One goal of Early Hearing Detection and Intervention (EHDI) programs is to identify permanent hearing loss in children by 3 months of age (Joint Committee on Infant Hearing, 2007). This task presents a unique situation because infants younger than 6 months of age are not developmentally capable of providing reliable head-turn responses during behavioral audiometry. Assessment guidelines for infants and children have stated that the most appropriate strategy for estimating hearing thresholds in young infants is the use of frequency-specific auditory brainstem response (ABR) procedures (AAA, 2007; ASHA, 2004).
In the assessment of young infants' hearing, ABR is a clinically feasible part of a comprehensive test battery that includes otoscopic examination, immittance measures with a high-frequency probe tone, and diagnostic otoacoustic emissions. The use of ABR also assists in meeting the second goal of EHDI programs—to initiate intervention by 6 months of age. Intervention with personal amplification can proceed by using threshold estimates obtained from the ABR procedure (Bagatto et al., 2005); however, use of these threshold estimates for hearing aid fitting requires an understanding of the differences between behavioral and electrophysiological tests of hearing.
Behavioral and Electrophysiological Hearing Tests
Frequency-specific ABR thresholds [in decibel normal hearing level (dB nHL)] are not directly equivalent to behavioral thresholds [in decibel hearing level (dB HL)] measured using visual reinforcement audiometry (VRA) techniques. Depending on the frequency, patients with sensorineural hearing loss have been shown to have ABR threshold estimates that are 5 dB to 30 dB higher than their VRA thresholds (Stapells, 2000a).
Reasons for this disparity include differences in calibration procedures; the part of the auditory system where the response is elicited; and the duration of signals being used, such as pure tones for VRA and tone bursts for ABR (Bagatto, 2008). Given these inherent differences, the disparity between behavioral and electrophysiological thresholds is not surprising. In addition, prescriptive algorithms for fitting infants and young children with hearing aids have traditionally used behavioral thresholds to calculate the prescription (i.e., National Acoustics Labs, Non-Linear, version 1 or NAL-NL1; Desired Sensation Level, version 4 or DSL [i/o] v4.1).
To ensure appropriate application of ABR data for the hearing aid fitting, thresholds in nHL are converted to estimates of true behavioral threshold in HL by applying threshold adjustment factors based on studies of the relationship between frequency-specific ABR thresholds and subsequent behavioral thresholds (e.g., Stapells, 2000a). This correction accounts for the differences between ABR and behavioral thresholds. The resulting thresholds are often referred to as “estimated hearing level” (eHL) thresholds and are appropriate for use in hearing aid fitting algorithms (Bagatto et al., 2005). It is important to note that some ABR systems have the threshold adjustments embedded within the system. For those that do not, adjustments to ABR threshold estimates must be applied prior to proceeding with the hearing aid fitting (see Bagatto, 2008).
ABR Threshold Estimates in Hearing Aid Fitting
When ABR threshold estimates have been obtained, the resulting nHL value must be adjusted to reflect a behavioral threshold. Given that ABR thresholds are typically higher than behavioral thresholds, a correction value is subtracted from the nHL value. The resulting eHL value represents a better estimate of the behavioral threshold for the purposes of hearing aid fitting. For example, a value of 10 will be subtracted from an ABR threshold of 50 dB nHL at 2000 Hz. This adjustment results in an estimated behavioral threshold of 40 dB eHL, which is used to calculate the hearing aid prescription at 2000 Hz. The correction values are frequency-specific and are generally larger in the low frequencies than in the high frequencies (Stapells, 2000b). They also will vary depending on the ABR measurement parameters being used to detect a response, which are closely tied to the values used to calibrate the ABR system (see Stapells, 2000b). Therefore, consideration of measurement parameters is important before applying a certain set of correction values.
It is also critical to ensure protocols are in place to ensure the corrections are applied once—and only once—to prevent over- or under-amplification. Some hearing aid prescriptive algorithms allow the option of clinician-applied versus software-applied nHL to eHL corrections (e.g., DSL m[i/o] v5.0a). In addition, consideration of the variability in ear canal acoustics in the pediatric population will provide a more accurate hearing aid fitting and allow for better comparison of hearing threshold measurements over time (Bagatto et al., 2005).
Meeting the EHDI goals requires the use of appropriate infant hearing assessment protocols and subsequent considerations for accurate hearing aid fitting. Infants younger than 6 months of age will have their hearing assessed using ABR procedures. If an adjustment is not already embedded into the system, ABR threshold estimates require adjustment prior to the calculation of the hearing aid prescription. Protocols should indicate the frequency-specific correction values and explicit statements about their application to ensure accurate and timely hearing aid fitting for infants with hearing loss.