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Abstract

To explore the correlation between the width of the bony cochlear nerve canal (CNC) and long-term auditory rehabilitation after unilateral cochlear implantation (CI) in pediatric patients with congenital deafness and bilateral cochlear nerve canal stenosis (CNCS). A retrospective review was performed on 10 patients with bilateral CNCS and bilateral congenital profound hearing loss who each underwent unilateral cochlear implantation. The width of the CNC was determined on computed tomography (CT) imaging and following CI, auditory and speech performance following CI were graded using categories of auditory performance (CAP), speech intelligibility rating (SIR), and the meaningful auditory integration scale (MAIS) at 24 months following implantation. No correlation was noted between CAP score and CNCS at 24 months post CI (P > .05). A positive correlation was noted between SIR score and CNC width (ρ = .81, P < .05). Similarly, a positive correlation was noted between MAIS and CNC width (ρ = .71, P < .05). The width of the CNC in patients with CNCS is positively correlated with some long-term auditory and speech outcomes after CI.

Introduction

The bony cochlear nerve canal (CNC) transmits the cochlear nerve from the fundus of the internal auditory canal (IAC) to the base of the cochlear modiolus. Cochlear nerve canal stenosis (CNCS), first reported in 2000, is associated with varying degrees of sensorineural hearing loss (SNHL), but presents most commonly as a profound loss.¹ CNCS is typically diagnosed on computed tomography (CT) imaging, where a threshold of less than 1.5 mm is generally used for the cochlear nerve canal (CNC) to be considered stenotic. MRI may show a hypoplastic or aplastic cochlear nerve.² Associated abnormalities include inner ear malformations and syndromic hearing loss, although these are present in a minority of patients with CNCS.³

Hearing rehabilitation in patients with CNCS depends in a large part on contralateral hearing status. When a bilateral profound loss is present, cochlear implantation (CI) is the preferred method of hearing rehabilitation.⁴ CI outcomes in patients with CNCS have not been fully elucidated. In an earlier report, patients with CNCS undergoing CI demonstrated increased electrocochleography thresholds, with many patients deriving significant benefit.⁵ This indicates that CIs are effectively able to stimulate the cochlea and produce a measurable neural response in these patients, but that they require a higher current to do so.

In this study, we discuss our experience with a series of patients with bilateral CNCS who underwent unilateral CI for hearing rehabilitation. We correlate long-term speech and language outcomes, as measured by categories of auditory performance (CAP), speech intelligibility rating (SIR), and infant-toddler meaningful auditory integration scale (IT-MAIS) with the diameter of the cochlear nerve canal.

Materials and Methods

Institutional review board approval was obtained for this study. Inclusion criteria were patients with bilateral profound hearing loss, bilateral CNCS on CT imaging, and normal inner ear morphology who underwent unilateral CI (shown in Figure 1). The choice of ear to be implanted was determined by a consensus between audiology and otology based on a variety of factors individualized to each patient within the study population. Factors included residual hearing amenable to hearing aid use in one or both ears and CNC diameter; in general, the ear with a larger CNC diameter was implanted. Exclusion criteria were medical comorbidities and presence of inner ear malformations. A retrospective review was performed of patients meeting criteria within the study period December 2012 to May 2017; 10 patients were identified (shown in Table 1).

Figure 1.

Stenosis of the bony cochlear nerve canal (CNC). In this patient, the CNC measures 1.1 mm (normal, >1.5 mm).

Table 1.

Data of 10 patients with CNCS.

