The identification of hearing loss during infancy and the prompt initiation of amplification and habilitation have become standards of care in pediatric audiology. In order to meet the time-based goals set forth by the Joint Committee on Infant Hearing, electrophysiological tests must be used to estimate hearing sensitivity.
The auditory brainstem response (ABR) has been established as the gold standard test for estimating hearing sensitivity in infants or young children who are unable to participate reliably in behavioral hearing assessments. Multiple studies with large numbers of subjects have demonstrated strong relationships between frequency-specific ABR thresholds measured with tone-burst stimuli and corresponding frequencies on later behavioral measures ( Ear Hear 2006;27:60-74 and Ear Hear 2009;30:350-368).
Given the consistent relationship between frequency-specific ABR and behavioral thresholds, methods have been developed to prescribe amplification for infants based on tone-burst auditory brainstem response ( Int J Audiol 2010;49[suppl 1]:S70-S79). These methods include the use of correction factors, as previously described in The Hearing Journal (February 2008 issue, page 10), to account for threshold differences between the brief stimuli used to elicit ABR and the relatively long-duration pure tones used to measure hearing sensitivity behaviorally.
Over the last 10 years, the auditory steady-state response (ASSR) has received a great deal of attention from researchers and clinicians as a potential alternative to ABR for estimating hearing sensitivity in infants.
Unlike the ABR, which measures the response of the auditory nerve and brainstem over time, the ASSR assesses the frequency of the response from the auditory system. With this approach, the stimuli are amplitude modulated at a specific frequency, usually between 70 and 110 Hz. The auditory system preserves the amplitude modulation in response to the stimuli.
When the frequency of the ASSR is recorded and analyzed, the response to a specific stimulus is apparent as a spike at the modulation frequency. The auditory steady-state response system detects a response automatically, eliminating the need for the clinician to determine its presence.
Different stimulus frequencies can be modulated at different rates, which allows for the presentation and recording of multiple stimulus frequencies simultaneously. For example, a 500-Hz tone could be modulated at 80 Hz, a 1,000-Hz tone at 90 Hz, a 2,000-Hz tone at 100 Hz, and a 4,000-Hz tone at 110 Hz.
The ability to test multiple frequencies in the same ear at the same time potentially represents a significant improvement in test efficiency. However, Jennifer Hatton and David R. Stapells have reported that the response amplitude of ASSR in infants is smaller when multiple-frequency stimulus conditions are used than when a single frequency is tested ( Ear Hear 2011;32:349-357).
This amplitude reduction is larger than that observed in adults and could result in elevated estimates of thresholds, particularly if the system's automated detection criterion is not adjusted to account for age and number of simultaneous stimuli. Importantly, the existence of amplitude reduction with multiple-frequency ASSR at near-threshold levels in infants who have hearing loss has not been reported in the research literature.
PRESENT AND FUTURE
Given the significant amount of research and discussion about auditory steady-state response in recent years, our clinic frequently receives questions from clinicians about whether or not we are currently using or recommending that others use ASSR to assess hearing in infants. Our clinic has the capability of performing ASSR but continues to use tone-burst ABR as the basis for electrophysiological assessment for several reasons.
First, auditory brainstem response sets a very high standard for predicting behavioral thresholds, and the relationship between tone-burst ABR thresholds and behavioral thresholds is documented. Similar studies of ASSR in large groups of children with a range of hearing losses are starting to emerge ( Eur Arch Otorhinolaryngol 2012;269:73-79), but more work is needed before we can approach ASSR with the same confidence we have in ABR.
ASSR also may have important limitations in the diagnosis of auditory neuropathy spectrum disorder (ANSD), which I wrote about in December ( HJ December 2012 issue, p. 10). Specifically, the characteristics of ANSD that are visible in the ABR, including an absent or significantly abnormal auditory brainstem response with cochlear microphonic, cannot easily be confirmed by an ASSR.
While the presence of otoacoustic emissions (OAEs) with a significantly abnormal ASSR would be a strong diagnostic indication of ANSD, not all children with this condition have present OAEs. Therefore, some children with ANSD would appear to have a profound hearing loss on auditory steady-state response and would likely be remediated accordingly without additional testing.
While some have argued that ASSR can be used in conjunction with tone-burst ABR, any advantages in terms of efficiency would be mitigated by the need to perform more testing. For these reasons, we have decided not to implement ASSR as part of our diagnostic test battery at the present time.
In the future, however, ASSR has the potential to be the diagnostic test we use for electrophysiological assessment of auditory function in infants. New studies are published every year, and specific research findings would increase our confidence in using the technique clinically.
For example, more data from large populations of children with varying types and degrees of hearing loss will further establish the relationship between ASSR and behavioral thresholds in clinical populations. Criteria that take into account the number of simultaneous stimuli and the age of the patient will help improve response detection with ASSR in infants. Methods of estimating the presence of auditory neuropathy spectrum disorder with ASSR are needed to ensure accurate diagnosis and appropriate intervention.
We owe it to our young patients to use assessment methods that we trust based on the available research. At the present time, auditory brainstem response remains our best option, but I suspect that auditory steady-state response may soon have the evidence base to support its clinical implementation.
References on Tap
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