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Hearing evaluation in infants


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Hearing evaluation in infants an update for pediatricians
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Sullivan, Janet E
University of South Florida
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Auditory perception in children   ( lcsh )
Auditory pathways   ( lcsh )
Hearing disorders in children   ( lcsh )
auditory development
otoacoustic emissions
Dissertations, Academic -- Audiology -- Doctoral -- USF   ( lcsh )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )


ABSTRACT: This paper provides an overview of developmental timetables relevant to hearing and of current pediatric audiological techniques and practices. The first sections summarize structural and functional development of the auditory pathway and the development of primary auditory processing. These developmental sequences appear to follow similar paths in humans and animals. Speech and music perception involve more complex processing and are strongly influenced by experience. Hearing disorders affect the perception of complex sounds in a variety of ways, depending on the site(s) of lesions. Early onset hearing impairment, including conductive loss from chronic otitis media, can seriously impede language development. Language cannot develop normally without adequate speech stimulation. Sensitive and inexpensive techniques are available for performing neonatal hearing screening, and early intervention has a positive effect on development of language skills in hearing-impaired children. Thus, the National Institute of Health has recommended nationwide universal newborns hearing screening. The rationale and methodology of universal screening programs is summarized in the chapter. Advances in the field of the genetics of hearing impairment are also reviewed
Includes bibliographical references.
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by Janet E. Sullivan.
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Professional research project (Au.D.)--University of South Florida, 2002.
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Hearing evaluation in infants
h [electronic resource] :
an update for pediatricians /
by Janet E. Sullivan.
[Tampa, Fla.] :
University of South Florida,
Professional research project (Au.D.)--University of South Florida, 2002.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
System requirements: World Wide Web browser and PDF reader.
Mode of access: World Wide Web.
Title from PDF of title page.
Document formatted into pages; contains 48 pages.
ABSTRACT: This paper provides an overview of developmental timetables relevant to hearing and of current pediatric audiological techniques and practices. The first sections summarize structural and functional development of the auditory pathway and the development of primary auditory processing. These developmental sequences appear to follow similar paths in humans and animals. Speech and music perception involve more complex processing and are strongly influenced by experience. Hearing disorders affect the perception of complex sounds in a variety of ways, depending on the site(s) of lesions. Early onset hearing impairment, including conductive loss from chronic otitis media, can seriously impede language development. Language cannot develop normally without adequate speech stimulation. Sensitive and inexpensive techniques are available for performing neonatal hearing screening, and early intervention has a positive effect on development of language skills in hearing-impaired children. Thus, the National Institute of Health has recommended nationwide universal newborns hearing screening. The rationale and methodology of universal screening programs is summarized in the chapter. Advances in the field of the genetics of hearing impairment are also reviewed
Adviser: Chisolm, Theresa
Auditory perception in children.
Auditory pathways.
Hearing disorders in children.
auditory development.
otoacoustic emissions.
Dissertations, Academic
x Audiology
t USF Electronic Theses and Dissertations.
4 856


Hearing Evaluation in Infants: An Update for Pediatricians Janet E. Sullivan A professional Research Project submitted to the Faculty of the Department of Communication Sciences and Disorders in Partial Fulfillment of the Requirements for the Degree of Doctor of Audiology Theresa Chisolm, Chair Lewis Barness Ann Barron April 18, 2003 Tampa, FL Key words: Auditory development, Otoac oustic emissions, Auditory brainstem response Copyright, 2003 Janet E Sullivan


Janet Sullivan i TABLE OF CONTENTS Abstract v Developmental Sequences 1 Prenatal Structural/Physiologic Development of the Auditory system 1 Development of Hearing Function 2 Frequency Resolution 3 Temporal Resolution 4 Intensity Resolution 4 Hearing Sensitivity 5 Complex Sound Perception 6 Speech Perception 7 Normal Sequence of Development of Sensory Systems 8 Auditory Experience and L earning in the Fetus 10 Hearing Impairment and Diagnostic Techniques 11 Epidemiology of hearing Impairment 11 Early Identification of Deafness 12 Historical Perspective 13 Methodology 15 Training 16 Data Management 16


Janet Sullivan ii TABLE OF CONTENTS, continued Types of Hearing Impairment 16 Conductive Hearing Impairment 17 Sensory (Cochlear) Hearing Impairment 18 Neural Hearing Impairment 18 Genetics of Deafness 20 Contributions of Genetic Studi es to Rehabilitation 22 Disorders of Central Auditory Processing 23 Hearing Screening and Diagnostic Methods 24 Assessment of Middle Ear Status 24 Tests of Cochlear Integrity 24 Tests of Neural Function 25 Tests of Central Auditory Function 25 Summary 26 References 27


Janet Sullivan iii LIST OF TABLES AND FIGURES Table 1: Nuclear gene loci for nonsyndromic hearing loss 36 Figure 1: Schematic representation of the inner and outer hair cells of the organ of Corti 38 Figure 2: Cross-section of the co chlea showing the position of the organ of Corti between the basilar membrane and the tectorial membrane. 39 Figure 3: Schematic representation of tympanometry 40 Figure 4: Schematic representation of the measurement of distortion product otoacoustic emissions 41 Figure 5: Schematic representation of the origin of the auditory brainstem response. 42


Janet Sullivan iv Hearing Evaluation in Infants: An Update for Pediatricians Janet E. Sullivan (ABSTRACT) This paper provides an overview of de velopmental timetables relevant to hearing and of current pediatric audiologi cal techniques and practices. The first sections summarize structural and functional development of th e auditory pathway and the development of primary auditory processing. These developm ental sequences appear to follow similar paths in humans and animals. Speech and music perception involve more complex processing and are strongly influenced by e xperience. Hearing disorders affect the perception of complex sounds in a variety of ways, depending on the site(s) of lesions. Early onset hearing impairment, including condu ctive loss from chronic otitis media, can seriously impede language development. Language cannot develop normally wit hout adequate speech s timulation. Sensitive and inexpensive techniques are available fo r performing neonatal hearing screening, and early intervention has a positive effect on de velopment of language skills in hearingimpaired children. Thus, the National Inst itute of Health has recommended nationwide universal newborns hearing screening. The rationale and methodology of universal screening programs is summarized in the chapter. Advances in the field of the geneti cs of hearing impairme nt are also reviewed