No.	Age implanted (years)	Ear	Implant type	Width of CNC (cm)	Postoperative 24 months
No.	Age implanted (years)	Ear	Implant type	Width of CNC (cm)	CAP	SIR	IT-MAIS/MAIS
1	2	R	Freedom	.11	5	3	27
2	3	R	Freedom	.13	5	5	40
3	1	R	SONATA	.08	6	3	27
4	2	R	Freedom	.07	2	1	11
5	4	L	C40+	.12	6	3	40
6	1	L	SONATA	.00	2	1	14
7	1	R	CI512	.11	5	3	32
8	2	L	SONATA	.08	3	2	27
9	1	L	SONATA	.06	6	2	27
10	6	R	CI24RE-CA	.07	5	3	34

All patients underwent preoperative CT imaging, and CNC diameter on the implanted side was measured independently by a neuroradiologist as well as by the primary author (SZ). Although interrater reliability was not calculated, there was minimal discrepancy between the measurements. All patients underwent CI via a standard facial recess approach. Patients underwent routine audiological assessments, and at 24 months post-implantation, all patients underwent evaluation for categories of auditory performance (CAP, shown in Table 2), speech intelligibility rating (SIR, shown in Table 3), and meaningful auditory integration scale (MAIS, for children over 3 years of age); the infant-toddler MAIS (IT-MAIS) was utilized for patients under 3 years of age at the time of evaluation (shown in Table 4).

Table 2.

Categories of auditory performance (CAP).

Grade	Decision Criteria
0	Can’t detect ambient sounds
1	Can detect ambient sounds
2	Demonstrates response to speech sounds
3	Able to identify ambient sounds
4	Some speech sounds can be distinguished without lip-reading
5	Understands common phrases without lip-reading
6	Understands a conversation in quiet without lip-reading
7	Able to have telephone conversation with a close contact
8	Able to understand a group of people talking in a room (such as a classroom or dining room) in the presence of noise
9	Able to have a telephone conversation with a stranger or when topic of conversation is unknown

Table 3.

Speech intelligibility rating (SIR).

Grade	Decision Criteria
1	Coherent speech can’t be understood and spoken words can’t be recognized. The main mode of daily communication of patients is gesture
2	Coherent speech cannot be understood, individual words in speech can be understood in combination with conversational context and lip-reading cues
3	Coherent speech can be understood by a particular listener but requires the, listener to understand the topic of the conversation, pay attention and lip-read
4	Coherent speech can be understood by 1 listener who is not familiar with deaf speech and does not need to listen hard
5	Coherent speech can be understood by all listeners and the child’s language can be easily understood in daily contexts

Table 4.

Infant-toddler meaningful auditory integration scale (IT-MAIS).

No	Question
1	When a child wears a hearing device, does his or her voice change?
2	Can a child speak syllables that can be identified as ‘speech’ or consecutive syllables?
3	Can a child spontaneously respond to being called by his or her name in a quiet environment, relying solely on auditory cues (no visual cues)?
4	Can a child respond spontaneously to a name being called to him or her in a noisy environment, relying solely on his or her hearing (no visual cues)?
5	Can a child spontaneously respond to sounds in the environment (barking dogs, toys, etc.), without prompting, at home?
6	Can a child spontaneously respond to environmental sounds in a new environment?
7	Can a child spontaneously recognize that auditory information is part of his or her daily life?
8	Can a child spontaneously distinguish between 2 people’s voices based solely on auditory cues (no visual cues)?
9	Can a child spontaneously distinguish between spoken and nonspoken sounds by hearing alone?
10	Can a child spontaneously perceive mood (anger, arousal, anxiety) just by listening?

Spearman correlation was performed with SPSS 22.0 (IBM, Armonk, New York) to investigate the relationship between CNC diameter and test parameters.

Results

Ten patients (7 male and 3 female) with bilateral CNCS and bilateral profound hearing loss underwent unilateral cochlear implantation at a mean age of 2.3 years (standard deviation [SD], 1.6 years). The mean CNC diameter on preoperative CT imaging as was .83 mm (SD 0.04 mm). Both Cochlear (NSW, Australia) and Med-EL (Innsbruck, Austria) devices were used. The right ear was implanted in 6 patients, and the left ear was implanted in 4. There were no perioperative complications noted.

