Differences in otoacoustic emissions in infants and adults with normal hearing

Introduction There has been an increasing interest in studying OAE in order to understand and investigate the cochlear function both in adults and infants, since the first description of OAE by Kemp in 1978. There still is no complete description of the most basic aspects of audition although studies of ear development were performed even before 1970 (Abdala 2000). It is believed that the study of OAE in human beings can serve as good criterion to investigate the physiological events of cochlear maturation, because OAE provide a glimpse into the human cochlea during the earliest segments of perinatal and postnatal life. The maturational processes underlying the acoustical and mechanical functioning of the human ear and the detectable functional differences in OAE responses represent a challenging and interesting scientific problem. There is evidence that OAE (TEOAE or/and DPOAE) differ in newborn compared to older children and adults with normal hearing (Norton et al 1990; Kok et al 1992). Studies have revealed that transient otoacoustic emissions change with age: TEOAE amplitude increase within the first days after birth (Kok et al 1992; Welch et al 1996) and remain statistically significant higher in the first year of life than in older children and adults and then rapidly decrease in the first 2-4 years of life (Norton et al 1990; Spektor et al 1991). Distortion product otoacoustic emissions measured in newborns are not adult-like in high-frequency range at 4-6 kHz (Groh et al 2006). The amplitude of otoacoustic emissions presents little variations during the first 6 months of life (Abdala et al 2008). Kon (2000) stated in his study that there are no significant changes of DPOAE Abstract. Objective: Otoacoustic emissions (OAE) are widely used as a noninvasive technique as the test can provide a glimpse into the human cochlea during the earliest segments of perinatal and postnatal life. Although the cochlea is structurally and functionally adult-like by term birth, both transient otoacoustic emissions (TEOAE) and distortion products (DPOAE) differ in newborn compared to older children and adults with normal hearing. We aim to compare the measurements of both TEOAE and DPOAE from infants that have passed the hearing screening test and to similar measurements from normal-hearing older subjects. Material and Methods: This cross-sectional study included 165 participants (126 children and 39 adults) during a period of 5 months. The infant data were obtained from children aged 0-6 months old, divided into three age groups: 0-1 month (group 1), 1-3 months (group 2) and 3-6 months (group 3). The infant subjects included in the study groups have passed the hearing screening test, were with good general health and had normal otoscopic results. The otoacoustic emissions measurements were performed during the physiological sleep. The adult data were obtained from adults (group 4) with normal hearing as shown by the pure tone audiometry and normal functioning of the middle ear and cochlea. We studied the changes of the TEOAE signal/noise ratio (SNR) and of multiple parameters of the DPOAE responses: DPOAE levels, DPOAE noise floor and DPOAE SNR. Results: We discovered significant differences of TEOAE SNRs between pediatrics groups and adult group at the frequencies of 1 and 1.5 kHz in favor of the adult group (for the right ear when comparing group 2 and 3 to group 4 at 1 kHz, p<0.001, respectively p=0.006; for the left ear when comparing group 1 and 2 to group 4 at 1.5 kHz, p<0.001, respectively p=0.001). The differences in TEOAE SNRs remained significant at the frequencies of 3 and 4 kHz in favor of the infant groups (for the right ear when comparing group 2 to 4 at 4 kHz, p=0.004; for the left ear when comparing group 1, 2 and 3 to group 4, p=0.01, p.001, respectively p=0.004). Our results recorded significant differences at 1 and 1.5 kHz for DPOAE level for both ears when comparing pediatric groups to adult group (for the right ear, between group 2 and 4 at 1 kHz, p<0.001 and at 1.5 kHz, p=0.004 and between group 3 and 4, p=0.005; for the left ear, between group 1 and 2 compared to group 4 at 1 kHz, p=0.005, respectively p=0.02 and at 1.5 kHz, for all pediatrics groups compared to adult group: group 1 and 2, p<0.001, group 3 p=0.03). There were statistically significant differences for DPOAE noise between all pediatric groups and adult group at frequency range of 2-6 kHz for both ears. When comparing DPOAE SNRs of the pediatric groups to adult group, we observed significant differences for both ears at all the frequencies tested (p<0.05) with the exception at 1 kHz for the left ear when comparing group 1 to group 4. Conclusion: The infant otoacoustic emissions remain larger at high frequencies than adult for both transient otoacoustic emissions and distortion products.