Keywords
cochlear nerve deficiency - electrical stapedial reflex - post operative - children
- cochlear implant
Introduction
Cochlear nerve deficiency (CND) is described as a small or absent cochlear branch
of the vestibulocochlear nerve with the clinical manifestation of sensorineural hearing
loss (SNHL) of unknown etiology and pathophysiology. Cochlear nerve deficiency is
present in 2.5 to 21.2% of patients with congenital SNHL.[1]
[2] Inner ear malformations and CND, detectable with computed tomography (CT) and magnetic
resonance imaging (MRI), contribute to 15 to 39% of pediatric SNHL cases.[3]
[4] In recent years, high-resolution MRI has been used to observe the neuromorphology
and development of the internal auditory meatus (IAM) and cochlear nerve (CN). Researchers
have proposed a new IAM nerve grading system and a CN classification system based
on MRI findings, which are as follows: grades 0 to III indicated zero, one, two, and
three nerve bundles observed in the IAM (aplasia); grade IV, four nerve bundles in
the IAM with a hypoplastic CN (hypoplasia); and grade V, four nerve bundles in the
IAM with a normal-sized CN and normal position of the nerves.[5] Cochlear implants (CIs) in patients with CND remain controversial, as some studies
have reported very poor results,[6] while others have reported limited speech detection and discrimination.
Objective measures are crucial in defining the levels used for CI mapping. The association
between CI outcomes and electrically evoked compound action potential (eCAP) measures
has been evaluated in previous studies.[7]
[8]
[9] He et al.[10] reported eCAP being recorded at all test electrodes in children with normal-sized
CN. In contrast, the eCAP could not be recorded at any electrode site in 4 out of
23 children with CND. For all other children with CND, the percentage of electrodes
with measurable eCAPs decreased as the stimulating site moved in a basal-to-apical
direction. In a similar study, researchers examined the eCAP thresholds and their
correlation with the behavioral thresholds in the map for children with CND.[11] The findings indicated a significant correlation at the basal, middle, and apical
electrodes, with the eCAP thresholds being either equivalent to or greater than the
behavioral T-levels. Additionally, these thresholds were within the dynamic range
of the map for ∼ 90% of the electrodes studied.[12]
Another objective approach that could be useful in mapping is electrically evoked
stapedial reflex threshold (eSRT). According to Raine et al. (1997), eSRT can be measured
postoperatively using an immittance meter in the implanted or nonimplanted ear to
gauge the response to electrical stimulation through the implant.[12] Intraoperatively, eSRT could be recorded by visually observing the stapedial contraction
during surgery. The brainstem mediates the neuromuscular reaction known as the stapedial
reflex. Jerger et al. (1988) found a link between eSRT and comfort levels (C or M
levels) of a map and concluded that eSRT can be used to predict C levels.[2]
[13] Bresnihan et al. (2001) discovered that the eSRT approach consistently produced
lower results than behavioral strategies, and that children who used map with eSRT
wore their implant for longer periods of time with fewer instances of discomfort.[14] There is a strong association between CI programs developed using behavioral techniques
and eSRT recording to assess C levels.[15]
[16] Thus, eSRT methods have become crucial in mapping protocol. Another study evaluated
how well eSRT levels measured using different modes of stimulation to predict the
clinically mapped M-levels of each patient's daily use program obtained behaviorally
in the course of standard clinical care.[17] They identified progressively lower eSRT levels measured for 1-, 4-, and 15-electrode
stimulation conditions in that order. Average eSRT levels measured with 15-electrode
stimulation were closest to and not statistically different from the average clinically-based
M-levels from the patients' clinical programs. Their results suggested that 1- or
4-electrode stimulation can be used to measure eSRT on a subset of electrodes, and
15-electrode stimulation can be used as an upper bound of stimulation during global
adjustment of the speech coding map to a comfortable loudness level. However, global
stimulation of the electrodes has not been much reported in the literature. The present
study explored the use of global stimulation using live speech in eliciting electrical
stapedial reflexes in individuals with CND fitted with CI.
Methods
Participants
The present retrospective study was conducted by reviewing the CI database in Custom
Sound software, version 7.0 (Cochlear India Private Limited) from January 2015 to
December 2023 to identify patients with CND. The patient records were confirmed through
their medical history and imaging reports from the CI unit at the Department of Hearing
Studies, Dr S R Chandrasekhar Institute of Speech and Hearing, Bengaluru, India. The
study was approved by the Institutional Ethical Committee of Dr. S. R. Chandrasekhar
Institute of Speech and Hearing with the number BSHRF/RC/IERC/IM/IS/O2/2024. Patient
records were included using the following criteria: (1) diagnosed with bilateral sensorineural
hearing loss with CND before implantation; (2) implanted with a Cochlear Nucleus device
(Cochlear Ltd); (3) regularly programmed at our center. The exclusion criteria included:
(1) children with any other malformations with or without CND. [Table 1] shows the details of the demographics of the participants along with the eSRT and
behavioral C levels (of basal electrode number 1 as reference) of patients with CND.
