Diseases

Most permanent hearing loss is due to damage/malfunction of the nerve that transmits sound from the inner ear to the brain (auditory nerve). For those infants in whom a cause is determined, approximately half have a genetic condition and the remaining half have an acquired condition to explain their hearing loss.

Hearing in infants can be tested using two different methods: the auditory brainstem response (ABR) evaluations or the otoacoustic emission (OAE) measures. Both tests are accurate, noninvasive, automated, and do not require any observable response from the infant. Which test is used depends on the screening program's choice of instrumentation and training. For a screening tool, both methods are extremely effective. There are, however, some distinct differences in how the hearing is measured using an ABR versus an OAE.

Most hospital screening programs will refer infants who failed the initial screening test to a secondary center that specializes in more complete testing for diagnosis. Sometimes, simple problems, such as too much residual amniotic fluid and vernix in the ear canal, will resolve prior to the rescreen and the infant will pass the second test. The rescreen is an important step in determining whether the infant is able to hear, so it should not be taken lightly. If an infant does not pass the rescreen, then a full diagnostic test will be necessary. This test may be completed at the rescreen site or at another facility, depending on several factors.

The ability of an infant to compensate for the hearing loss will depend on both the type and the degree of hearing loss. The type of hearing loss refers to where in the ear the hearing loss is located and what is causing it. There are two basic types of hearing loss, conductive and sensorineural. A conductive loss is caused by problems in the outer or middle ear. This is the type of loss that results when a child has a middle-ear infection, trapped fluid from birth, impacted wax in the outer ear or malformation of the ear and associated structures. It is usually correctable with medical treatment or surgery. Occasionally, a conductive loss cannot be corrected with surgery. However, these children typically do extremely well with hearing aids. A sensorineural loss is indicative of a problem in the inner ear or somewhere along the nerve to the ear (auditory nerve). This type of loss is typically permanent and cannot be corrected with surgery. A hearing aid or a cochlear implant may be utilized in this situation. The degree of hearing loss refers to the severity of the hearing loss, which can range from mild to profound. Although the term "mild" sounds relatively benign, a mild hearing loss in a child who is trying to develop speech and language can have a huge impact on his/her success. If the hearing loss, whether it is conductive, sensorineural, or a combination of both, is determined to be uncorrectable by a physician, then the next step is to fit the infant with hearing aids. This is accomplished by a pediatric audiologist who can adjust the hearing aids according to the baby's hearing loss and monitor the infant's progress through regular visits. The infant should also be enrolled in an early childhood intervention program that is often provided through the school system. In addition to the early medical or surgical treatment of the hearing loss, parental involvement is essential. Parents need to monitor the child's progress and facilitate and encourage the use of the hearing aids and other therapeutic exercises that are designed to help the child become a careful listener and talker. Research has found that the one common denominator among successful hearing-impaired children is the parent's willingness to help the child throughout his/her lifetime. Summary & Recommendations (from the American Academy of Pediatrics Preventative Series Task Force -- July 2008) Importance: Children with hearing loss have increased difficulties with verbal and nonverbal communication skills, increased behavioral problems, decreased psychosocial well-being, and lower educational attainment compared with children with normal hearing. Detection: Because half of the children with hearing loss have no identifiable risk factors, universal screening (instead of targeted screening) has been proposed to detect children with permanent congenital hearing loss. There is good evidence that newborn hearing screening testing is highly accurate and leads to earlier identification and treatment of infants with hearing loss. Benefits of detection and early treatment: Good quality evidence shows that early detection improves lingual outcomes.

Both tests have advantages and disadvantages when used for screening, and depending on the program and experience of the audiologist, either one can be utilized successfully. The OAE is easy and cost-effective. However, the false-positive rate (for example, an infant fails a hearing test but actually has normal hearing) may be higher for an OAE than for an ABR. The false-positive rate for ABR testing is approximately 4% when testing is done during the first three days of life. The false positive rate for OAE testing is 5%-21% for testing done during the first three days of life. This large variation between ABR and OAE testing is commonly felt to reflect the OAE testing device's increased sensitivity to residual amniotic fluid and vernix that is commonly found in the neonate's ear canal. The two tests, however, rely on different mechanisms of hearing for the screening. For in-depth testing and a complete hearing evaluation of infants, these tests work best together as a complement to each other.

Hearing loss in a newborn can be caused by a number of conditions. Some of the known risk factors include high bilirubin levels (jaundice), drugs that are toxic to the ears (for example, medicines that are given to the newborn to battle a serious infection may damage hearing as a side effect), prolonged mechanical ventilation, low Apgar scores, meningitis, prematurity, and/or low birth weight. Malformed structures in the middle or outer ear can also lead to hearing loss. Viral illness during the pregnancy, such as rubella (German measles) or cytomegalovirus (CMV), can be passed to the newborn and result in hearing loss. Hearing loss can sometimes be inherited in abnormal genes passed from the parents to the newborn or be the result of a gene mutation that occurred during fetal development. Genetic counseling is often recommended for parents to determine if heredity is the cause of the hearing loss. In approximately half of all cases of hearing loss, the cause is never determined.

