Pathophysiology of Facial Nerve Stimulation and Its Implications for Electrical Stimulation in Cochlear Implants

OBJECTIVES

A small number of cochlear implant (CI) users experience facial nerve stimulation (FNS), which can manifest as facial twitching. In some patients, this can be resolved by adjusting the electrical stimulation parameters. However, for others, facial stimulation can significantly impair CI outcomes or even prevent its use. The exact mechanisms underlying FNS are unclear and may vary among patients.

DESIGN

Transimpedance measurements were used to assess lateral and longitudinal spread of current within 15 cochlea of nucleus CI recipients with FNS (13 unilateral recipients and 1 bilateral recipient). We compared the transimpedance measurements with programming parameters from clinical visits and pre- and postoperative temporal bone computed tomography (CT) scans to identify factors that may contribute to FNS in each CI ear.

RESULTS

In nine ears, transimpedance curves showed inflection, which suggests a localized current sink within the cochlea. This indicates a low-impedance pathway through which current exits the cochlea and stimulates the labyrinthine segment of the facial nerve canal. Electrodes near this current sink were disabled or underfit to minimize facial stimulation. In the other seven ears, current flow peaked toward the basal end of the cochlea, suggesting that current exits through the round window or other structures near the basal end of the cochlea, stimulating the tympanic segment of the facial nerve.

CONCLUSIONS

Objective transimpedance measurements can be used to elucidate the mechanisms of FNS and to develop strategies for optimizing electrical stimulation parameters and speech coding to minimize or eliminate FNS in a small subset of CI users.

The Effects of Multi-Mode Monophasic Stimulation with Capacitive Discharge on the Facial Nerve Stimulation Reduction in Young Children with Cochlear Implants: Intraoperative Recordings

Facial nerve stimulation (FNS) is a potential complication which may affect the auditory performance of children with cochlear implants (CIs). We carried out an exploratory prospective observational study to investigate the effects of the electrical stimulation pattern on FNS reduction in young children with CI. Ten ears of seven prelingually deafened children with ages up to 6 years old who undergone a unilateral or bilateral CI surgery were included in this study. Electromyographic (EMG) action potentials from orbicularis oculi muscle were recorded using monopolar biphasic stimulation (ST1) and multi-mode monophasic stimulation with capacitive discharge (ST2). Presence of EMG responses, facial nerve stimulation thresholds (T-FNS) and EMG amplitudes were compared between ST1 and ST2. Intra-cochlear electrodes placement, cochlear-nerve and electrode-nerve distances were also estimated to investigate their effects on EMG responses. The use of ST2 significantly reduced the presence of intraoperative EMG responses compared to ST1. Higher stimulation levels were required to elicit FNS with ST2, with smaller amplitudes, compared to ST1. No and weak correlation was observed between cochlea-nerve and electrode-nerve distances and EMG responses, respectively. ST2 may reduce FNS in young children with CI. Differently from the electrical stimulation pattern, the cochlea-nerve and electrode-nerve distances seem to have limited effects on FNS in this population.

Cochlear re-implantation with the use of multi-mode grounding associated with anodic monophasic pulses to manage abnormal facial nerve stimulation

Objectives: To investigate the outcomes of cochlear re-implantation using multi-mode grounding stimulation associated with anodic monophasic pulses to manage abnormal facial nerve stimulation (AFNS) in cochlear implant (CI) recipients. Methods: Retrospective case report. An adult CI recipient with severe AFNS and decrease in auditory performance was re-implanted with a new CI device to change the pulse shape and stimulation mode. Patient's speech perception scores and AFNS were compared before and after cochlear re-implantation, using monopolar stimulation associated with cathodic biphasic pulses and multi-mode stimulation mode associated to anodic monophasic pulses, respectively. The insertion depth angle and the electrode-nerve distances were also investigated, before and after cochlear re-implantation. Results: AFNS was resolved, and the speech recognition scores rapidly increased in the first year after cochlear re-implantation while remaining stable. After cochlear re-implantation, the e15 and e20 electrodes showed shorter electrode-nerve distances compared to their correspondent e4 and e7 electrodes, which induced AFNS in the first implantation. Conclusions: Cochlear re-implantation with multi-mode grounding stimulation associated with anodic monophasic pulses was an effective strategy for managing AFNS. The patient's speech perception scores rapidly improved and AFNS was not detected four years after cochlear re-implantation.