At 24-month postoperative follow-up, CAP scores ranged from 2 to 6. There was no correlation observed between CAP score and CNC diameter (rs = .334, P = .35). SIR scores ranged from 1 to 5; a positive correlation between SIR and CNC diameter was noted (rs = .808, P = .005). MAIS/IT-MAIS scores ranged from 11 to 40 and similarly a positive correlation between MAIS/IT-MAIS and CNC diameter was noted (rs = .712, P = .021; shown in Table 5).

Table 5.

Correlation between width of cochlear nerve canal and CAP, SIR, and MAIS/IT-MAIS scores.

	Postoperative 24 months
	CAP	SIR	IT-MAIS/MAIS
Coefficient of association (rs)	—	.808	.712
P	.334^*	.005^▽	.021^▽

* P>.05, ▽- P<.05.

Discussion

Cochlear nerve canal stenosis (CNCS) is associated with congenital hearing loss and can occur unilaterally or bilaterally. When CNCS is present bilaterally, the most common presentation is congenital deafness. CNCS has been associated with cochlear nerve hypoplasia and aplasia as well as labyrinthine abnormalities.

In this series, patients with bilateral CNCS, normal inner ear morphology, and without medical or syndromic comorbidities underwent unilateral cochlear implantation for rehabilitation of deafness. We found that these patients generally derived significant benefit from unilateral CI. Performance in this cohort approximated results seen in CI performed in congenitally deaf peers without CNCS.⁶

We further sought to correlate post-implant auditory and speech performance with the diameter of the CNC. The speech intelligibility rating scale and meaningful auditory integration scale were both significantly positively correlated with cochlear nerve canal diameter (rs=.81 and .71, respectively). Categories of auditory performance demonstrated no correlation. This suggests that patients with a larger cochlear nerve canal diameter may demonstrate better post implant performance. This finding is similar to the observation that patients with smaller CNC diameters demonstrate poorer word performance.⁷ Although the observed correlation does not imply causation, it is possible that a larger CNC diameter transmits more intact cochlear nerve fibers from the internal auditory canal to the cochlea. This would certainly be a possible explanation for the improved results noted in patients with larger CNC diameters. Another explanation is that electrophysiologic differences within the cochlea and/or auditory nerve associated with CNC diameter, independent of cochlear nerve fiber count, lead to better auditory performance.

MAIS/IT-MAIS is a scale used for children over 3 years of age and children under 3 years of age, respectively, to evaluate meaningful use of sound in daily life by hearing-impaired children. SIR is a scale used to rate the overall intelligibility of a patient’s speech. CAP is a scale used to rate the outcomes of pediatric CI patients in daily life. These 3 scales provide complementary information about the specific and global benefits of implantation in children. Our findings demonstrated a positive correlation between CNC diameter and MAIS/IT-MAIS, SIR and no correlation between CNC diameter and CAP. This may be explained by the fact that our patient population remains relatively heterogeneous and auditory perceptive performance demonstrated significant variability across subjects. However, better overall performance, as well as better speech, was noted for patients with larger CNC diameters. This may generally suggest improved overall outcomes after implantation with larger CNC diameters.

We did not note any perioperative or postoperative complications in this short series. Previous research has suggested that patients with CNCS undergoing CI have a higher incidence of facial nerve stimulation, but this was not observed in our study.⁸

The present research is valuable for patient counseling. In clinical practice, if a cochlear nerve canal significantly smaller than the generally accepted threshold of 1.5 mm for CNC stenosis is encountered, a patient’s family may be counseled that cochlear implant outcomes are more guarded. However, contrary to some prior reports, CNCS is not an absolute contraindication to cochlear implantation. Rather, the findings in the present work are in agreement with previous research, suggesting that cochlear implant outcomes in patients with CNCS lie along a spectrum of varying degrees of performance.⁹

One limitation of the study is that magnetic resonance imaging (MRI) was not performed for the patients. If MRI results had been uniformly available, the observed performance could also be correlated with the appearance or diameter of the cochlear nerve within the IAC. While the authors’ institutions now routinely utilize MRI as part of the evaluation process for pediatric patients undergoing CI, our referral network includes many practice locations where MRI is difficult to impossible to perform prior to the patients arriving at our tertiary centers. Therefore, it is beneficial for referring centers without MRI access to note that pediatric patients with profound loss and CNCS on CT imaging may well be CI candidates, regardless of MRI status. Another limitation of the study is the lack of consistent longitudinal follow-up on these patients, including long-term follow-up after 24 months post-implantation, as this could potentially reveal eventual improvements in poor performers. Finally, a prospective investigation of patients with CNCS with hearing loss could shed further light on the overall auditory performance and help select the best candidates for cochlear implantation in this population.

