Otoacoustic emissions in preterm infants: indications for cochlear development?

Hearing Loss Diagnosis and Early Hearing-Related Interventions in Infants With or at High Risk for Cerebral Palsy: A Systematic Review

AIM

To synthesize published evidence regarding hearing impairment diagnosis and interventions in infants with or at high risk for cerebral palsy in the first year after birth.

METHOD

Nine databases were searched for MeSH terms up to February 2020. Included studies were published in English, enrolled infants with or at high risk for cerebral palsy, and addressed hearing evaluation/rehabilitation within the first year after birth. Quality of evidence was evaluated using RTI Item Bank and QUADAS-2.

RESULTS

Eighteen articles met inclusion criteria. Quality of the evidence ranged from low to high, revealing variability in diagnostic assessment methodologies and adherence to diagnostic schedules. Concerns for bias included lack of recognition of cerebral palsy effects and etiologies on functional hearing assessment methods and results. Two interventions (hearing aid and cochlear implantation) were identified; however, reported use was inconsistent.

INTERPRETATION

Hearing screening in infants with or at high risk for cerebral palsy requires evaluation of the entire auditory pathway preferentially using comprehensive electrophysiological panels of assessments. For infants with perinatal neural insults, pediatric neurologists are uniquely positioned to recommend adherence to systematic surveillance and comprehensive audiology assessments, regardless of comorbidities and motor impairments.

Frequency shifts with age in click-evoked otoacoustic emissions of preterm infants

A previous study [Brienesse et al. (1997). Pediatr. Res. 42, 478-483] demonstrated a positive shift with increasing postmenstrual age (PMA) in the frequencies of synchronized spontaneous otoacoustic emissions (SSOAEs) in preterm infants. We used a mixed model approach to describe a shift with PMA in the spectra of click-evoked otoacoustic emissions (CEOAEs) measured in a group of 22 preterm infants. The rate in shift in CEOAE spectral components was found to be frequency dependent, with a mean estimate of 10 Hz/week for frequencies around 2 kHz and 30 Hz/week for frequencies around 4.25 kHz. This rate decreased with increasing PMA. Because SSOAEs are often part of the CEOAE response, a comparison was made between the shifts in SSOAEs and CEOAEs in a sub-group of 16 preterm infants. The results indicate that the shifts found for both types of OAE are similar, which supports a common mechanism for this change in OAE-characteristic. At present it is not clear to what extent developmental processes in the cochlea and the middle ear can account for these frequency shifts in the spectra of CEOAEs and SSOAEs during the preterm period.

Maturation of the auditory system: 1. Transient otoacoustic emissions as an index of inner ear maturation

The transiently evoked otoacoustic emission amplitude of 42 preterm babies (84 ears; post-conceptional age [PCA] 30-36 weeks) was compared with the TEOAE amplitude of 39 full-term babies (78 ears; PCA 37-45 weeks) in order to trace the inner ear maturation characteristics. An ILO-92 otoacoustic emission recording system was used with linear clicks of 70 dB peak equivalent SPL. The results obtained indicated: (1) There was no statistically significant difference between preterm and full-term ears; (2) There was no significant difference between males and females; (3) There was a significant difference between left and right ear TEOAE amplitude; (4) The interaction of ear with age in relation to TEOAE amplitude was statistically significant; (5) Noise and stimulus parameters did not reveal any significant differences between right and left ears; (6) A positive correlation existed between birthweight and TEOAE amplitude; and (7) A negative correlation existed between aminoglycoside treatment and TEOAE amplitude. The results indicate subtle changes in TEOAE amplitude over time, showing a natural development of the inner ear function during the process of maturation.

Otoacoustic emissions latency difference between full-term and preterm neonates

Transiently evoked otoacoustic emissions (TEOAEs) were recorded from full-term and preterm neonates. The responses were decomposed, by means of an adaptive approximation method, into waveforms of defined frequencies, amplitudes, latencies and time spans. Statistically significant differences in the latency values were found between the tested groups. Differences were also found in the time spans of the TEOAEs components. For the preterm neonates the contribution of long-duration components (i.e. long-time span) was higher. Those components were characterized by narrow frequency band and contrary to the short-time span components their latencies did not depend on frequency. The removal of the long-duration components, from the pool of analyzed data, decreased the latency differences between the tested groups. The results indicate that the origin of the longer latency values for preterm neonates (with a post conceptional age up to 33 weeks) in respect to full-term neonates can be attributed to the presence of long-lasting components. The correspondence, which was found between frequencies of long-duration components and the spectral peaks of spontaneous otoacoustic emissions (SOAEs), suggests that those components may be connected with SOAEs.

