Auditory steady-state responses Maturation of bone-conduction hearing Infant bone-conduction thresholds

The objective of this study was to compare boneconduction (BC) auditory steady-state responses (ASSR) for infants and adults with normal hearing to investigate the time course of maturation of BC hearing sensitivity. Bone-conduction multiple ASSRs were recorded in 0 11-month-old (n 35), and 12 24-monthold infants (n 13), and adults (n 18). Low-frequency BC ASSR thresholds increased with age, whereas, highfrequency ASSR thresholds were unaffected by age except for a slight improvement at 2000 Hz. Compared to adults, BC ASSR amplitudes for young infants were larger for low frequencies, whereas, their amplitudes were smaller or similar for high frequencies. Compared to adults, young infants are much more sensitive to low-frequency BC stimuli, and probably more sensitive to high-frequency BC stimuli; these differences between infants and adults persist until at least two years of age. Different ‘normal levels’ for infants of different ages must be used and are proposed in this study. Sumario El propósito de este estudio fue comparar las respuestas por vı́a ósea (BC) de estado estable (ASSR) en niños y adultos con audición normal, para investigar el proceso de maduración temporal de la sensibilidad auditiva de la BC. Se registraron respuestas múltiples ASSR por BC en niños de 0-11 meses de edad (n 35) y de 12-24 meses (n 13) además de adultos (n 18). Los umbrales de frecuencias graves ASSR para BC aumentaron con la edad mientras que los umbrales ASSR de frecuencias agudas no se afectaron por la edad excepto por una ligera mejorı́a en 2000 Hz. Comparado con los adultos, las amplitudes BC ASSR en niños pequeños fueron más grandes en las frecuencias graves mientras que fueron menores o similares en las frecuencias agudas. En comparación con los adultos, los niños pequeños son mucho más sensibles a los estı́mulos por BC en las frecuencias graves y probablemente más sensibles con los estı́mulos por BC en las frecuencias agudas. Estas diferencias entre niños y adultos persisten por lo menos hasta los dos años de edad. Deben usarse y se proponen en este estudio diferentes ‘‘niveles normales’’ para niños de edades diferentes. Elevation of hearing thresholds to air-conduction stimuli in infants may result from a sensorineural, conductive, or mixed hearing loss. In this case, it is necessary to obtain boneconduction thresholds, both to distinguish between sensorineural, conductive, and mixed hearing losses, and to determine the magnitude of the air-bone gap. This is routinely done in adults and should also be done when testing infants. Although bone-conduction testing is known to be essential, many clinicians continue to use only air-conduction stimuli when estimating thresholds in infants using the auditory brainstem response ISSN 1499-2027 print/ISSN 1708-8186 online DOI: 10.1080/14992020802055284 # 2008 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society Susan A. Small School of Audiology & Speech Sciences, The University of British Columbia, 5804 Fairview Avenue, Vancouver, BC, V6T 1Z3, Canada. E-mail: [email protected] Received: March 22, 2007 Accepted: March 10, 2008 D ow nl oa de d B y: [A nn e S m al l, S us an ] A t: 16 :0 8 24 M ay 2 00 8 (ABR) or auditory steady-state response (ASSR). Despite the importance of bone-conduction testing, there are only a limited number of published studies in infants that have recorded boneconduction thresholds to frequency-specific stimuli using either ABR (Cone-Wesson & Ramirez, 1997; Foxe & Stapells, 1993; Stapells & Ruben, 1989) or ASSR (Small et al, 2007; Small & Stapells, 2005b, 2006) techniques. Collectively, the ABR studies have investigated bone-conduction thresholds for 22 neonates at 500 and 4000 Hz (Cone-Wesson & Ramirez, 1997), and 36 sixmonth-old infants at 500 and 2000 Hz (Foxe & Stapells, 1993; Stapells & Ruben, 1989). Our earlier ASSR studies estimated bone-conduction thresholds in 29 pre-term infants (Small & Stapells, 2006) and, across three studies, a total of 35 infants 0 11 months of age (Small et al, 2007; Small & Stapells, 2005b, 2006) at 500, 1000, 2000, and 4000 Hz. There are no published frequency-specific bone-conduction threshold data (ABR or ASSR) for infants older than 11 months of age. To establish normal bone-conduction hearing levels for infants and to understand the time course of maturation for bone-conduction hearing, comparisons of bone-conduction hearing thresholds at the same frequencies for large groups of young and older infants and adults are needed. From research to date, we know that infant-adult differences exist for both ABRs and ASSRs to bone-conduction stimuli, and that these differences are age and frequency dependent. Stuart et al (1993) reported that bone-conduction click-ABR (i.e. response elicited from a broad range of frequencies along the cochlear partition) thresholds in neonates are 17.5 dB (in dB nHL) better than those obtained for adults, and they thus suggested that the delivery of a bone-conducted signal is more effective in neonates than in adults (Stuart et al, 1990, 1993; Yang et al, 1987). Cone-Wesson and Ramirez (1997) also reported better bone-conduction click-ABR thresholds in neonates than adults. In contrast, Cornacchia and Morra (1983) found no difference for bone-conduction click-ABR thresholds between older infants (16 20 months of age) and adults. For brief-tone stimuli, which are frequency specific, Foxe and Stapells (1993) found that bone-conduction ABR