromic.
Non-syndromic hearing impairment accounts for the vast majority of inherited hearing loss, approximately 70 percent. Autosomal- recessive inheritance is responsible for about 80 percent of cases of non-syndromic hearing impairment, while autosomal-dominant genes cause 20 percent, less than two percent of cases are caused by X-linked and mitochondrial genetic malfunctions.
Syndromic (sin-DRO-mik) means that the hearing impairment is associated with other clinical abnormalities. Among hereditary hearing impairments, 15 to 30 percent are syndromic. Over 400 syndromes are known to include hearing impairment and can be classified as: syndromes due to cyotgenetic or chromosomal anomalies, syndromes transmitted in classical monogenic or Mendelian inheritance, or syndromes due to multi-factorial influences, and finally, syndromes due to a combination of genetic and environmental factors.
Variable expression of different aspects of syndromes is common. Some aspects may be expressed in a range from mild to severe or different combinations of associated symptoms may be expressed in different individuals carrying the same mutation within a single pedigree. An example of variable expressivity is seen in families transmitting autosomal dominant Waardenburg syndrome. Within the same family, some affected members may have dystopia canthorum (an unusually wide nasal bridge due to sideways displacement of the inner angles of the eyes), white forelock, heterochromia irides (two different-colored irises or two colors in the same iris), and hearing loss, while others with the same mutation may only have dystopia canthorum.
How Do Genes Work?
Genes are a road map for the synthesis of proteins, which are the building blocks for everything in the body: hair, eyes, ears, heart, lung, etc. Every child inherits half of its genes from one parent and half from the other parent. If the inherited genes are defective, a health disorder such as hearing loss or deafness can result. Hearing disorders are inherited in one of four ways:
Autosomal Dominant Inheritance: For autosomal dominant disorders, the transmission of a rare allele of a gene by a single heterozygous parent is sufficient to generate an affected child. A heterozygous parent has two types of the same gene (in this case, one mutated and the other normal) and can produce two types of gametes (reproductive cells). One gamete will carry the mutant form of the gene of interest, and the other the normal form. Each of these gametes then has an equal chance of being used to form the offspring. Thus the chance that the offspring of a parent with an autosomal dominant gene will develop the disorder is 50 percent. Autosomal dominant traits usually affect males and females equally.
Autosomal Recessive Inheritance: An autosomal recessive trait is characterized by having parents who are heterozygous carriers for mutant forms of the gene in question but are not affected by the disorder. The problem gene that would cause the disorder is suppressed by the normal gene. These heterozygous parents (A/a) can each generate two types of gametes, one carrying the mutant copy of the gene (a) and the other having a normal copy of the gene (A). There are four possible combinations from each of the parents, A/a, A/A, a/A, and a/a. Only the offspring that inherits both mutant copies (a/a) will exhibit the trait. Overall, offspring of these two parents will face a 25 percent chance of inheriting the disorder.
X-linked Inheritance: A male offspring has an X chromosome and a Y chromosome, while a female has two copies of the X chromosome only. Each female inherits an X chromosome from her mother and her father. On the other hand, each male inherits an X chromosome from his mother and a Y chromosome from his father. In general, only one of the two X chromosomes carried by a female is active in any one cell while the other is rendered inactive. This is why when a female inherits a defective gene on one X chromosome, the normal gene on the other X chromosome can usually compensate. As males only have one copy of the X chromosome, any defective gene is more likely to manifest into a disorder.
Mitochondrial Inheritance: Mitochondrias, small powerhouses within each cell, also contain their own DNA. Interestingly, the sperm does not have any mitochondria, and consequently, only the mitochondria in the egg from the mother can be passed from one generation to the next. This leads to an interesting inheritance pattern where only affected mothers (and not affected fathers as their sperms do not have mitochondria) can pass on a disease from one generation to the next. Sensitivity to aminoglycoside antibiotics can be inherited through a defect in mitochondrial DNA and is the most common cause of deafness in China!
In the last decade, advances in molecular biology and genetics have contributed substantially to the understanding of development, function, and pathology of the inner ear. Researchers have identified several of the various genes responsible for hereditary deafness or hearing loss, most notably the GJB2 gene mutation. As one of the most common genetic causes of hearing loss, GJB2-related hearing loss is considered a recessive genetic disorder because the mutations only cause deafness in individuals who inherit two copies of the mutated gene, one from each parent. A person with one mutated copy and one normal copy is a carrier but is not deaf. Screening tests for the GJB2 gene are available for at risk individuals to help them determine their risk of having a child with hearing problems.
