Genetic Hearing Disorders in Louisiana

Danielle Mercer¹, Fern Tsien², and Barbara Gordon-Wendt¹

¹Department of Communication Disorders, LSUHSC School of Allied Health Professions
²Department of Genetics, LSUHSC School of Medicine

Hearing Loss

There are 3 main types of hearing loss: conductive, sensorineural, and mixed. Conductive hearing loss occurs when there is a problem conducting sound through the outer ear or the middle ear (or both). Sensorineural hearing loss occurs when there is a problem in the inner ear or in transmission from the auditory nerve to the brain. Mixed hearing loss is a combination of both conductive and sensorineural hearing losses.

The most common cause of conductive hearing loss is fluid in the middle ear. This is also known as otitis media, and it may be due to an ear infection. The presence of fluid prevents sound from moving through the middle ear as effectively as it should. Holes in the eardrum (or tympanic membrane) and stiffening of the bones in the middle ear (malleus, incus, and stapes) are other causes of conductive hearing losses. Abnormalities of the outer ear can also cause a conductive hearing loss. An example would be an absent or narrowed ear canal, which would reduce the amount of sound making it to the eardrum. Missing or misshapen pinnae (the visible part of the outer ear) would also be considered outer ear abnormalities. Though absent or misshapen pinnae are not usually associated with a significant hearing loss, a conductive hearing loss could occur if the abnormality blocks the passage to the eardrum or promotes earwax accumulation.

Sensorineural hearing loss most commonly results from impairment in the cochlea. The cochlea is the sensory organ of hearing and is located in the inner ear. The cochlea receives sound from the middle ear and translates it to an electrical signal. The auditory nerve then transmits this electrical signal to the brain, where it is interpreted. Sensorineural hearing losses can result from problems in the cochlea or in the auditory nerve.

A mixed hearing loss contains components of both conductive and sensorineural hearing losses.

Degree of hearing loss

In addition to type, hearing loss can be classified based on degree of severity. In increasing levels of severity, hearing loss is classified as mild, moderate, moderately severe, severe, or profound.

Effects of Hearing Loss on Child Development

Good hearing is essential to normal language development. Though everyone will have difficulty understanding spoken language if they have hearing loss, children will be most affected by a hearing loss. Young children who are learning language will miss sounds, often causing them to misunderstand words and sentences. Adults will use their experience with language to help fill in the gaps when there are sounds they don't catch. Since children do not yet have a large knowledge base of language, their progress can be slowed down by a hearing loss. This is why hearing loss should be detected and treated as early as possible. A hearing loss present at birth should be addressed by no later than 6 months of age.

How severe does the hearing loss have to be to affect language development?

Surprisingly, language delays can occur with even a very mild hearing loss, and this effect has been confirmed by many research studies. In addition, children with untreated hearing loss tend to have a harder time in school. This demonstrates why it is so important to diagnose hearing loss in children. An audiologist is the sole professional trained to evaluate and diagnose any type of hearing loss. If you have concerns about your child's hearing, visit an audiologist or discuss your concerns with your child's pediatrician. Many schools have hearing screening programs. Ask your child's school if they perform annual hearing screenings.

For information on audiology services at LSU Health Sciences Center, visit the following link:

http://alliedhealth.lsuhsc.edu/clinics/audiologyclinic.aspx

To find an audiologist in your area, visit the following links:

http://webportal.audiology.org/Custom/FindAnAudiologist.aspx

http://www.asha.org/proserv/

What causes hearing loss?

There are many different causes of hearing loss. Some of these include:

infections
head injury
malformations of any part of the ear
loud noise exposure
exposure to certain "ototoxic" medications
the aging process
fluid in the ear
foreign object in ear canal
too much earwax
acquired diseases
genetics

This site will focus on the genetics causes of hearing loss. Other causes of hearing loss, listed above, should be ruled out before investigating genetics causes.

Genetic Causes of Hearing Loss

How are traits passed on?

Each of us is a product of our two parents, passed down to us in the form of genes. Our genes contain a code of instructions for everything about us, such as eye color, height, and blood type. Each of these traits is passed down from parent to child. For any given gene, you have two copies: one received from your mother and one received from your father. We call these two gene copies alleles. The two alleles of a gene can interact in different ways. If the alleles are different, one allele may overpower the other allele.

1) Dominant When one allele is dominant, it masks, or “covers up” the other allele. The dominant trait will be the trait seen in the individual. If we are talking about a gene that causes a disease, one copy of the defective gene is enough to cause a disease. Most people with a dominantly inherited disease received the disease gene from one parent. In a minority of cases, a new mutation occurring in the individual can cause the disease.

2) Recessive The recessive allele will be the one masked, or “covered up” by the dominant allele. The recessive trait will not be seen unless the individual receives two recessive alleles, one from each parent. In this case, the recessive trait will be seen because there is no dominant allele covering it up.

For more information on dominant and recessive alleles, click here.

Genes are attached to chromosomes. Each person has 46 chromosomes, 23 from each parent. Two of these 46 chromosomes are sex chromosomes, so named because they determine a person's gender. Males have one X chromosome and one Y chromosome. Females have two X chromosomes. A disorder of a gene that is located on the X chromosome is called an X-linked disorder. X-linked disorders are more likely to affect males because they only have one X chromosome. Since females have two X chromosomes, a normal allele on one X chromosome can often mask the abnormal allele on the other X chromosome.

The other 44 chromosomes that are not sex chromosomes are called autosomes. A disorder of a gene that is located on an autosome is referred to as autosomal.

A small number of genes can be passed on yet a different way, through mitochondria. Mitochondria are often called the “powerhouses” of cells because a major function is generating energy for the cell. Mitochondria are found in egg cells, but not in sperm cells. So, mitochondrial genes are only inherited from the mother.

To recap, here are the major ways a genetic disorder can be inherited:

Autosomal dominant
Autosomal recessive
X-linked
Mitochondrial

Syndromic vs. nonsyndromic

There are many different genetic forms of hearing loss. Most of these are associated with a syndrome. A syndrome is a pattern of symptoms or characteristics that occur together. Hearing loss is found as a part of over 400 different syndromes (syndromic hearing loss). Some of the more common ones are discussed below. When a genetic form of hearing loss occurs in isolation, it is referred to as nonsyndromic hearing loss. A person with nonsyndromic hearing loss has hearing loss with no other symptoms. Among genetic forms of hearing loss, approximately 30% of cases are syndromic and 70% are nonsyndromic.

Syndromes

Nonsyndromic hearing loss

See the following link for genetic services in Louisiana:

http://www.ncbi.nlm.nih.gov/sites/GeneTests/clinic/location/?db=genetests&state=LA

To locate providers of genetic services outside Louisiana, please visit the following link:

http://www.ncbi.nlm.nih.gov/sites/GeneTests/clinic?db=genetests