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Ear & Hearing:
doi: 10.1097/AUD.0b013e31829db914

Wideband Acoustic Immittance Measures: Developmental Characteristics (0 to 12 Months)

Kei, Joseph1; Sanford, Chris A.2; Prieve, Beth A.3; Hunter, Lisa L.4

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1Hearing Research Unit for Children, School of Health and Rehabilitation Sciences, University of Queensland, Division of Audiology, Queensland, Australia; 2Department of Communication Sciences and Disorders Idaho State University, Pocatello, Idaho, USA; 3Department of Communication Sciences and Disorders, Syracuse University, Syracuse, New York, USA; and 4Communication Sciences Research Center Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA.

ACKNOWLEDGMENTS: The authors declare no conflict of interest.

Address for correspondence: Joseph Kei, Division of Audiology, School of Health and Rehabilitation Sciences, University of Queensland, Brisbane, QLD 4072, Australia. Email:

Received February 22, 2013

Accepted May 26, 2013

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Measures of the external and middle ear function in humans such as tympanometry (single frequency or multifrequency), acoustic stapedial reflex (ASR) test, and wideband acoustic immittance (WAI) measures are influenced by the dynamic characteristics of the peripheral auditory system. In particular, the results of these measures in healthy newborns (0 to 4 weeks) and young infants (1 to 12 months) are different from those of normally hearing adults. Such differences may be attributed to the developmental aspects of the peripheral auditory system. Evaluation of the function of the peripheral auditory system of newborns and young infants requires an appreciation of the maturation of the auditory system from birth.

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The developmental changes of the external and middle ear are described in detail by Wilson (2012). Briefly, the external and middle ear of a neonate is not mature at birth. The peripheral auditory system undergoes a fast developmental period from birth to 12 months. The external auditory canal is surrounded by a thin layer of elastic cartilage at birth (Mclellan & Webb 1957). On pressurization as in tympanometry, the diameter of the external auditory canal may increase by an average of 18.3% under positive pressure or decrease by an average of 28.2% of its original value under negative pressure (Holte et al. 1990). With maturation, ossification of the inner two thirds of the external auditory canal occurs, thereby increasing the stiffness of the external auditory canal. The length of the external auditory canal increases with age, resulting in a decrease in resonance frequency of the ear canal (Kruger 1987). The tympanic membrane decreases in thickness, increases in size, and changes its orientation with respect to the external auditory canal (Anson et al. 1955; Eby & Nadol 1986; Ikui et al. 1997; Qi et al. 2006). The mass and resistance of the middle ear decrease due to changes in bone density of the ossicles and loss of mesenchyme and fluids in the middle ear (Richany et al. 1954; Olszewski 1990). The stiffness of the middle ear increases owing to changes in the orientation and fiber structure of the tympanic membrane, fusion of the tympanic ring, and tightening of the ossicular joints. The combined effect of the decrease in mass and increase in stiffness of the middle ear results in an increase in the lowest resonance frequency of the external and middle ear with age (Holte et al. 1991; Homma et al. 2009).

Reports of the physical size of the middle ear and mastoid air cells in humans vary considerably among studies. A review of large consecutive studies by Cinamon (2009) revealed that the middle ear and mastoid air cell system is well developed at birth with a volume of 0.7 to 1.1 mL. From 1 to 6 years of age, the volume increases linearly. Thereafter, the rate of growth saturates, reaching an average adult size of 6.5 to 8.0 mL (range = 2 to 22 mL) (Molvaer et al. 1978; Ensari et al. 1999). Sade et al. (2006) found that the volume of the middle ear and mastoid was reduced in children who had a history of chronic secretory otitis media.

The Eustachian tube of a neonate is short (30 mm), almost horizontal (approximately 10°) and surrounded mainly by glandular tissue (Proctor 1967). Its tubal cartilage can only be acted on by the tensor palate muscle, but not by the levator palate muscle which is farther away. The Eustachian tube opens sharply but closes more gradually, resulting in tubal inefficiency. The Eustachian tube develops slowly taking about 7 years to reach full maturation with increased length and greater inclination (45°). This explains, at least in part, the higher prevalence of otitis media associated with upper respiratory tract infections in infancy and early childhood (Holborow 1970, 1975).

These maturational changes will likely affect WAI measures in infants during the first 12 months of life. There is a need to distinguish between variations in WAI measures attributable to maturation aspects and those attributable to disorders in the sound conductive pathways (external and middle ear). To achieve this goal, age-specific normative WAI data for newborns and young infants are required. WAI findings with significant deviation from age-specific norms will require further investigation into the possible causes. This article provides a contemporary review of research in WAI measures in newborns and young infants from a developmental perspective.