Facial nerve stimulation in adult cochlear implant recipients with far advanced otosclerosis

Objectives

The objective of this study was to predict occurrence of facial nerve stimulation (FNS) in cochlear implanted patients for far-advanced otosclerosis (FAO) by correlating preoperative computed tomography (CT)-scan data to FNS and to evaluate FNS impact on hearing outcomes.

Methods

Retrospective analysis on 91 ears (76 patients) implanted for FAO. Electrodes were straight (50%) or perimodiolar (50%). Demographic data, extension of otosclerosis on preoperative CT scan, occurrence of FNS, and speech performance were analyzed.

Results

Prevalence of FNS was 21% (19 ears). FNS appeared during the first month (21%), 1-6 months (26%), 6-12 months (21%), and over 1 year (32%) postimplantation. Cumulative incidence of FNS at 15 years was 33% (95% CI = [14-47%]). Extension of otosclerotic lesions on preimplantation CT-scan was more severe in FNS ears compared to No-FNS (p < .05): for Stage III, 13/19 (68%) and 18/72 (25%) ears for FNS and No-FNS groups, respectively (p < .05). Location of otosclerotic lesions relative to the facial nerve canal was similar whatever the presence or not of FNS. Electrode array had no impact on FNS occurrence. At 1 year post-implantation, duration of profound hearing loss (≥5 years) and previous stapedotomy were negatively associated with speech performance. FNS did not impact hearing outcomes, despite a lower percentage of activated electrodes (p < .01) in the FNS group. Nevertheless, FNS were associated with a decrease of speech performance both in quiet (p < .001) and in noise (p < .05).

Conclusion

Cochlear implanted patients for FAO are at greater risk of developing FNS affecting speech performance over time, probably due to a higher percentage of deactivated electrodes. High resolution CT-scan is an essential tool allowing FNS prediction but not time of onset.

Level of evidence

2b, Laryngoscope Investigative Otolaryngology, 2022.

Management of Severe Facial Nerve Cross Stimulation by Cochlear Implant Replacement to Change Pulse Shape and Grounding Configuration: A Case-series

OBJECTIVES

To investigate the combined effect of changing pulse shape and grounding configuration to manage facial nerve stimulation (FNS) in cochlear implant (CI) recipients.

PATIENTS

Three adult CI recipients with severe FNS were offered a replacement implant when standard stimulation strategies and programming adjustments did not resolve symptoms. Our hypothesis was that the facial nerve was less likely to be activated when using anodic pulses with "mixed-mode" intra-cochlear and extra-cochlear current return.

INTERVENTION

All patients were reimplanted with an implant that uses a pseudo-monophasic anodic pulse shape, with mixed-mode grounding (stimulus mixed-mode anodic)-the Neuro Zti CI (Oticon Medical). This device also allows measurements of neural function and loudness with monopolar, symmetric biphasic pulses (stimulus MB), the clinical standard used by most CIs as a comparison.

MAIN OUTCOME MEASURES

The combined effect of pulse shape and grounding configuration on FNS was monitored during surgery. Following CI activation, FNS symptoms and performance with the Neuro Zti implant were compared with outcomes before reimplantation.

RESULTS

FNS could only be recorded using stimulus MB for all patients. In clinical use, all patients reported reduced FNS and showed an improvement in Bamford-Kowal-Bench sentences recognition compared with immediately before reimplantation. Bamford-Kowal-Bench scores with a male speaker were lower compared with measurements taken before the onset of severe FNS for patients 1 and 2.

CONCLUSIONS

In patients where CI auditory performance was severely limited by FNS, charge-balanced pseudo-monophasic stimulation mode with a mixed-mode grounding configuration limited FNS and improved loudness percept compared with standard biphasic stimulation with monopolar grounding.

Cochlear implantation among patients with otosclerosis: a systematic review of clinical characteristics and outcomes

PURPOSE

There have been considerable advancements in cochlear implants in different clinical scenarios; however, their use in patients with otosclerosis remains challenging. This review aimed to investigate the surgical and clinical outcomes of cochlear implantation in patients with otosclerosis.

