Apparent threshold variation for certain frequencies can occur during the course of initial detection and characterization of an infant's hearing loss. Implementation of an effective universal hearing screening program to detect unilateral or bilateral hearing loss of varying severity in newborns and young infants entails the use of objective physiologic measures such as transient-evoked (TEOAE) or distortion-product (DPOAE) otoacoustic emissions or auditory brainstem response (ABR). Because the presence of normal otoacoustic emissions (OAEs) is believed to be indicative of intact outer hair cell function, their absence can be used to detect sensory (ie, inner ear) hearing loss [1,21]. In neonates, OAEs can be reliably recorded in response to stimuli with frequencies above 1500 Hz. External ear canal obstruction and middle ear effusion, with resultant transient dysfunction of the conductive mechanism, can result in the absence of OAEs (a positive test result) in the presence of normal cochlear function [3]. Because OAEs are generated within the cochlea by outer hair cells, OAE testing is not effective in detecting isolated neural (ie, eighth nerve or auditory brainstem pathway) dysfunction such as auditory neuropathy. In contrast, screening with the auditory brainstem response, generated by electrical activity within the cochlea, auditory nerve, and auditory brainstem pathways, can detect underlying auditory dysfunction, including isolated auditory neuropathy or neural conduction disorders in the absence of peripheral (eg, middle ear or cochlear) hearing loss (Joint Committee on Infant Hearing, Year 2000 (JCIH) Position Statement). Potential errors associated with screener bias and individual test interpretation can be obviated by utilizing infant hearing screening technologies incorporating automated response detection [4-8]. False negative results may be observed with both ABR and OAE screening paradigms, particularly when hearing loss configurations include normal or near normal hearing for one or more frequencies in the target test range, as is the case with isolated low-frequency (ie, <1000 Hz) hearing loss (ie, a rising audiometric configuration) or a steeply sloping high-frequency (le, >2000 Hz) loss. ABR thresholds elicited with a broadband click stimulus correlate best with pure-tone behavioral thresholds in the 2000- to 4000-Hz region for the same ear but not for frequencies of 1000 Hz or less. Overall, auditory-evoked potential measurements are simply not as sensitive as behavioral threshold measures. Behavioral thresholds for clicks typically occur at 30- to 36-dB peak sound pressure, yet few normal-hearing patients demonstrate ABRs to such low intensity stimuli in clinical test conditions. If behavioral thresholds are measured with long-duration stimuli (such as those employed in pure-tone audiometry), differences between ABR and behavioral thresholds may be as great as 25 dB. The two major shortcomings of click-evoked ABRs-namely, sensitivity and utility in predicting low-frequency behavioral thresholds-would not be alleviated by using tone-burst stimuli in cases of severe to profound hearing loss. Tone burst-evoked ABRs tend to be less sensitive than behavioral thresholds by 10 to 30 dB, even when 4096 stimulus repetitions are included in each averaged ABR response and each averaged response is replicated once [9]. Accordingly, it may be inappropriate to presume that an ear is anacusic solely because no click-evoked ABR is observed at maximum stimulation levels, such as 100 dB HL. In one study, aided and unaided behavioral pure-tone thresholds were measured in pediatric patients who demonstrated no click-evoked ABR when the stimulus was delivered at the upper limits of the ABR equipment (100 dB HL,) [9]. The likelihood of observing either an unaided or aided behavioral response in an ear diminished as the test frequency was increased. For example, 58% of all ears had unaided behavioral responses of 100 dB HL or better at 250 Hz, declining to 12% at 4000 Hz). Seventy percent of all ears had unaided speech awareness thresholds (SATs) of 100 dB or better, and approximately 45% of these ears had aided SATs of 50 dB or better. Significant percentages of ears with no click-evoked ABRs demonstrated aided behavioral thresholds better than 60 dB HL. Assessing benefit from conventional amplification is a dynamic longitudinal process during which the interplay of multiple variables (eg, type of hearing aid, volume settings, adjustment to amplification, auditory responsiveness, otitis media history, parental cooperation, aural rehabilitation program, additional handicaps) must be monitored and weighed against evaluation results. Reliable aided thresholds may emerge slowly over a period of months as the child learns to wear a hearing aid and attend to soft acoustic stimuli.