Temporal bone study of development of the organ of Corti: correlation between auditory function and anatomical structure

OBJECTIVE

To study the development of the organ of Corti in the human cochlea, and to correlate our findings with the onset of auditory function.

MATERIAL AND METHODS

Step sections of 81 human fetal temporal bones were studied, from eight weeks of gestation to full term.

RESULTS

By the end of the 10th week, the tectorial membrane primordium could be traced even in the most apical turns. Individual hair cells became identifiable at the basal turn at 14 weeks. At the same time, a small but well formed oval space was observed between the inner and outer hair cells in the basal turn. This does not correspond to the tunnel of Corti, as is erroneously quoted in the literature, as the individual pillar cells develop at later stages. Between 14 and 15 weeks, Hensen's cells were recognised for the first time. Individual pillar cells were identifiable at 17 weeks and the tunnel of Corti opened at 20 weeks. By 25 weeks, the cochlea had reached its adult size, but continued to develop until full term.

DISCUSSION AND CONCLUSIONS

A temporal coincidence of different developmental events is responsible for early fetal audition at 20 weeks, including growth of pillar cells, opening of the tunnel of Corti and regression of Kollicker's organ, with the subsequent formation of the inner spiral sulcus and then separation of the tectorial membrane. The fine structures of the organ of Corti continue to develop well after the 25th week, and this may well alter the mechanical properties of the vibrating parts of the cochlea, which may in turn account for the frequency shift observed in preterm infants. These changes will have to be taken into account in the development of prenatal hearing screening tests.

Cochlear maturation and otoacoustic emissions in preterm infants: a time–frequency approach

Click-evoked otoacoustic emissions (CEOAEs) from preterm infants were analyzed to characterize developmental changes of cochlear active mechanisms. Due to their strong time-varying properties, CEOAEs were studied with a time-frequency approach--the wavelet transform (WT). By means of the WT, CEOAEs were decomposed into 12 frequency bands, spanning the 0.25-6.25 kHz range. For each band, the root-mean-square (RMS) level and latency were studied as functions of both frequency and age. Because CEOAEs were averaged using the non-linear mode of acquisition, the developmental changes in observed in this study are related to the non-linear component (which is actually the most predominant component of the active cochlear response) of CEOAEs, the linear one being mostly canceled out by non-linear averaging. In our study, there was evidence that properties of CEOAE non-linear components are related to the post-conception age (PCA) in that the levels and latency of CEOAE frequency components changed until the age of about 38 weeks post-conception, whereas after 38 weeks, CEOAE features were very similar to those of term newborns. In particular, the CEOAE levels increased and latency decreased with age. The observed changes in CEOAE properties seem to reveal a development of cochlear active mechanisms, although contributions from outer and middle ear development cannot be excluded. Also, in agreement with previous physiological and behavioral findings, our results revealed that the development of CEOAE properties was not the same for all the frequencies, being greater for frequencies 4 kHz, and resembled the development of the cochlear partition, which proceeds from base to apex.

Emissões otoacústicas evocadas transientes (EOAET): amplitude da resposta em recém-nascidos a termo e pré-termo

As emissões otoacústicas evocadas transientes (EOAET) têm sido a técnica mais empregada nas triagens auditivas neonatais. Nos últimos anos, uma análise mais detalhada das EOAET tem evidenciado a presença de características até então pouco exploradas do sistema auditivo periférico. OBJETIVO: O objetivo deste trabalho foi o de estudar o comportamento da amplitude de resposta das EOAET em recém-nascidos a termo e pré-termo em função dos parâmetros lados da orelha, sexo, espectro de freqüência e idade pós-concepcional. FORMA DE ESTUDO: Clínico prospectivo. MATERIAL E MÉTODO: A amostra compôs-se de 526 recém-nascidos, sendo 440 nascidos a termo e 86 nascidos pré-termo. Estes últimos foram subdivididos em um grupo com idade pós-concepcional na data do teste variando entre 31 e 36 semanas e outro grupo com idade pós-concepcional na data do teste variando entre 37 e 44 semanas. O teste das EOAET foi realizado em ambiente sem tratamento acústico com o analisador de emissões otoacústicas ILO 88 da Otodynamics, no modo "Quickscreener". RESULTADO: Ao analisarmos os resultados, observamos assimetria significante da amplitude média de resposta a favor da orelha direita, do sexo feminino, das bandas de freqüências altas e dos recém-nascidos com maior idade pós-concepcional. Estas assimetrias sugerem que o parâmetro amplitude, além de fornecer evidência da presença das EOAET e portanto da integridade do sistema auditivo periférico, também pode ser tomado como um indicador de maturação do sistema auditivo periférico em recém-nascidos.