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Keefe et al. (2000) conducted a study to investigate the feasibility of obtaining WAI results from newborns. A total of 4163 ears were assessed using a custom-made device with a broadband chirp (see Rosowski et al., this issue, pp. 9S–16S, for a review of the principles of WAI measures). Calibration was performed using six cylindrical tubes of different lengths but having the same diameter (4.85 mm). The calibration procedure calculated the Thevenin pressure and impedance associated with the probe and measurement system by fitting the measured pressure responses to modeled impedance functions based on cylindrical-tube geometry.

WAI data from a selected subset of 2081 ears were analyzed. The median reflectance measured across a frequency range of 0.25 to 8 kHz varied from 0.1 to 0.25, with two minima at 2 and 6 kHz and two maxima at 1 and 4kHz. This large-scale study provides evidence on the possible applications of the WAI measures in newborns for detection of hearing loss. Keefe et al. (2000), however, noted the difficulty in maintaining a proper probe seal during the measurements in some neonates. More research is needed to evaluate methods for obtaining a proper probe seal when using WAI measures with the neonatal population (Merchant et al. 2010).

Shahnaz (2008) successfully measured WAI in 31 newborns (65 ears) cared for in the neonatal intensive care unit (NICU) using a Mimosa Acoustics system. The gestational age of the participants ranged from 32 to 51 weeks with a mean of 37.8 weeks. Reflectance and normalized admittance were measured in 49 ears that had successfully completed and passed all tests, including automated auditory brainstem response, transient-evoked otoacoustic emission (TEOAE), and 1 kHz tympanometry tests. Testing was conducted in an NICU ward where the ambient noise level could not be controlled. In view of the high noise level (65 dBA) in the NICU, WAI measures in the low frequencies were affected. Hence, WAI data between 0.45 and 6 kHz were analyzed.

Figure 1 shows that newborns cared for in the NICU exhibited mean reflectance ranging from 0.18 to 0.64 with a minimum of 0.18 at 2 kHz and a maximum of 0.56 at 4.5 kHz. The high reflectance of 0.64 at 0.45 kHz may be due to the high level of ambient and physiologic noise when the measurement was made. When compared with reflectance obtained from 1-month-old infants (Keefe & Levi 1996), newborns cared for in the NICU showed higher overall reflectance across frequency. The smaller reflectance in 1-month-old infants may be due to receding amniotic fluid and mesenchyme in the middle ear. Despite these discrepancies, both groups had minimum reflectance at around 2 kHz, indicating maximum efficiency of the conductive pathway (outer and middle ear) at this frequency. Interestingly, the newborns cared for in the NICU had greater mean reflectance than 1-month-old infants in the high frequencies (3.5 to 6 kHz). These findings may be attributed, at least partly, to a greater mass loading of the middle ear in NICU babies because mass elements control the conduction of the high-frequency response of the middle ear. Shahnaz (2008) found no significant difference in mean reflectance between left and right ears.

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Sanford et al. (2009) established normative WAI data obtained from newborns who passed a screening test using distortion product otoacoustic emissions (DPOAEs). WAI data were acquired using a prototype system developed by Liu et al. (2008). On day 1 of the hospital-based screening, 375 ears passed DPOAEs and 80 were referred. Median absorbance, measured under ambient pressure condition, in the pass in DPOAEs (DP-Pass) group varied between 0.32 and 0.65 across the frequency from 0.25 to 8 kHz. The median absorbance attained two maxima: 0.52 at 1.6 kHz and 0.65 at 7 kHz. On day 2, 67 of the 80 ears referred on day 1 were assessedwith the result that 53 ears passed and 14 failed the DPOAE screen. Median absorbance obtained from the 53 ears showed a similar pattern of results obtained on day 1 with two maxima at the same frequencies. The median absorbance, ranging from 0.35 to 0.82, was slightly greater than that obtained on day 1. Based on the results obtained on day 1 and day 2, Sanford et al. suggested that the transient sound conduction effects, presumed to have been a contributing factor that led to an initial refer on day 1, had begun to resolve by day 2.

