Miriam Geal-Dor

Development of hearing in neonatal rats: Air and bone conducted ABR thresholds

While the human full-term neonate can hear at birth, in the rat the onset of auditory function as monitored by recording auditory nerve-brainstem evoked responses (ABR) has been reported to begin on post-natal day (PND) 12-14 and reaches adult thresholds at about 22 days. In order to determine the factors involved in this late onset and then rapid threshold improvement in rats, the ABR to both air conducted (AC) and bone-conducted (BC) auditory stimulation was determined in neonatal rats. ABR to maximal intensity BC stimuli (55 dB above adult rat ABR threshold--55 dB HL*) could be recorded from PND 7-8 while AC responses to 80 dB HL* stimuli, only from PND 11. The air-bone gap (a measure of conductive immaturities only) disappeared on PND 15. This shows that there are both conductive (external and middle ear--Air-bone gap) and sensori-neural (inner ear--BC threshold) immaturities in the neonatal rat; the conductive factors are resolved by PND 15 while the sensori-neural continue after that. With respect to conductive factors, it seems that the state of the ear canal is not important while the chief conductive factors involved probably include mesenchyme resorption and/or ossicular ossification. The chief sensori-neural factor may be the development of the endocochlear potential. It is likely that the human fetus in-utero undergoes similar stages of development.

Thyroid hormone induces earlier onset of auditory function in neonatal rats

The effect of thyroid hormone injection on the development of auditory function in neonatal rats was evaluated using auditory nerve-brainstem evoked responses (ABR). The hormone induced earlier onset of auditory function. In order to differentiate between conductive and sensorineural factors, both air-conducted (AC) and bone-conducted (BC) ABR responses were recorded. Neonatal rats were injected with thyroxine (T4), or with saline (control animals), from day of birth (post-natal day-PND-0), daily, until PND 9. AC- and BC-ABRs were recorded from PND 6 up to PND 20. It was found that both AC- and BC-ABR thresholds were lower in the T4-injected rats up to PND 15, after which no difference was found between the two groups. This indicated earlier maturity of both conductive (external and middle ears) and sensorineural (inner ear) factors and is probably due to the earlier appearance in the blood of higher T4 levels, following injection, than that occurring naturally during the neonatal period in these animals.

Human fetal auditory threshold improvement during maternal oxygen respiration

It has been suggested that the near full-term fetus in-utero has a sensori-neural hearing loss compared to the neonate due to the relative hypoxia resulting from placental oxygenation compared to pulmonary oxygenation. This hypothesis was tested by estimating the threshold of the fetus to vibrio-acoustic stimulation applied to the maternal abdomen while the mother was breathing room air and again when breathing oxygen. Fetal response was assessed by maternal perception of fetal movement and by objective demonstration of movement by ultrasound. It has been shown that the fetal responses are to the acoustic component of the stimulus, that the acoustic stimulus is not overly attenuated or masked, and that maternal oxygen inhalation enhances fetal oxygenation. The results showed that the threshold was lower and/or the response was stronger when the mother was breathing oxygen compared to when she was breathing room air. Thus it is confirmed that in-utero the fetus has an hypoxia-induced sensori-neural hearing loss. At birth, with the shift to more efficient pulmonary oxygenation, there is an improvement in auditory threshold.

Comparison of two hearing screening programs in the same population: Oto-acoustic emissions (OAE) screening in newborns and behavioral screening when infants

OBJECTIVE Hearing screening programs in infancy should identify hearing impairment as early as possible. The two common programs utilize either objective neonatal tests (oto-acoustic emissions (OAE) or automatic auditory brainstem responses (aABR)) or behavioral screening at 7-9 months of age. Most countries employ only one of these options. The uniqueness of this study is the comparison of both hearing screening programs on the same group of children. METHODS The study was conducted on 1545 children born between the years 1999 and 2003 who were followed up in public well baby clinics in the Jerusalem district. The children were tested with transient oto-acoustic emissions (TEOAE) before discharge from the neonatal ward, and later, at the age of 7-9 months, underwent a behavioral hearing screening test in a public well baby clinic. The results of both hearing screening programs were compared. RESULTS The compliance rates were 99.7% for the neonatal testing and 83% for the 7-9 months behavioral testing (p=0.0001). The failure rate was 4-6% in both screening programs; failure of OAE testing was unilateral in 65% of newborns; at 7-9 months bilateral failure was more common (56%). There was an 11.2% disagreement (kappa coefficient 0.03) between the outcomes of both tests. In another group of 49 known hearing-impaired children, 27 who had undergone newborn screening were diagnosed before the age of behavioral testing. Twelve children had failed either both tests or the only test they underwent. In nine cases, the children had passed one of the hearing screening tests and had failed the other, and one child had passed both tests. CONCLUSIONS Newborn hearing screening has the advantages of objectivity, early identification, and higher compliance. The major advantage of the later behavioral test is identification of later onset or progressive hearing impairment as well as auditory neuropathy spectrum disorder. Each screening test is testing different entities; hence they are complementary and not interchangeable or superfluous. We recommend a comprehensive two-step hearing screening plan (newborn and later behavioral) with close cooperation between the health care providers involved.

