A two-step scenario for hearing assessment with otoacoustic emissions at compensated middle ear pressure (in children 1–7 years old)
Introduction
Otoacoustic emissions (OAEs) are low intensity sounds generated within the normal cochlea. For screening and assessment of congenital and early-acquired sensorineural hearing loss, measurement of OAEs is widely accepted.
The recording of transient evoked otoacoustic emission (TEOAE) is influenced by a number of factors including both inner and middle ear pathology. The middle ear is not only important in the transmission of the evoking stimulus to the cochlea (forward transmission), but also in the delivery of the emission through the middle ear to the ear canal (retrograde transmission). TEOAEs are reduced when sound conduction through the middle ear is compromised [1]. Middle ear pathology, as otitis media or a negative middle ear pressure, is highly prevalent among infants and young children. In ears with abnormal (negative) middle ear pressure, caused by Eustachian tube dysfunction, TEOAEs are attenuated [2]. Owens reported that in ears with a normal or near normal hearing, even a small amount of negative middle ear pressure resulted in a reduced OAE amplitude and reproducibility [3]. This suggests that negative middle ear pressure may account for many of the false positive rates in screening for permanent hearing loss.
The predictive power of the tympanogram for hearing loss is poor [4]. Furthermore, the type of a tympanogram is not correlated with the presence or absence of emissions [4], [5]. Therefore, evidence of abnormal middle ear function obtained using conventional tympanometry seems to be a poor predictor of TEOAE status.
In a screening procedure in children of 0–5 years old, a combination of TEOAE measurement and tympanometry showed an additional value in the choice for further hearing analyses. Both goals of screening, detection of sensorineural and conductive hearing loss, were satisfactory reached [6]. A system combining TEOAE measurement and tympanometry should have the possibility to compensate an abnormal (negative) middle ear pressure. Once pressure equilibration is achieved, the tympanic membrane, maybe also the labyrinthine window membranes and basal membrane becomes more relaxed and sound transmission should improve [7]. This could lower the number of false positives in referred patients and could make an important contribution to the practical application of TEOAEs.
The audiometric assessment in very young children, using pure tone audiometry, tympanometry and auditory brain stem responses is time consuming. TEOAE measurements are objective, rapid and non-invasive. An initial assessment with TEOAE in a paediatric outpatient ward could limit the number of patients in need for further assessment. The ‘presence’ of OAEs however, needs to be defined as little is known about the effect of the chosen definition on the pass or fail outcome of a TEOAE measurement. The definitions we found varied from less specified as “50% reproducibility” [8] to more strict definitions for reproducibility such as “75% and 3 dB signal-to-noise (S/N) ratios for different frequency bands” [9] or “50% reproducibility for at least 3 test frequencies” [10]. Often the pass/fail definitions were not documented. One should expect that a more rigid definition result in a lower pass rate.
In this study we investigated whether measuring TEOAEs at compensated middle ear pressure (CMEP) in children from 1–7 years old with mild middle ear disease resulted in higher pass rates than at ambient pressure. Secondly, as there seems to be no consensus on the pass/fail definition, we defined 12 definitions based on correlation responses, overall signal-to-noise ratios and signal-to-noise ratios for specific frequency bands (Table 1), and we studied the influence of these definitions on our results.
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Subjects
One hundred and eleven children with hearing complaints, aged between 1 and 7 years (mean 4 years and 5 months), were seen on the out patient ward of the University Hospital Maastricht, The Netherlands. As part of the ENT diagnostic procedure, they were scheduled for audiometric assessment.
The standard audiometric assessments were performed with the clinical audiometer (Camea, ADH Hoensbroek, The Netherlands). It included a pure tone audiogram for the older children and conditioned orientation
TEOAE measurement at ambient pressure and at CMEP
Fig. 2 shows a result of TEOAEs measured at ambient pressure, at compensated pressure and the tympanogram for the same ear. At ambient pressure, the stimulus level was 78 dB pe SPL resulting in an emission amplitude of 13 dB SPL (Fig. 2b). The middle ear pressure, obtained from the peak in the tympanogram, was −181 daPa. At this negative pressure the second TEOAE measurement was performed with the same stimulus level of 78 dB pe SPL (Fig. 2c). The overall emission amplitude was increased to 19 dB
Discussion and conclusion
The aim of this study was to investigate whether measuring of TEOAEs at CMEP in children, from 1 to 7 years old with mild middle ear disease, results in higher pass rates than at ambient pressure.
We found that a significant reduction of the number of ears failing TEOAE registration can be obtained when measuring TEOAEs at CMEP. In this study population with a high prevalence of middle ear disease, TEOAEs at ambient pressure were absent in 55–84% of the ears assessed.
Measuring TEOAEs in a
Acknowledgements
This work was supported by the Heinsius Houbolt Foundation and GN Otometrics.
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