Table 1
Details of the participants
|
Patient nr.
|
Chronological age
|
Age of implant
|
Implant age
|
Implant
|
Implanted ear
|
MRI findings
|
C level (reference basal electrode number 1)
|
eSRT (reference basal electrode number 1)
|
Difference
|
|
1
|
6.8 years
|
2.5 years
|
4.3 years
|
CI24RE (CS)
|
Right
|
Bilateral cochlear nerve deficiency
|
203
|
Absent
|
NA
|
|
2
|
3.4 years
|
2.2 years
|
1.2 years
|
CI24RE (CS)
|
Right
|
Bilateral cochlear nerve deficiency
|
177
|
Absent
|
NA
|
|
3
|
4.8 years
|
3.1 years
|
1.7 years
|
CI24RE (CA)
|
Left
|
Atretic bilateral internal auditory canals with hypoplastic vestibulo-cochlear nerves
and normal 7th nerves
|
182
|
Absent
|
NA
|
|
4
|
6.9 years
|
4.8 years
|
2.1 years
|
CI24RE (CA)
|
Right
|
Agenesis of bilateral 8th cranial nerve, normal 7th cranial nerve
|
137
|
138
|
-1
|
|
5
|
1.2 years
|
1.0 years
|
2 days
|
CI24RE (CA)
|
Right
|
Hypoplastic right, absent left 8th cranial nerve, normal right and left hypoplastic
7th cranial nerve with aberrant course, coursing antero- Medial & superior to common
cavity
|
197
|
201
|
-4
|
|
6
|
4.2 years
|
4.0 years
|
1.2 months
|
CI422
|
Left
|
Aplasia of right and hypoplasia of left 8th cranial nerve, normal 7th cranial nerve
|
160
|
Absent
|
NA
|
Abbreviations: MRI, magnetic resonance imaging; NA, not applicable as eSRT is absent.
Behavioral C Levels
All the mapping sessions were done by a trained audiologist with the Custom Sound
Software Version 5.1 (Cochlear Ltd). Prior to eSRT measurement, the behavioral responses
were measured by selecting 5 electrodes (1, 6, 12, 16, 22) and interpolating them.
Here, stimulation began at 5 CL above the previously measured behavioral C levels.
If the child showed no response, then the level of stimulus was raised by 5 programming
units, and the response was observed. If the child showed discomfort, three programming
units were reduced and then increased or decreased by one programming unit until the
audiologist perceived that the patient was hearing comfortably.
eSRT Measurements
The eSRT measurements postcochlear implantation were performed using standard procedure
by trained audiologists in the CI unit at the institute. A GSI Tympstar or GSI Tympstar
Pro middle ear analyzer (Grason-Stadler, Inc.) was used for tympanometry. Participants
having a 'type-A' tympanogram curve underwent eSRT. A 226-Hz probe tone was introduced
into the target ear to record the stapedius reflex, and the eSRT was obtained using
an implant speech processor coupled to an HP (Hewlett-Packard Enterprise) or a Fujitsu
(Fujitsu Limited) laptop with custom sound software.
In the programming software, the option “live” was chosen, which allowed the participant
to hear live speech. The response was measured by presenting live speech at a conversational
level. The stimuli used to elicit reflexes were monosyllables spoken by the implant
audiologists at a moderate level at three inches from the processor microphone. The
eSRT responses were collected within the same time window using manual reflex decay
for 15 seconds. The stimulation was started at 5 programming units above the previously
mapped behavioral C levels. If no reflex was seen, then the current level was raised
by 5 programming units. If a clear and time-locked downward deflection was present,
the stimulus level was decreased by 2 programming units until no deflection was observed.
Then, 1 programming unit was increased or decreased to derive the accurate value.
The lowest stimulation level at which a deflection was obtained on three repeated
presentations was considered as eSRT. [Fig. 1] depicts the deflection as seen during the measurement of eSRT in a CI recipient.
The increase in stimulation level was restricted to not more than 10 programming units
above the participants' behavioral C levels. If the child showed any sign of discomfort,
the testing was terminated.
