Portuguese Version

Year:  2003  Vol. 69   Ed. 1 - (14º)

Artigo Original

Pages: 84 to 91

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Transient evoked otoacoustic emissions (TEOAE): response amplitude in term and pre-term neonates

Author(s): Mônica C. A. Bassetto,
Brasília M. Chiari,
Marisa F. Azevedo

Keywords: acoustical stimulation, cochlea, neonate, otoacoustic emissions, neonatal screening.

Abstract:
Transient evoked otoacoustic emissions (TEOAE) have been widely used in neonatal hearing screening programs. In the last years, a closer analysis of TEOAE has shown some unexplored characteristics of the peripheral auditory system. The aim of this research was to study the TEOAE response amplitude in term and pre-term neonates, as a function of the ear side, gender, frequency spectrum and post-conceptional age. The sample was composed by 526 newborns, 440 born at term and 86 pre-term neonates divided in two groups: one with post-conceptional age varying from 31 to 36 weeks and the second one with post-conceptional age varying from 37 to 44 weeks. The TEOAE test was performed in an environment without acoustical treatment with the ILO 88 - Otodynamics Otoacoustic Emission Analyzer, in the Quickscreener mode. The results showed a statistically significant asymmetric mean amplitude response in favor of the right ear, the females, the high frequency bands and the newborns with higher post-conceptional age. These differences observed between the mean amplitudes of the TEOAE of term and pre-term newborns, suggest that the amplitude of the TEOAE, besides providing an evidence of the integrity of the peripheral auditory system, may be used as an indicator of the maturation of the peripheral auditory system in newborns.

INTRODUCTION

In the past 15 years, the scientific community has witnessed significant findings in the area of cochlear physiology: the old concept of the cochlea as a passive organ was gradually replaced by the concept of a mechanically active organ, capable of generating energy and promoting accurate frequency selectivity.

The identification of the phenomenon of otoacoustic emissions have contributed enormously to evidence the active and non-linear characteristics of the cochlear mechanism and, subsequently, the introduction of a device clinically capable of recording them has had a huge impact in the audiology field.

A reliable, quickly and non-invasive assessment of the pre-neural function of the inner ear met the needs of neonatal hearing screening programs, which had recognized for a long time the potential benefits of universal screening, but did not count on the appropriate technique to this end.
Otoacoustic emissions are sounds recorded from the external acoustic canal either spontaneously or evoked by acoustic stimulation. When evoked, emissions are commonly classified based on the generating stimulus: transient, stimulus-frequency or distortion product.

The most widely used and recommended technique in neonatal hearing screening is transient evoked otoacoustic emissions1-8, since they use acoustic stimuli of weak intensity, comprising a wide range of frequencies (clicks that range from 400 to 6400Hz) and the recording can be completed in a short period of time, normally lasting one minute per ear.

However, despite the technical reliability of the technique as an instrument for neonatal hearing screening, the criteria adopted for interpretation of parameters of transient otoacoustic emissions differ in each program9, suggesting the need for better exploration of intrinsic properties in order to unify the criteria.

Even though there is no consensus about the topic yet, some research studies involving transient evoked otoacoustic emissions in newborns have suggested the existence of asymmetry of response amplitude between the two ears10, 11, between gender10-12, and different conceptional ages13, 14. The study of these asymmetries could reveal characteristics that are unknown to the peripheral hearing system of the newborn, whose understanding has been limited so far by lack of appropriate instruments to measure the pre-neural function of the inner ear in this age range.

The process of maturation of the peripheral hearing system, an object of controversy, could be well understood by the comparative analysis of amplitude of response in transient evoked otoacoustic emissions (TEOAE) and its distribution in the spectrum of frequencies in term and preterm neonates; this knowledge would provide input to the preparation of a single criterion to interpreter TEOAE in neonatal hearing screening programs.

Considering the importance of the analysis of response amplitude in transient otoacoustic emissions in term and preterm newborn, both for clinical and scientific purposes, the objective of the present study was to investigate the response amplitude parameter in transient otoacoustic emissions in term and preterm neonates concerning the variables: ear, gender, frequency spectrum and post-conceptional age.

