Early Diagnosis and Prevention of Hearing Loss

8.1

Normal Human Auditory Development

There are gradients of maturation in the auditory system. One gradient is characterized by early maturation of the brainstem and reticular activating system (RAS) pathways, followed by a later and very extended maturation of thalamocortical and intracortical connections. It is, however, clear that the specific lemniscal and extralemniscal auditory pathways mature at different and slower rates than the nonspecific RAS. The parallel processing in these three pathways may offer ultimately a top–down influence on processing of auditory information ( ) and involve top–down mechanisms of perceptual learning (see chapter: Brain Plasticity and Perceptual Learning ). Another gradient is that of the maturation of cells and axons in cortical layers, initially in layer I followed by layers IV – VI and then upward to the superficial layers II – III ( ).

The peripheral to cortical maturational processes suggest that the roughly two decades of human auditory maturation can be divided into several periods dictated by structural or functional temporal landmarks. One may divide anatomical development into a perinatal period (third trimester to 6 months postnatal), early childhood (6 months to 5 years), and late childhood (5–12 years) as done by . Although the division into these periods has its merits, explored a separation of the maturational sequence as reflecting two major auditory processes: discrimination and perception of sound. The first developmental period is manifested by early auditory discrimination and is characterized by maturation in the brain stem, the RAS, and cortical layer I. This process is determined by increasing axonal conduction velocity and is largely complete at age 1 year, though fine-tuning occurs into the second year of life ( Fig. 8.2 ).

Infants younger than the age of 6 months have the ability to discriminate phonemic speech contrasts in nearly all languages, a capability they later lose when raised in a one-language environment. In contrast, the histology of the brain in the first half-year of life indicates only a poor and very partial maturation of the auditory cortex. This discrepancy suggests either that infants rely largely on subcortical processing for this discrimination, or that the methods used in quantifying the structural and physiological properties of the auditory system are incomplete, or at least insensitive. It is likely that the cortical input in this period is mainly due to that provided by the early maturing RAS. A detailed argument is provided in the work of .

The second major maturational period reflects the development of auditory perception, the attribution of meaning to sound, with its neural substrate in cortical maturation. This process depends on synapse formation and increasing axonal conduction velocity and has a maturational onset between 6 months and 1 year. The age of 6 months is a behavioral turning point, with changes occurring in the infant’s phoneme discrimination. This is more or less paralleled by regressive changes in the constituent makeup of layer I axons in the auditory cortex and the onset of maturation of input to the deep cortical layers. One could entertain the idea that, at about 6 months of age, the cortex starts to exert a modulating or gating influence on subcortical processing via efferents from the maturing layers IV – VI, resulting among others in the loss of discrimination of foreign language contrasts. The period between 2 and 5 years of age, the time of development of perceptual language, is characterized by a relatively stable level of cortical synaptic density that declines by 14 years of age ( ). In later childhood, a continued improvement of speech in reverberation and noise, and of sound localization, is noted. At the end of the maturational timeline, one usually considers the hearing of young adolescents as completely adult like. However, speech perception in noise and reverberant acoustic environments does not mature until around age 15. This should be considered in the design of class rooms and the management of sound levels during class.

The maturation of auditory anatomy and behavior is reflected in progressive changes in electrophysiological responses. At approximately 2 years of age, the electrophysiological measures of auditory function in the form of the auditory brain stem response, middle latency response, the late P ₂ component of the cortical auditory-evoked potentials, and the mismatch negativity are fully mature. At about 6 years of age, the long-latency (~100 ms) N ₁ component of the CAEP is typically not recordable with stimulus repetition rates above 1/3 s. Reliability improves over the next 5 years, and the N ₁ is detectable in all 9- to 10-year-olds at stimulus rates of approximately 1/s. Age 12 and up is characterized by major transient changes in the cortical evoked potentials that are likely related to the onset of puberty ( ) and functional aspects of this perceptual process continue to change well into adulthood ( Fig. 8.3 ). Although behavioral measures of auditory perception are mostly adult-like by age 15, the maturation of long-latency CAEPs continues for at least another 5 years thereafter. This may suggest the need for additional behavioral studies in adolescents and provides yet another example of the relative strengths and weaknesses of alternative methods in the evaluation and interpretation of human auditory maturation. However, not all detectable electrophysiological or structural changes need to be behaviorally relevant. Early diagnosis as provided by universal newborn hearing screening (UNHS) is the most relevant and will be discussed in Section 8.3 .