In addition to establishing normative ambient WAI data for newborns, Sanford et al. (2009) also reported normative WAI data under pressurized ear canal conditions (e.g., wideband tympanometry). The results were plotted in a three-dimensional diagram with absorbance plotted against frequency and ear canal pressure. The median absorbance for the DP-Pass group was higher than that of the refer in DPOAEs group between 1 and 8 kHz. The effect of the direction of pressure sweeps on absorbance was also investigated. The results showed that the median absorbance was greater in the descending sweep than in the ascending sweep, especially at negative ear canal pressures (see Sanford et al., this issue, pp. 65S–71S).

A similar study, which aimed to establish normative WAI data in newborns, was conducted by Hunter et al. (2010). A total of 324 newborns (493 ears) aged 3 to 102 hr were assessed using WAI measures, DPOAEs, and 1 kHz tympanometry at two test sites. Ears were classified into DP-Pass (352 ears) and refer in DPOAEs (141 ears) groups. Reflectance was measured using a Mimosa Acoustics system with chirp and tonal stimuli delivered at 60 dB SPL to the neonate’s ear. The median reflectance obtained from the DP-Pass group varied between 0.3 and 0.55 across the frequencies from 1 to 6 kHz. An important finding from this study was that reflectance at 2 kHz decreased significantly with age during the first 4 days after birth, which in turn, translated into higher DP-Pass rates with age during this neonatal period. The association between reflectance and DP-Pass rate is expected in view of the receding fluid and mesenchyme which improves the absorption of energy by the middle ear. Hunter et al. did not find any significant difference in reflectance with regard to gender, ear, birth type, birth weight, stimulus type, and test site.

Acknowledging that DPOAEs per se may have limitations in serving as a reference standard for normal middle ear function, Aithal et al. (2013) adopted a reference standard based on a battery of tests including 1 kHz tympanometry, ASR, TEOAE, and DPOAE tests. Sixty-six of 195 healthy neonates (13.3 to 116.5 hr old; mean 46.0 hr) passed the battery of tests in one or both ears. A prototype Reflwin research system developed by Interacoustics A/S was used for WAI measurements. The system consists of a Windows-based computer coupled to an AT235 acoustic immittance instrument (Interacoustics A/S, Assens, Denmark). Wideband clicks at 55 dB SPL were delivered to the neonate’s ear. Responses from 16 clicks were averaged for each measurement of reflectance from 0.25 to 8 kHz. The 1/3 octave–averaged reflectance results at 16 selected frequencies were reported for several percentiles from 0 to 100. The reflectance data revealed no significant difference between ears or genders. On a graphical display, the median reflectance results showed a pattern with two maxima at 0.5 and 4 kHz and two minima at 1.5 and 6 kHz (see Fig. 2). The median reflectance varied from 0.21 to 0.59.

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Comparison of Reflectance in Newborns With 1-Month-Old Infants

Figure 2 shows a comparison of reflectance results obtained from newborns cared for in the NICU (Shahnaz 2008), healthy newborns (Hunter et al. 2010; Aithal et al. 2013), and 1-month-old infants (Keefe et al. 1993). While the pattern of reflectance results appears to be the same for the three groups, the frequencies at which the minimum and maximum points occur are different. The newborns cared for in the NICU showed minimum and maximum reflectance at 1.7 and 4.1 kHz, respectively, which are higher than 1.5 kHz and 3.9 kHz obtained by the other two groups. The reflectance of the newborns cared for in the NICU is greater than that of the other two groups between 0.45 and 1.2 kHz, indicating slightly greater stiffness of the conductive pathway of the newborns cared for in the NICU than the other two groups. From 1.7 to 3.5 kHz, the healthy newborns attained higher reflectance than the other two groups. Beyond 3.5 kHz, the maximum reflectance of the 1-month-old infants is lower than that of the other two groups, indicating a smaller mass loading effect on the conductive pathway of the 1-month-old infants due to the receding fluid and mesenchyme.

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A close examination of reflectance results between the Aithal et al. (2013) and Hunter et al. (2010) study showed slightly lower reflectance across most frequencies for the Aithal et al. study. This may suggest that using a more stringent reference standard (requiring a pass in 1 kHz tympanometry, ASR, TEOAE, and DPOAE tests) as adopted by the Aithal et al. study could have excluded those cases with less efficient conductive pathways than using a single-test reference standard (e.g., DPOAE only).