MATERIAL AND METHOD

The sample consisted of 526 newborns who participated in the program of neonatal hearing screening conducted in a large private hospital in the region of the Greater Sao Paulo, between January and December 1997. The children were selected from an initial group of 594 newborns, which had some subjects excluded owing to failure in hearing screening. Parents were informed about the program after the baby was born, and they participated spontaneously. The project was analyzed and approved by the Ethics Committee of Federal University of São Paulo - EPM.

Among the 526 neonates in the sample, 440 children were born at term, gestational age equal or greater than 37 weeks, and 86 were preterm neonates with gestational age below 37 weeks.
Preterm babies were distributed in the study according to maturation stage, taking into account post-conceptional age on the screening day, that is, time in weeks from fecundation up to the current date, after birth. Thus, there were three groups:

 GT group, or Term Group - comprising 440 newborns from the normal nursery care, gestational age equal or greater than 37 weeks;
 GPTA, or Preterm Group A - comprising 42 preterm newborns, whose gestational age and post-conceptional age on the test date were below 37 weeks;
 GPTB, or Preterm Group B - comprising 44 preterm newborns whose gestational age was below 37 weeks and post-conceptional age on the test date was equal or superior to 37 weeks.

Upon the formation of the group, there were 202 (45.9%) male babies and 238 (54.1%) female babies in the GT; 22 (52.4%) male babies and 20 (47.6%) female babies in the GPTA, and 15 (34.1%) male and 29 (65.9%) female babies in the GPTB.

The procedure adopted for hearing screening was transient evoked otoacoustic emissions (TEOAE) test associated with observation of hearing behavior15, and tympanometry conducted with a portable immittanciometer HandtympTM, by Danplex. These procedures were used to facilitate the identification of possible middle ear or central hearing abnormalities7, 15. All selected newborns presented results for transient evoked otoacoustic emissions, hearing behavior and tympanometry within the normal range15, 16.

Term newborns were tested in the ambulatory of pediatrics, on their mothers' laps, preferably during post-prandial sleep. Preterm newborns who were hospitalized during the screening were tested on their regular crib, in the high-risk nursery setting, before hospital discharge. The chronological mean age of newborns on the test date was 11 to 20 days.

The level of background noise in the test room recorded by a sound pressure meter model 2700 Quest Technologies ranged from 58 to 70dBSPL in the pediatric ambulatory room and from 64 to 67dB SPL in the common crib in the high risk nursery.

TEOAE were recorded for both ears17. We used the equipment ILO 88 - Otoacoustic Emissions Analyzer (Otodynamics Ltda, Version 3.9), coupled to a portable microcomputer Sharp PC-3060. For the procedure, we used Quickscreener mode, indicated for neonatal hearing screening. The program consists of a standard non-linear mode, that is, three stimuli of a specific intensity and a fourth stimulus three times more intense, but with opposite polarity, with click stimuli presented at a speed twice faster than the standard one, to maximize efficiency of data collection. Clicks last 80ms in the frequency range of 400 to 6000Hz. There was no gain adjustment for the acoustic stimuli and intensity was programmed for 80dB SPL peak equivalent, ranging from 66 to 83dB SPL peak equivalent. We assessed 260 sets of responses with this standard but in case the newborn was agitated, the test was concluded after a minimum of 80 sets of responses had been recorded.

The acoustic pressure response in the external acoustic canal was recorded by a microphone coupled to a probe, submitted to the analysis of Fourier (FFT), which enabled separation of otoacoustic emissions and floor noise, expressed as level of response in dB SPL. This level of response, treated in this study as amplitude of response, was expressed in general value and in bands of frequency centered in 800Hz, 1600Hz, 2400Hz, 3200Hz and 4000Hz. The time of analysis of the response was 12ms and the initial 2.5ms were eliminated from data treatment to avoid contamination of external and middle ear response.

Responses were alternatively compiled in two channels, A and B, to assess correlation expressed as index of reproducibility. The program provides an index of general reproducibility and by bands of frequency.