8.2

Effects of Early Hearing Loss on Speech Production

The value of early determination of hearing loss was reported by . They found that early identification of hearing loss and early intervention resulted in significantly better language development. This was exemplified by children whose hearing losses were identified by 6 months of age that demonstrated significantly better language scores than children identified after 6 months of age. drew the conclusion that “identification of hearing loss by 6 months of age, followed by appropriate intervention, is the most effective strategy for the normal development of language in infants and toddlers with hearing loss. Identification of hearing loss by 6 months can only be accomplished through universal newborn hearing screening.” compared the vocalizations 21 infants with normal hearing and 12 early-identified infants with hearing loss over a period of 14 months (from 10 to 24 months of age). They found that children with hearing loss were delayed in the onset of consistent canonical babble relative to age-matched controls. This suggested to them that consonant development in infants with hearing loss was delayed but not qualitatively different from children with normal hearing. extended this to the transition from babble to words and found that both groups increased the purposeful use of voice between 16 and 24 months of age. However, the hearing loss group was much slower to develop expressive vocabulary and demonstrated larger individual differences than the normal hearing group. The delay had significant effects on expressive vocabulary development. looked at the effects of the age of hearing aid or cochlear implant fitting. They measured potential predictors such as age at fitting of amplification (1 month to 6 years), degree of hearing loss in the better hearing ear (ranging from mild to profound), and cochlear implant use. Age at fitting of amplification showed the largest influence and was a significant factor in all outcome models. The degree of hearing loss and cochlear implant use were important factors in the modeling of speech production and spoken language outcomes. These studies strongly suggest the need for early identification of hearing loss.

8.3

Early Detection

Early studies into the feasibility of newborn hearing screening include who evaluated the use of ABRs in 321 high-risk newborns. Of those, 234 received predischarge tests and 200 had follow-up tests. Screening ABR with 40 dB nHL clicks appeared appropriate in the nursery. Screening sensitivity was good, and only 8% of babies failed. Follow-up ABR after 3 months confirmed hearing loss in eight babies. screened an extremely high-risk group of 137 infants from a neonatal intensive care unit (NICU). Of the 137 infants tested, 82 passed the initial ABR, 22 conditionally passed, and 34 failed. Follow-up behavioral and audiometric testing was done in 82 infants. Four infants had severe sensorineural hearing loss (SNHL), each of whom had failed the initial ABR. None of the infants who initially passed or conditionally passed the ABR had SNHL on follow-up testing. An important precursor for UNHS was the study by who screened 1850 infants from the well-baby nursery and NICU using transient evoked otoacoustic emissions (TEOAEs) (see chapter: Hearing Basics ) in a two-stage process. Infants referred from the first stage prior to being discharged from the hospital were rescreened 4–6 weeks later. Those who did not pass the second-stage TEOAE screening were referred for diagnostic ABR and/or behavioral audiological evaluation for confirmation of hearing loss, fitting with amplification, and enrollment in early intervention programs. They identified 11 infants with unilateral or bilateral SNHL more than 25 dB (a prevalence of 5.95 per 1000) and 37 with unilateral or bilateral recurrent conductive hearing loss more than 25 dB (a prevalence of 20.0 per 1000). emphasized critical periods in neural development of hearing as an important incentive for UNHS. In another prequel to UNHS procedures, assessed 149 newborns who had normal automated ABRs and also had distortion product otoacoustic emissions (DPOAEs; chapter: Hearing Basics ) measured. They found that the DPOAE pass rates were lower at low frequencies, likely due to lower signal-to-noise ratios, and that test results could be improved by eliminating frequencies below 2.0 kHz.

A Joint Committee on Infant Hearing statement ( ) had “endorsed the goal of universal detection of infants with hearing loss and encourages continuing research and development to improve techniques for detection of and intervention for hearing loss as early as possible.”