Merchant et al. (2010) investigated changes in WAI within the newborn period (birth to 1 month). WAI was assessed in seven newborns (12 ears) and eleven 1-month-old infants (19 ears) who passed newborn hearing screening and DPOAEs using a Mimosa Acoustics system. The reflectance pattern revealed a maximum of 0.6 at 500 Hz, decreased with frequency to reach a minimum at around 2 kHz, and then increased with frequency beyond 2 kHz. Some ears in each group exhibited sharp maxima in the 4 to 6 kHz range. The newborn group showed greater mean reflectance at 2 kHz than the 1-month-old group (0.18 versus 0.09). While there were no gender effects, there were small but significant differences between the left and right ears at three frequency bands between 0.5 and 4 kHz. Despite the small sample size, the reflectance results showed similar patterns between the two age groups, consistent with the findings of Aithal et al. (2013), Hunter et al. (2010), and Keefe et al. (1993) from a developmental perspective.

When the newborn reflectance data of the Merchant et al. (2010) study are compared with those of Aithal et al. (2013), Hunter et al. (2010), and Sanford et al. (2009), there are subtle differences across a wide range of frequencies. The discrepancies may be attributed to differences in subject samples, middle ear characteristics, and methodological differences such as use of different probe tips, different criteria used to ensure a good seal, and calculation of reflectance based on different estimates of the cross-sectional area of earcanals (see Table 1 for comparison of findings expressed in both reflectance and absorbance).

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WAI Measures in Infants (1 to 12 Months)

An early attempt to study the maturation of the external and middle ear of infants was conducted by Keefe et al. (1993) using a custom-made WAI system. A number of WAI measures including impedance level, impedance phase, resistance, reactance, and energy reflectance (ER) were measured at ambient pressure across a frequency range of 0.125 to 10.7 kHz. Participants were healthy full-term infants aged 1 month (n = 15), aged 3 months (n = 18), aged 6 months (n = 11), aged 12 months (n = 23), and aged 24 months (n = 11); 10 adults with normal hearing served as controls. The WAI results for the 1- and 3-month-old babies showed a distinctive pattern from that of the other age groups (see Fig. 3). As shown in Figure 3, the mean impedance levels (in decibels) for 1-month-old infants are higher than those of other age groups, with two maxima at 0.7 and 4 kHz and two minima at 0.25 and 2 kHz. The high impedance may be explained by the small cross-sectional area of the earcanal of 1-month-old infants because the characteristic acoustic impedance is inversely proportional to the cross-sectional area. Keefe et al. showed that impedance decreases with age over the first 12 months of life as the infants’ ear canal area increases with age. After compensating for the cross-sectional area effects, the normalized impedance data show a different pattern of results with smaller differences in normalized impedance between the infant groups (see Fig. 4). The authors concluded that growth of the ear canal diameter is an important factor accounting for the observed variation of impedance level with age. While there is a high degree of overlap from 0.7 to 8 kHz, there are clear divergences in normalized impedance below 500 Hz across the younger infant groups (1, 3, and 6 months).

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Fig. 4.
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Keefe et al. (1993) also compared reflectance across all age groups (see Fig. 5). The reflectance was large at low frequencies at all ages, indicating that most of the energy was reflected at the eardrum. In particular, the reflectance of the 1-month-old infants appeared to be lower than those of other age groups for frequencies up to 2 kHz. Between 1 and 4 kHz, frequencies that are important for speech perception, the ER attained minimum values. This suggests that energy transmission into the middle ear is most efficient in this range. At high frequencies, ER was high. In conclusion, Keefe et al. suggested that development of the infant’s external and middle ear strongly influences impedance and reflectance measures. Factors contributing to such developmental changes include ear canal growth (both length and cross-sectional area), growth in the middle ear cavities, and resonance of the ear canal walls up to approximately 6 months of age. Keefe et al.’s results indicate that maturation of the peripheral auditory system is not complete by 24 months of age.

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Sanford and Feeney (2008) provided further evidence of maturation effects based on WAI findings obtained under various ear canal pressure conditions. A total of 60 healthy full-term infants, with 20 in each age group of 4, 12, and 24 weeks, were assessed. Infants passed DPOAE and 1 kHz tympanometry tests for inclusion in the study. Infant data on reflectance, admittance, admittance phase, and conductance, measured across a frequency range of 0.25 to 8 kHz at tympanometric peak pressure (TPP), and ±200, ±100, ±50, and ±25 daPa relative to TPP were compared with data obtained from 20 adults with normal hearing.