In addition, the program recorded the level of floor noise in dB SPL present in the external acoustic canal during the test, the necessary time in seconds to conduct the test and the stability index of the probe, in % of the external acoustic canal.

The criteria employed to consider the presence of response in the transient evoked otoacoustic emissions were: response amplitude (signal/noise ratio) equal or superior to 3dB SPL, central frequency band of 1600Hz; response amplitude (signal/noise ratio) equal or greater than 6dB SPL in central frequency bands of 2400, 3200 and 4000Hz; general reproducibility and central frequency band of 1600Hz, equal or greater than 50%; reproducibility for bands with central frequency of 2400Hz, 3200 and 4000Hz greater than 70%; probe stability equal or superior to 70%; overlapping of waves A and B by visual inspection.

Once defined the presence of TEOAE by the criteria described above, we selected, according to our objectives, the response amplitude parameters whose study took into account the following variables: ear, gender, frequency of spectrum and post-conceptional age.

For the statistical analysis of results, we employed the following tests: 1. Mann-Whitney test for independent samples to compare mean amplitude between male and female newborn babies for each combination of ear side. 2. Wilcoxon test for paired samples to compare mean amplitude between both sides for each combination of gender. 3. Kurskal-Wallis test to compare mean amplitude values among the groups for each combination of gender and ear side. This test is indicated when comparing three or more sets of information with level of numerical measurement and independent samples. 4. Friedman test to compare the values of mean amplitude between the different bands of frequency, for each gender and ear side. When statistically significant differences were observed, we conducted multiple comparisons to identify where the difference was. This test is indicated when comparing three or more sets of information, with numerical measurement level and paired samples.

In all tests, we fixed as 0.05 or 5% (£ 0.05) the level of rejection of null hypothesis, marking significant values with an asterisk.

RESULTS

Initially, we studied the parameter of amplitude and noticed that there was asymmetry of response concerning ear and gender for each studied group. Since we detected statistically significant differences for amplitude and ear and gender, these variables were not grouped. Therefore, in the following analyzes, the comparison of amplitude by frequency bands and the comparison of amplitude in the studied groups was made for each ear side and gender.

We studied amplitude of response of TEOAE for each ear in newborns from the three groups (Tables 1 to 6). Thus, we noticed the presence of asymmetry of amplitude of TEOAE favoring the right ear in male and female patients.

Next, we detected the difference in amplitude response and genders, for each ear in the three groups (Tables 7 to 12). In GT, we recorded greater amplitude of response for female patients.
In the third step, we compared mean amplitude of different bands of frequency for each gender and side in the groups (tables 13 to 18). Since differences between higher frequency bands could have been masked by the presence of bands centered in 1.6kHz, owing to the background noise in the test room, the comparisons were conducted excluding this band. We observed greater amplitude in higher frequency bands.

Finally, Tables 19 to 22 compare the mean amplitude values among the groups Term, Preterm A and Preterm B for each combination of gender and ear side, in order to examine the effect of post-conceptional age and amplitude of TEOAE. The results showed that the greater the post-conceptional age, the greater the response amplitude for TEOAE.

GT - Male
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Right     13.5     9.3     15.9     17.8     16.4
Left      12.5      9.2     15.9     18.2     16.4

p = 0.0000* 0.8949 0.9692 0.4054 0.7557

Table 2. Mean, general and frequency band amplitudes of TEOAE for female neonates in the Term Group (GT), by ear side.

GT - Female
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Right     13.8     9.4     15.3     18.0     18.1
Left      13.3      8.5     14.5     17.8     17.2

p = 0.0143* 0.0063* 0.0525 0.5556 0.0570

Table 3. Mean, general and frequency band amplitudes of TEOAE for male neonates in the Pre-Term A Group (GPTA), by ear side.

GPTA - Male
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Right     13.7     9.4     16.8     15.9     16.4
Left      11.9     10.0     13.4     13.4     13.1

p = 0.0534 0.6677 0.0074* 0.0967 0.0178*

Table 4. Mean, general and frequency band amplitudes of TEOAE for female neonates in the Pre-Term A Group (GPTA), by ear side.

