Prematurity and the Auditory System: Considerations for Audiologists

Abstract

Editor's note: Research discussed from the author's lab was funded in part by Auburn University Intramural Grants Program and by the Hearing Health Foundation Emerging Research Grant Prematurity is a widespread health issue that affects babies and families worldwide. Approximately 15 million preterm births, defined as delivery prior to 37 weeks, occur globally each year, with 380,000 annual preterm births recorded in the United States alone. The prevalence of premature birth has increased in recent years, with estimates as high as one out of every 10 infants. Furthermore, data from 2015 to 2017 reveal preterm birth rates among African American women (14%) to be about 49 to 50 percent higher than that among Caucasian women (9%), making it a health disparity issue.1-3 The earlier the gestation at the time of delivery increases the likelihood of significant health problems and long-term disabilities. Various conditions may contribute to premature birth, but the cause is often unknown. In about half the cases of preterm birth, spontaneous preterm labor occurs due to an unknown etiology. Other factors that play a role in preterm birth include chronic medical illnesses (i.e., diabetes, high blood pressure, etc.), family history of preterm births, pregnancy with multiples, pre-eclampsia, placental abruption, maternal age (< 17 years or > 35 years), lack of prenatal care, environmental factors, smoking, and/or drug use.3 Having a better understanding of the impact of prematurity on children's auditory skills development can help hearing care professionals in providing intervention recommendations and long-term outcomes of children with a premature birth history.iStock/Ondrooo, audiology, pediatrics, premature.Table 1: Acoustic Gap Between the NICU and Womb Environments.5DEVELOPMENT OF THE CENTRAL AUDITORY SYSTEM Many factors, such as length of gestation, nutrition, medications, and environment, play critical roles in brain development during the third trimester of pregnancy. These factors are very different when the infant is born prematurely.4, 5At about 23 weeks of gestation, babies in the womb can hear sounds via bone conduction. Following the onset of hearing, the myelination of the auditory pathway occurs, allowing the ability to discriminate sounds. The capability to process different frequencies (discrimination, representation, and resolution) starts at the beginning of the third trimester and continues until age 1. Between the gestation weeks of 26 and 29, babies can process the intensity cues of sounds, marking the development of their loudness perception.6, 7 Following a full-term birth, the central auditory pathways and processing abilities are influenced heavily by rapid development from birth through the first six months of life; thereafter, this maturation continues gradually and is considered complete by age 7 for most auditory skills.8 Although the inner ear has matured and is functional at full-term birth, the presentation levels required to elicit activity from the distal portion of the auditory nerve (i.e., auditory brainstem response wave I) show that peripheral processing in infants is still immature compared with that in adults.8-10This immaturity may be due to the conductive component of the outer and middle ear, which reduces the amount of sound energy transmitted through the auditory system.11-13Despite known peripheral immaturity, an infant, quickly after birth, is able to localize sounds and utilize binaural processing; such processing skills continue to develop until age 5.6, 8, 14 Refinement of discrimination processing, which begins developing preterm, also continues to mature up until age 6.8-10 However, the skills required for binaural integration and recognition of degraded speech stimuli are still developing, and approach adult-like performance around 13 years of age and may continue to mature.15-20 The development of the central auditory system inside the womb versus outside in the neonatal intensive care unit (NICU) occurs differently. During the last trimester, the hair cells of the cochlea and their connections undergo fine-tuning.5, 7While inside the womb, infants hear more low- and mid-frequency sounds, potentially accounting for why the afferent synaptic transmissions below 6,000 Hz are mature at birth. Following birth, they are exposed to a broader range, including high-frequency sounds and noise, and then their synaptic transmission matures for frequencies above 6,000 Hz.21, 22 It is theorized that early exposure to noise in the NICU could influence the fine-tuning of cochlear hair cells and connections. Infants required to stay in the NICU were found to have been exposed to excessive daily doses of ambient noise and have significantly less exposure to language than their full-term peers. Table 1 shows the details of a NICU environment.5The more premature an infant, the greater impact the NICU environment might have on the infant's central auditory system.5, 