Development of inner ear (cochlear and vestibular) function in the fetus-neonate.

The development of function in the various receptors in the inner ear was studied in the neonatal rat, which is altricious with respect to hearing, using short latency evoked potentials, both auditory (ABR) and vestibular (VsEP). It was found that VsEPs could be recorded from all the vestibular end-organs by post natal day (PND) 8, whilst ABR could only be recorded from all animals on PND 14, showing the earlier onset of vestibular function in the inner ear. These results are discussed with relation to onset of inner ear function in the human fetus.

Cooling induces a decrease in middle ear compliance.

The effects of cooling rats from 37 degrees C to 27 degrees C and rewarming to 37 degrees C on the conductive mechanism of the middle ear was studied by means of acoustic impedance measurements. Cooling reduced middle ear compliance reversibly, without an effect on external canal volume and middle ear pressure. These results provide evidence for an increase in the stiffness of the tympanic membrane and/or of the ossicular chain and/or a decrease in stapes mobility. Thus a small part of the decrease in the magnitude of otoacoustic emissions during cooling is due to an effect on the conductive mechanism of the middle ear.

Bone Conduction Thresholds without Bone Vibrator Application Force

BACKGROUND Osseous bone conduction (BC) stimulation involves applying the clinical bone vibrator with an application force of about 5 Newton (N) to the skin over the cranial vault of skull bone (e.g., mastoid, forehead). In nonosseous BC (also called soft tissue conduction), the bone vibrator elicits hearing when it is applied to skin sites not over the cranial vault of skull bone, such as the neck. PURPOSE To gain insight into the mechanisms of osseous and nonosseous BC. RESEARCH DESIGN In general, thresholds were determined with the bone vibrator applied with about 5 N force directly to osseous sites (mastoid, forehead) on the head of the participants, as classically conducted in the clinic, and again without direct physical contact (i.e., 0 N force) achieved by coupling the bone vibrator to gel as in ultrasound diagnostic imaging, on the same or nearby skin sites (nonosseous BC). The participants were equipped with earplugs to minimize air-conducted stimulation. STUDY SAMPLE In the first experiment, 10 normal-hearing participants were tested with stimulation (5 and 0 N) at the forehead; in the second experiment, 10 additional normal-hearing participants were tested with stimulation at the mastoid (about 5 N) and at the nearby tragus and cavum concha of the external ear (0 N). RESULTS The mean thresholds with 0 N were much better than might be expected from classical theories in response to stimulation by a bone vibrator, in the absence of any application force. The differences between the mean thresholds with the 0 N and the 5 N forces depended on condition, site, and stimulus frequency of the comparisons. The difference was 1.5 dB at 1.0 kHz on the forehead; ranged between 10 and 12.5 dB at 1.0 kHz on the cavum and tragus (versus on the mastoid) and at 2.0 and 4.0 kHz on the forehead; 17 and 19 dB at 2.0 kHz on the cavum and tragus (versus on the mastoid); reaching 32 dB only in a single condition (forehead at 0.5 kHz). CONCLUSIONS As it is unlikely that threshold intensity stimulation delivered with 0 N application force could have induced vibrations of the underlying or nearby bone, inducing osseous BC, the relatively low thresholds in the absence of any application force, together with the small differences between the thresholds with 0 N (gel/soft tissue, nonosseous) and 5 N force (osseous BC) lead to the suggestion that in most situations, the BC thresholds actually represent the nonosseous (soft tissue conduction) thresholds at the stimulation site.

Auditory Behavior in Everyday Life (ABEL) questionnaire in Hebrew and in Arabic and its association with clinical tests in cochlear-implanted children.

Abstract Background: The aim of this study was to describe the results of the Auditory Behavior in Everyday Life (ABEL) questionnaire adapted to Hebrew and to Arabic and its association to clinical test results in children with cochlear implants. As assessment of hearing by audiometry does not always adequately reflect performance in daily life, questionnaires have been developed to assess functioning in natural surroundings and to track progress. In order to evaluate cochlear-implanted children’s verbal and communicative abilities, the parental ABEL questionnaire was developed in 2002. The advantages of the ABEL questionnaire are that it is intended for a wide age range, is quick to administer, and is filled out by parents themselves. Methods: The ABEL questionnaire was translated into Hebrew and into Arabic and routinely used in the clinic. A total of 61 questionnaires were thus filled out by parents of children with cochlear implants (ages 3.9–14.3 years) when they came for routine mapping. Retrospectively, data were analyzed and questionnaire results were compared with performance with the implant on several clinical tests: audiometric thresholds, discrimination (percentage) of vowel-consonant-vowel nonsense syllables, and results of speech perception tests with monosyllabic and bisyllabic words and with sentences in quiet and in noise. Results: A correlation was found between the different sections of the questionnaire, and age at implantation had a significant effect on questionnaire scores. However, correlations between questionnaire score and clinical tests were found only for speech perception tests in noise and not in quiet or to audiogram and speech reception threshold. Conclusions: As has been reported previously, self-evaluation or parental evaluation does not always correlate with all measured results of hearing performance. However, the subjective information collected through questionnaires can be valuable for evaluation of progress, for counseling and rehabilitation training, as well as for mapping.