The reflectance results, shown in Figure 6 (left panel), reveal an increase in reflectance from 0.25 to 2 kHz with pressure for all age groups irrespective of the direction of pressure change. The increase in reflectance indicates an increase in stiffness of the external and middle ear. For 4-week-old infants, the reflectance at 0.25 kHz increased by approximately 30% in response to changes in ear canal pressure, whereas a smaller increase in reflectance was observed in older infants and adults. This indicates that there is less change in reflectance as an infant develops. The decreasing effect of pressure on reflectance with age may reveal ossification of the ear canal wall. Between 2 and 6 kHz, negative pressures caused an increase in reflectance, while positive pressures produced a decrease in reflectance for 4-week-old infants only. This phenomenon, not observable in older infants (≥12 weeks of age) and adults, is demonstrated by the difference in reflectance obtained under applied positive or negative pressure and TPP, as shown in Figure 6 (right panel). The opposite effects of the pressure on reflectance between 2 and 6 kHz may be explained in terms of the mechanics of the ossicular chain when the tympanic membrane changes its orientation with respect to the earcanal. Sanford and Feeney (2008) suggested that positive pressure may enhance ossicular coupling due to the ossicles being pushed more closely together, enabling a more efficient sound conduction pathway and resulting in decreased reflectance. Conversely, negative pressure may be pulling the tympanic membrane outward, changing effective articulation of the ossicular chain.

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Sanford and Feeney (2008) also showed that wideband admittance, a measure of the admittance of the earcanal and middle ear across a frequency range of 0.25 to 8 kHz, increased with age. A double-peaked pattern with two maxima between 2 and 6 kHz was seen for infants aged 4, 12, and 24 weeks, in contrast to adults, who showed a single peak at around 3 kHz. Admittance phase between 0.25 and 0.5 kHz recorded at TPP for 4-week-old infants was around +45°, indicating equal contributions of conductance and susceptance components. The phase increased with age, consistent with increasing stiffness during the maturation process. The phase between 1 and 3 kHz increased significantly with positive pressures of +100 and +200 daPa for 4-month-old infants; this change in phase with pressure diminished with age. In summary, data from Sanford and Feeney provided valuable information on developmental aspects of WAI measures in infants. This information will aid in the interpretation of WAI findings in infants in clinical settings.

Keefe and Levi (1996) conducted further investigation on infants and adults, documenting maturation of the external and middle ear. A 226 Hz reflectance tympanogram for a normal-hearing adult was plotted when ear canal pressure was varied from +250 to −300 daPa. When calculating the reflectance, the ear canal area was estimated assuming a circular cross section with a range of diameters of 0.8, 0.9, 1.0, and 1.1 cm. The results showed that reflectance attained a minimum value at 0 daPa and that the minimum value increased with increasing ear canal diameter. Reflectance increased to 0.99 at extreme positive and negative static pressures (+250 and −300 daPa), thus validating the assumption for 226 Hz tympanometry that almost all acoustic energy is reflected at these extreme pressures. However, when the same test was conducted on a 3-month-old infant, the reflectance calculated using ear canal diameters of 0.4, 0.5, 0.6, and 0.7 cm did not asymptote to unity at extreme static pressures, indicating energy loss in the infant’s ear canal wall. The authors recommended that 226 Hz admittance tympanometry should not be used with young infants and that a higher probe tone frequency should be used because the nonrigid ear canal wall of infants is less responsive to high-frequency probe tones.

Werner et al. (2010) analyzed WAI data obtained from infants in an attempt to investigate the effects of age, gender, and ear on WAI measures. Subjects included 198 infants aged 2 to 3 months, 260 infants aged 5 to 9 months, and 210 healthy adults served as controls. Traditional tympanometry (226 Hz) was used to screen all infants because 1 kHz tympanometry was not available (data were gathered between 1992 and 1995). WAI measures including reactance, resistance, and reflectance were recorded using a custom-made prototype device. The WAI results for the three age groups were in agreement with those reported by Keefe et al. (1993). A significant age effect for reflectance from 0.25 to 3 kHz was reported, with reflectance increasing progressively with age. At high frequencies, adults had significantly higher reflectance than infants, but reflectance for the two infant groups did not differ; no gender effect was found. Reflectance in the right ear was significantly greater than that in the left ear, although the mean difference in reflectance across all frequencies was small (0.02). Resistance and reactance were greater for females than males, but these WAI measures were not different between left and right ears.