Janet Sullivan 1 Recent advances in the field of auditory physiology coupled with longstanding concerns about delayed identi fication of hearing impairment have precipitated public health initiatives (National Institute of H ealth, 1993) and legislation for neonatal hearing screening programs (Blake & Hall, 1990). Pediatric audiology, once more “art” than science, is now largely based on physiologic methods rather than observed behavior. With current techniques, it is not only possible to detect hearing impairment at birth but also to determine the site of the lesion and to obtain close estimate s of hearing threshold at specific frequencies (Werner, Folsom & Mancl, 1993). Habilitative measures, including amplification, can begin within week s of birth. Protocols for the management of hearing impairment are guided not only by the site of the lesion but by the developmental sequences and interactions among all of the child’s sensory modalities. This chapter provides an overview of developmental timetables relevant to hearing and of current pediatric audiolog ical techniques and practices. DEVELOPMENTAL SEQUENCES PRENATAL STRUCTURAL/PHYSIOLOGIC DEVELOPMENT OF THE AUDITORY SYSTEM For obvious reasons, most studies of structural and physiologic development of the auditory system have been performed in animals. However, gross anatomical development of the auditory system in humans appears to follow the same sequence as that of many animal species (Pujol., Lavigne-R ebillard & Uziel, 1990). Avian species provide particularly useful models, b ecause their behavior, including embryonic behavior, has been studied extensively, and th eir hearing develops si milarly to that of


Janet Sullivan 2 humans. Comparative studies show that the relationship between the onset of hearing and time of birth varies among animal spec ies, but the proportion of time between conception and the onset of audito ry function (referred to as the silent period ) is consistent across species. Morey and Carlile (1990) demonstrated similarity in this proportion among species by recording the time of onset of the firs t cochlear response (the cochlear microphonic). Although the time of appearance of the cochlear microphonic is not known in humans, othe r morphologic and physiologic measures indicate that the silent peri od in humans is remarkably similar to that of other animals (Pujol, Lavigne-Rebillard, & Uz iel, 1990; Bredberg, 1968). The otic placode can be identified around 23 days of gestation (Streeter, 1917), and differentiation of the organ of Corti begins around the tenth week of gestation. As in other mammals, structural development follows a base-to-apex grad ient, corresponding to the lowand highfrequency regions of the organ of Corti (Bredberg, 1968). Differentiation of hair cells, se paration of the tectorial membra ne from the organ of Corti, and innervation in the human cochlea at 24 w eeks gestation are similar to those of other mammals at the onset of their cochlear f unction. Action potentials elicited from the eighth nerve at 27 weeks gestation in preter m infants (Stockard, Stockard,& Coen, 1983) confirm that the cochlea is functional and the neural connection is present at that time. DEVELOPMENT OF HEARING FUNCTION Structurally, the cochlea appears capable of function around thr ee months before birth (Lavigne-Rebillard & Pujol, 1988). However, assessing responsivity in utero is complicated by the attenuation of environm ental sounds by the mother’s abdomen and


Janet Sullivan 3 amniotic fluid and by the high level of ambient noise in utero. Gross motor responses (Wedenberg, 1965). or heart rate changes (T anaka & Arayama, 1969) can be elicited by intense sound at 26 to 28 week s gestational age. Lower intensities are not effective. Response thresholds could be elevated by mask ing noise, by the lack of air conduction of sound, or by cochlear immaturity. Behavior al studies of preterm infants do not shed much light on the question. There have been no attempts to measure behavioral threshold to sound in preterm infants, because behavioral responsivity at this age is not a reliable index of hearing (Gerber, Lima & Copriviza, 1983). Primary auditory processing refers to the extraction and co ding of the physical attributes of sound, while secondary audito ry processing can be thought of as the selection of combinations of both the quantit ative and qualitative attributes of sound to accomplish a “perceptual goal” (Werner, 1996) tasks which might include the detection of sound, discrimination among sounds and understanding of connected speech. In children, perceptual goals would be revealed by preferential listening tasks. The following sections on the resolution of physi cal attributes summarize development of primary auditory processing. These fundamental abilities appear to be less influenced by experience than are secondary auditory processing tasks. Frequency Resolution. Frequency resolution or t uning within the auditory system refers to the specificity with which structures in the aud itory system respond to sounds of a particular frequency, a primary function necessary for discrimination of frequencies. In the prenatal period, cochlear and neural maturation are the main contributors to development of frequency


Janet Sullivan 4 resolution. Tuning within the cochlea is mature as early as it has been measured (Abdala & Sininger, 1996). but within neural structures, it con tinues to develop up to six months of age (Folsom & Wynne, 1987). After that, attention may be the primary factor for the continued improvement in freque ncy resolution throughout childhood (Werner & Marean, 1996). Temporal Resolution. Temporal resolution refers to abilities to detect changes in the timing of sound such as detection of brief gaps in sound, discrimi nation of sounds of varying duration, and improvement of performance with increas ing duration of sound. It has a longer developmental course than intensity or fre quency resolution, possibly because it is more dependent upon attention and memory (Werner & Marean, 1996). Early in development, factors such as myelination, fiber diameter and synaptic efficiency are significant contributors to development of temporal resolution. Intensity resolution The dynamic range of auditory neurons is restricted in developing mammals. This might be expected to influence an infant’s ability to detect small changes in stimulus intensity or to perceive loudness in a mature fashion. The amplitude of the auditory brainstem response would be a reasonable appro ach to measuring this ability in infants, but its variability is too large. Thus, the few studies performed to date have relied primarily on behavioral methods, and little is known about the time course of the development of intensity resolution.


Janet Sullivan 5 Hearing Sensitivity. Developmental studies using physiologic estimates of hearing threshold reveal that there is a period of rapid improvement from birth to six months, followed by a slower phase of development until around ten years of age (Werner & Marean, 1996). Several mechanisms could explain the improvemen t in hearing thres hold with age. Maturation of external and middle ear st ructures is an important contributor. Studies of energy reflectance have shown that energy tr ansfer through the external and middle ear continues to mature beyond 24 months of ag e (Keefe, Ling & Bulen, 1992). A caudal to rostral progression of maturation occurs within the auditory pathway: two months for the eighth nerve, two to three years for the aud itory brainstem pathway, and the teen-years for cortical pathways (P onton et al, 1996). In children mature enough to understand and cooperate, behavioral measures are used for threshold assessment. These tests are highly dependent upon listening strategies, motivation and attention, whic h also change with age. Prenatal factors other than auditory experience may also in fluence the course of auditory development. Newborns of moth ers who smoke are less readily aroused by auditory stimuli than those of non-smokers (Franco et al, 1999). Not surprisingly, maternal undernutrition can result in delaye d development of the auditory brainstem pathway. One study suggested more rapid development of brainstem auditory conduction time in breastfed vs formula-fe d newborns (Amin et al, 2000).