METHODS

An electronic literature search was performed using four main databases through February 2021 to identify original studies of cochlear implantation in patients with otosclerosis for inclusion in this systematic review. The study protocol was registered with the Prospectively Registered Systematic Reviews and Meta-analyses (reference number: CRD42021234753).

RESULTS

A total of 23 studies including 3162 patients were enrolled. Of these patients, only 392 had otosclerosis and underwent cochlear implantation. The duration of deafness was reported in only eight studies, extending up to 50 years. Far-advanced otosclerosis was observed in 153 patients. A total of 56 patients used hearing aids. Stapedectomy and stapedotomy were performed in 118 and 63 patients, respectively. In three studies, the temporary success of stapedectomy and stapedotomy was 6 (43%) and 5 (71%) patients, respectively. Computed tomography was used as a preoperative assessment tool in most studies (n = 14, 60.9%). Incomplete implant insertion occurred in 17 patients, while facial nerve stimulation occurred in 36 patients after implantation.

CONCLUSION

Cochlear implantation is a relatively safe modality that can provide promising audiological outcomes in patients with otosclerosis. However, several factors, including cochlear ossification, duration of deafness, and previous operations, can affect its outcomes. Further studies with a larger sample population are recommended.

Risk Factors for Facial Nerve and Other Nonauditory Side Effects Following Cochlear Implantation

OBJECTIVE

The purpose of this study was to characterize a cohort of patients with nonauditory side-effects (NASx) following cochlear implant (CI) surgery.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

One hundred twenty three multichannel CI recipients with intraoperative facial nerve stimulation (FNS).

INTERVENTIONS

Intraoperative electrical auditory brainstem responses (eABR) during CI surgery.

MAIN OUTCOME MEASURES

Nonauditory side effects post-CI activation.

RESULTS

Intraoperative FNS was identified in 2.26% of patients (123/5441), of whom, 34% (42/123) experienced VII stimulation on CI activation. Pain was experienced by 22% (27/123) and vestibular dysfunction was experienced by 4% (5/123) of cases. All case who experienced pain and/or vestibular NASx also experienced VII stimulation. The majority of cases were managed by CI remapping or observation and habituation.Significant relationships were found between etiology of hearing loss and presence of FNS upon initial activation (p < 0.05). No significance was found between FNS intraoperatively and at initial activation for all assumed mechanisms of hearing loss (p > 0.05) with the exceptions of acquired hearing loss of undetermined etiology and toxic etiology group (p < 0.05).There was no significant impact of implant array design (p > 0.05).

CONCLUSIONS

This study has characterized patients with NASx in a large cohort of CI patients. One third of cases identified with FNS intraoperatively, developed NASx post-CI activation. Risk factors for NASx postactivation include high-risk etiologies and intraoperative objective measures (i.e., eABR). This may assist surgeons and audiologists to identify at-risk patients who may need modifications in CI program planning.

Anatomic features of the fetal round and oval windows, and their relations with the tympanic nerve

OBJECTIVE

The study aimed to examine morphometric properties of the round window (RW) and oval window (OW) and to show their relation with the tympanic nerve (the Jacobson's nerve, JN) in human fetuses from the otologic surgeon's perspective.

METHODS

Thirty temporal bones of 15 fetal cadavers (8 males, 7 females) aged with 24.40 ± 3.71 weeks were included in the study. The height, width and surface area of the RW and OW and also distance from the JN to the OW and RW were measured.

RESULTS

The height, width and surface area of the RW in this work were measured as 1.48 ± 0.25 mm, 1.57 ± 0.37 mm, and 2.05 ± 0.69 mm², respectively. The RW was detected as round-shaped (8 cases, 26.7%), oval-shaped (15 cases, 50%), and dome-shaped (7 cases, 23.3%). The height, width and surface area of the OW were measured as 1.42 ± 0.26 mm, 2.90 ± 0.44 mm, and 3.63 ± 0.74 mm², respectively. The OW was observed as oval-shaped (15 cases, 50%), kidney-shaped (10 cases, 33.3%), D-shaped (4 cases, 13.3%), and trapezoid-shaped (1 case, 3.3%). The JN was found 1.21 ± 0.60 and 1.18 ± 0.54 mm away from the RW and OW, respectively.