Conclusion

Cochlear nerve canal stenosis (CNCS) is associated with profound hearing loss. When deafness associated with bilateral CNCS is present, an increased CNC diameter in the ear to be implanted is associated with better auditory performance. The CNC is easily seen on computed tomography imaging, and should be measured prior to implantation to aid in patient counseling.

Footnotes

Author Contributions

Si Zhang provided substantial contributions to the conception of the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Line Wang provided substantial contributions to the design of the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Sushing Gong provided substantial contributions to the acquisition, analysis, and interpretation of the data for the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Pao Liu provided substantial contributions to the acquisition, analysis, and interpretation of the data for the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Xinyu Song provided substantial contributions to the acquisition, analysis, and interpretation of the data for the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Qimei Wang provided substantial contributions to the conception of the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Wie Wang Zhang provided substantial contributions to the design of the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Hannah Landsberger provided substantial contributions to the conception of the work, revised the work critically for important intellectual content, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Kevin A. Peng provided substantial contributions to the conception and design of the work, drafted the work, gave final approval of the version to be published, and agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported in part by Beijing Municipal Science and Technology Commission (Applied Technology Research and Development Project) (No. Z191100007619043).

Ethical approval

This study was conducted ethically in accordance with the World Medical Association Declaration of Helsinki. This study protocol was reviewed and approved by the IRB of the Beijing Friendship Hospital, Capital Medical University, approval number 2020-P2-125-01.

Informed consent

Written informed consent was obtained from the subjects’ parents.

ORCID iD

Hannah Landsberger

References

Fatterpekar

Mukherji

Alley

Lin

Castillo

. Hypoplasia of the bony canal for the cochlear nerve in patients with congenital sensorineural hearing loss: initial observations. Radiology. 2000;215(1):243-246.

Miyasaka

Nosaka

Morimoto

Taiji

Masaki

. CT and MR imaging for pediatric cochlear implantation: emphasis on the relationship between the cochlear nerve canal and the cochlear nerve. Pediatr Radiol. 2010;40(9):1509-1516.

Glastonbury

Davidson

Harnsberger

Butler

Kertesz

Shelton

. Imaging findings of cochlear nerve deficiency. Am J Neuroradiol. 2002;23(4):635-643.

Govaerts

Casselman

Daemers

De Beukelaer

Yperman

De Ceulaer

. Cochlear implants in aplasia and hypoplasia of the cochleovestibular nerve. Otol Neurotol. 2003;24(6):887-891. doi:10.1097/00129492-200311000-00011.

Han

Wang

Zhang

Peng

. Neural response telemetry thresholds in patients with cochlear nerve canal stenosis. Otolaryngol Head Neck Surg. 2015;153(3):447-451.

Zhong

Dong

Lyu

Liu

Chen

. The analysis of reliability and validity of the IT-MAIS, MAIS and MUSS. Int J Pediatr Otorhinolaryngol. 2017;96:106-110. doi:10.1016/j.ijporl.2017.03.006.

Purcell

Iwata

Phillips

Paladin

Sie

Horn

. Bony cochlear nerve canal stenosis and speech discrimination in pediatric unilateral hearing loss. Laryngoscope. 2015;125(7):1691-1696. doi:10.1002/lary.25087.

Rah

Yoon

Chang

, et al. Facial nerve stimulation in the narrow bony cochlear nerve canal after cochlear implantation. Laryngoscope. 2016;126(6):1433-1439. doi:10.1002/lary.25655.

Chung

Jang