Behavioral Bone-Conduction Thresholds for Infants with Normal Hearing

Abstract

Bone-conduction thresholds have been used in audiologic assessments of both infants and adults to differentiate between conductive and sensorineural hearing losses. However, air- and bone-conduction thresholds estimated for infants with normal hearing using physiological measures have identified an "air-bone gap" in the low frequencies that does not result from conductive hearing impairment but, rather, from maturational differences in sensitivity. This maturational air-bone gap appears to be present up to at least 2 yr of age. Because most infants older than 6 mo of age are clinically assessed behaviorally, rather than physiologically, it is necessary to determine whether a similar maturational air-bone gap is present for behavioral air- and bone-conduction thresholds. The purpose of this study was to estimate behavioral bone-conduction thresholds for infants using a standard clinical visual reinforcement audiometry (VRA) protocol to determine whether frequency-dependent maturational patterns exist as previously reported for physiological bone-conduction thresholds. Behavioral bone-conduction minimum response levels were estimated at 500, 1000, 2000, and 4000 Hz using VRA for each participant. Young (7-15 mo; N = 17) and older (18-30 mo; N = 20) groups of infants were assessed. All infants were screened and considered to be at low risk for hearing loss. Preliminary "normal levels" were determined by calculating the 90th percentile for responses present as a cumulative percentage. Mean bone-conduction thresholds were compared and analyzed using a mixed-model analysis of variance across frequency and age group. Linear regression analysis was also performed to assess the effect of age on bone-conduction thresholds. Results of this study indicate that, when measured behaviorally, infants under 30 mo of age show frequency-dependent bone-conduction thresholds whereby their responses at 500 and 1000 Hz are significantly better than those at 2000 and 4000 Hz. However, thresholds obtained from the younger group of infants (mean age of 10.6 mo) were not significantly different from those obtained from the older group of infants (mean age of 23.0 mo) at any frequency. The findings of the present study are similar to the results obtained from previous physiological studies. Compared to previously documented air-conduction thresholds of infants using similar VRA techniques, a maturational air-bone gap is observed in the low frequencies. Therefore, differences between infant and adult bone-conduction thresholds persist until at least 30 mo of age. As a result, different "normal levels" should be used when assessing bone-conduction hearing sensitivity of infants using behavioral methods.