CONCLUSION

This study containing morphological data about the shapes, diameters and area of the RW and OW may be useful to predict surgical difficulty, and to select implants of suitable size preoperatively for the windows. Knowing the relationship between the JN and the windows can be helpful to avoid iatrogenic injuries of the nerve.

Piloting the recording of electrode voltages (REVS) using surface electrodes as a test to identify cochlear implant electrode migration, extra-cochlear electrodes and basal electrodes causing discomfort

OBJECTIVES

To determine if Electrode Voltage (EV) measurements are potentially suitable as a test for detecting extra-cochlear electrodes in cochlear implants (CIs).

METHODS

EV measurements were made using surface electrodes in live mode in 17 adult cochlear implant (CI) users. Repeatability, the effects of stimulation level, CI active electrode position, (active) recording electrode position and stimulation mode (for Nucleus devices) were investigated.

RESULTS/DISCUSSION

Recordings made in monopolar mode showed good repeatability when the active recording electrode was placed on the ipsilateral earlobe; voltages increased linearly with stimulation level as expected. EVs for basal electrodes differed greatly between partially inserted/migrated devices, fully inserted devices with all electrodes activated, and those with deactivated basal electrodes [χ²(2) = 10.2, p < 0.05 for the most basal electrode]. EVs for Nucleus devices were small for electrodes on the array when compared to those for monopolar return electrodes, except for the participant with extra-cochlear electrodes. We argue that fibrosis around the electrode array facilitated current flow across the round window in this case.

CONCLUSION

The test appears to be a viable approach to detect electrode migration and extra-cochlear electrodes in adult CI users and may also be sensitive to discomfort caused by current leakage from the basal end of the cochlea.

Facial nerve stimulation in a post-meningitic cochlear implant user: using computational modelling as a tool to probe mechanisms and progression of complications on a case-by-case basis

Facial nerve stimulation (FNS) is a side-effect of cochlear implantation that can result in severe discomfort for the user and essentially limits the optimal use of the implant. Three-dimensional cochlear implant modelling research has led to the progression from generic models to user-specific models with one of the intentions to develop model-based diagnostic tools. The objective of this study is to investigate the mechanisms that underlie the manifestation of FNS in the post-meningitic cochleae of a specific CI user through computational modelling. Bilateral models were created using a method previously developed for the construction of a three-dimensional user-specific volume conduction model of the cochlea and was expanded to include the facial nerve geometry. Reduced temporal bone density based on bone densitometry, cochlear duct ossification and degenerate auditory neural fibres were incorporated into a comprehensive FNS model. Auditory and facial nerve thresholds were predicted with the models showing good correspondence to perceptual thresholds and the user's FNS experience. Ossified cochlear ducts appear to aggravate the increase in thresholds caused by the otic capsule's decreased resistivity. This translational case study demonstrates the application of computational modelling as a clinical instrument in the assessment and management of complications with cochlear implantation.

Laser tympanic neurectomy for post-cochlear implant pain: A new technique

OBJECTIVES

To present the case histories and management by LASER tympanic neurectomy of two patients who presented with pain as their only symptom after cochlear implantation, avoiding the need for device removal.

CLINICAL PRESENTATION

Two of our patients presented with otalgia, which appeared 6 months after cochlear implantation and resulted in their refusing to use the device. The pain was not controlled by repeated remapping or medical management. Neither patient showed evidence of infection or inflammation around their device. X-rays showed that there were no extracochlear electrodes or evidence of extrusion. One patient had current leakage from two electrodes, which were switched off, but the pain persisted.

INTERVENTION

Both patients received an intratympanic injection of 1 ml of 0.5% Bupivacaine to anaesthetize the tympanic plexus in the middle ear and were then observed for a day, found to have relief of their pain and were able to use the implant with audiological benefit for this short time, until the effect of the anaesthetic had worn off. Therefore we planned and performed a tympanic neurectomy on both patients using CO2 laser.

CONCLUSION

Tympanic neurectomy removed the pain in two cochlear implant patients who presented with pain which was present only when the implant was switched on.

Cochlear implantation in patients profoundly deafened after head injury

OBJECTIVE

There is very little information in the literature regarding outcomes of cochlear implantation in patients profoundly deafened after head injury. The aim of this study was to assess outcomes in this group of patients.