A newborn who fails an initial hearing screen may not necessarily have a permanent hearing loss or a hearing loss at all. There are many possible reasons why an infant may fail a hearing screening test. One common reason is that fluid from the birth may still be present in the ear canal. This fluid blocks the sound stimulus, preventing it from reaching the inner ear, and therefore causes the newborn to fail. Similarly, fluid in the middle-ear space behind the eardrum (a common site for infection in children) can also block the sound stimulus and lead to a false failed test. After these problems resolve, the infant usually passes the rescreen. Therefore, it is important to have at least one week between the initial hearing screen and the rescreen to allow the newborn a chance to "dry out." Another possible reason for a false failure is excessive noise or movement from the infant during the test. The responses that are recorded with an ABR or OAE are very, very small. Any movement or crying from the infant can prevent the equipment from detecting the response. Therefore, it is important that the newborn is quiet or sleeping for the hearing screen. Feeding the infant just prior to the screening is often very helpful. Although neither test is painful, they are novel experiences for the newborn and can be momentarily upsetting. If it becomes evident that an infant has a hearing loss, then a full diagnostic exam is necessary to determine the type and amount of hearing loss.

Newborn infant hearing screening programs are designed to identify hearing loss in infants shortly after birth. All states have implemented these screening protocols within hospitals and birthing clinics. Most hearing screening tests are done prior to discharge from the hospital or birthing clinics. Typically, nurses or medical assistants are trained extensively on how to operate automated equipment for testing infants. Prior to discharge, each newborn has his/her hearing tested. If, for some reason, the newborn does not pass the screen, a rescreen is usually done. If the infant does not pass the second hearing test, he/she is referred to a specialist for further testing. Specialists who are experts at testing hearing are called audiologists. Audiologists have had training that emphasizes diagnostic hearing testing techniques as well as hearing rehabilitation of children and adults. Their postgraduate academic training requires a minimum of a master's degree.

In order to process sounds, electrical impulses are transmitted through nerves from our ears to the brainstem at the base of the brain. An auditory brainstem response (ABR) is a physiological measure of the brainstem's response to sound. It tests the integrity of the hearing system from the ear to the brainstem. The test is performed by placing four to five electrodes on the infant's head, after which a variety of sounds is presented to the infant through small earphones. As the hearing nerve fires, the sound stimulus travels up to the brain. This electrical activity generated by the nerve can be recorded by the electrodes and is represented as waveforms on a computer screen. The audiologist can then present different loudness levels of each sound and determine the softest levels at which the infant can hear. For infant-screening purposes, only one sound is used to test the hearing, commonly referred to as a "click." The click is a grouping of several sounds to test a wider area of the hearing organ at one time. The click is typically presented at a loud level and a soft one. If a healthy response is recorded, then the infant has "passed" the hearing screen. Testing usually takes five to 15 minutes to complete.

An otoacoustic emission test (OAE) measures an acoustic response that is produced by the inner ear (cochlea), which in essence bounces back out of the ear in response to a sound stimulus. The test is performed by placing a small probe that contains a microphone and speaker into the infant's ear. As the infant rests quietly, sounds are generated in the probe. Once the cochlea processes the sound, an electrical stimulus is sent to the brainstem. In addition, there is a second and separate sound that does not travel up the nerve but comes back out into the infant's ear canal. This "byproduct" is the otoacoustic emission. The emission is then recorded with the microphone probe and represented pictorially on a computer screen. The audiologist can determine which sounds yielded a response/emission and the strength of those responses. If there is an emission present for those sounds that are critical to speech comprehension, then the infant has "passed" the hearing screen. Testing generally takes about five to eight minutes.

The basic difference between a diagnostic test and a hearing screen is the amount of information gathered during the session. For example, if an infant fails the hearing screen, it is unknown if there truly is a hearing loss, how much hearing loss is present, or whether or not the hearing loss is permanent or correctable. A diagnostic test can usually answer these questions. The diagnostic test session is understandably longer and requires more interaction with the infant. Typically, a more extensive ABR is conducted, using a variety of test stimuli. OAEs are also performed to cross-check the results of the ABR. To complete a thorough test, the infant needs to sleep in the office for upward of 45 minutes. The more information that can be collected, the more complete the results will be.

Significant hearing loss is the most common disorder at birth. Approximately 1%-2% of newborns are affected. Several national committees, including the National Institutes of Health, the American Academy of Otolaryngology/Head and Neck Surgery, and the American Academy of Pediatrics, have recommended that hearing loss in infants be identified, and when possible treated, prior to 6 months of age. This recommendation is based on studies that have shown that children identified with hearing loss prior to 6 months of age have a better chance of developing skills equivalent to their peers by the time they enter kindergarten. Children not identified until later (for example, it is very common to first identify hearing impaired children at age 2 to 3 years) may ultimately suffer from irreversible and permanent impairments in speech, language, and cognitive abilities when compared to their peers. Prior to the implementation of hearing screen programs, it was customary to only test those newborns who had known significant risk factors for hearing loss. This group included infants whose mothers suffered from illness during pregnancy, those who had a family history of hearing loss, or those who were exposed to drugs known to affect hearing. In addition, infants with the following conditions were included for hearing screening:

• low birth weight and/or prematurity, or oxygen deprivation or breathing difficulties at birth;
• high bilirubin levels (yellow color);
• syndromes associated with hearing loss;
• abnormal head or face structures;
• infections such as cytomegalovirus, syphilis, herpes, or toxoplasmosis; or
• low Apgar scores (which assess several health factors at one and again at five minutes after birth).

However, despite the testing of all infants who fell into this "high-risk registry," over half of all newborns with hearing loss were missed! In order to identify this large group of hearing-impaired infants not identified with current testing protocols, it is now recommended that all newborns have a hearing test prior to discharge from the hospital. The goal of this program is to identify all hearing-impaired infants at an early age, thereby increasing these children's chance at healthy and more productive lives.