| Hearing
impairment represents the most common sensory deficit in children.
Sensorineural hearing loss (SNHL) or "nerve deafness"
represents the most common form of congenital hearing impairment
and a significant health care problem. Over 40,000 children are
born in the United States with significant hearing impairment,
with about 4,000 of these profoundly deaf. The incidence is estimated
at about 1 in every 1000 live births. Therefore, the otolaryngologist
and parents will frequently be challenged with deciding on the
appropriate diagnostic tests for these children. About 50% of
hearing impairment is genetic (familial) in origin. The Auditory
Genetics Laboratory of the Center functions to provide current
state-of-the-art diagnostic and research testing for children
with hearing loss and their families.
On the horizon of our diagnostic evaluation in children with
SNHL is molecular genetic testing. The ability to unlock genetic
information that can be attributed to deafness is truly a revolutionary
development. Any means to diminish the number of afflicted individuals
who remain undiagnosed would appear to offer a distinct improvement
to current medical care. Most patients with hearing loss have
not other medically related problems (i.e. nonsyndromic) and
have few other family members (i.e. recessive inheritance) with
a similar problem. Autosomal recessive (a genetic pattern in
which the parents are carriers and each give a mutation to the
child) nonsyndromic hearing loss accounts for 70% of all genetic
types and typically have a severe form of hearing loss. Dominant
forms (only one mutation is required as one parent is a carrier
for the mutation and passes it along to the hearing impaired
child) of hearing loss typically have at least several affected
persons in the family and they are usually not completely deafened.
Syndromic forms of hearing loss are generally easier to identify,
because of the other medical problems the patients exhibit.
Although, in several types of syndromes, the hearing loss may
be the initial problem that brings the patient to the attention
of a physician. Over 70 types of nonsyndromic hearing loss have
been localized, with only a small number of these have actually
been completely identified.
Family Research Testing
If initial evaluation and testing cannot find a cause for the
hearing loss, the laboratory can also evaluate certain families
with specialized genetics techniques. This research testing
is easy to perform and can possibly determine the cause of a
families hearing loss. This type of research requires all (or
as many as possible) family members or parents to sign a consent
form, obtain a hearing test, and have a small amount of blood
drawn to isolate their genetic material (DNA). This service
is provided at little or no expense to the family. The DNA is
tested against known causes of hearing loss and if none is found,
an extensive search is then begun to identify a new form of
genetic hearing loss. The results of the study may take several
months to several years to complete. A designated family member
will be updated on any new or significant findings. As for our
routine diagnostic testing, counseling on the results is provided
by the Center. For more information, contact Dr. John Greinwald.
The Auditory Genetics Laboratory is currently engaged in research
projects seeking the identification of the gene for DFNB17 (a
rare cause of nonsyndromic recessive deafness), the interaction
with mitochondrial mutations and animoglycoside antibiotic exposure
and the genetic factors that are important for the susceptibility
to noise-induced hearing loss.
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