Transient Otoacoustic Emissions and Auditory Brainstem Responses in Low-Risk Cohort of Newborn and One-Month-Old Infants: Assessment of Infant Auditory System Physiology in the Prenatal Alcohol in SIDS and Stillbirth Network Safe Passage Study

Abstract

Background: The Prenatal Alcohol and Sudden Infant Death Syndrome and Stillbirth Network, known as the "Safe Passage Study," enrolled approximately 12,000 pregnant women from the United States and South Africa and followed the development of their babies through pregnancy and the infant's first year of life to investigate the role of prenatal alcohol exposure in the risk for sudden infant death syndrome (SIDS) and adverse pregnancy outcomes, such as stillbirth and fetal alcohol spectrum disorders.

Purpose: Auditory system tests were included in the physiologic test battery used to study the effects of prenatal alcohol exposure on neurophysiology and neurodevelopment, as well as potential causal relationships between neurodevelopmental disorders and SIDS and/or stillbirth. The purpose of this manuscript is to describe normative results when using the auditory test battery applied.

Research design: The test battery included the auditory brainstem response (ABR) and transient-evoked otoacoustic emissions (TEOAEs). Data were collected on individual ears of newborns and 1-month-old infants.

Study sample: From a cohort of 6,070 with auditory system exams, a normative subsample of 325 infants were selected who were not exposed prenatally to alcohol, cigarette smoke, or drugs nor were they preterm or low birthweight. The subsample is small relative to the overall study because of strict criteria for no exposure to substances known to be associated with SIDS or stillbirth and the exclusion of preterm and low birthweight infants. Expectant mothers were recruited from general maternity at two comprehensive clinical sites, in the northern plains in the United States and in Cape Town, South Africa. These populations were selected for study because both were known to be at high-risk for SIDS and stillbirth.

Data collection and analysis: ABR and TEOAE recordings were stored electronically. Peak latency and amplitude analysis of ABRs were determined by study personnel, and results were evaluated for differences by age, sex, test site, race, and ear (left versus right).

Results: TEOAE findings were consistent with existing literature including the increase in signal-to-noise (SNR) over the first month of life. The SNR increase is due to an increase in amplitude of the emission. TEOAE amplitude asymmetry favoring the right ear was found, whereas SNR asymmetry was not, perhaps because of the small sample size. A nonsignificant trend toward larger responses in female babies was found; a result that is generally statistically significant in studies with larger samples. Latencies were found to be shorter in ABRs elicited in the right ear with amplitudes that were slightly bigger on average. An expected decrease in wave V latency was observed from birth to 1-month of age, but the finding was of borderline significance (p = 0.058).

Conclusions: One month is a short time to judge development of the auditory system; however, the ABR and TEOAE findings were consistent with current literature. We conclude that the auditory system data acquired for the Safe Passage Study, as reflected in the data obtained from this cohort of "unexposed" infants, is consistent with published reports of these auditory system measures in the general population.

Figure 1.

Mean narrow band SNRs of transient OAEs are shown with ±1 SE bars. Wideband responses for each ear and condition appear to the right of the frequency-specific data. TEOAE data from all infants appear in the middle of the graph and are depicted in red. Infants evaluated in the newborn period have open symbols and those evaluated at 1 month are shown as filled symbols. As expected, the subset of infants referred to as “bilateral pass” (shown in green) have larger SNRs and those with bilateral fail ratings (shown in blue) have the lowest TEOAE SNRs.

Figure 2.

TEOAE amplitude and noise from the “bilateral pass” group, after averaging left and right ears, plotted by infant’s age. Based on ANOVA, the change in amplitude with age was significant at p < 0.001, whereas the effect of age on noise was nonsignificant, p = 0.99.

Figure 3.

TEOAE measures shown separately by site. The NP5 site is omitted because of the small number of participants.

Figure 4.

Left-right ear comparisons for TEOAE background noise, amplitude, and SNR for bilateral pass participants. Scatter plots of data from left versus right ear are shown with superimposed mean values and error bars indicating ±1 standard deviation. Both noise and amplitude are statistically larger in the right compared with the left ear, but SNRs are balanced across left and right ears (symmetrical).

Figure 5.

TEOAE amplitude by frequency band demonstrates consistently larger values for the right ear. Only infants in the “bilateral pass” group are included.

Figure 6.

Example of a newborn ABR recording. An ABR recording from a typical newborn is shown at the top. Each ABR is the average of two buffers shown below, one elicited with a condensation and the other with a rarefaction click stimulus. Buffers are comprised of 1,024 sweeps each and added to create the final average. The early peaks of the buffers (<2 msec) are out of phase and constitute the cochlear microphonic (Starr et al, 2001). Stimuli used were 80 dB nHL clicks presented at 13/sec. Response peaks are labeled conventionally as Waves I, III, and V. Peaks are defined by their poststimulus latency in msec and amplitude from peak to the following trough in μV.

Figure 7.

Display of mean peak amplitudes by latency for ABR tests performed in the newborn period and at 1 month of age. As expected, latencies are shorter, and amplitudes are larger at 1 month compared with the newborn, but only the amplitude changes achieve statistical significance (see Tables 10 and 11). Error bars indicate ±1 standard deviation.

Figure 8.

Display of mean peak amplitude by ABR latencies for Wave I, II, and III for male and female infants. Although mean amplitudes are greater for females and latencies are slightly shorter, these differences did not reach statistical significance based on repeated measures ANOVA (see Tables 10 and 11). Error bars indicate ±1 standard deviation.

Figure 9.

Display of ABR mean peak amplitude by latency for infants by maternal race. No significant effect of maternal race was found for latency or amplitude (see Tables 10 and 11). Error bars indicate ±1 SD.

Figure 10.

Display of ABR mean peak amplitude by latency for the left and right ears. Error bars indicate ±1 standard deviation. The latencies in the right ears are significantly shorter for peaks I (p = 0.003), III (p = 0.014), and V (p = 0.019); however, amplitudes did not differ by ear (see text).

Figure 11.

ABR Waves I and V compared by infants’ TEOAE pass/fail status (see text for definition) to investigate whether TEOAE status is reflected in ABR latencies or amplitudes. Bivariate displays of ABR Wave V amplitude versus latency are shown for participants who “passed” the TEOAE (open symbols) and those whose response was recorded as a “fail” (closed). The data are further divided by right versus left ear. Significant differences (see Table 13) were found for the Wave V latency by TEOAE status in the right ear (p = 0.023) and for Wave I amplitude by status also in the right ear (p = 0.049).