Pendred Syndrome (cont.)
What causes Pendred syndrome?
Pendred syndrome can be caused by changes, or
mutations, to a gene known as SLC26A4 (also referred to as the PDS gene) on
chromosome 7. Because it is a recessive trait, a child needs to inherit two
mutated SLC26A4 genes—one from each parent—to have Pendred syndrome. The child's
parents do not need to have Pendred syndrome to be a carrier of a mutation in
the SLC26A4 gene.
Couples who are concerned that they might be able to pass Pendred syndrome on
to their children may seek genetic testing. A possible sign that a person may be
a carrier of a mutated SLC26A4 gene is a family history of early hearing loss.
Another sign is a family member who has both a goiter and hearing loss. A
mutation in the SLC26A4 gene can be determined by genetic testing that uses a
blood sample.
The decision to have a genetic test is complex. Most people receive
assistance from a genetic counselor trained to help them weigh the medical,
emotional, and ethical considerations. A genetic counselor is a health
professional who provides information and support to individuals and families
who have a genetic disease or who are at risk for such a disease.
How is Pendred syndrome diagnosed?
A physician called an otolaryngologist or a
clinical geneticist will consider a person's hearing, inner ear structures, and
sometimes the thyroid in diagnosing Pendred syndrome. The specialist will
evaluate the timing, amount, and pattern of hearing loss. He or she will ask
questions such as:
- "When did the hearing loss start?,"
- "Has it worsened over
time?," and
- "Did it happen suddenly or in stages?."
Early hearing loss is one of
the most common characteristics of Pendred syndrome; however, this symptom alone
does not mean a child has the condition.
The specialist uses inner ear imaging techniques known
as magnetic resonance imaging (MRI) or computed tomography (CT or CAT) to look for two key
characteristics of Pendred syndrome. One characteristic might be a cochlea with
too few turns. The cochlea is the spiral-shaped part of the inner ear that
converts sound into electrical signals that are sent to the brain. A healthy
cochlea has two-and-a-half turns, but the cochlea of a person with Pendred
syndrome may have only one-and-a-half turns.
A second characteristic of Pendred syndrome is enlarged
vestibular aqueducts (see figure). The vestibular aqueduct is a bony canal that
runs from the vestibule (a part of the inner ear between the cochlea and the
semicircular canals) to the inside of the skull. Inside the vestibular aqueduct
is a fluid-filled tube called the endolymphatic duct, which ends at a balloon-shaped
endolymphatic sac. When the vestibular aqueduct is enlarged, the endolymphatic
duct and sac grow large with excess fluid in comparison to their normal sizes.
The function of the vestibular aqueduct is not well understood.
Picture of the Inner Ear
When screening for Pendred syndrome, it is not recommended to test the blood
for thyroid hormone because the amount usually is the same whether someone has
Pendred syndrome or not. Some people may receive a "perchlorate washout test," a
test that determines whether the thyroid is functioning properly. Although this
test is probably the best test for determining thyroid function in Pendred
syndrome, it is not used often and may be replaced by genetic testing.
Individuals who have a goiter may be referred to an endocrinologist, a doctor
who specializes in glandular disorders, to determine whether the goiter is due
to Pendred syndrome or to another cause. Goiter is a common feature of Pendred
syndrome, but many individuals who develop a goiter do not have Pendred
syndrome. Conversely, many people who have Pendred syndrome never develop a
goiter.
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