Janet Sullivan 6 Complex Sound Perception The preceding sections have dealt with the most fundamental ab ilities to distinguish quantitative features of simple sounds. Bu t, the most important properties of hearingfunction involve the analysis of complex e nvironmental sounds varying in frequency and amplitude over time. Our ability to perceive the qualitative attributes of sound, such as loudness, pitch and timbre, are more difficu lt to measure but are essential to the understanding of speech and perception of music. The pitch of tones in sequence gives ri se to melody, while timbre evokes a quality of sharpness or brightness. The vowels in speech can be described as harmonic complexes of different timbres (Werner & Marean, 1996). While adults can be asked to rate various attributes of complex sound, we can only infe r infants’ abilities and preferences from discrimination studies based on behavior. From such studies, it appe ars that infants are particularly responsive to speech stimuli (Hutt et al, 1968) and that newborns prefer their mother’s voice over that of anot her female (DeCasper & Fifer, 1980) .. Furthermore, infants are more responsi ve to “baby talk” or infant directed speech characterized by high fundamental frequency and unique patter ns of intonation (Fernald, 1985). When presented with tonal, non-speech stimuli mimicking the intonation-contour of infantdirected speech, infants prefer these stimu li over tonal stimuli wit hout the unique contour (Fernald & Kuhl, 1987). Some investigators hypothesize that infant directed speech facilitates language acquisition either by provi ding cues to syntactic structure and by segmenting ongoing speech to separate individual words (Morgan, Meier & Newport, 1987) or simply by promoti ng social interaction (Snow, 1993). Infants have poor low-


Janet Sullivan 7 frequency discrimination (Olsho, 1984; Olsho, Koch & Halpin, 1982) and this ability continues to develop well into childhood (Maxon & Hochberg, 1982). Whether this contributes to their preference for the large intonation contours of infant-directed speech is not known. In contrast, high frequency di scrimination, an important ability for the perception of subtle differences among the c onsonants of speech, de velops quickly during infancy. Three month old infants appear to have an adult-like perception of pitch as it relates to octaves (Demany & Armand, 1984), in that, as one tone approaches an octave above a second tone, the two tones are perceived as similar. When multiple tones are harmonically related, we perceive them as one pitch, which is relate d to the fundamental or lowest frequency. If the fundamental fr equency is removed from the complex, leaving only the harmonics, the perception of pitch is unchanged. This virtual pitch perception appears to be present in infants at least by 7 months of age (Clarkson & Clifton, 1985). but It is not known whether lear ning and experience play a role Very little is known about the development of musi cal pattern perception prior to six months of age. It appears that infants are able to process di fferent aspects of music simultaneously. For example, 6 month olds are sensitive to cha nges in rhythm even when melody and tempo are changed simultaneously (Trehub & Thorpe, 1989). Speech Perception. The smallest segments of speech, which assign or change meaning in words, can be identified by adults regardless of the voi ce-variations among speakers. Therefore, invariant acoustic cues exist, which label these segments or phonemes All of the


Janet Sullivan 8 specific abilities relating to in tensity, frequency and timing di scussed in previous sections are recruited in the development of detecti on of these cues for speech perception. For example, the phonemes /b/ and /p/ differ in voice onset time by a matter of milliseconds, but the small timing difference allows us to di stinguish the two sounds in words. This example of categorical perception is present in early infanc y, but the boundaries between categories in the native language shar pen with development (Aslin, 1981). Young infants demonstrate the ability to distinguish phonemes, which do not exist in their native language (Aslin, 1981; Lasky, Syrdal-Lasky & Klein. 1975). They become less sensitive to non-phonemic differences among consonants between 6 and 12 months of age, apparently as a result of lingusitic experience (Werker & Lalone, 1988). The loss of this ability is permanent. American adults can discriminate the English phonemes /r/ and /l/, but the acoustic difference between these so unds in Japanese does not signal a change in meaning and is not discrimina ted by mature native Japanese (Miyakawa et al, 1975). Similarly, vowels vary greatly in spectral characteris tics among speakers, yet adults are able to sort them into equivalent cla sses based upon a general sp ectral contour, where relative pitch is attended to over absolute p itch. Two-month-old infants are apparently able to categorize at least some vowels (Marean, Werner & Kuhl, 1992). NORMAL SEQUENCE OF DEVELOPMENT OF SENSORY SYSTEMS Sensory systems in all animal s develop in a predictabl e order: somatosensory, vestibular, olfactory, auditory, and visual (G ottleib, 1971). Animal st udies indicate that interactions between auditory structures and the environment are critical to normal development. Knowledge of the effects of abnormal sequencing of sensory events is derived from animal data, but human preterm bi rth or the absence of auditory stimulation


Janet Sullivan 9 in the deaf infant can approxima te those experimental conditions. It has been proposed that sensory stimulation, which occurs earlier in development than usual, can interfere with learning from other sensory modalities in immature animals (Turkewitz & Kenny, 1982). Fo r example, visual structures mature in early intrauterine life, but patt erned visual stimuli are not avai lable in that environment. In contrast, auditory structur es, which are nearly mature in late intrauterine life, do receive patterned stimulation, primarily from biologic noise and the voice of the mother. Early auditory perception is normally free to develop in the absence of competition from the developing visual system. In the pret erm infant, however, patterned visual stimuli are experienced much earlier than usual. Animal studies have clearly demonstrated deficits in auditory learning as a result of improper sequencing of sensory stimulation. Gottlieb et al demonstrated that ducklings hatched at a normal time and exposed to mother’s vocalizations recogni zed and “preferred” her vocali zations over other sounds. However, ducklings exposed to light prior to normal hatching did not learn to recognize the mother’s call (Gottleib, 1980; Gottlei b, Tomlinson & Radell, 1989; Radell & Gottleib, 1992). Unnatural stimulation of an earlier developing system, in this case the vestibular system, interferes with auditory learning in duck embryos. When embryos and hatchlings were exposed to rapid oscillati ons of a waterbed, those exposed prior to hatching were unable to recogni ze or learn the mother’s call, whereas hatchlings exposed to the same stimulation recognized the call (R adell & Gottleib, 1992). It is not known whether these differences result from cha nges in neural organization (Trukewitz & Kenny, 1982) or from altered attention Gottle ib, Tomlinson & Radell, 1989) The normal sequence of patterned stimulation in various modalities is certainly interrupted in


