Portuguese Version

Year:  2001  Vol. 67   Ed. 5 - (6º)

Artigos Originais

Pages: 644 to 648

Distortion product otoacoustic emissions in normal hearing neonates

Author(s): Gláucia G. Raineri 1,
Carmen Z. V. Coube 2,
Orozimbo A. Costa Filho 3,
Kátia F. Alvarenga 4

Keywords: evoked otoacoustic emissions, distortion product, hearing, neonates

Abstract:
Introduction: Distortion product otoacoustic emissions (DPOAE) researches on neonates shed a new light into early diagnosis of sensorial hearing losses. Study design: Prospective results clinical not randomized. Aim: The purpose of the present study was to typify DPOAE in 20 normal hearing neonates of both sex, according to occurrence, amplitude and duration of test, through DP-Gram and utilizing ILO 92 OAE System-Otodynamics Analyzer. Results: Seventeen out of twenty neonates who volunteered for the research were evaluated, totaling 33 ears. DPOAE occurred in 57.5% in 1 kHz; 93.9% in 1.5 kHz and 3 kHz; 96.9% in 2 kHz and 100% in 4 kHz and 6 kHz. Conclusion: The DPOAE average amplitude fluctuated from 9.9 to 20.3 dB SPL. The average duration of the test was 82 seconds.

INTRODUCTION

In recent years, a significant technological development has been noticed in the area of audiology, resulting in more precise assessment procedures for the diagnosis of hearing loss.

At the end of the 1970's, a technique to detect otoacoustic emissions started to be researched, using two pure tones of different frequencies as stimuli (f1 and f2) and recording responses from the external acoustic canal, denominated distortion product evoked otoacoustic emissions (DPOAE) (Kemp, 1979).

Despite the fact that they do not quantify the hearing loss, the otoacoustic emissions enable the detection of damages located in Corti's organ (Kemp, 1978). Therefore, applicability of DPOAEs has been widely discussed, involving differential diagnosis of hearing loss, monitoring of subjects exposed to auditory harmful agents (industrial noise and ototoxic drugs, for example), up to intra-operative monitoring in otological surgeries and hearing screening (Lonsbury-Martin, Martin,1990; Kemp and Ryan,1991; Lonsbury-Martin, Mccoy; Whitehead; Martin, 1993 and 1995).

The test of DPOAEs may be carried out in two different ways: DP-gram, varying frequency at a predetermined intensity, and DP-growth rate, keeping the primary tone and varying intensity. However, intensity of stimulus for DPOAEs should not exceed 80dNSPL to prevent the firing of stapedial reflex.

Norton and Slover (1994) concluded that there was no consensus among researchers about the best parameters for clinical use of DPOAEs. Professionals involved in the area are still concerned about clinical investigations of DPOAEs in specific otological populations and defined criteria to analyze the results.

As to the best intensity level to perform the test, Coube (2000), in a recent study developed in our center, showed that the use of L1=L2 at 70dBSPL provides excellent performance of the test, with high sensitivity and specificity, by clearly separating normal hearing from sensorineural hearing loss subjects.

Another controversial issue is whether age influences otoacoustic emissions, possibly due to cochlear biomechanics and/or loss of external hair cells, observed throughout life (Collet, Moulin, Gartner, Morgon, 1990; Lonsbury-Martin, Cutler, Martin, 1991; Roede, Harris, Probst, Xu, 1993; Stover; Norton, 1993; Kimberley, Hernadi, Lee, Brown, 1994). This piece of data is extremely important if we consider that otoacoustic emissions have been used in hearing screening programs.

Lafreniere, Jung, Smurzynski, Leonard, Kim and Sasek (1991) conducted a study with a sample, of 23. neonates with no history of hearing loss risk and they found two regions of peaks of amplitude and one of valley, but the difference in frequency from peaks and valleys could be partly explained by acoustic impedance of the probe. DPOAEs were present in all neonates, similarly to adults, when using the same devices and probes.

Bonfils, Avan, François, Trotoux and Narcy (1992) recorded DP-growth rates referring to primary tones with geometrical mean of 867Hz to 8kHz in 23 neonates. DPOAEs were recorded in all ears in frequencies above 1kHz, in six ears in 1kHz and only in one ear in 867Hz. The fact was justified by the contamination of the noise produced by the neonates in low frequencies (noisy breathing). DPOAEs recorded for the neonates were higher than in adults, mean value of 6dB, regardless of frequency.

Marco, Morant, Caballero, Ortells, Paredes and Brines (1995) carried out a study of DPOAEs in a group of 12 neonates with no risk factor for hearing loss and previously pass status at transient evoked otoacoustic emissions (TEOAE) screening. They used the ratio f2/fl = 1.2 and constant intensity, both for fl and f2 (75 dBSPL) to record the DP-gram. It was observed that the occurrence of DPOAEs decreased considerably for low frequencies, but after 3kHz, DPOAEs were detected in 100% of the ears. Neonatal DP-gram showed characteristics similar to that of adults, with two maximum amplitude peaks for frequencies 2kHz and 5 and 6 kHz and decrease of amplitude in middle frequencies. When background noise was analyzed, it was noticed that it was louder in low frequencies (0.7 to 1kHz).