GPTA - Female
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Right     14.7     8.7     15.4     15.8     14.4
Left      13.7      8.9     14.7     13.8     16.0

p = 0.2627 0.7404 0.4094 0.2352 0.3720

Table 5. Mean, general and frequency band amplitudes of TEOAE for male neonates in the Pre-Term B Group (GPTB), by ear side.

GPTB - Male
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Right     13.4     8.1     14.3     16.5     15.5
Left      14.2      9.3     14.1     17.5     16.8

p = 0.5936 0.4420 0.6155 0.7798 1.000

Table 6. Mean, general and frequency band amplitudes of TEOAE for female neonates in the Pre-Term B Group (GPTB), by ear side.

GPTB - Female
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Right     12.6     8.6     14.7     18.6     16.6
Left      13.3      9.2     16.9     19.3     19.5

p = 0.2275 0.7731 0.0585 0.5538 0.0154*


Table 7. Mean, general and frequency band amplitudes of TEOAE for the right ear of neonates in the Term Group (GT), by gender.

GT - Right Ear
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Male      13.5      9.3     15.9     17.8     16.3
Female    13.8    9.4     15.3     18.0     18.1

p = 0.3143 0.7946 0.2878 0.3762 0.0015*

Table 8. Mean, general and frequency band amplitudes of TEOAE for the left ear of neonates in the Term Group (GT), by gender.

GT - Left Ear
Amplitude (dB NPS)
Ear General / 1,6 / 2,4 / 3,2 / 4,0 kHz
Male      12.5      9.2     15.9     18.2     16.4
Female    13.3    8.5     14.5     17.8     17.2

p = 0.0332* 0.1920 0.0576 0.9157 0.0526

Table 9. Mean, general and frequency band amplitudes of TEOAE for the right ear of neonates in the Pre-term A Group (GPTA), by gender.

GPTA - Right Ear
Amplitude (dB NPS)
Gender General / 1.6 / 2.4 / 3.2 / 4.0 kHz
Male      13.7      9.4     16.8     15.9     16.4
Female    14.7    8.7     15.4     15.8     14.4

p = 0.5708 0.6030 0.3768 0.7812 0.1939

Table 10. Mean, general and frequency band amplitudes of TEOAE for the left ear of neonates in the Pre-term A Group (GPTA), by gender.

GPTA - Left Ear
Amplitude (dB NPS)
Gender General / 1.6 / 2.4 / 3.2 / 4.0 kHz
Male     11.9     10.0     13.4     13.4     13.1
Female    13.7    8.9     14.7     13.8     16.0

p = 0.1307 0.9696 0.4631 0.9799 0.1890

Table 11. Mean, general and frequency band amplitudes of TEOAE for the right ear of neonates in the Pre-term B Group (GPTB), by gender.

GPTB - Right Ear
Amplitude (dB NPS)
Gender General / 1.6 / 2.4 / 3.2 / 4.0 kHz
Male     13.4     8.1     14.3     16.5     15.5
Female    12.6    8.6     14.7     18.6     16.6

p = 0.4958 0.5340 0.9505 0.1269 0.5104

Table 12. Mean, general and frequency band amplitudes of TEOAE for the left ear of neonates in the Pre-term B Group (GPTB), by gender.

GPTB - Left Ear
Amplitude (dB NPS)
Gender General / 1.6 / 2.4 / 3.2 / 4.0 kHz
Male     14.2     9.3     14.1     17.5     16.8
Female    13.3    9.2     16.9     19.3     19.5

p = 0.6468 0.9203 0.1566 0.3456 0.1144


Table 13. Mean amplitudes of TEOAE in central frequency bands of 2.4 KHz, 3.2 KHz and 4.0KHz in male neonates in the term group (GT) by ear side.

GT - Male
Mean Amplitude ( dB NPS )
Ear / 2.4 KHz / 3.2 KHz / 4.0KHz / p / Resultado
Right     15.9     17.8     16.3     < 0.0001 *     2.0k = 4.0< 3.2KHz
Left      15.9      18.2     16.4     < 0.0001 *     2.0k = 4.0< 3.2KHz

Table 14. Mean amplitudes of TEOAE in central frequency bands of 2.4 KHz, 3.2 KHz and 4.0KHz in female neonates in the term group (GT) by ear side.