Janet Sullivan 10 infants born prematurely, but the cons equences are not known at this time. The organization of neurons in the auditory areas of the cerebral cortex into clusters occurs in response to stimulation. The peri od during which the development of sensory areas of the cortex can be compromised by s timulus-conditions is referred to as the sensitive period Premature infants at 23 weeks gest ation through the earl y months of life in the neonatal intensive care unit are in th e sensitive period for cochlear development. The critical period for language development spans th e first 2-3 years of life during which adequate speech stimulation is required. Conge nitally deaf children who are not provided early amplification should be considered at risk for perman ent structural and functional changes in development. Neuropathologic studies of seven profoundl y deaf humans revealed that cell size in the cochlear nucleus was inversely correlated with the duration of deafness. Similarly, auditory deprivation in animals has been shown to alter functional and structural development in the peripheral and central auditory pathways (Batkin, Groth & Watson, 1970; Webster & Webster, 1977; Moore, 1990; Tierney, Russell & Moore, 1997). Deprivation of “meaningful” or patterne d sounds results in Impaired auditory discrimination and processing in animals. Structural changes in the central auditory pathway can also result from unilateral deafness. All of th ese findings have implications for global developmental processes in children with hearing impairment. AUDITORY EXPERIENCE AND LEARNING IN THE FETUS Although data are limited, it appears that prenatal experience influences auditory development in humans as well as in animal s. Although sounds ar e attenuated by the


Janet Sullivan 11 abdomen and fluid, normal conversation ne ar the pregnant abdomen is probably recognizable in pitch and rhythm and can be appreciated by the term infant (Stein, Spieker & MacKain, 1982). New borns prefer their mother’s voice over other voices. They can be conditioned to suck on a pacifier at a particular rate in order to initiate the sound of mother’s voice (DeCasper & Fifer, 1980) However, they show no preference for their father’s voice with which they had little or no pren atal experience – over another male voice (DeCasper & Prescott, 1984). Infants exposed for the last six weeks in utero to mothers’ reading a st ory with a distinctive caden ce preferred hearing it over another story shortly after birt h. Newborns who did not have that prenatal experience showed no preference (DeCaspe r & Spence, 1986). HEARING IMPAIRMENT AND DI AGNOSTIC TECHNIQUES EPIDEMIOLOGY OF HEARING IMPAIRMENT As estimated by the most recent surv ey of the prevalence of “serious” hearing impairment in children in this country (CDC 1997), the average annual prevalence rate is 1.1 per thousand children (aged 3-10 years). Two-thirds of the children with impairment had a sensorineural loss that did not result fr om a postnatal cause. Half of those with prenatal/neonatal onset were di agnosed after the age of three. The survey included only those children with hearing loss >40dBHL. However, a loss of 30-40 dBHL in the speech frequencies would render most consonants in conversational level speech inaudible to prelingual children. Thus, th e reported prevalence from this study is probably not an accurate indication of significant childhood hearing impairment. Niskar and associates (1998) tested a sample of 342 children, and found that 17% had


Janet Sullivan 12 a hearing loss of at least 25 dB HL at 1, 2, 4, and/or 6k in one or both ears. Other prevalence studies including bilateral hear ing impairment of this magnitude have revealed rates of 3-5 per thousand (Sorri & Rantakllio, 1985; Watkin, Baldwin & Laoide, 1990). Data from groups of hospitals with universal hearing sc reening programs have provided specific information on the prevalen ce of hearing impairment among newborns. In Colorado, over 41,000 infants were test ed and bilateral co ngenital hearing loss requiring amplification was found to occur in 2 per thousand infants (Mehl & Thompson, 1998). The Rhode Island study of over 53,000 in fants also yielded a rate of 2 per thousand (Vohr, Carty & Moore, 1998). A study of 54,228 newborns in Texas yielded a rate for bilateral sensorineu ral hearing loss of 3.14 per t housand (Finitzo, Albright & O’Neal, 1998). In Hawaii, 10,372 newborns were screen ed, and the incidence of bilateral hearing impairment was 1 per thousand and 5 per t housand in the well-baby and the intensive care unit populations, resp ectively (Mason & Herrman, 1998). Van Naarden and Decoufle (1999) used the CDC surveillance da ta to estimate that 19% of cases of presumed congenital hearing impairment in th e sample were associated with low birth weight. Black infants, partic ularly males, had a higher prev alence rate than other races, even when birth weight was normal. EARLY IDENTIFICATION OF DEAFNESS Language cannot develop normally wit hout adequate exposure to speech-stimuli during the first three years of life. This unde rscores the urgency of early identification of hearing impairment in children. The prev alence of hearing impairment in infancy


Janet Sullivan 13 exceeds that of all other handicapping condi tions for which mandated neonatal screening programs exist (Johnson, Mauk & Takakawa, 1993) Yet despite the relative frequency of its occurrence, there remains an aver age delay of two to three years in the identification of neonatal-onset deafness (H arrison & Roush, 1996), because most infants with severe hearing impairment will startle to loud sounds, will laugh and learn to babble at appropriate ages. Less than 10% of parent s of infants with mode rate-to-severe hearing loss were concerned about the child’s heari ng at the time of diagnosis (Garganta & Seashore, 2000). Delayed habilitation throughout the cr itical period for language development virtually ensures a language defi cit in children with early-onset deafness, and that language delay can re sult in severe learning deficits including read ing problems which are resistant to remediation. Historical Perspective The release of the position statement of the Joint Committee on Infant Hearing (1982) served as the impetus for the development of a number of hearing screening programs for “high-risk” neonates in this country. Methodology for achieving the recommended timetable for identification was not specifie d, because technology had not yet provided a sensitive, reliable and efficient tool for id entifying infants with hearing impairment. Since the publication of that position statem ent, the milieu in which screening programs were evolving was altered by the introduction of two new techniques: auditory brainstem responses (ABR) and evoked otoa coustic emissions (OAE). The latter offered a rapid, inexpensive screen for hearing impairme nt (Kemp & Ryan, 1993), while the former provided an estimate of hearing threshold in neonates (Picton, Oulette & Hamel, 1979).


Janet Sullivan 14 With these developments, cost-effective identification and diagnosis became attainable goals. Once the financial hurdle was lowered, interest shifted toward universal screening rather than screening limited to high-risk neona tes. Over half of deaf neonates have no identifiable risk factor for hearing loss, thus would not be detected by a risk-based screening program (ASHA, 1989). This, along w ith the advent of otoacoustic emissions, prompted the National Institute of Health to recommend universal neonatal hearing screening using otoacoustic em issions as the first-level screen with confirmation by auditory brainstem responses (NIH, 1993). The wisdom of universal hearing screening was challenged by Bess and Paradise (!994) who argued that empirical evidence supporting more fa vorable outcomes in deaf children identified earlie r as compared with those identified later was lacking. However, Yoshinaga-Itano and her associ ates (1998) have since demons trated a clear association between age of identification and outcome in hearing impaired children. Children identified before 6 months of age have expressive and recept ive language quotients significantly higher than those of children identified after 6 months. The impact of early identification is present re gardless of gender, presence of secondary disability, socioeconomic status or age at testing. The same group has ca lculated the actual cost of public services to affected children, the av erage cost to affected families, and the estimated cost of a screening program in Colorado, thus laying the groundwork for a cost-benefit analysis of universal hearing screening. It was es timated that the direct costs of a screening program in Colorado woul d be recovered within ten years of implementation (Mehl & Thompson, 1998). Fa lse-negative rates are negligible, and