7 Significant differences have been found in how the brain processes sound at the electrophysiological level in premature infants and children compared with infants and children born at fullterm.23-28 Significant differences have also been found in how preterm infants discriminate sounds compared with their fullterm counterparts. The ability to discriminate sounds is critical to the development of speech, language, and auditory skills, which build children's foundation for learning to read.29, 30 EVALUATING THE AUDITORY SYSTEM Monitoring the hearing of children born prematurely is critical, particularly for those who have spent their first days, weeks, or months in the NICU. The Joint Committee on Infant Hearing (JCIH) recently updated its position statement to include that infants with an NICU history (more than five days) should have a diagnostic hearing evaluation by 9 months of age.31,32 This is a significantly earlier evaluation than that recommended in JCIH's 2007 position statement.31,32Typically, the follow-up schedule for monitoring this at-risk population is set by the audiologist and may vary by setting. Some protocols call for annual evaluations following verification of normal (peripheral) hearing status up until the age of 3 or 5 years.32,33 At present, no protocol is suggested for the assessment of central auditory processing skills of premature children. Ongoing assessment of hearing and auditory skills is essential to identifying later-onset hearing loss and/or delayed auditory skills.32 Studies in our lab found that 8- to 11-year-old children with gestational age of less than 37 weeks exhibited reduced auditory temporal processing and patterning skills (i.e., frequency pattern test and gaps-in-noise) and significantly different binaural interaction abilities (i.e., masking level difference).34 The results suggest that compared with their full-term peers, they exhibited a reduced ability to analyze acoustic events over time, as well as processing intensity or time differences of binaural acoustic stimuli. These differences were not seen in older cohorts (age 12-15 years) on the same auditory processing tests, suggesting a lag in the maturity of those auditory skills.35 The older cohort, however, still presented with significant differences in binaural integration (i.e., two-digit pair dichotic digits) and speech-in-noise testing (Listening in Spatialized Noise Test [LiSN]).36These findings reflect immaturity of higher-level processing of auditory information required for binaural integration and complex listening, which relies on binaural interaction skills and recognition of degraded speech stimuli. Moreover, these skills are typically adult-like in children aged 12 to 14 years.23-28 With studies showing that the development of central auditory processing skills occur differently in this population, it would be prudent for audiologists to consider evaluating these skills in children as early as possible. Prior research indicates that children born prematurely are at risk for underdeveloped auditory discrimination, which is critical to speech, language, auditory processing, and reading development.29, 30 This risk remains evident through critical school-age years, supporting the need to consider and develop protocols and norms that take into consideration gestational age when assessing the auditory skills of young children. Such considerations have been incorporated into norms established for newborn infant hearing screenings (NIHS) but not in central auditory processing assessments. Central auditory processing tests are available to evaluate children as young as 42 months (i.e., Auditory Skills Assessment [ASA]).37Historically, many experts and clinicians would wait until a child is approximately age 7 to conduct a diagnostic test of the central auditory processing system due to established norms. However, in the past 10 years, more norms are available to test younger children, with audiologists, speech-language pathologists, and related professionals advocating for assessment as early as possible. Monitoring the development of auditory milestones and completing auditory processing skill assessments as soon as test norms allow will facilitate early intervention. Norms that consider earlier intervention and prematurity can prevent long-term deficits caused by immature auditory processing skills in young children. Making progressive steps toward widespread auditory skill assessments may improve the hearing health care for premature children who are at a higher risk for immature auditory skills. To make this possible, practicing audiologists working with the pediatric population should strive to (1) meet the recommended diagnostic hearing evaluations at 9 months of age for premature children,32 (2) inform parents during the 9 month visit of typical auditory skills development milestones, and (3) recommend auditory skills screening before age 4 to promote universal pre-school hearing screenings.

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