Acquisition of early auditory milestones with a cochlear implant

OBJECTIVE Speech acquisition after cochlear implant is a long process. Various studies have followed the auditory milestones in the early period after implantation. The aim of the present study was to track the development of hearing skills in the early period after cochlear implantation and evaluate which factors influence the process. METHODS 195 records of children implanted in the Hadassah Medical Center were examined retrospectively. Data on etiology, age at implantation and type of implant were collected. In addition, information on the rate of progress was measured: the first time that there was detection and identification of Ling sounds, the first time it was possible to obtain SDT (speech detection threshold), SRT (speech reception threshold) and an audiogram, and the first accurate repetition of VCV (vowel consonant vowel) sounds. RESULTS Results show a consistent pattern of auditory milestone acquisition similar to that of normal development, from milestones that do not require decoding beginning with SDT, detection of Ling sounds followed by an audiogram which requires cooperation, to tasks that involve decoding starting with SRT and repetition of Ling sounds and finally VCV repetition. The children implanted before 24 months of age achieved the auditory milestones later than children implanted between 2 and 6 years, apparently since these tasks involve cognitive abilities which are not yet developed in the youngest children. Previous hearing experience improved the rate of acquisition of the auditory milestones and progress was faster in the second implanted ear compared to the first implanted ear. CONCLUSION More research is needed to address the relationship between acquisition of early auditory milestones and performance with the cochlear implant later on in life.

Inner Ear Excitation in Normal and Postmastoidectomy Participants by Fluid Stimulation in the Absence of Air- and Bone-Conduction Mechanisms

Background: Hearing can be induced not only by airborne sounds (air conduction [AC]) and by the induction of skull vibrations by a bone vibrator (osseous bone conduction [BC]), but also by inducing vibrations of the soft tissues of the head, neck, and thorax. This hearing mode is called soft tissue conduction (STC) or nonosseous BC. Purpose: This study was designed to gain insight into the mechanism of STC auditory stimulation. Research Design: Fluid was applied to the external auditory canal in normal participants and to the mastoidectomy common cavity in post‐radical mastoidectomy patients. A rod coupled to a clinical bone vibrator, immersed in the fluid, delivered auditory frequency vibratory stimuli to the fluid. The stimulating rod was in contact with the fluid only. Thresholds were assessed in response to the fluid stimulation. Study Sample: Eight ears in eight normal participants and eight ears in seven post‐radical mastoidectomy patients were studied. Data Collection and Analysis: Thresholds to AC, BC, and fluid stimulation were assessed. The postmastoidectomy patients were older than the normal participants, with underlying sensorineural hearing loss (SNHL). Therefore, the thresholds to the fluid stimulation in each participant were corrected by subtracting his BC threshold, which expresses any underlying SNHL. Results: Hearing thresholds were obtained in each participant, in both groups in response to the fluid stimulation at 1.0 and 2.0 kHz. The fluid thresholds, corrected by subtracting the BC thresholds, did not differ between the groups at 1.0 kHz. However, at 2.0 kHz the corrected fluid thresholds in the mastoidectomy patients were 10 dB lower (better) than in the normal participants. Conclusions: Since the corrected fluid thresholds at 1.0 kHz did not differ between the groups, the response to fluid stimulation in the normal participants at least at 1.0 kHz was probably not due to vibrations of the tympanic membrane and of the ossicular chain induced by the fluid stimulation, since these structures were absent in the mastoidectomy patients. In addition, the fluid in the external canal (normal participants) and the absence of the tympanic membrane and the ossicular chain (mastoidectomy patients) induced a conductive hearing loss (threshold elevation to air‐conducted sounds coming from the bone vibrator), so that AC mechanisms were probably not involved in the thresholds to the fluid stimulation. In addition, as a result of the acoustic impedance mismatch between the fluid and skull bone, the audio‐frequency vibrations induced in the fluid at threshold would probably not lead to vibrations of the bony wall of the meatus, so that hearing by osseous BC is not likely. Therefore, it seems that the thresholds to the fluid stimulation, in the absence of AC and of osseous BC, represent an example of STC, which is an additional mode of auditory stimulation in which the cochlea is activated by fluid pressures transmitted along a series of soft tissues, reaching and exciting the inner ear directly. STC can explain the mechanism of several auditory phenomena.