GT - Female
Mean Amplitude ( dB NPS )
Ear / 2.4 KHz / 3.2 KHz / 4.0KHz / p / Resultado
Right     15,3     18,0     18,1     <0,0001*     2,4 < 3,2 = 4,0KHz
Left      14,5      17,8     17,2     <0,0001*     2,4 < 3,2 = 4,0KHz

Table 15. Mean amplitudes of TEOAE in central frequency bands of 2.4 KHz, 3.2 KHz and 4.0KHz in male neonates in the pre-term A group (GPTA) by ear side.

GPTA - Male
Mean Amplitude ( dB NPS )
Ear / 2.4 KHz / 3.2 KHz / 4.0KHz / p / Resultado
Right     16,8     15,9     16,4     0,6420     2,4 = 3,2= 4,0KHz
Left      13,4      13,4     13,1     0,4887     2,4 = 3,2= 4,0KHz

Table 16. Mean amplitudes of TEOAE in central frequency bands of 2.4 KHz, 3.2 KHz and 4.0KHz in female neonates in the pre-term A group (GPTA) by ear side.

GPTA - Female
Mean Amplitude ( dB NPS )
Ear / 2.4 KHz / 3.2 KHz / 4.0KHz / p / Resultado
Right     15,4     15,8     14,4     0,8936     2,4 = 3,2 = 4,0KHz
Left      14,7      13,8     16,0     0,0715     2,4 = 3,2 = 4,0KHz

Table 17. Mean amplitudes of TEOAE in central frequency bands of 2.4 KHz, 3.2 KHz and 4.0KHz in male neonates in the pre-term B group (GPTB) by ear side.

GPTB - Male
Mean Amplitude ( dB NPS )
Ear / 2.4 KHz / 3.2 KHz / 4.0KHz / p / Resultado
Right     14.3     16.5     15.5     0.2466     2.4 = 3.2 = 4.0KHz
Left     14.1     17.5     16.9     0.1423     2.4 = 3.2 = 4.0KHz

Table 18. Mean amplitudes of TEOAE in central frequency bands of 2.4 KHz, 3.2 KHz and 4.0KHz in female neonates in the pre-term B group (GPTB) by ear side.

GPTB - Female
Mean Amplitude ( dB NPS )
Ear / 2.4 KHz / 3.2 KHz / 4.0KHz / p / Resultado
Right     14.7     18.6     16.6     0.0081 *     2.4k < 3.2 = 4.0 KHz
Left      16.9      19.3     19.5     0.0175 *     2.4k < 3.2 = 4.0 KHz

Table 19. Mean, general and band frequency amplitudes of TEOAE on the right ear of male neonates by groups Term (GT), Pre-term A group (GPTA) and Pre-term B group (GPTB).

Male - Right Ear
Amplitude
Mean (dB NPS) / GT / GPTA / GPTB / p / Resultado
General Amplitude      13.5     13.7     13.4     0.9204     T=PTA=PTB
Amplitude 1.6 KHz     09.3     09.4     08.1     0.6229     T=PTA=PTB
Amplitude 2.4 KHz     15.9     16.8     14.3     0.3799     T=PTA=PTB
Amplitude 3.2 KHz     17.8     15.9     16.5     0.3205     T=PTA=PTB
Amplitude 4.0 KHz     16.3     16.4     15.5     0.8304     T=PTA=PTB

Table 20: Mean, general and band frequency amplitudes of TEOAE on the left ear of male neonates by groups Term (GT), Pre-term A group (GPTA) and Pre-term B group (GPTB).

