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 (1).

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 (1).

Genetic factors are believed to cause 33 percent of cases of hearing impairment in infants and young children (1). Scientists believe that mutations (changes) in as many as 400 genes may contribute to hearing impairment (1).

Genetic causes (1) 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.

• 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 (1).

Are there different types of hearing impairment?
Yes. When sound enters the outer ear (auricle or pinna), it moves through the ear canal to the eardrum (tympanic membrane). Incoming sound causes the eardrum to vibrate, which moves three small bones (ossicles) in the middle ear. In this way, the ear canal, the eardrum and the middle ear transmit sound from the outside to the inner ear (cochlea). Within the inner ear, thousands of tiny hair cells detect the incoming vibrations and convert them into signals that are relayed to the auditory nerves, which send neural impulses to the hearing center in the brain.

Hearing impairment can occur in different parts of the hearing pathway.

• Conductive hearing impairment occurs when something interferes with sound passing through the outer or middle ear. A blockage in the ear canal, damage to the eardrum, and fluid or an infection in the middle ear (called otitis media) are examples of conditions that can cause conductive hearing impairment. This type of hearing impairment is usually temporary and often can be corrected with medication or drainage.
  
• Sensorineural hearing loss occurs when the hair cells in the inner ear cannot detect all incoming vibrations or when neural impulses are not transmitted to the brain. Prenatal infections or genetic factors can cause this type of hearing impairment. Sensorineural hearing impairment is generally permanent. However, many children can be helped with devices that amplify sound. Sensorineural hearing impairment also can result from damage to the brain's auditory center.
  
• Mixed hearing impairment occurs when a child who has a sensorineural hearing impairment also has a conductive impairment (such as fluid in the middle ear). Early and proper treatment of acute ear infections is crucial to protecting children from hearing impairment.

In the otoacoustic emissions (OAE) test, a small microphone is placed in the baby's ear. The microphone, connected to a computer, sends soft clicking sounds or tones into the ear and records the inner ear's response to sound.

In the automated auditory brainstem response (AABR) test, soft clicking sounds are presented to the ear through small earphones. Sensors placed on the head and connected to a computer measure brain wave activity in response to sound.

• The test should be repeated or
• The baby should be referred either to a hearing specialist (audiologist) or to an ear, nose and throat (ENT) specialist (also called an otolaryngologist). The specialist will do more extensive tests to determine if the baby has a hearing impairment.

The screening tests are only a general indication that there may be a problem. While up to 10 percent of babies have abnormal results on their hearing screening test, some will be found normal on additional tests (1).

What tests are used to diagnose hearing impairment after the newborn period?
The most common hearing test for infants under 6 months of age is the diagnostic auditory brainstem response test. It is similar to the automated AABR, but it provides more information and must be administered by a specialist.

Children between 6 months and 2 years of age often are tested with visual reinforcement audiometry (VRA). During VRA testing, a series of sounds are presented to the child through earphones or speakers. The child is trained to turn toward any sound and is rewarded with an entertaining visual image for responding.

Children between 2 and 4 years of age are tested with conditioned play audiometry (CPA). Children are asked to perform a simple play activity (like placing a ring on a peg) when they hear a sound. This is similar to the test for older children and adults who press a button or raise their hand when they hear a sound.

These tests also may be recommended:

• If a child was not screened as a newborn
• If a child has had persistent ear infections, meningitis or other illness that can cause hearing loss
• If a child has been diagnosed with a syndrome that can include hearing loss 
• If a parent suspects the child is not responding normally to sounds

• Failure to startle at loud sounds
• Not turning toward the sound of a voice or imitating sounds after about 6 months of age
• Lack of babbling at 9 months of age
• Not using single words by 18 months of age
• Using gestures instead of words to express needs

Parents should be concerned about hearing impairment in older children if they:

• Develop vocabulary more slowly than their peers
• Have speech that is difficult to understand or that is too loud or too soft
• Often ask for words to be repeated
• Turn on the TV too loud
• Appear inattentive at school and have difficulties learning to read or perform simple mathematics