Journal of the International Telemedicine Academy

Technological advances in Neonatal Hearing Screening

Stavros HATZOPOULOS

Department of Audiology, University of Ferrara, Italy

Key words: Automated Otoacoustic Emissions, AOAE, Automated Auditory Brainstem response. AABR, Neonatal Hearing Screening, Auditory State Steady Response, hearing threshold

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Introduction

In 2006 Otoacoustic Emissions (OAEs) celebrate a life span of 28 years (after the first OAE publication by David Kemp in 1978). The most significant contribution of OAEs is in the area of Neonatal Hearing Screening (NHS). Within the last decade, numerous new objectives have been presented in the NHS area such as the quality of the automated OAE responses, the estimation of a hearing threshold etc. To respond to the clinical demands of these objectives, several new methodologies have been introduced in the clinical practice the last few years and the aim of this technical note is to provide information on these latest technological trends.

Automated Auditory Brainstem Responses

In the early 2002, the first 4rth generation OAE devices appeared in the market and provided the possibility to numerous clinical realities to integrate information from automated OAE (A-OAE) and automated ABR (A-ABR) recordings. The combined screening protocols (A-OAE + A-ABR) targeted the identification of auditory neuropathy cases most prevalent in the NICU environment. Nevertheless, the presence of portable ABR equipment provided the possibility to conduct studies in real screening environments (and not in various simulations in ideal ambient conditions) where the hearing threshold was assessed with both portable and clinical equipment. A pilot study conducted by our group (Giorba et al, 2006) in the context of a regional project in Emilia-Romagna (Project CHEAP) have suggested that the portable ABR and OAE technologies are converging in terms of time requirements. The data collected in the above study has suggested : (i) the average time for a AOAE responses is clearly less than 10 s in a cooperative subject, and less that 120 s (2 min) in non-cooperative subjects. (ii) test times of A-ABR in cooperative subjects were less than 120 s, while uncooperative subjects were tested within 10 min (per ear). While it takes some minimum expertise to properly handle and position the OAE probe, the ABR electrode placement presents more complications especially in cases where the subject shows high electrode impedance. In the latter case the AABR testing is difficult to complete and the test times are unavoidably longer.

Theoretically a 2-stage approach (ie A-OAE + A-ABR) eliminates the risk of not identifying infants with Auditory Neuropathy and assures that the screening sensitivity is high . Contrary to these hypotheses recent data from an American study (White et al, 2005) suggest that this is not the case. The study assessed information from 86634 infants and for the infants who were screened for hearing loss using a typical 2-stage OAE/A-ABR protocol, approximately 23% of those with permanent hearing loss at 8-12 months of age would have passed the A-ABR. The data suggest that stringent criteria should be incorporated in the final evaluation of the current OAE and ABR automated devices.

Auditory Steady State Responses in Neonatal Screening

Both OAE and ABR technologies utilize as stimuli electrical clicks and the acquired information is clearly more related to the audiometric frequencies of 1.0 and 2.0 kHz. Within this context, there has been a speculation of whether other measurements technologies could be used in a fast hearing assessment of neonates, children and adults. A group of similar electrophysiological measurements to OAEs and AABR includes electro-cochleography (EcoG), and Middle latency (ML) and Steady State Responses (SSR). From this group the latter category has shown interesting characteristics due to fact that by alternating the modulation frequency (i.e. increasing it) of the stimuli one can get responses from the Auditory cortex (low modulation frequencies around 40 Hz) or from the Brainstem (Cone-Wesson et al; 2002; Dimitrijevic et al , 2002: John and Picton, 2002). The SSR protocol has already passed to an automated one (ASSR) and for the last two years numerous publications have been devoted to the threshold estimation via the ASSR technique. The ASSR protocols have been greatly optimized, (Gorga et al, 2004) and the SSR responses are detected in the frequency domain by robust probabilistic algorithms.

In 2002 Conne-Wesson et al proposed the use of ASSR as a hearing screening tool, with the objective that ASSR could substitute the A-ABR. A few reports have been available since (Stueve and O'Rourke, 2003; Luts et al, 2004; Swanepoel et al; 2004) indicating a good agreement between ASSR and A-ABR at 2.0 kHz and various differences at 0.5, 1.0 and 4.0 kHz. Most studies recommended the use of the SSR technique in the clinic but the point of substituting the A-ABR with ASSR is not supported yet by the available data. The factors which affect the A-ABR (ambient noise and electrode impedance) interfere with the ASSR recordings as well. In order to resolve these issues Vivosonic has presented a new line of devices using preamplifiers at the level of the scalp-electrodes (called amplitrodes) which suppress the level of ambient noise and provide very clean A-ABR and ASSR traces. It is to be seen how these electrodes will be intergraded in the normal clinical reality since the pre-amplifiers require electrical energy which translates into changing batteries every x tests. In the context of neonatal screening, an ASSR screening protocol might target initially a few frequency points (ie 1.0 & 2.0 kHz or 2.0 & 4.0 kHz) which show immunity to ambient noise (Figure 1 & Figure 2) . Nevertheless the ASSR protocol requires significant optimizations before becoming a member of the neonatal hearing screening battery of tests.

