The Auditory Characteristics Of Children With Inner Auditory Canal Stenosis And The Outcomes Of Cochlear Implants For Children With Cochlear Nerve Deficiency

Auditory neuropathy (AN), as a specific form of hearing loss, is recognized by more and more clinicians. Patients with AN usually complain the fluctuating hearing sensitivity, and unexpected speech perception performance that is disproportion to the hearing loss if the hearing impairment is origin to cochlea. AN is diagnosed electrophysiologically by the presence of otoacoustic emissions (OAEs) and/or cochlear microphonics (CM) in conjunction with absent or severely distorted auditory brainstem responses (ABRs). This suggests that those patients with AN usually have normal outer hair cell function but disordered neural conduction. There are still some debates about the etiology and the lesion sites of this disorder. It is generally acknowledged that the lesion of the inner hair cell, the synapse between the inner hair cell and the auditory nerve and the auditory nerve itself contribute to the AN.Internal auditory canal (IAC) stenosis is a relatively rare disorder that can lead to sensorineural hearing loss (SNHL). Although the exact incidence of SNHL is unknown, it is estimated that approximately 20% of congenital SNHL have abnormalities in the inner ears, among which 12% have IAC stenosis. That is to say, less than 2% of congenital SNHL have IAC stenosis. The normal diameter of IAC ranges from 2 mm to 8 mm, with an average of 4 mm. However, there are some controversies about the definition of the IAC stenosis. Some scientists define the IAC stenosis as less than 2 mm, while the others define it as less than 3 mm. Congenital IAC stenosis is usually associated with some other inner ear abnormalities, such as cochlear nerve deficiency, cochlear malformation, semicircular malformation and soA stenostic IAC was considered as a contraindication of cochlear implantation before, because the majority of patients with IAC stenosis are associated with cochlear nerve difficiency (CND), including aplasia and hypoplasia of cochlear nerve. However, it has now become no longer contraindicative, particularly with the development of the radiological technology. More and more cochlear implants (CI) with abnormalities (including IAC stenosis) in the inner ear are reported. But the participants in those studies were not that many because of the relatively low prevalence of IAC stenosis. In addition, the auditory measurement is limited because most of those children with IAC stenosis have speech retardation.Cochlear nerve deficiency (CND) refers to those cases in which the cochlear nerve is either small or absent on MRI. The nerve is considered absent when it could not be identified on axial, coronal, or reconstructed coronal oblique IAC images. When the nerve is evident but substantially smaller than the other nerves in the IAC or smaller than the cochlear nerve in the contralateral ear, it is considered small.So far, some studies had reported on outcomes of CI in the ears with CND. It showed that the quantity of the participants in all the studies was not that large because of the relative rarity of CND. Only a few studies reported the speech perception because of the retardation of speech development for the children with CND. There were some differences among these studies.In the current study, we describe the auditory characteristics of children with inner auditory canal stenosis to assess the relationship between the IAC stenosis and AN. We retrospectively review the audiotory records of children with bilateral cochlear nerve deficiency before and after cochlear implantation, to evaluate whether they can benefit from cochlear implantation.PART IThe Auditory Characteristics of Children with Inner Auditory Canal StenosisObjectiveTo describe the auditory characteristics of children with inner auditory canal stenosis as well as to assess whether the narrow inner auditory canal is associated with auditory neuropathy (AN), with special emphasis on the relationship between inner auditory canal (IAC) stenosis and AN.DesignA total of 21 children, with inner auditory canal stenosis (5 unilateral,16 bilateral), including 9 females and 12 males aged between 11 months and 6.0 years, participated in this study. The medical records of all the participants were retrospectively reviewed, in terms of middle ear impedance, auditory brainstem response (ABR), cochlear microphonic (CM), distortion-product otoacoustic emission (DPOAE) and behavioral audiometry (BA). Meanwhile, a comparative study was conducted on the auditory characteristics of AN based on whether the children were associated with isolated IAC stenosis. Ten children (16 ears) were included in the isolated IAC stenostic group,10 children (20 ears) who had AN without apparent congenital malformation in middle and inner ears with AN served as age-matched controls.ResultsIn the ABR tests, Wave V was not observed in all the patients. CM response was detected in 30 of 37 (81.1%) ears with stenostic IAC. Sixteen of 19 (84.2%) ears with isolated IAC stenoses had CM response present on ABR waveforms. According to the criteria of AN (presence of OAEs and/or CM; absent or altered ABRs), the 30 of 37 (81.1%) ears with stenotic IAC presented AN characteristics. There was no significant difference in the presentation of AN characteristics between the children with and without isolated IAC stenoses. There was no significant difference between the subjects with isolated IAC stenosis and the controls in some audiological examinations, such as middle ear impedance and ABR. However, there were significant differences between the two groups in the degree of hearing loss and DPOAEs. As the behavioral threshold shown, all the ears with isolated IAC stenosis had profound hearing loss, whereas, only 20% ears in the controls groups had profound hearing loss(P=0.0000). None of the 16 ears had positive DPOAEs in most frequencies (from 500Hz to 8000Hz), whereas in the controls,60% ears tested had a normal DPOAE (P=0.0057).ConclusionWe show that the prevalence of AN in the children with IAC stenoses is much higher than those without IAC stenoses, regardless whether they have other inner ear anomalies. In addition, the auditory characteristics of AN with IAC stenosis are significantly different from AN without any middle and inner ear malformations.PART IIThe Outcomes of Cochlear Implants for Children with Cochlear Nerve DifficiencyObjectiveWe try to retrospectively review the audiotory records of children with bilateral cochlear nerve deficiency before and after cochlear implantation. We aim to evaluate the outcomes of the implantation for the children with CND.DesignEleven children with bilateral CND were included in the study. All of them received cochlear inplants (CIs) in one ear according to radiological findings and auditory results. We retrospectively reviewed the auditory records pre-implantation. And we evaluated all the children by categories of auditory performance (CAP), speech Intelligibility Rating (SIR), electrically-evoked compound active potential (ECAP) and behavioral testing aimed with CI after they used CI for a year. The auditory records pre-implantation and post-implantation were compared.ResultsEight of 11 children with bilateral CND accompanied with AN in the ear with CI. The mean threshold of behavioral testing aided with implantation was significantly decreased at all frequencies, compared with the mean threshold pre-implantation.7 children with cochlear nerve aplasia also had significant improvement in the behavioral aided testing. The majority of their CAP scores were higher than that of pre-implantation. However, only 3 of the SIR scores were higher than that of pre-implantation, most of children had the same SIR score regardless they had used implant for a year.ConclusionChildren with CND have improved awareness of sound; They show worse awareness of sound and speech perception, compared with children with normal cochlear nerve; They can benefit from CIs, regardless there is only limited improve in speech development after 1 year of using implantation; Children who are identified cochlear nerve aplasia on MRI, if the auditory testing identify the completed auditory pathway, may benefit from implantation.