Fig. 1 Presence of time-locked deflection of the reflex with the presentation of live speech
stimuli in reflex decay mode of immittance instrument.
Results
From the CI database, a total of 273 recipients were identified, among whom 6 children
had CND whose eSRT levels were recorded. All 6 CI recipients with CND underwent global
stimulation of the electrodes using speech stimuli through Custom Sound Software to
measure their eSRT. Among the six, two individuals had electrically-evoked stapedial
reflexes. The current levels at the basal electrodes were chosen as a reference to
compare the C levels and eSRT levels. Global stimulation yielded reflexes at 138 and
201 current levels, respectively, for the two recipients. For both children, the behavioral
C levels across the electrodes were lower than the eSRT thresholds obtained through
global stimulation, as shown in [Table 1]. According to radiographic findings, the first child (Patient number 4) had bilateral
agenesis of the 8th cranial nerve, and the second child (Patient number 5) presented
with hypoplastic right and absent left 8th cranial nerve. Activation of individual
electrodes with pulsatile stimuli had not produced reflexes in any of the participants.
Discussion
Cochlear implantation in children with CND, particularly those with CN aplasia, has
been contentious. Patients with CND would have limited nerve stimulation from electrical
impulses given by CI electrodes. In turn, this would reduce the amount of neurological
activity produced at higher centers along the auditory pathway and its related regions.
Thus, in theory, the benefit of CI may be compromised if the CN is absent or hypoplastic.
However, successful implantation outcomes show that even children with CND can benefit
from CI.[5]
[18]
[19]
[20]
[21]
[22] Electrically-evoked stapedial reflex threshold is one objective tool to measure
the outcome of CI, and the current study explored the use of eSRT in CND.
In the current study, CND was identified in 2% of the CI recipients from the database.
This is in accordance with the literature, which reports retrospective findings of
CI recipients.[23] The research has noted a strong relationship between eSRTs and comfort levels determined
by subjective assessments.[13]
[24]
[25]
[26] In the current study, two out of the six patients presented with eSRT, and the difference
between the eSRT and behavioral C levels was in accordance with the literature. Thus,
when the stapedius reflex can be identified postoperatively, the eSRT may be effective
for mapping speech processor,[19] which can be applicable for patients with CND. Most of the studies are performed
using pulsatile stimulation of individual electrodes, which could be the reason for
the lack of reports on eSRT in CND. On the other hand, it has been speculated that
global stimulation of electrodes using live speech would increase the chances of eliciting
a reflex. This is the same principle applied to acoustic reflex thresholds, which
are lower in response to broadband stimulation.[27] Charoo et al. supported this speculation through their study, which showed a direct
relationship between the number of electrodes stimulated and the M levels obtained
behaviorally. In their study, there was no statistical difference between the eSRT
obtained through 15 electrode stimulation and the M levels of the patient's map.[17] Although there was not enough data to perform a statistical measure in the current
study, global stimulation of the electrodes using live speech elicited reflexes in
2 out of 6 patients, whereas individual electrode stimulation of apical, medial, and
basal electrodes elicited no eSRT in any patient.
Radiologic indicators, including IAM diameter, bony cochlear nerve canal patency,
cochlear aperture status, and cochlear anatomy, cannot reliably predict whether the
CN is present or absent. Even though a diameter of less than 1.5 mm can more frequently
be connected to a lack of CN, CND can occur in both narrow and normal-sized IAMs.
However, normal-appearing cochlea and IAM do not corroborate the presence of a CN
sufficient for CI.[23] The MRI findings of the participants, as shown in [Table 1], are also contraindications for CI, but the candidacy was established in these patients
using electrical auditory brainstem response (eABR). Thus, the presence of postoperative
eSRT in the two patients is a positive indicator confirming electrical stimulation
of the auditory nerve.
Conclusion
Our study indicates that global stimulation of electrodes may be useful to elicit
post-CI eSRT and supports its use in mapping CND over individual electrode stimulation.
However, the study included a small sample of 6 patients, out of which eSRT was obtained
for only 2 patients. Thus, further research is essential to validate the use of global
stimulation versus individual electrode stimulation on a larger group of patients
with CND. This would aid in appropriate rehabilitation measures as well as improved
counselling on the prognosis.
Bibliographical Record
Megha Sasidharan, Anil Kumar, Pratik Agarwalla. Electrical Stapedial Reflex Thresholds
in Cochlear Implant Recipients with Cochlear Nerve Deficiency: A Retrospective Study.
Int Arch Otorhinolaryngol 2025; 29: s00451808276.
DOI: 10.1055/s-0045-1808276
