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I. Patterns of development

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  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 ...

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