Figure 1. ASSR response from a well baby who was crying using the AUDERA device from VIASYS. The lowest tested frequency was not available due to noise. The length of the testing procedure was 22 min (14 min longer than the successfully completed A-ABR test). Despite the theoretical noise immunity at 2.0 and 4.0 kHz the size of the error bars indicate that the measurements are too variable to be considered. The "x" symbols indicate the mean threshold level of the measurements.

Figure 2. ASSR response from a well baby using the AUDERA device. The length of the test was also longer that the A-ABR ( 16 vs 7 min). The A-ABR suggested a REFER probably due to conductive complications suggested by the AASR outcome. In this case the 2.0 and 4.0 kHz frequencies show good noise immunity (suggested by the small size of the error bars). The "x" symbols indicate the mean threshold level of the measurements.

Threshold estimation via DPOAE measurements

An interesting challenge for otoacoustic emissions has been the relationship between the amplitude of the OAE response and the hearing threshold (Whitehead et al 1995a; 1995b; Shera et al, 1999). For cases where no conductive losses are present there is a good agreement between OAEs and the hearing threshold. In such cases Input-Output distortion product OAE (DPOAE) protocols may offer more information (Whitehead et al, 1995a, Janssen et al, 1998; Dorn et al, 2001; Gorga et al, 2003b). Besides the relationship to pure-tone thresholds, DPOAE I/O-functions provide an estimate of the compression related to the outer hair cell amplifier. Data supporting this hypothesis are available from animal studies where the hearing of the animals was impaired with acute furosemide intoxication (Mills and Rubel, 1996) and human studies with subjects suffering from cochlear hearing loss (Janssen et al., 1998; Kummer et al., 1998; Boege and Janssen, 2002; Neely et al., 2003). In these studies the slope of the DPOAE I/O-function increased with increasing hearing loss revealing a loss of compression of the outer hair cell amplifiers. In this context by using numerous combinations of I/O DPOAE recordings one can obtain very precise information related to the status of the cochlear amplifier (Gorga et al, 2003a, 2003b). Recently, extrapolated DPOAE I/O-functions were constructed from neonates to estimate pure-tone threshold levels and the corresponding cochlear compression values (Janssen et al., 2003). The estimated hearing threshold was found to be increasing within the early postnatal period (average age: 3 days), predominantly at the higher frequencies, and to be normalized in a follow-up measurement (after four weeks). However, the slope of the DPOAE I/O-functions obtained in the first and second measurement was unchanged revealing normal cochlear compression. Consequently, these findings were interpreted as temporary conductive hearing losses due to the presence of amniotic fluid and/or Eustachian tube dysfunction. In this clinical scenario, especially during the first days of life, a hearing screening test may lead to false positive results due to a temporary conductive hearing loss. The use of the slope of DPOAE I/O-functions could be used as an index of conductive losses which might result in less false positives an in less time spent for audiological clinical diagnostics. According to the data of Jenssen et al (2003) the values of the DPOAE slope can discriminate and differentiate conductive from sensorineural hearing losses. In addition DPOAE I/O-functions have been reported to be correlated with loudness (Neely et al. 2003), so DPOAE I/O information would also offer the potentiality of assessing information to basic hearing aid fitting.

The research findings from Janssen et al (2003) and Gorga et al (2003a) have been commercialized in a device called Cochlea- Scan (Osvald et al, 2003) by Fischer-Zoth. Hearing threshold can be extrapolated up to values relative to 50 dB HL in the frequency range from 1.5 to 6 kHz. Figure 3 shows two data acquisition sequences. At present the Cochlea-Scan device offers a platform for a third generation OAE testing (TEOAEs, DPOAEs), I/O DPOAE estimation with hearing threshold extrapolation and Pure Tone Audiometry measurements.

Figure 3. Cochlea-scan displays during the threshold estimation (top panel) and threshold extrapolation (bottom panel) from a well baby. The audiometric notch between 2 .0 to 4.0 kHz is probably enhanced by the standing waves in the external auditory meatus which influence the values of the employed DPOAE measurements.

Acknowledgements

The author would like to thank Maddalena Rossi and Giorgio Rossi for technical assistance with the CochleoScan and AUDERA equipment.

Appendix

The reader interested in additional information than the one presented might visit the OAE Portal (http://www.otoemissions.org) and the OAE Portal Forum.

References