Reformatted parasagittal oblique MRI images. (a) A normal cochlear nerve of larger diameter than the facial nerve. F facial nerve, C cochlear nerve, SV super. (b) A hypoplastic cochlear nerve of smaller diameter than the facial nerve (red triangle). (c) Facial and vestibulocochlear (red triangle) nerves are identified, but cochlear nerve is not separated. (d) Absence of vestibulocochlear nerve. Only facial nerve is recognized
8.2 Measurement and Reading of EABR
8.2.1 Measurement of Intracochlear EABR
EABRs were recorded on electrodes within the cochlea. The responses were recorded with the Neuropack (Nihon Kohden Co., Tokyo, Japan) electrodiagnostic system and were triggered externally by the stimulus output of each CI company’s software and the interface unit. The interface unit was also connected to a stock speech processor and the subject’s headpiece; the stimulus signal was transmitted across the skin to the implanted device. The electrically evoked brainstem potentials were recorded by using needle electrodes placed on the forehead (different electrode), the nape of the neck (indifferent electrode), and the contralateral earlobe (reference electrode). The recording of electrical activity included two or three replications of 1000 sweeps at each stimulus level with a time window of 10 ms for each stimulus condition. Frequency cutoffs of 100 and 1000 Hz were used. The pulse duration was set to 30 ms and the stimulation amplitude for a single recording fell from high to low current. If no response was detected, pulse duration was increased.
8.2.2 EABR Waves of Patients Without Inner Ear Malformation
220.127.116.11 Case No. 1
In this case, hearing loss was found by newborn hearing screening. Congenital cytomegalovirus infection was confirmed by polymerase chain reaction for cytomegalovirus DNA in his umbilical cord. CT and MRI studies showed normal inner ear structure. He was fitted with hearing aids bilaterally, but his hearing loss progressed to profound sensorineural hearing loss. At the age of 3 years, he underwent implantation with a CONCERTO Flex28 (MED-EL, Innsbruck, Austria). All electrodes were inserted, and further assessment via telemetry showed good ECAP and EABR responses via the cochlear implant (Fig. 8.2a, b). After cochlear implantation, his hearing recovered well, and he achieved an IT-MAIS score of 34 at 6 months after implantation.
(a) ECAP waves of case no. 1. (b) EABR waves of case no. 1
The basal electrodes have higher thresholds and longer wave eV latencies than the apical and middle electrodes. It may be that the higher thresholds and longer wave eV latencies of the most basal electrodes are the result of the greater distance from the neural elements compared to the more apical electrodes, which are located further along the scala tympani. According to our series, the mean wave eIII latencies of the ears without a malformation for apical and basal electrodes were 2.29 ± 0.22 and 2.40 ± 0.24 ms, and the mean wave eV latencies of the ears without a malformation for apical and basal electrodes were 4.26 ± 0.40 and 4.55 ± 0.32 ms . All children without malformations had EABR wave eV latencies of less than 5 ms. The patients were divided into three groups according to their EABR responses. The typical response group included all patients showing reproducible wave eV responses with EABR eV latencies of less than 5 ms. The atypical response group was defined as those patients who presented with reproducible wave eV responses that were measured in only a limited number of electrodes and/or that showed EABR eV latencies of more than 5 ms pulse duration. In the no response group, no identifiable wave eV responses could be seen in any of the electrodes, even with longer pulse duration.
8.3 EABR Waves of Patients with Modiolus Present Type of Inner Ear Malformation
8.3.1 Modiolus Present and Cochlear Nerve Present Type
18.104.22.168 Case No. 2
This is a case of congenital progressive hearing loss with bilateral enlarged vestibular aqueduct (EVA) (Fig. 8.3a, b). SLC26A4 mutations were confirmed. At the age of 3 years, she underwent implantation with a CONCERTO Flex soft (MED-EL) in her right ear. All electrodes were inserted, and further assessment via telemetry showed good ECAP and EABR responses via the cochlear implant (Fig. 8.3c). After cochlear implantation, her hearing recovered well.