Male - Left Ear
Amplitude
Mean (dB NPS) / GT / GPTA / GPTB / p / Resultado
General Amplitude     12.5     11.9     14.2     0.4189     T=PTA=PTB
Amplitude 1.6 KHz     09.2     10.0     09.3     0.9440     T=PTA=PTB
Amplitude 2.4 KHz     15.9     13.4     14.1     0.0793     T=PTA=PTB
Amplitude 3.2 KHz     18.2     13.4     17.5     0.0042*     T > PTA
Amplitude 4.0 KHz     16.4     13.1     16.8     0.1139     T=PTA=PTB

Table 21: Mean, general and band frequency amplitudes of TEOAE on the right ear of female neonates by groups Term (GT), Pre-term A group (GPTA) and Pre-term B group (GPTB).

Female - Right Ear
Amplitude
Mean (dB NPS) / GT / GPTA / GPTB / p / Resultado
General Amplitude      13,8     14,7     12,6     0,3115     T=PTA=PTB
Amplitude 1,6 KHz     09,4     08,7     08,6     0,7985     T=PTA=PTB
Amplitude 2,4 KHz     15,3     15,4     14,7     0,7806     T=PTA=PTB
Amplitude 3,2 KHz     18,0     15,8     18,6     0,1994     T=PTA=PTB
Amplitude 4,0 KHz     18,1     14,4     16,6     0,0080*     T>PTA

Table 22: Mean, general and band frequency amplitudes of TEOAE on the left ear of female neonates by groups Term (GT), Pre-term A group (GPTA) and Pre-term B group (GPTB).

Female - Left Ear
Amplitude
Mean (dB NPS) GT GPTA GPTB p Result
General Amplitude      13,3     13,7     13,3     0,8614     T=PTA=PTB
Amplitude 1,6 KHz     08,5     08,9     09,2     0,6861     T=PTA=PTB
Amplitude 2,4 KHz     14,5     14,7     16,9     0,0503     T=PTA=PTB
Amplitude 3,2 KHz     17,8     13,8     19,3     0,0055*    T=PTB>PTA
Amplitude 4,0 KHz     17,2     16,0     19,5     0,1070     T=PTA=PTB

DISCUSSION

The adoption of TEOAE technique as an instrument for the neonatal hearing screening has been determined by the development of studies in the area, especially in the past decade. However, some properties of TEOAE still require elucidation for us to be able to adopt clearly defined parameters to assess the results of TEOAE in different age ranges and test situations.

In the study of amplitude of TEOAE and ear side (Tables 1 to 6), we observed significantly greater mean amplitudes on the right ear of newborns in the GT and GPTA. The same finding was described in the literature10, 11, 18. It is assumed that mechanisms that involve asymmetry of occurrence of spontaneous otoacoustic emissions between the right and left ears19, asymmetry of intensity of the efferent olivocochlear system inhibition10 18, and morphology asymmetry between the right and left cranial-facial regions20 are related to aural asymmetry.

Upon the study of the variable gender (tables 7 to 12), we observed mean amplitudes of TEOAE statistically greater in female newborns of the GT. Other authors11, upon the study of term newborns, have also reported asymmetry of TEOAE amplitude between genders. Such finding has been attributed to morphometric differences in cochlear length in different genders21.

Since otoacoustic emissions are a product of the non-linear intrinsic mechanic activity of the cochlea, the evidence that amplitude is related to ear and gender suggests the need for more studies in order to understand the exact origin of these asymmetries and how they could influence the results of the TEOAE test in neonates.

We observed that the band centered in 1600Hz presented significantly lower amplitude than others in term and preterm babies, for all combinations of ear and gender. The reduced amplitude in the 1600Hz band was probably due to the influence of noise in the test environment since newborns were tested in an environment with no acoustic treatment, with marked background noise. TEOAE amplitude, especially in newborns, suffers significant interference of the level of background noise2.

The greatest amplitude of TEOAE in this study was recorded in frequency bands centered in 3200Hz and/or 4000Hz (Tables 13 to 18), confirming the literature data22, 23.

The predominance of response amplitude of TEOAE in this frequency range in newborns has been primarily attributed to influence of external and middle ear2, and also to the cochlear amplification mechanism24 and inhibition of the efferent auditory system25.

