1. 
   

   Incidence

   
   

   Infants admitted to the neonatal intensive care unit (NICU) have a relatively high incidence of perinatal complications and risk factors associated with congenital hearing loss.

   
   
   
   
   1. 
      

      Infants admitted to the NICU have 30-fold increased risk of congenital hearing loss compared to well-baby population.

      
      
   2. 
      

      A prevalence of 3.2% of unilateral or bilateral congenital hearing loss was found among a cohort of NICU infants.

   
   
2. 
   

   Embryology

   
   

   The auditory system develops from the first trimester of pregnancy onward and continues to mature until 6 to 12 months after birth.

   
   
   
   
   1. 
      

      During the first trimester the basic structures of the cochlea, cochlear nerve, brainstem, and auditory cortex are formed.

      
      
   2. 
      

      In the second trimester rapid maturation of the cochlea and cochlear nerve occurs. The auditory nuclei in the brainstem increase rapidly in size during the second trimester. By the end of the second trimester, the cochlea has a mature appearance, with the exception that synaptic terminals formed by efferent brainstem axons are smaller and less numerous than in the adult cochlea.

      
      
   3. 
      

      At the beginning of the third trimester the first myelination occurs in the cochlear nerve and the brainstem. Myelin formation is of great importance for rapid and synchronized nerve conduction.

      
      
      
      
      1. 
         

         Movement of the fetus in response to sound occurs for the first time around 25 weeks’ gestation and becomes more consistent around 28 weeks.

         
         
      2. 
         

         This is the time when in preterm infants a recordable auditory brainstem response (ABR) appears.

      
      
   4. 
      

      Final maturation of the auditory system continues from the perinatal period until 6 to 12 months of age.

      
      
      
      
      1. 
         

         Full-functional cochlear maturity is achieved a few weeks before term birth.

         
         
      2. 
         

         Auditory neurons reach about 50% to 60% of their adult size at time of birth.

         
         
      3. 
         

         During the perinatal months, rapid growth occurs in the brainstem.

         
         
      4. 
         

         The axonal myelin density in the cochlear nerve and brainstem increases rapidly and becomes adult-like by 6 to 12 months of age.

   
   
3. 
   

   Pathophysiology

   
   

   During the preterm and perinatal period the auditory system is in full development and, therefore, highly vulnerable.

   
   
   
   
   1. 
      

      This is especially true in preterm birth, during the period of fast myelination (between 30 and 34 weeks’ postconceptional age) that can result in delayed auditory maturation.

      
      
   2. 
      

      Perinatal complications may impair the auditory part of the brainstem or delay its maturation.

   
   
4. 
   

   Types of hearing loss

   
   

   Hearing loss may occur due to abnormal development or pathology in different parts of the auditory system. Two main types of hearing loss can be distinguished.

   
   
   
   
   1. 
      

      A conductive hearing loss is located somewhere in the external or middle ear. Temporary hearing loss in infants is usually of conductive (middle ear effusion) origin.

      
      
   2. 
      

      A sensorineural hearing loss (SNHL) is located in the cochlea or the auditory pathway to the brain. In infants, permanent hearing loss is usually of sensorineural origin.

      
      
   3. 
      

      A combination of both conductive and sensorineural hearing loss can also be found.

      
      
   4. 
      

      Auditory neuropathy spectrum disorder (ANSD)

      
      
      
      
      1. 
         

         NICU infants have an increased risk of delayed auditory maturation or neural pathology compared to healthy infants. ANSD is an example of such a condition in which transmission of sound to the brain is abnormal. Children who suffer from this condition experience difficulties with speech perception, especially in noise, and the development of language skills.

There are several different tests available to diagnose and evaluate hearing loss in infants. The following tests can be used to diagnose hearing loss in infants: otoacoustic emission (OAE), tympanometry, and ABR measurement.

1. 
   

   Otoacoustic emission

   
   

   An otoacoustic emission is a low-intensity sound, which is generated within the inner ear. These sounds are produced by the cochlea, most likely the outer hair cells.

   
   
   
   
   1. 
      

      OAEs measure only the peripheral auditory system, which includes the outer ear, middle ear, and cochlea. The response originates from the cochlea, but the middle and outer ear must be able to transmit the emitted sound to the recording microphone introduced in the ear canal.

      
      
   2. 
      