Janet Sullivan 15 false-positive rates in long-standing programs range from 0.3%% to 7%. Methodology Equipment with automated “pass/ refer” decision-making capability is now commercially available for neonatal screeni ng by otoacoustic emissions and by auditory brainstem responses. In selecting equipment, consideration should be given only to those systems employing a statistically-proven algorithm for pass/refer decisions. Because Eustachian tube function is in efficient, reabsorption of middle ear fluid and mesenchymal tissue is incomplete in newborns. Ear canals are typically full of debris in the first day(s) of life. Most “false-positive” hearing screens in newborns can be attributed to altered middl e ear function or obstruction (Stockard & Curran, 1990). Either OAE or ABR can be used succe ssfully in universal sc reening programs. OAE equipment is less expensive, easier to operate, and faster. It is una ffected by electrical noise. On the negative side, refer rates are higher because of OAE’s sensitivity to middle ear dysfunction. High levels of ambient noise (a s in the NICU) can interfere with testing. Infants in the NICU are at increased risk of eighth nerve and/ or brainstem dysfunction, which would not alter OAEs. ABR can detect such lesions. This test is also less affected by transient middle ear dysfunction and ambient noise, so refer rates are lower than those of OAE-based programs. But, start-up costs are higher, screening requires more time, and electrical noise can interfere with the te st. When the nursing st aff is responsible for screening, the time factor becomes critical. The ideal model is a two-level inpatie nt screen with an OAE-screen followed by ABR in those infants failing the OAE. Babies who fail the second level screen are referred to a


Janet Sullivan 16 physician for medical management, an audiolog ist for diagnosis and aural rehabilitation and an early intervention program, if available in the community. Training Reimbursement rates for screening are lo w (or non-existent in some states). For that reason, existing hospital staff, low pay-scal e employees or volunteers usually serve as screeners. Nurses may be re sentful of additional responsibi lities, turn-over rates are high for poorly paid employees, and volunteers may not be sufficiently committed to the screening program. Efficient and effective tr aining programs are therefore essential to a national implementation plan. Data Management Tracking of children who were discharged without a screen or who failed the screen is a critical component of an eff ective program. It is not suffici ent to inform a parent of the possibility of deafness. In mass screen ing programs, data management must be computer-based. Software packages designed specifically for univers al hearing screening are available, the most commonly used being Oz Screening Information Management Solution (SIMS) and HI*TRACK. Most automated OAE or ABR systems are compatible with one or both of these database programs. TYPES OF HEARING IMPAIRMENT Encoding of acoustic information occurs at multiple levels of the auditory pathway, and lesions from the end organ upward can disrupt the processing of auditory


Janet Sullivan 17 information. Consequently, the nature of a nd severity of symptoms vary widely as a function of the site of the le sion in the auditory pathway. Conductive hearing impairment Normally, low to moderate intens ity sound reaches the cochlea via air conduction through the external auditory canal, where it impinge s sequentially on the tympanic membrane, the ossicular chain, and the oval window of the cochlea. The middle ear normally serves as a mechanical amplifier. Any condition, which impedes the flow of air in the canal or the movement of the structures of the middle ear will reduce the efficiency of this amplifier and result in attenuation of sound intensity wi th less distortion of quality than other types of hearing impairment. Hi gher intensity sounds (>60 dB) will still reach the cochlea via vibration of the bones of the skull, bypassing the middle ear system. Mild, transient conductive hearing impairment is relatively inconsequential in an older child or adult. In a young child, mild to moderate conductive loss may render imperceptible many of the consonants in convers ational-level speech t hus interfering with speech and language development. If the loss persists over long periods during the critical period for language acquisition, it can have serious structural and functional consequences for language development. For example, 4 to 5 year old children with histories of chronic otitis me dia are less able to distingu ish words signaled by different voice onset times, regardless of intelligen ce or hearing sensitivity on the day of testing (Clarkson, Eimas & Marean, 1989). Hearing loss associated with chronic otitis media is usually fluctuating in nature, interfering with normal binaural hearing expe rience. This could result in impaired sound


Janet Sullivan 18 localization ability and speech processing in noisy environments. Binaural function has been shown to be abnormal in 5 to 7 year old children with past histories of chronic otitis media (Gunnaron & Finitzo, 1991 ). Brier and Gray (1993) were not able to demonstrate any improvement in sound localization or speech processing abilities following surgical correction of unilateral atresia in children, s uggesting that th e effects were permanent. Sensory (cochlear) hearing impairment The fluid within the cochlea can be set into motion either by vibration of the skull (bone conduction) or by movement of the stap es in the oval window of the cochlea (air conduction via the middle ear). The fluid motion results in shearing of the stereocilia of the sensory cells (hair cells), which, in turn, initiates the firing of single nerve fibers terminating in the eighth nerve (Figure 1). Coding of frequency-information begins in the cochlea. Injuries at this level are irre versible and can have devastating effects on both the loudness and quality of sound. Even unilateral hearing loss in children affects speech perception, learning, sel f-image and social skills. Hearing aids are usually effective in cases of pure cochlear hearing loss if there is residual hearing in the speech frequencies. Unfortunately, in many cases, neurons within the cochlea begin to deteriorate, and eight h nerve dysfunction comp licates rehabilitation. Neural Hearing Impairment Axons of the single nerve fibers of th e cochlea assemble to form the auditory portion of the eighth cranial nerve. Their con centric organization by frequency provides evidence of the nerve’s function as a second-le vel coding device for acoustic information. Complex analyses of the intensity, frequency and temporal information in speech are