STUDY DESIGN

Retrospective case review.

SETTING

The Manchester Cochlear Implant Programme, University of Manchester.

PATIENTS

Profoundly deafened patients after head injury who proceeded to cochlear implantation for auditory rehabilitation.

MAIN OUTCOME MEASURES

Mean age and duration of deafness at implantation. Preimplantation and postimplantation speech perception outcomes were measured using Bench Kowel Bamford (BKB) sentences in quiet and noise, City University of New York sentences with lip reading and Arthur Boothroyd words scoring the percentage phonemes correct.

RESULTS

Twenty patients received 23 cochlear implants. Mean age at implantation was 51 years (standard deviation, 12 yr). Mean duration of deafness at implantation was 12 years (range, 1-30 yr).Preimplantation BKB score in quiet of 0%. Mean postimplantation BKB score in quiet was 64% (range, 0%-100%) and in noise was 61% (range, 0%-97%). Three were nonusers, and 1 required reimplantation. There was a moderately negative correlation between outcome and age at implantation (r = -0.41, p < 0.05) and between outcome and duration of deafness (r = -0.52, p < 0.05).

CONCLUSION

Cochlear implantation is an effective method for hearing rehabilitation in profoundly deafened patients after head injury. However, negative factors, such as significant injury to the central auditory pathway, basal turn obliteration, long duration of deafness, and nonauditory stimulation, should be considered in the preoperative assessment of these patients.

Symptomatic postsurgical traumatic neuromas

HYPOTHESIS

Growing nerve fibers from the stumps of amputated sensory nerves can form traumatic neuromas within inner or middle ear postsurgical fibrosis and may produce symptoms commensurate with the normal function of the nerve involved, that is, balance or pain.

BACKGROUND

Microscopic traumatic neuromas have been identified in postoperative middle and inner ear fibrosis in the temporal bones of patients complaining of intractable pain or imbalance.

METHODS

Postsurgical temporal bones having inner or middle ear traumatic neuromas were reviewed. Of 20 bones with inner ear fibrosis after a variety of neurotologic surgeries, 12 were found to have traumatic neuromas, most from the utricular nerve or lateral canal. Five ears in 4 patients with middle ear fibrosis after chronic ear surgery had traumatic neuromas arising from Jacobson nerve. An additional 58 bones from chronic ear surgery patients with no neuromas served as a control group. Neurofilament immunohistochemistry labeling substantiated the presence of nerve fibers. Clinical symptoms noted from the clinical records were compared between those with and without traumatic neuromas.

RESULTS

Of the 12 patients (75%), 9 with inner ear traumatic neuromas clinically reported constant disequilibrium postsurgery lasting for years. None (0%) without neuromas reported new symptoms postoperatively (p <or= 0.001). All 5 ears with middle ear traumatic neuromas experienced otalgia postsurgery, whereas none of the other 58 patients with no neuroma reported this problem.

CONCLUSION

Postoperative intractable disequilibrium or pain may occur as a result of the formation of traumatic neuromas in scar tissue in the inner or middle ears.

Trends and outcomes in the Manchester adult cochlear implant series

The adult cochlear implant programme in Manchester was established in 1988, initially using funding obtained from the HEAR (Help Ear and Allied Research: charity number: 519784) charity before government resources became available in the mid-1990s. Manchester was the first centre in the UK to implant multichannel devices on a regular basis. To date, over 250 adults have been implanted, including nine bilateral and eight deaf-blind patients. All the patients have a postlingual onset of severe-profound hearing loss; 73% (n = 175) of the implants performed used a Nucleus multichannel implant and 24% (n = 58) used a Medel multichannel implant. In addition, the team has implanted three Medel single channel devices, two Ineraid devices and one Clarion High Focus II device. This study is a retrospective analysis of the trends and outcomes in implant fitting during the first 14 years (1988-2002) of the programme. The paper describes the patient demographics and audiological complications for 240 implantations performed on 214 patients. Speech perception outcomes are reported for a subset of the patients. The average score for the Bench, Kowal, Bamford sentence test at the post-18-month stage of implant use is 66% and for Arthur Boothroyd words 53%. Trends in the series are analysed with respect to the change in criteria for adult implantation, the move towards bilateral implantation and the rate of uptake of cochlear implants by different ethnic groups.