thresholds to brief tones for six-month-old infants were similar to adult thresholds at 500 Hz, but 5.5 dB poorer compared to adults at 2000 Hz. They also found that these infants had bone-conduction ABR thresholds to 500-Hz brief tones that were significantly better (10.5 dB) compared to 2000 Hz. ConeWesson and Ramirez (1997) reported that bone-conduction ABR thresholds in neonates were 20 dB better at 500 Hz and 5 dB better at 4000 Hz compared to adults (n 3). They also showed that bone-conduction ABR thresholds for neonates were 14 dB better at 500 Hz compared to 4000 Hz [it is also noteworthy that their 500-Hz thresholds for the neonates were 25 dB better than those reported by Foxe and Stapells (1993)]. We found similar results for bone-conduction ASSRs in our previous studies of pre-term and 0 8-month-old post-term infants; bone-conduction ASSRs thresholds were significantly better at 500 and 1000 Hz compared to 2000 Hz (Small et al, 2007; Small & Stapells, 2006). Foxe and Stapells (1993) found no significant difference in 500and 2000-Hz ABR thresholds (in dB nHL) in adults; similarly, bone-conduction ASSR thresholds (in dB HL) in adults show little or no difference across frequency (Small & Stapells, 2005). Frequency-specific bone-conduction thresholds in infants older than 11 months of age are needed to investigate when bone-conduction thresholds become adult-like. The pattern of amplitudes of bone-conduction ASSRs in young infants as a function of frequency are similar to previously reported bone-conduction ABR amplitudes in infants. Low-frequency bone-conduction ABR amplitudes are larger than those to high frequencies (Cone-Wesson & Ramirez, 1997; Foxe & Stapells, 1993; Stapells & Ruben, 1989); lowfrequency bone-conduction ASSR amplitudes are also larger than those to high-frequency stimuli for preand young postterm infants (Small & Stapells, 2006; Small et al, 2007). Direct comparison of ABR and ASSR amplitude differences across age groups cannot be made because of the additional contribution of the 40-Hz response to the adult ABR (when using a 40/second rate) that is not present in the infant ABR (Foxe & Stapells, 1993). However, comparisons between infants and adults can be made for 80-Hz ASSR amplitudes; adults have larger boneconduction ASSRs than infants at 40 dB HL for 2000 and 4000 Hz but show little difference compared to young infants at 500 and 1000 Hz (Small & Stapells, 2006). The purpose of the present study was to compare multiple ASSR thresholds to bone-conduction stimuli in infants of different ages to adults*all with normal hearing*to investigate the time course of maturation of bone-conduction thresholds across frequency. It was also the purpose of this study to establish ‘normal levels’ for bone-conduction ASSRs for young and older infants. To this end, new data as well as previously published bone-conduction ASSR data for three different age groups were combined and compared in this study. Boneconduction ASSRs were investigated for a group of ‘older’ infants, a group not previously assessed. A larger group of ‘young’ infants was created for this study by pooling data from the smaller groups of young infants presented in previously published studies (Small et al, 2007; Small & Stapells, 2005b, 2006). Finally, a larger group of adult subjects was formed in order to compare infant and adult bone-conduction ASSRs by pooling new bone-conduction ASSR threshold data collected in this study with previously published adult data (Small & Stapells, 2005). Materials and Methods Participants Two groups of infants who passed a hearing screening in both ears and a group of adults with normal hearing (pure-tone behavioural airand bone-conduction thresholds 525 dB HL at 500 4000 Hz) participated. ASSRs to bone-conduction stimuli were recorded in 35 ‘young’ infants [age range of 0.5 44.0 weeks; mean age of 16.0 weeks], 13 ‘older’ infants [age range of 12 24 months; mean age of 18.2 months], and 18 adults [age range of 19 48 years; mean age of 22.9 years] recruited from the community. Data for the 35 young infants were previously reported as small groups in three separate studies (nine infants from Small et al, 2007; 14 infants from Small & Stapells, 2006; 12 infants from Small & Stapells, 2005b); these small groups were combined to obtain one large group of young infants. Data for 10 of the 18 adults presented in the present study were also previously reported (Small & Stapells, 2005) and combined with new data from eight additional adults to form a larger group of adults. Bone-conduction ASSRs in older infants were recorded 2 International Journal of Audiology, Volume 00 Number 0 D ow nl oa de d B y: [A nn e S m al l, S us an ] A t: 16 :0 8 24 M ay 2 00 8 for the first time in this study. (Note: The ages of the infants from 0 24 months were not evenly distributed, therefore, the infants were divided into two age groups, 0 11 and 12 24 months of age. The age distribution for the infants tested is shown in Figure 4). Four of the infants recruited from the community were screened using an automatic auditory brainstem response (AABR) screening test at 35 dB nHL. The hearing of the other infants was screened using a distortion-product otoacoustic emissions (DPOAE) screening test. The pass criterion for the DPOAE screening was a signal-to-noise ratio 5 dB at 2000, 3000, and 4000 Hz in both ears. Infants who passed the AABR or DPOAE hearing screening test in both ears were considered to be at low risk for significant hearing loss and thus included in the study.