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Wednesday, 21 November 2007

Genetics and hearing impairment

Hearing impairment is the decreased ability to hear and discriminate among sounds. It is one of the most common birth defects. Each year in the United States, about 12,000 babies (3 in 1,000) are born with significant hearing impairment (1). Hearing impairment that is present at birth is called congenital hearing impairment. Hearing impairment also can develop later in childhood or during adulthood.
The Centers for Disease Control and Prevention (CDC) recommends that all babies be screened for hearing impairment before 1 month of age, preferably before they leave the hospital (1). This is because language and communication develop rapidly during the first two to three years of life, and undetected hearing impairment can lead to delays in developing these skills. Without newborn screening, children with hearing impairment usually are not diagnosed until 2 to 3 years of age.
The goal of early screening, diagnosis and treatment is to help children with hearing impairment develop language and academic skills equal to those of their peers. Most states have an Early Hearing Detection and Intervention Program to help ensure that all babies are screened, and that infants who do not pass the screening receive the follow-up care they need. The March of Dimes, the American Academy of Pediatrics, the Maternal and Child Health Bureau, the CDC and others strongly support these programs.
What causes hearing impairment in babies and children?Hearing impairment can be inherited (genetic) or nongenetic. Nongenetic causes include illness or injury occurring before, during or after birth. In some cases, the cause of hearing impairment is not known. About 90 percent of babies with congenital hearing impairment are born to hearing parents.
Genetic factors are believed to cause 33 percent of cases of hearing impairment in infants and young children. Scientists believe that mutations (changes) in as many as 400 genes may contribute to hearing impairment.
Genetic causes of hearing impairment can be:
Syndromatic: One feature of a group of birth defects that occur together. This type of impairment accounts for about 30 percent of cases.
Nonsyndromatic: A solitary birth defect. About 30 percent of cases of nonsyndromatic hearing impairment are caused by a mutation in a gene called Connexin 26.About one-third of cases of hearing impairment are caused by nongenetic factors. They include illnesses during pregnancy, such as:
Rubella (German measles)
Cytomegalovirus infections
Toxoplasmosis
Herpes infection
Syphilis
Preterm birth (before 37 completed weeks of pregnancy) also can be a cause.
After birth, head injuries and childhood infections (such as meningitis, measles or chickenpox) can cause permanent hearing impairment. Certain medications, such as the antibiotic streptomycin and related drugs, also can cause hearing impairment. Ear infection (otitis media) may cause temporary hearing impairment. Frequent and poorly treated ear infections can cause damage sufficient to impair hearing.
The causes of the remaining third of cases of hearing impairment in infants and children are unknown.

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