(a) Axial computed tomography imaging study showing enlarged vestibular aqueduct malformation. (b) Axial MRI study showing enlarged vestibular aqueduct malformation and normal cochlear nerves. (c) EABR waves of case no. 2
The presence of enlarged vestibular aqueduct (EVA) in the presence of normal cochlea, vestibule, and SCCs is a typical case of modiolus present and cochlear nerve present type of inner ear malformation. This type of inner ear malformation shows as good EABR and CI performance as those without malformation. In the cochlear malformation cases in which the modiolus was present, the basal electrodes have higher thresholds and longer wave eV latencies than the apical and middle electrodes. These are similar threshold and latency patterns to those observed in the patients without malformations.
8.3.2 Modiolus Present and Cochlear Nerve Deficiency Type
22.214.171.124 Case No. 3
This child was 10 years old and had progressive hearing loss (Fig. 8.4a). She has a very thin cochlear nerve canal in CT (Fig. 8.4b), and her cochlear nerves could not be seen on MRI (Fig. 8.4c), but she had obvious auditory response on both ears. She had cochlear implantation (MED-EL CONCERTO Flex soft) on the left ear. Her ECAP showed a threshold at 600 CU (current unit) with a 30-ms pulse duration (Fig. 8.4d), which is the usual pulse width. Meanwhile, her EABR threshold was 800 CU with a 55-ms pulse duration (Fig. 8.4e), which means that we need to nearly double the intensity to obtain the EABR threshold as compared with ECAP. Now, her category of auditory performance (CAP) score is 6, and she is very satisfied with CI.
(a) Pure tone audiometric result for case no. 3 before cochlear implantation. (b) Parasagittal oblique MRI study showing the absence of cochlear nerve. (c) Axial computed tomography imaging study showing cochlear nerve canal stenosis. (d) ECAP waves of case no. 3. (e). EABR waves of case no. 3
Even the patient has cochlear nerve deficiency, if she has obvious auditory response with hearing aids, she can be a good indication for cochlear implantation. Because obvious auditory response implies that the cochlear nerve is functionable. Cochlear nerve canal stenosis cases have normal spiral ganglion cells, so ECAP shows good responses with the usual intensity. However, because a high intensity is needed to go through the thin auditory nerve, the EABR threshold is high. It may be better to use modiolar-hugging electrodes, because peri-modiolar electrode placement reduces the spread of excitation of CI stimulation. These reduced nerve stimulation thresholds may result in improved speech discrimination by implant users with modiolus presence and cochlear nerve deficiency.
8.4 EABR Waves of Patients with Modiolus Absent Type of Inner Ear Malformation
8.4.1 Modiolus Absent and Vestibulocochlear Nerve Present Type
126.96.36.199 Case No. 4
This child has congenital profound hearing loss with bilateral common cavity (CC) malformation (Fig. 8.5a). He underwent cochlear implantation with PULSAR Standard (MED-EL) at 2 years old in his right ear and CONCERTO Standard (MED-EL) at 4 years 8 months old in his left ear. In the right ear, no ECAP response and variable EABR responses were obtained; in the left ear, variable ECAP and EABR responses were obtained (Fig. 8.5b, c). His IT-MAIS score was 35 at 1 year after first implantation.
(a) Axial computed tomography imaging study showing common cavity malformation. (b) Parasagittal oblique MRI study showing vestibulocochlear and facial nerves. (c) ECAP waves of case no. 4. (d) EABR waves of case no. 4
The type of cochlear malformation characterized by modiolus absence and vestibulocochlear nerve presence is CC or incomplete partition type I (IP-I) with open fundus of the internal auditory canal (IAC). ECAP recordings depend largely on spinal ganglion cells, which are very often defective in modiolus deficiency-type malformed cochlea. EABR can be obtained in modiolus deficiency-type implant users because the measures are not dependent on the implant having telemetry capabilities and because the wave eV of EABR, which occurs at a later latency than ECAP, is easier to isolate from the stimulus artifacts. The cochlear malformation cases with modiolus deficiency did not exhibit threshold and latency differences between electrodes. The auditory nerve tissues in modiolus deficiency malformations are supposed to be in the inner ear wall, and so the distances from each electrode to the auditory nerve tissue should not be different in modiolus deficiency-type malformations.