Abdala (1996) recorded 2f1-f2 DPOAEs in adults and term and pre-term neonates to define the ideal f2/fl ratio and level separation fl-f2. The investigated frequencies were 1500 and 6000Hz, with constant f2 and variation of fl to produce 13 different frequency ratios. The separation of primary tone levels varied from 15 to 0dB, at 5dB intervals, and the results showed that the ideal average of the frequency ratio for the generated DPOAEs was comparable in adults and neonates. The separation level of 15 or 10 dB (fl>f2) produced wider amplitude of DPOAEs for adults and term neonates, considering that the DPOAEs of preterm neonates seemed to be relatively insensitive to separation of primary tone levels.

Lopes Filho and Carlos (1996) investigated TEOAE and DPOAEs in 30 neonates (60 ears), but two of them were excluded. They observed that DPOAEs were not recorded below the normal ranges reported by the literature for young adults, and they were more precise than TEOAE, since the latter were not recorded in neonates.

Qian and Jiang (1997) recorded DPOAEs from 20 neonates (40 ears) and concluded that when fl was below 1kHz, the presence of DPOAEs was very low, whereas when it was between 1400 and 4000HZ, the occurrence was greater than 90%. Similarly, amplitude of DPOAEs was lower within the area of low frequencies compared to frequencies 1 to 4 kHz, in which amplitude was greater. The authors concluded that DPOAEs in neonates was easily affected by noise, stimulus intensity and middle ear function.

Lasky (1998) studied DPOAEs in neonates and adults observing that because of anatomical and acoustic characteristics of external and middle ear, the same input signal resulted in louder sound pressure level stimulus for neonates, in broad frequency band, recorded at the occluded external ear. The levels of noise at the external acoustic canal were 5 to 15 dB less loud for adults, in frequencies below 3kHz. In high frequencies, all subjects had similar amplitude in 2f1/f2. Amplitude of DPOAEs in 2f1/f2 and f2/f1 in adults and neonates were consistently similar, despite reliability differences.

These studies have shown the interest of investigators in using not only transient evoked otoacoustic emissions as a hearing screening tool, but also both types of distortion product due to its frequency-specificity. Therefore, the objective of the present study was to analyze distortion product evoked otoacoustic emissions according to occurrence, amplitude and duration of test using DP-gram in normal hearing neonates.

MATERIAL AND METHOD

We selected 20 healthy neonates (40 ears), 11 female and 9 male babies, born at term and with no hearing loss risk, according to the anamnesis and the criteria recommended by the joint Committee on Infant Hearing (1995). The assessment included otological clinical inspection, tympanometry, transient and distortion product evoked otoacoustic emissions. In order to analyze data, we excluded seven ears because of significant contamination of background noise, which impaired the analysis of the emissions.

We used the distortion product analyzer ILO 92 OAE System-Otodynamics Analyzer.

We carried out the Distortion-Product Gram (DPgram) testing the geometrical means close to 1, 1.5, 2; 3, 4 and 6 kHz, with stimuli at 70 dBSPL (L1=L2), at three points/ octave steps. F2/F1 ratio was 1.22.

DPOAEs were considered present if they had 3dB or greater amplitude over background noise, after the calculation of means, medians and standard deviation of medians for all variables. We also used Student's t statistical test, significance level of 1%, to compare gender and right and left ear.

RESULTS

Out of 33 studied ears, only 19 (57.5%) presented DPOAEs in 1 kHz, 31 (93.9%), in 1.5 kHz and 3 kHz, 32 (96.9%), in. 2 kHz and 33 (100%), in 4 kHz and 6 kHz.

At a significance level of 1% for the comparison of gender and side, we did not find statistically significant differences. Therefore, the analysis was performed as a whole, as shown in Table 1, presenting values of means, medians and standard deviation.

As Graph 1 shows, amplitude peaks of DPOAEs were in frequencies at about 2 kHz and 6 kHz, in which amplitude means were respectively 17.4 dB SPL and 20.3 dB SPL; there were also two decreased regions in the range of 1kHz, with amplitude mean of 9.9 dBSPL and another region at 3kHz with mean amplitude of 12.7dBSPL. The mean amplitude of background noise showed a peak area in frequencies 1 and 1.5 kHz, with mean values of 2.1 and 2.7 dB SPL, followed by decrease as it approached higher frequencies, with mean values of -3.6 and -8.5 dB SPL, up to the frequency of 4 kHz and an extra value for the frequency of 6 kHz, reaching 1,2 dB SPL.

The mean duration of the test for each side was 82 seconds (maximum duration of 133 seconds and minimal duration of 16 seconds).

DISCUSSION

As to occurrence of DPOAEs, in the present study we noticed that for the frequency of 1kHz, it was less frequent than 60%, but it was present in more than 90% of the cases in the remaining frequencies, a result similar to that described by Qian and Jiang (1997). Marco et al. (1995) also found descending occurrence of DPOAEs for low frequencies, but they observed occurrence of 100% above 4kHz.