Janet Sullivan 19 based in part on firing patterns of the eighth nerve. Auditory nerve pathology can lead to total deafness or to such severe distortion that speech sounds cannot be discriminated. Extraction of meaningful sounds from bac kground noise becomes extremely difficult. Hearing aids are less effective or ineffective. Sensory hearing loss may progress to sensorineural hearing loss, when neurons within the cochlea begin to degenerate. This progr ession helps to explain the variability in reported efficacy of hearing ai ds among individuals with similar audiograms. In fact, hearing thresholds to pure tones are extremely poor pred ictors of the success of amplification-devices. Until recently, sensory and neural hearing impairments were not distinguishable by objective testing techniques, thus the term se nsorineural hearing impairment was applied to virtually all permanent hearing disorders. Since the development of techniques for separate assessment of the eighth nerve and the hair cells of the cochlea, auditory neuropathy has been recognized as a distinct aud itory disorder. It is characterized by poor speech discrimination out of proportion to the pure tone hearing thresholds (Starr, Picton & Sininger, 1996), which may be with in normal limits. Binaural processing of sound, frequency discrimination and intensity discrimination are impaired. Ambient noise severely alters speech intell igibility. Temporal aspects of auditory perception are compromised in these patient s (Sininger, Doyle & Moore, 1999). They are unable to detect brief gaps in sounds normally. The symptoms are attributed to abnormal neural synchrony which is important for the perception of acoustic cues in speech. They may act behaviorally deaf as infants and then de monstrate responsivity at later ages, despite the fact that responses cannot be recorded from the eighth nerve or auditory brainstem


Janet Sullivan 20 pathway. Cochlear responses (otoacoustic emissions and cochlear microphonics) are present but may deteriorate with age, possibly as a result of retrograde degeneration of the cochlea. Auditory neur opathy is frequently associat ed with severe neonatal hyperbilirubinemia, which tends to affect structures above the level of the cochlea. Hearing aids are not helpful in these patients. GENETICS OF DEAFNESS. Inherited syndromic deafness does not pr esent the same identific ation-dilemma as that of non-syndromic deafness, because related anom alies usually lead to early testing. New techniques in molecular genetics have grea tly increased our unde rstanding of monogenic inherited disorders. Linking to a single gene requires analysis in large affected families, and long-term studies of several families are currently underway. Based on the symptoms of a disorder in a family a nd existing knowledge about the fu nction of specific genes, a candidate gene is first analyzed in the family fo r abnormal sequencing. Genes serve as templates for the creation and regulation of proteins, guidi ng development, providing for renewal of tissues and regulati ng the function of organs at th e biochemical level. Fifteen genes with various mutations responsible for non-syndromic hearing loss have been identified. The inheritance of monogenic c ongenital hearing loss is autosomal recessive in 75%, autosomal dominant in 20%, x-linke d in 5% and mitochondr ial in less than 1% (Willems, 2000). The POU family of genes is responsible for en coding transcription factors. Mutations on chromosome xq21 in or around the nuclear gene POU3F4 are responsible for x-linked deafness type 3 (DFN3), with progressive hearing loss and fixation of the stapes.


Janet Sullivan 21 DFN15, an autosomal dominant form of progre ssive hearing loss, is related to POU4F3 gene on chromosome 5q which is responsible for transcription of target genes important for the survival of cells in the organ of Corti. Table 1 lists the all of the known loci for non-syndromic hearing loss. Figure 2 shows a cross-section of th e cochlea and sites where genetic defects can occur. The two ducts flanking the cochlear duc t are filled with perilymph. Potassiumrich endolymph fills the cochlear duct housi ng the organ of Corti be tween the basilar and the tectorial membranes, which serve as res onators. The relative movement of these membranes leads to the influx of potassi um ions through channels on the myosincontrolled filaments linking the tips of the stereo cilia of the hair cells. Depolarization of the hair cells results in an electrical signal, which is transmitted to the eighth nerve. Potassium ions probably then flow out through potassium channels in the lateral wall of the hair cells to surrounding s upport cells, to cochlear fibrocyt es and the stria vascularis via connexin channels where they are secr eted back into endolymph through another potassium channel. Mutations have been identified in genes affecting middle ear and cochlear bone development, structural characteristics of th e tectorial membrane, myosin (present in the stereocilia), diaphanous (involved in maintenan ce of the actin-filled stereocilia of the hair cells), fibrocytes (associated with ion channels), and connexin (Willems, 2000). Connexins are channel-forming proteins in me mbranes, fibrocytes and supporting cells of the cochlea. Mutations in the gene for connexin-26 accounts for as much as 50% of autosomal recessive hearing loss (Willems, 2000).


Janet Sullivan 22 The mechanism by which the mutation resu lts in a disorder may be investigated by developing a mutant strain of animal (A nderson, Herrup & Breakfield, 1996). Several animal models of genetic deafness have b een developed, contributing greatly to our understanding of variability in the type, seve rity and time-course of inherited hearing disorders. For example, one x-linked m ouse model produced prof oundly deaf mice with no gross cochlear anatomical defects but with a marked reduction in the endocochlear potential. Fibrocytes, which play a role in the regulation of cochlear potassium homeostasis, are severely affected by the mutation. Another mutant strain has been developed which blocks the normal developm ent of the bony labyrinth and the ossicles. Data from both humans and animals ma y lead to gene therapies for preventing or reversing hearing loss. Contributions of Genetic St udies to Rehabilitation. Studies in families with progressive hearing loss have been particularly useful in revealing patterns of de struction of the cochlea and neur ons within the cochlea and their relationship to speech intelligib ility, effects of noise and efficacy of hearing aids and cochlear implants. Temporal bone studies ha ve been performed in many cases (Halpin, Herrmann & Wheaty, 1996). As mentioned earlier, pure tone thresholds do not correlate well with hearing aid efficacy, probably becau se the detection of simple sounds do not require as many functional channels of intact hair cells, sing le nerve cells and functional connections among auditory structures as are required for speech perception and extraction of signals from noise. When sp eech intelligibility deteriorates following depopulaton of neurons within the cochlea, hearing aids become less effective, and


Janet Sullivan 23 cochlear implants or sign la nguage must be considered. Disorders of Central Auditory Processing Structural orga nization by frequency or tonotopic organization is characteristic of every level of the auditory pathway. Functiona lly diverse eighth nerve fibers branch to the separate frequency-regions of the cochlear nuclei of the brainstem. These structures the first waystations in the brainstem auditory pathway demonstrate selective vulnerability to bilirubin t oxicity (Gerrard, 1952 ) and perinatal hypoxic/ischemic injury (Hall, 1964), thus represent a common site of lesion in perinatall y acquired auditory dysfunction. In many cases, children with lesions of the brainstem auditory pathway will demonstrate deficiencies in the processing of auditory stimuli rather than reduced sensitivity to sound in a quiet environment, and learning disabilities are common in these children. Cortical deafness is a clinical rarity not readily dia gnosed, because patients tend to show inconsistent responsivity to sound with poor speech production and understanding, in spite of normal physiologic responses from the peripheral and brainstem auditory pathways. In behavioral audiometric testi ng with pure tones in a sound room, they may exhibit normal “hearing” thresholds. Bilatera l temporal lesions and cortical deafness have been described secondary to fever (H ood, Berlin & Allen), congenital malformation (Landau, Goldstein & Kleffner, 1957), meningitis (Lechevalier, Rosa & Estache, 1984), or cerebral infarcts (Jerger, Weilers & Shar brough, 1969). Amplification in such patients is ineffective and inappropriate. Rather training in sign language should begin immediately upon diagnosis.


