Genetice Evaluation and
The Purpose of a Genetic Evaluation
Counseling for Hearing Loss
(Source: Boystown Research Registry)
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. In order to give the most accurate answers to these questions, a genetic evaluation is done to try to determine the cause of the hearing loss.
Hearing loss may be caused by things that are not genetic, such as infection, but in about 50% of cases of severe to profound deafness, the cause is genetic. There are many different types of genetic conditions. They may be "syndromic", involving other medical or physical findings in addition to the hearing loss, or "non-syndromic", meaning that there are no other significant features besides the hearing loss. There are several ways in which hearing loss can be inherited: autosomal dominant, autosomal recessive, X-linked (dominant or recessive), multifactorial, or chromosomal. Genetic hearing loss may be present at birth ("congenital") or it may start later in life. It may be stable or progressive; it may be conductive, sensorineural, or mixed.
What Happens in a Genetic 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 counselor who either has a Masters degree or Ph.D. in genetics. The medical history (including records from the audiologist and Ear-Nose-and-Throat doctor) and family history are reviewed and a specialized 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. A careful physical examination is done to look for 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 kidney problems; a white forelock could be part of Waardenburg syndrome; nightblindness or tunnel vision could indicate Usher syndrome, or severe nearsightedness could be a sign of Stickler syndrome.
The counselor goes over the family history and draws a "family tree", usually covering about three generations. For example, in the case of a child with hearing loss, the family history information would cover the child´s brothers and sisters, parents and grandparents, and aunts, uncles, and cousins. Any relative with hearing loss is noted, along with any other physical or learning problems they may have had. In addition, there are a number of specific conditions that should be asked about in any relative, whether or not they have hearing loss. This is because syndromes include one or more additional findings in addition to the hearing loss. A person with a syndrome does not necessarily have all of the findings. Thus, the counselor will ask about such things as kidney problems, ear pits, white forelocks, nightblindness, nearsightedness, and so on.
Typically, both parents and any brothers and sisters are asked to have audiological testing, and medical reports or tests may be needed from other family members as well. In addition, the geneticist may recommend specific medical tests such as ultrasound of the kidneys, an X-ray, or a blood or urine test. An examination by an ophthalmologist is often recommended (if it has not been done already), and an ERG (electroretinogram) may be done to screen for Usher syndrome, which also involves progressive visual problems due to retinitis pigmentosa. An EKG (electrocardiogram) may be done to look for a specific abnormality in the heartbeat that is seen in the Jervell and Lange-Nielsen syndrome. This would be done especially if there is any history of fainting. A CAT scan, which is a type of X-ray, may be recommended in order to look at the structure of the middle ear, cochlea, and vestibular system. An ABR (auditory brainstem response; sometimes called a BER, or brainstem evoked response) can be done to detect a hearing loss in a baby or a person who cannot perform a regular behavioral audiological evaluation. The ABR can also see if a sensorineural hearing problem lies in the cochlea, the auditory nerve, or in the part of the brain that first receives the sound information. Vestibular testing can tell if the vestibular system in the inner ear which contributes to the sense of balance has been affected as well. In some genetic forms of hearing loss, the vestibular system may not have developed, and meningitis can also damage the vestibular system. In a very young child, tests for viral infections may show that the child had a congenital infection (that is, an infection before birth) such as CMV (cytomegalovirus) or rubella which could have caused the hearing loss. Blood and urine tests that look at kidney function are also important because of the genetic conditions that involve both the ears and kidneys. A blood test for thyroid function may be done to screen for Pendred syndrome, which includes goiter.
Chromosome tests may be done in some cases when a person has other physical or developmental problems. Chromosomes are the tiny pieces of DNA in each cell of the body that carry the genes. The chromosomes can be examined under the microscope to see if a chromosome or part of a chromosome is missing or duplicated. However, in order for an abnormality to be seen under the microscope, it must be fairly large and involve many genes. At this time, geneticists cannot see individual genes, such as the gene for Waardenburg syndrome, so this test cannot be used to diagnose a hearing loss caused by a single gene.
The Results of the Evaluation:
If it´s genetic:
When the whole evaluation process has been completed, the results are explained to the family. If the hearing loss is genetic, the way it is inherited will be discussed. If a syndrome is identified, the geneticist describes the medical or physical features, and if there are any, concerns for future medical problems and how they can be managed. In some cases, there are support groups for individuals or families with the syndrome.
If it´s still unknown:
Unfortunately, the cause of the hearing loss may still be unknown even after an extensive medical genetic evaluation. This is particularly true when there is only one person in the family with hearing loss, and neither they nor anyone else in the family have any of the signs of a syndrome. In this case, the hearing loss might have been caused by something which is not genetic, but it also could be genetic. The most common genetic cause would be autosomal recessive inheritance, in which the person with the hearing loss has two genes for hearing loss. Each of the parents would have only one gene for hearing loss, which would not affect their hearing. They would have inherited that gene from one of their parents, and so on back through generations; only when both parents have the gene, and both pass it on to a child, would the hearing loss show up. There would be a 25% chance with each pregnancy that they could have a child with hearing loss. A person with autosomal recessive hearing loss would have a very small chance of having a child with hearing loss, unless they married someone with exactly the same genes for hearing loss or with a dominant form of hearing loss.
It would also be possible that a person with hearing loss could have a dominant gene that is new to him or her and which arose through the process of mutation. No one else in the family, including the parents, would have the gene, but the person with the new hearing loss gene would have a 50% chance of passing it on to a child. Finally, if the person with the hearing loss is a male, it is possible that the hearing loss is X-linked. The mother could "carry" the gene, or it could have appeared for the first time in the boy with hearing loss.
What is the chance it will occur again?
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 profoundly deaf, the chance that the next child would be deaf is about 10% [F.R. Bieber & W.E. Nance. 1978. L.G. Jackson, R.N. Schimke, (Eds) Clinical Genetics: A source book for Physicians. NY; Wiley.] If both parents are deaf, and the cause of their deafness is unknown, the chance that their first child would be deaf would also be about 10%.
This brings up some very important points about genetic counseling. Genetics professionals in the United States support the idea of "non-directive" counseling, which means that genetic counseling is meant to be informative and supportive. It is not meant to tell people what to do or whether or not to have children. These principles have been adopted by the major professional societies for genetics professionals.
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.
Date Originally Created: Fall of 1990.
The information presented here first appeared in publications of the Boys Town National Research Register for Hereditary Hearing Loss, the National Institute on Deafness and Other Communication Disorders (NIDCD), Hereditary Hearing Impairment Resource Registry (HHIRR), or the Boys Town Research Registry for Hereditary Hearing Loss.
The Boys Town Research Registry for Hereditary Hearing Loss
The Boys Town Research Registry for Hereditary Hearing Loss (Registry) is designed to foster a partnership between families, clinicians and researchers in the area of hereditary hearing loss/deafness through three primary functions. First, the Registry disseminates information to professionals and families about clinical and research issues related to hereditary deafness/hearing loss. Second, the Registry collects information from individuals interested in supporting and participating in research projects. This information is used to support the third function of the Registry - matching families with collaborating research projects.
For more information, contact:
Research Registry for Hereditary Hearing Loss
555 N. 30th Street
Omaha, NE 68131
800 320-1171 (V/TDD)
402 498-6331 (FAX)