On the spectral periodicity of transient-evoked otoacoustic emissions from normal and damaged cochleas

The spectral quasi-periodicity of transient-evoked otoacoustic emissions (TEOAE) is well acknowledged since Zwicker described a preferred spacing of 0.4 bark between consecutive peaks in the spectrum of otoacoustic emissions from normal ears. While there is scarce evidence of any anatomical reason for this regularity, several functional models of the cochlea have predicted that the structure of emission spectra reflects important characteristics of cochlear filters. In an attempt to check such predictions, the average regularity of TEOAE spectra was studied in three groups of human subjects, normally hearing adults, healthy neonates, and adults suffering from noise-induced hearing loss. Significant differences in emission periodicities were found. Around 1 kHz, the preferred spacing was close to 130 Hz in normally hearing adult ears and neonates. In contrast, no clear periodicity was found in the group of damaged ears, even though they had clinically normal pure-tone audiometry below 2 kHz. Around 4 kHz, the preferred spacing was close to 240 Hz in normal adults and neonates, whereas TEOAEs were absent in many impaired ears. A phenomenological model assuming that TEOAEs stem from the responses of a slightly disarrayed bank of highly tuned filters predicts that the filter width would be the same in healthy young adults and neonates. In contrast, ears suffering from high-frequency hearing loss could exhibit early damaged filters. The proposed method might provide an objective assessment of parameters otherwise difficult to evaluate, especially in neonatal cochleas.

Development of human cochlear active mechanism asymmetry: involvement of the medial olivocochlear system?

To study the functional development of the medial olivocochlear system, transient-evoked otoacoustic emission suppression experiments were conducted in 73 ears of 38 pre-term and 11 full-term neonates. The continuous contralateral stimulation was a broad band white noise, presented at 70 dB SPL. Efferent suppression was determined by subtracting the without-contralateral stimulation condition from the with-contralateral stimulation condition. Across this population, a mean suppression effect of contralateral stimulation on transient-evoked otoacoustic emissions was found, with most of the suppression effect observed after 8 ms. The amount of suppression is linearly, positively correlated with the conceptional age. In the subgroup of bilaterally tested neonates, the suppression of transient-evoked otoacoustic emissions is similar in the right ear and the left ear in subjects whose conceptional age is less than 36 weeks and significantly higher in the right ear than in the left ear in older neonates. This last observation was seen at frequencies where transient-evoked otoacoustic emission amplitudes became higher in the right ear than in the left ear as the conceptional age increased, a finding already reported in adults. This study shows that the functional adult pattern of the medial efferent system, probably involved in the detection of signals in noise such as speech sounds, seems to appear gradually in neonates and represents one of the several arguments in favor of functional auditory lateralization in humans, with a right ear advantage.

Long-term and short-term variations in amplitude and frequency of spontaneous otoacoustic emissions in pre-term infants

In pre-term infants, spontaneous otoacoustic emission (SOAE) frequencies show an upward shift with time. The present study aimed to monitor the SOAE amplitude variation during this frequency shift. A long-term observation of 87 SOAE frequencies from 18 pre-term infants yielded a positive frequency shift of 0.72 per cent per week, which was not accompanied by a simultaneous amplitude shift, as the mean variations in SOAE amplitude were practically zero. Furthermore, there was no relationship between the short-term SOAE amplitude variation and the infant's post-conceptional age. Only the absolute amount of SOAE amplitude variation seemed to grow with time. Comparison with induced variations in SOAE frequency argues against a middle ear influence on the SOAE frequency shift. In our view, the absence of any amplitude shift during the upward SOAE frequency shift further suggests cochlear development during the last period of gestation.

Frequency shift of individual spontaneous otoacoustic emissions in preterm infants

In adults, spontaneous otoacoustic emissions (SOAE) have shown a considerable frequency stability. In preterm infants, however, the SOAE proved to show an apparent and consistent upward shift of frequency at increasing postconceptional age (PCA). In 25 ears of 14 preterm infants (PCA, 29.1-41.3 wk) a total of 66 SOAE frequencies were monitored, ranging from 1611 to 5774 Hz. All but one of the SOAE frequencies shifted toward higher frequency. The SOAE frequency shift rate in Hertz per week was proportionally constant relative to the SOAE frequency. The mean shift rate was 0.74 +/- 0.39%/wk. At increasing PCA, the SOAE frequency shift rate tended to slow down. A linear fit through the data predicted the SOAE frequency to stop at about 45-50-wk PCA. The frequency dependence and time course of the SOAE frequency shift strongly suggest cochlear maturation during the last period of gestation.