Janet Sullivan 24 HEARING SCREENING AN D DIAGNOSTIC METHODS Assessment of Middle ear Status Tympanometry and acoustic reflex (s tapedius muscle) testing are collectively known as immittance audiometry. The probe of the tympanogram contains a miniature microphone to measure the intensity of a tone which, when introduced into the ear canal, reflects off the tympanic membrane (TM) and tr avels back to the probe. The air pressure in the ear canal is systematically changed from positive to negative in order to alter the compliance of the TM which, in part, determin es the amount of sound energy that will be reflected versus absorbed (Figure 3). Tympanometry is a sensitive test for the presence of fluid in the middle ear, retraction of the TM (negative pressure), disarticulation of the ossicles or perforation of the TM in children 7 months of age or older. Before that age, the walls of the ear canal are cartilaginous and may expand when air pressure is increased in the canal, resulting in a falsely normal reading. Acoustic reflexes are ab sent or elevated in threshold when there is a sensorineural hearing loss or middle ear dysfunction. The acoustic reflex was at one time used to assess brainstem integrity, but more sens itive tests have si nce replaced it. Tests of Cochlear Integrity Before the discovery of evoked oto acoustic emissions, no dire ct physiologic test of cochlear integrity existed. Clin ical application of this hair cell response, in combination with the auditory brainstem response, now pe rmits the separate assessment of cochlear-


Janet Sullivan 25 and the eighth nerve-function. The OAE is ra pid (4-5 minutes), inexpensive, reliable, and objective. It can be recorded in the pret erm / term newborn (awake or asleep) and is highly sensitive to moderate hearing impai rment of either conductive or sensory (cochlear) origin. Evoked otoacoustic em issions are thought to be generated by the elongation and contraction of the outer hair ce lls of the cochlea in response to either repetitive clicks (transient evoked otoacoustic emissions or TEOAEs) or to two-tone stimulation (distortion product otoacoustic emissions or DPOAEs). DPOAEs can be elicited at specific frequencie s to assess cochlear function at specific locations along the basilar membrane on which the hair cells ar e situated. A small probe placed at the entrance to the ear canal presents the t ones and delivers the response to a microphone (Figure 4). Computer software generates either a series of clicks or a “sweep” of two tones across a wide range of frequencies. The same software generates a graph of the spectrum of the emissions record ed in response to the stimuli. These tests do not provide an estimate of hearing threshold. Tests of Neural Function The eighth nerve action potential can be recorded non-invasively from the early preterm period onward. It is us ually recorded in combination with the auditory brainstem response, using the same electrode confi guration and signal averaging equipment. Tests of central auditory function Auditory brainstem responses (ABRs) are used both for the objective assessment of the brainstem auditory pathway (Figure 5) and for the estimation of hearing threshold.


Janet Sullivan 26 The latter permits the selecti on of appropriate amplification at an early age. Electrodes are taped to the scalp, and re petitive clicks or brief t ones are presented through an earphone. Brief samples of the electroencephalogram are collected following the presentation of each stimulus, stored in a computer, and then summated by a signal averaging system. Noise is nulled through av eraging, leaving only th e response, which is time-locked to the stimulus. The patient mu st be sleeping during threshold estimation. Sensitive, objective and simple tests of function of higher levels of the auditory pathway have not been developed at this time. Behavioral batteries of tests of central auditory processing have not proven to be sens itive or specific (Singer, Hurley & Preece, 1998). SUMMARY Hearing impairment of any type in early ch ildhood can have serious permanent effects on development. It can be detected and diagnosed in the newborn period, and early intervention significantly improves language de velopment and reading abilities. Chronic otitis media during the criti cal period for language devel opment may result in altered production and perception of consonants of speech. An aggressive approach to identification, diagnosis and prompt treatment of all types of a uditory disorders is recommended.


Janet Sullivan 27 REFERENCES Abdala, C, & Sininger, Y. (1996). The develo pment of cochlear frequency resolution in the human auditory system. Ear Hear 17 (5), 374-385. American Speech and Hearing Association Committee on Infant Hearing. (1989). Audiologic screening of infants w ho are at risk for hearing impairment. ASHA 31 (3), 61-64. Amin, S.B., Merle, K.S., Orlando, M.S.& Dalze ll, L.E. (2000) Brainstem maturation in premature infants as a function of enteral feeding type. Pediatrics, 106 (2), 318-322. Anderson, J., Herrup, K. & Breakfield, X. (1992) Creation of transgenic mice that overexpress MAO-B neuronally. Ann Am Acad Sci 648, 178-188. Aslin, R.N.. (1981). Experiential influences and sensitive periods in perceptual development: A unified model. In RN Aslin, JR Alberts, MR Petersen (Eds): Development of Perception: Psychological Perspectives Academic Press, New York, pp 45-94. Batkin, S., Groth, H., Watson, J.R., & Ansb erry, M, (1970). Effects of auditory deprivation on the development of auditory sensitivity in albino rats. Electroencephalogr Clin Neurophsiol 28 (4), 351-359. Bess, F.H. & Paradise, J.L. (1994). Universa l screening for infant hearing impairment: not simple, not risk-free, not nece ssarily beneficial, and not presently justified. Pediatrics, 93, 330-334. Blake, P.E. & Hall, J.W. (1990). The stat us of statewide policies for neonatal hearing screening. J Am Acad Audiol 1 : 67-74.


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Janet Sullivan 33 Pujol, R., Lavigne-Rebillard, M. & Uziel, A. (1990). Physiolo gical correlates of development in the human cochlea. Seminars in Perinatology 14 275-280 Radell, P.L., Gottleib, G. (1992). Developmen tal intersensory interference: Augmented prenatal sensory experience interfer es with auditory learning in duck embryos. Dev Psychol, 28 (5): 795-803. Singer, J., Hurley, R.M., Preece, J.P. (1998) Effectiveness of central auditory processing tests with childre n. Am J Audiol, 7: 1-11.l Sininger, Y.S., Doyle, K.J., Moore, J.K. ( 1999). The case for early identification of hearing loss in children: auditory system development, experimental auditory deprivation and development of speech and hearing. Pediatr Clin N Am 1999; 46, 1-14. Snow, C.E. (1993). Understanding soci al interaction and language development: sentences are not enough. In M Bornstein and J Bruner (Eds). Interaction in Human Development Hillsdale, NJ: Erlbaum.27. Sorri, M. & Rantakallio, P. Prevalence of hearing loss at the age of 15 in a birth cohort of 12,000 children from northern Finland. Scand Audiol, 14 :203-207. Starr, A., Picton, T.W., Sininger, Y., Hood ,L .J. & Berlin, C.I. Auditory neuropathy. Brain, 11 741-753. Stern, D.N., Spieker, S., MacKain, K. Intona tion contours as signals in maternal speech to prelinguistic infants. Dev Psychol 18 (5): 727-735. Stockard, J.E., Stockard, J.J., Coen, R.W. Auditory brain stem re sponse variability in infants. Ear Hear 4(1): 11-23.