The frequency that was the most difficult to record answers was 1kHz (19 out of 33 ears). Bonfils et al. (1992) recorded responses in six ears, from a total of 46; this difficult recording may be attributed to contamination of noise in low frequencies produced by the babies. Marco et al. (1995) also reported difficulties to record DPOAEs in 1kHz.

The findings of the present study showed two regions of peak of amplitude and two valley regions, one in I kHz and another in 3kHz, in-between the peak regions. Other authors, such as Lafreniere et al. (1991) and Marco et al. (1995), also mentioned two regions of peak and one of valley between them. It was observed that peak regions were recorded for frequencies 2 to 6kHz, with peaks of 17.4 and 20.3 dB SPL, and Marco et al. (1995) agreed on the peak frequencies, finding means of 17.8 and 22.4 dB SPL, respectively. Coube and Costa Filho (1998) found the same configuration of two peak regions and an intermediary valley region in adults. Conversely, Qian and Jiang (1997) disagree because they found shorter amplitudes for low frequencies than for high frequencies up to 4kHz, with no valley region reported by them.

Bonfils et al. (1992) reported that means of DPOAEs for neonates were greater, regardless of the frequencies. In general, Lafreniere et al. (1991) observed that DPOAEs were similar for neonates and adults when examined with the same devices and probes. Lopes Filho and Carlos (1996) found neonates DPOAEs within and above normal ranges described for young adults. In the present study, we did not compare amplitude of DPOAEs in adults and neonates, as the authors referred above. However, by comparing our findings in neonates we noticed that the DPOAEs means were lower only in 3kHz.

Table 1. Amplitude of DPOAEs and background noise.

Key: DPOAE - distortion product otoacoustic emissions; dB SPL -II decibel sound pressure level; DP - distortion product.



Graph 1. Amplitude of medians of distortion product otoacoustic emissions (DPOAE) and background noise.

As to background noise, Marco et al. (1995) reported that it was more evident in low frequencies, decreasing as the frequencies became higher. The same was noticed in the present study; however, for the frequency of 6 kHz, there was increase in amplitude. Lonsbury-Martin et al. (1990) observed that in high frequencies, as from 4kHz, the increase in amplitude of noise followed the increase in amplitude of DPOAEs, which is in accordance with our findings.

CONCLUSIONS

We concluded that background noise (noisy breathing, for example) was the main obstacle to recording distortion product in low frequencies, especially in 1kHz.

The results of the present study led us to the conclusion that the neonates (33 ears) studied with DPgram at 70dB stimuli presented: a) 57.5% occurrence of DPOAEs in 1kHz, 93.9% in 1.5 kHz and 3 kHz, 96.9% in 2 kHz and 100% in 4 kHz and 6 kHz; b) there were no statistically significant differences at 1% for both genders and right and left ears concerning occurrence and characteristics of DPOAEs amplitude; c) there were amplitude peaks in frequencies 2 and 6 kHz and decrease in 1 and 3 kHz; d) the mean amplitude of background noise showed a peak region in the frequencies 1 and 1.5kHz and decrease as the frequencies became higher, with a new peak in 6kHz. The mean duration of DP-gram was 82 seconds.

Although it is a relatively quick test, when dealing with neonates we should bear in mind that there are difficulties, such as having to interrupt the test because the baby is not calm enough.

ACKNOWLEDGEMENT

To Professor Jose Roberto Pereira Lauris, who carried out the statistical analysis of the present study.

REFERENCES

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1 Specialist in Educational and Rehabilitative Audiology, Hospital de Reabilitação de Anomalias Craniofaciais, Universidade de São Paulo (HRAC-USP), Bauru /SP.
2 Ph.D. in Sciences (Human Communication Disorders), HRAC - USP, Bauru /SP; Professor, School of Speech and Hearing Pathology, Faculdade de Odontologia de Bauru, Universidade de São Paulo (FOB-USP); Researcher of Centro de Pesquisas Audiológicas (CPA), HRAC-USP, Bauru /SP.
3 Full Professor in Otorhinolaryngology, School of Speech and Hearing Pathology, FOB-USP; Otorhinolaryngologist, Coordinator of Centro de Pesquisas Audiológicas, HRAC - USP, Bauru/ SP.
4 Ph.D. in Human Communication Disorders, Universidade Federal de São Paulo - Escola Paulista de Medicina; Professor, School of Speech and Hearing Pathology, Faculdade de Odontologia de Bauru, Universidade de São Paulo (FOB-USP); Post-Doctorate studies at University of Manchester (England) and University of Michigan (United States).

Study carried out at Centro de Pesquisas Audiológicas, Hospital de Reabilitação de Anomalias Craniofaciais, Universidade de São Paulo (HRAC-USP), Bauru-SP, approved by the Ethics Committee of the Hospital, and presented as the Dissertation of Specialization in Educational and Rehabilitative Audiology.
Address correspondence to:: Centro de Pesquisas Audiológicas do HRAC-USP - Rua Silvio Marchione, 3-20 - 17043-900 Bauru /SP - Tel.: (55 14) 235-8433.
Article submitted on March 22, 2001. Article accepted on April 11, 2001.

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