It is important to highlight that the analysis of frequency bands has contributed significantly to the study since many investigated properties could be observed only in frequency bands and not in general amplitude. This fact is due to the general amplitude being strongly influenced by the low components of response, which were masked by the background noise in our study. Thus, we consider it important that the analysis of TEOAE amplitude responses in newborns be conducted by frequency bands instead of by general amplitude, considering that the greatest amplitudes are found in high frequencies.

Upon the analysis of the effect of post-conceptional age increase on TEOAE amplitude of response by comparing the three groups (Tables 19 to 22), we noticed that there was statistically significant difference between mean amplitude of responses in high frequency bands of GT and GPTA. This increase in response amplitude caused by increased post-conceptional age can be interpreted as an indication of the fact that the peripheral auditory system goes through a process of maturation up to the term period.

In the last two months of gestation, which correspond to the post-conceptional ages of neonates in this study, there is the growth process of external and middle ears and the maturation of inner ear that explain these differences of response amplitude observed in different post-conceptional ages.
Asymmetries (between ear sides, genders and frequency bands) found in the present study increased as a result of increased post-conceptional age, since they could always be observed in the GT, but not in GPTA and GPTB.

The study of TEOAE amplitude of response between GT, GPTA and GPTB intended to check the influence of post-conceptional age increase in amplitude of TEOAE. Despite the fact that the literature describes that this increase is subtle and difficult to be detected in transversal studies26, we observed statistically significant differences between mean amplitude of GT and GPTA (on the left in males and on the right and left ears in females) in the high frequency bands.

In longitudinal studies, the association between increase in post-conceptional age and amplitude of TEOAE has been reported by some authors13, 14, 26, 27. However, only one study27 reported a statistically significant difference between mean general amplitude in term and preterm newborn in a transversal study.

This comparison between increase of post-conceptional age and amplitude of TEOAE reveals that peripheral auditory mechanisms involved in the generation and transmission of TEOAE would still be under maturation in the last weeks of gestation. Among the mechanisms, we can highlight the growth of the external acoustic canal and the middle ear and maturation of cochlear amplifiers. As the middle ear (acoustic reflex) and inner ear (outer hair cells) play the role of selective filter of frequencies privileging high frequencies acoustic energy under the regulating action of the efferent olivocochlear system28, the maturation process evidenced in the high frequencies of this study could involve the maturation of the efferent system.

These differences observed in mean amplitude of TEOAE in term and preterm neonates suggest that this parameter not only provides indication of the presence of TEOAE, and thus, of the integrity of the peripheral hearing system, but can also be interpreted as an indicator of peripheral hearing system maturation in newborns.

CONCLUSIONS

Based on the critical analysis of results, we could conclude the following:

In term newborns, there was predominance of greater amplitude of TEOAE to the right side, for female and for high frequency bands. In preterm babies, assessed between 31 and 36 weeks of post-conceptional age, there was predominance of greater amplitude of TEOAE to the right side. In preterm babies, assessed between 37 and 44 weeks of post-conceptional age, there was predominance of greater TEOAE amplitude for high frequency bands. In the comparative study of the three groups, there was statistically significant correlation between mean amplitude of TEOAE and post-conceptional age, since the greater the post-conceptional age, the greater the mean amplitude of TEOAE in high frequencies bands.

REFERENCES

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1 Ph.D. in Human Communication Disorders, UNIFESP/EPM.
2 Joint Professor, Course of Speech and Voice Therapy and Audiology, UNIFESP/EPM.
3 Joint Professor, Course of Speech and Voice Therapy and Audiology, UNIFESP/EPM.

Address correspondence to: Monica Bassetto - Rua Almirante Tamandaré, 322 apto 12 Centro Santo André - SP 09040-040 - Tel (55 11) 4437-1174 - E-mail: fmbassetto@uol.com.br

Summary of the Doctorate dissertation thesis submitted to the Division of Human Communication Disorders, Department of Otorhinolaryngology and Human Communication Disorders, UNIFESP/EPM, November 1998.

Study presented at XXV International Congress of Audiology, The Halle, Holland, in August 2000.

Article submitted on August 08, 2002. Article accepted on January 17, 2003.

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