Janet Sullivan 34 Stockard, J.E. & Curran, J.S. Transient elev ation of threshold of the neonatal auditory brain stem response. Ear Hear, 11 (1): 21-28. Streeter, L. (1917). The development of the scala tymapni, scala tymapni and perioticular cistern in the human embryo. Am J Anat 21, 299-320. Tanaka, Y. & Arayama, T. (1969). Feta l responses to acoustic stimuli. Practica in Oto-RhinoLaryngol, 31 : 269-273. Tierney, T.S., Russell, F.A. & Moore, D.R. (1997). Susceptibility of developing cochlear nucleus neurons to deafferentation-indu ced death abruptly ends just before the onset of hearing. J Comp Neurol, 378 : 295-306. Trehub, S.E. & Thorpe, L.A. (1989). Infants’ perception of rhythm Categorization of auditory sequences by temporal structure. Canadian J Psychol 43, 217-229. Turkewitz, .G.,& Kenny, P.A. (1982). Limitations on input as a basis for neural organization and perceptual deve lopment: A preliminary theoretical statement. Dev Psychobiol, 15 (4): 357-368. Van Naarden, K. & Decoufle, P. (1999). Relati ve and attributable risks for moderate to profound bilateral sensorineu ral hearing impairment associated with lower birth weight in children 3 to 10 years old. Pediatrics 104 : 905-910. Vohr,, B..R.., Carty, L..M., M oore, P..E..,& Letourneau, K. (1998). The Rhode Island a ssessment program: Experience with statewide hearing screening (1993-1996 ). J Pediatr, 133 (3): 353-358. Watkin, P.M., Baldwin, M. & Laoide, S. (1990) Parental suspicion an d identification of hearing impairment. Arch Dis Child 65: 846-850.


Janet Sullivan 35 Webster, D.B. & Webster, M. (1977). Ne onatal sound deprivation affects brain stem auditory nuclei. Arch Otolaryngol 103 (7), 392-396. Wedenberg, E. (1965). Pren atal tests of hearing. Acta Otolaryngolgica 206, 27-31. Werker, J.F. & Lalone, C.E. (1988). Cr oss-language speech perception: initial capabilities and developmental change. Devel Psychol 24: 672-683. Werner, L.A. (1996). The development of audito ry behavior (or what the anatomists and physiologists have to explain, Ear Hear 17(5): 438-446. Werner,, L.A., Folsom, R.C. & Mancl, L.R.. (1993). The relationship between auditory Brainstem response and behavioral th resholds in normal hearing infants and adults. Hear Res 68 (1): 131-141. Werner, L..A. & Marean, G..C. (1996). Human Auditory Development Westview Press, Boulder, CO. Willems, P.J. (2000). Genetic causes of hearing loss. NEJM 342 (15), 1101-1108. Yoshiniga-Itano, C., Sedey, A.L., Coulter, D. K &, Mehl AL. (1998). language of earlyand later-identified cildren with hearing loss. Pediatrics 102 : 1161-1171


Janet Sullivan 36 Table 1. Nuclear Gene Loci for Non-Syndromic Hearing Loss Locus Gene Chromosomal Location Autosomal dominant (DFNA) DFNA1 Diaphanous 5q31 DFNA2 Connexin 31 1p34 And KCNQ4 DFNA3 Connexin 26 13q12 DFNA4 19q13 DFNA5 ICERE-1 7p15 DFNA6 4p16 DFNA7 1q21-23 DFNA8 a-Tectorin 11q22-24 DFNA9 COCH 14q11-13 DFNA10 6q22-23 DFNA11 Myosin 7A 11q12-21 DFNA12 a-Tectorin 11q22-24 DFNA13 6p21 DFNA14 4p16 DFNA15 POU4F3 5q31 DFNA16 2q24 DFNA17 22q DFNA18 3q22 DFNA19 10 Autosomal recessive (DFNB) DFNB1 Connexin 26 13q12 DFNB2 Myosin 7A 11q13 DFNB3 Myosin 15 17p11 DFNB4 Pendrin 7q31 DFNB5 14q12 DFNB6 3p14-21 DFNB7 9q13-21 DFNB8 21q22 DFNB9 Otoferlin 2p22-23 DFNB10 21q22 DFNB11 9q13-21 DFNB12 10q21-22 DFNB13 7q34-36 DFNB14 7q31 and 19p13 DFNB15 3q21-25 DFNB16 15q21-22 DFNB17 7q31 DFNB18 11p14-15 DFNB19 18p11 DFNB20 11q25-ter


Janet Sullivan 37 Table 1 continued DFNB21 a-Tectorin 11q X-linked recessive (DFN) DFN 1* DDP xq22 DFN 2 xq22 DFN 3 POU3F4 xq21 DFN 4 xp21 DFN 5 Withdrawn DFN6 xp22


Janet Sullivan 38 Figure 1: Schematic representation of the inner and outer hair cells of the organ of Corti


Janet Sullivan 39 Figure 2: Cross-section of the co chlea showing the position of the organ of Cor ti between the basilar membrane and the tectorial membrane.


Janet Sullivan 40 Figure 3: Schematic representation of tympanometry


Janet Sullivan 41 Figure 4: Schematic representation of the measurement of distortion product otoacoustic emissions


Janet Sullivan 42 Figure 5: Schematic representation of the origin of the auditory brainstem response.


Filename: ThesisManuscript.WPD1.doc Directory: C:\Documents and Settings\jvanoss\Local Settings\Temporary Internet Files\OLK21 Template: C:\Documents and Settings\jvanoss\Application Data\Microsoft\Templates\ Title: Recent advances in the field of auditory physiology coupled with longstanding concerns about delayed identification of heari Subject: Author: Infectious Disease Keywords: Comments: Creation Date: 4/7/2003 10:21 AM Change Number: 15 Last Saved On: 4/8/2003 10:45 AM Last Saved By: janet E Sullivan Total Editing Time: 319 Minutes Last Printed On: 4/8/2003 10:57 AM As of Last Complete Printing Number of Pages: 47 Number of Words: 9,301 (approx.) Number of Charac ters: 53,016 (approx.)