A study aimed to establish and compare normative WAI data in infants was conducted by Hunter et al. (2008). Ninety-seven infants and children aged between 3 days and 47 months were recruited from a pediatric clinic and assessed using DPOAEs, tympanometry (0.226 and 1 kHz), and WAI measures. The study sample had an equal proportion of males and females and an ethnic mix of White, Asian, Hispanic, African American, and Pacific Islander backgrounds. Due to various reasons such as lack of cooperation by the child, inadequate signal level for DPOAE testing, ear blockage (wax), poor probe seal, or artifact in the recordings, only 81 participants (159 ears) completed all tests. A total of 138 ears were classified as having normal ear status, while 21 ears were classified as having poor ear status. WAI data were collected using a Mimosa Acoustics System with a chirp or sine wave stimulus delivered at 55 dB SPL. The stimulus intensity was increased up to 65 dB SPL as necessary to ensure that the stimulus in the earcanal was above the noise floor.

No significant differences were found for reflectance as a function of age group except at 6 kHz. In contrast, Keefe et al. (1993) and Sanford and Feeney (2008) reported a significant age effect from birth to adult age ranges, with the largest change found from birth to 6 months of age. Hunter et al. (2008) remarked that differences in probe design and calibration method might account for such discrepancies. The power reflectance data showed no gender or ear differences. Not surprisingly, Hunter et al. found that ears with TPP in excess of ±100 daPa showed slightly higher reflectance than ears with TPP in the normal range (−99 to +99 daPa). This is a potential confounding factor in interpreting reflectance results in infants when ambient pressure reflectance or absorbance alone is used.

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To date, WAI studies in newborns have developed normative data that are fairly consistent across studies, despite differences in study sample, methodology, and instrumentation (Shahnaz 2008; Sanford et al. 2009; Hunter et al. 2010; Merchant et al. 2010; Aithal et al. 2013; Prieve et al. 2013). One confounding factor that may affect the establishment of normative WAI data in newborns and young infants is the presence of fluid in either the earcanal or middle ear and mesenchyme that clings to the middle ear ossicles in normal infants (Piza et al. 1996). Hence, there is an urgent need to define normal ear status for newborns and infants, given the lack of a gold standard. Previously, some studies have included minimal or no audiologic assessment (e.g., Keefe et al. 1993; Merchant et al. 2010), while others have used evoked otoacoustic emissions and 1 kHz tympanometry to check middle ear function in newborns (e.g., Shahnaz 2008; Sanford et al. 2009; Hunter et al. 2010). Aithal et al. (2013) used a more stringent set of inclusion criteria that required newborns to pass a battery of tests. Prieve et al. (2013) used auditory brainstem response (air and bone conduction) to determine the status of the external and middle ear in newborns and infants. Undoubtedly, there is an urgent need to establish consensus on what the minimal test battery should be for establishing normative WAI data in newborns and infants.

At present, there is a lack of normative WAI data for newborns specific to gender, ear, race, and ethnicity. While significant gender and ear effects have not been consistently reported, those studies that have reported significant changes in WAI across genders or ears found small differences that may not be of clinical significance (e.g., Keefe et al. 1993). As normative data reflect the population from which they are developed, special consideration should be given to demographic characteristics such as race and ethnicity of the participants.

To accurately identify conditions in the sound conduction pathway in newborns and infants using WAI measures, clinicians should be cognizant of variations in WAI measurements caused by developmental changes in the anatomy and physiology of the external and middle ear of an infant. Recent findings provide strong evidence for the effects of maturation on the peripheral auditory system as reflected in WAI measures Keefe et al. 1993; Keefe & Levi 1996; Sanford & Feeney 2008). Keefe et al. (1993) and Sanford and Feeney (2008) found large differences in WAI measures between healthy infants aged 1 and 12 months. However, Hunter et al. (2008) and Werner et al. (2010) did not find significant differences in WAI findings between different infant groups. Further research is needed to replicate these findings with properly calibrated instruments and appropriate reference standards for normal ear status.

Additional normative WAI data for infants during the fast development period from birth to 12 months are needed. Keefe et al. (1993) and Sanford and Feeney (2008) provided WAI normative data for age-specific groups of 1, 3, 4, 6, and 12 months, while Hunter et al. (2008) and Werner et al. (2010) provided data for infants aged 0 to 2, 2 to 3, 3 to 5, 5 to 9, and 6 to 11 months. In particular, age-specific longitudinal and cross-sectional normative data for infants from birth to 1 year are currently lacking. Furthermore, it is desirable to establish WAI norms for diseased ears of the above age groups. Continued investigations in this area will enable clinicians to make correct clinical decisions by comparing an infant’s WAI results against normative data.

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