Hearing Loss Treatment
As the parent of a child with newly diagnosed hearing loss, you will have many questions and concerns regarding the nature of this problem, its effects on your child’s future, treatment options, and resources. This brief guide will give you necessary initial information, and provide guidance about the availability of resources, and the respective roles of different care providers.
It is always difficult for parents to receive bad news about any aspect of their child’s health. Reacting with anger, grief, and even guilt are not unusual when finding out that your child is hearing-impaired. These feelings are best managed by discussing them with a family member, close friend, clergy, or mental health professional. At times, the feeling may also result in a degree of denial. Feel free to seek a second opinion, but it is unadvisable to delay further recommended diagnostic evaluations for your child. The best treatment for hearing loss of any degree is appropriate early intervention. Significant delays may result in irreversible harm to your child’s hearing, speech, language, and eventual educational development.
You will come into contact with many healthcare and rehabilitation specialists during the long-term management of your child’s hearing loss. Some of them will be involved early in the journey and again at intervals. Others may step in later on. The following are professionals you will encounter and the role each of them will play in managing your child’s hearing loss.
The audiologist is likely to be the first professional you encounter, and possibly the one who gives you the initial news regarding your child’s hearing loss. The audiologist will carry out behavioral or objective testing (such as auditory brainstem responses) or a combination of these approaches to determine the degree and type of hearing loss. The audiologist will also eventually recommend appropriate amplification, following a medical consultation. The audiologist will also provide your child with well-fitting ear molds along with the hearing aids, as he or she grows. The audiologist may also be the professional who provides you with information and referral to an early intervention program. Over time, the audiologist will provide periodic follow-ups to chart your child’s progress and to monitor his or her hearing loss.
Otologist, Otolaryngologist, or Pediatric Otolaryngologist (ENT Physician)
Upon diagnosis of hearing loss, your child will be referred to an ear, nose, and throat specialist, (otolaryngologist), or one who specializes in childhood ear and hearing problems. This physician’s initial role is to determine the specific nature of the underlying problem that may be at least partially causing the hearing loss. Additionally, the physician will also determine if the problem is medically or surgically treatable, and if so, provide the necessary medical or surgical treatment. Such treatments could include something relatively simple, like the placement of eardrum ventilation tubes, or more complex surgical procedures. The ENT specialist may also refer your child for additional diagnostic procedures such as imaging studies (X-rays, CT-scans, MRI scans) to further define the type and source of hearing loss. The doctor will also provide clearance for hearing aid fitting, after determining if no other intervention is indicated. If it is determined that your child needs a cochlear implant, the otolaryngologist, along with the audiologist, will carry out further tests and examinations, and will carry out the implant surgery.
Early Intervention Specialist
This professional is typically is someone with an education background. He or she can help you find resources in your community, define family members’ roles in early intervention and management of the hearing loss, and can help you deal with questions regarding future educational placement. This specialist will also help you deal with your observations and concerns about your child and give you information and support regarding your child’s educational needs in the future.
Speech/ Language Pathologist (SLP)
This professional will evaluate the impact of your child’s hearing loss on speech/language development, and monitor his/her progress, noting if progress with that development is falling behind. If this happens, the SLP may refer back to the audiologist or otolaryngologist to determine if any changes have occurred in your child’s hearing. The SLP will also help your child to learn proper speech production, including correct articulation of speech sounds. If you choose oral communication for your child, in addition to the speech language pathologist your child may also be treated by an auditory-verbal therapist, who can help your child acquire the full range of speech sounds and guide the family to additional medical or audiological treatments. The auditory-verbal therapist will also help the child’s family become familiar with appropriate speech/language, auditory, and cognitive developmental milestones you may expect for a child with hearing loss.
Finally, many other people can provide additional assistance for your hard-of-hearing child. Parents of older hard-of-hearing children, and hard-of-hearing adults, can share their experiences with you and may have suggestions for educational and recreational resources in the community.
For more information on Hearing Loss, call Parkway ENT and Allergy, PA in Katy, TX at (281) 712-7241!