      OAEs are often used to screen hearing in infants and can partially estimate hearing sensitivity within a limited range. In general, the presence of an OAE suggests that hearing sensitivity should be below 30 dB nHL.

      
      
   3. 
      

      OAEs can also be used to differentiate between the sensory and neural components of SNHL. For example, in ANSD transmission of sound from the cochlea to the brain is abnormal. ANSD is characterized by normal OAEs (outer hair cell function) and severe abnormalities on ABR measurement. The normal function of the outer hair cells, in combination with severe abnormalities on ABR measurement, indicates neural dysfunction.

   
   
2. 
   

   Tympanometry

   
   

   Tympanometry is an examination used to evaluate the mobility of the tympanic membrane and the ossicular chain.

   
   
   
   
   1. 
      

      It describes the relation between the air pressure in the external ear and movement of the tympanic membrane and ossicular chain.

      
      
   2. 
      

      A tympanogram provides information on the compliance of the middle ear, ear canal volume, and middle ear pressure.

      
      
   3. 
      

      In infants, it is typically used to diagnose otitis media with effusion, which is a common cause of temporary conductive hearing loss.

   
   
3. 
   

   Auditory brainstem response

   
   

   A diagnostic ABR provides an accurate evaluation of the type and degree of hearing loss.

   
   
   
   
   1. 
      

      ABR measurement is the most important tool in diagnosing hearing impairment in infants.

      
      
   2. 
      

      Its peaks reflect the conduction of a neural signal as a result of a sound stimulus along the auditory nerve and different levels of the brainstem (Figure 48-1).

      
      
   3. 
      

      Peaks I and II reflect the cochlea and auditory nerve (peripheral response) and peaks III, IV, and V are generated more centrally, ie, by brainstem structures. It is assumed that peak III reflects the ascending auditory pathway or the cochlear nuclei in the ventral acoustic striae. Peak V reflects activity toward the inferior colliculus, most likely the lateral lemniscus.

      
      
   4. 
      

      The degree of hearing loss is estimated with the ABR response threshold.

      
      
   5. 
      

      The type of hearing loss can be estimated from the latency-intensity curves.

      
      
   6. 
      

      In infants ABR latencies are age dependent; therefore, age-adjusted normal values are required.

      
      
      
      
      1. 
         

         The ABR response in human development first appears around 25 weeks’ gestational age.

         
         
      2. 
         

         This response matures during the first years of life, resulting in decreased latencies of most of the response peaks.

         
         
      3. 
         

         In very preterm infants, interpreting the results of ABR measurement is a special challenge since various peaks of the ABR response are poorly detectable.

   
   
4. 
   

   Visual reinforcement audiometry

   
   
   
   
   1. 
      

      Visual reinforcement audiometry (VRA) is a behavioral audiometric test that can be used for children from about 6 months of age up to about 3 years.

      
      
   2. 
      

      In VRA, behavioral responses to frequency-specific tones and other stimuli are recorded.

      
      
   3. 
      

      The child will sit on the parent’s lap or in a chair between two calibrated loudspeakers or using earphones.

      
      
      
      
      1. 
         

         When the test is performed through loudspeakers, the best hearing ear is measured.

         
         
      2. 
         

         When a child tolerates earphones, both ears can be tested separately.

      
      
   4. 
      

      When a sound such as a tone at a specific frequency, speech, or music is presented, the child’s eye-shift or head-turn response toward the sound source is rewarded with a visual stimulus.

      
      
   5. 
      

      After about 3 years of age, children will easily be bored by this type of hearing test and play audiometry should be performed.

   
   
5. 
   

   Neonatal hearing screening

   
   

   The aim of the universal newborn hearing screening (UNHS) is to identify a conductive or sensorineural hearing loss with an average hearing loss of at least 40 dB in one or both ears before the age of 3 months.

   
   
   
   
   1. 
      

      Intervention and counseling are aimed to start before the age of 6 months in accordance with the screening guideline of the Joint Committee on Infant Hearing (JCIH) to prevent future problems with speech and language development.

      
      
   2. 
      

      There are different multiple stage screening methods using OAE and automated auditory brainstem response (AABR) testing.

      
      
   3. 
      

      In NICU infants, an AABR screening method is recommended because of the higher incidence of neural pathology, such as ANSD. In primary OAE screening, cases of ANSD may be missed.