Triphasic Pulses in Cochlear Implant Patients With Facial Nerve Stimulation

Hearing Performance in Cochlear Implant Users Who Have Facial Nerve Stimulation

Introduction  Facial nerve stimulation (FNS) is a complication in cochlear implant (CI) when the electrical current escapes from the cochlea to the nearby facial nerve. Different management to reduce its effects are available, although changes might result in a less-than-ideal fitting for the CI user, eventually reducing speech perception. Objective  To verify the etiologies that cause FNS, to identify strategies in managing FNS, and to evaluate speech recognition in patients who present FNS. Methods  Retrospective study approved by the Ethical Board of the Institution. From the files of a CI group, patients who were identified with FNS either during surgery or at any time postoperatively were selected. Data collection included: CI manufacturer, electrode array type, age at implantation, etiology of hearing loss, FNS identification date, number of electrodes that generated FNS, FNS management actions, and speech recognition in quiet and in noise. Results  Data were collected from 7 children and 25 adults. Etiologies that cause FNS were cochlear malformation, head trauma, meningitis, and otosclerosis; the main actions included decrease in the stimulation levels followed by the deactivation of electrodes. Average speech recognition in quiet before FNS was 86% and 80% after in patients who were able to accomplish the test. However, there was great variability, ranging from 0% in quiet to 90% of speech recognition in noise. Conclusion  Etiologies that cause FNS are related to cochlear morphology alterations. Facial nerve stimulation can be solved using speech processor programming parameters; however, it is not possible to predict outcomes, since results depend on other variables.

Cochlear implantation programming characteristics and outcomes of cochlear nerve deficiency

PURPOSE

Due to the specificity of cochlear implantation (CI) programming parameters and outcomes in cochlear nerve deficiency (CND) patients, this study aimed to investigate the correlation between programming parameters and outcomes and further compare the difference between normal and CND groups.

METHODS

Ninety (95 ears) CND patients (normal cochlea, 39; malformed cochlea, 56) and seventy-nine (81 ears) normal cochlea patients who underwent CI surgery with either Med-El or Cochlear devices were included. The programming parameters and outcomes evaluated by the questionnaires were collected and compared among the normal CND, malformed CND, and normal groups in the two device groups, and their correlation was analyzed.

RESULTS

In the CND group, a reduced stimulation rate, higher pulse width, and triphasic pulse were needed in some cases. The stimulus levels of the CND group were significantly higher than that of the normal group (p < 0.05), but the outcomes of the CND group were significantly worse than that of the normal group (p < 0.05), and the stimulus level was significantly correlated with the outcomes (p < 0.05). However, there was no difference between normal and malformed CND groups. The non-auditory response was observed in the CND group, especially the ones with malformations.

CONCLUSION

The CI programming parameters of some CND patients need to be adjusted, and a slower stimulation rate and higher pulse width are required sometimes. CND patients need a higher stimulus level than normal patients but their outcomes are poorer. Non-auditory response should be noticed in CND patients during programming.

Risk factors and management strategies of inadvertent facial nerve stimulation in cochlear implant recipients: A systematic review

Objectives

To systematically review the prevalence and risk factors of inadvertent facial nerve stimulation (FNS) after cochlear implant (CI) surgery. And to report the different management strategies used for reducing and resolving FNS.

Data Source

Web of Science, Scopus, PubMed, Cochrane Library, and Virtual Health Library (VHL) of the World Health Organization (WHO).

Review Methods

A systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) on studies that reported FNS as a complication after CI. A comprehensive electronic search strategy was used to identify the relevant articles. We extracted the data on the prevalence of FNS after CI activation, the reported grades, and the management strategies. The number of associated electrodes; cause of deafness; co-anomalies; and duration of hearing loss and their relationships with FNS were also studied.

Results

Twenty-one relevant articles were included in this review. The prevalence of FNS among the CI populations was 5.29% (175/3306 patients). Among those whose ages were reported, 58.3% (95/163) were adults, and 41.7% (68/163) were pediatrics. Modifying the different fitting parameters was the most used strategy, as it successfully resolved FNS in 85.5% of the patients (142/166). The second commonly used management strategy was surgical intervention (reimplantation or explantation), which was reported in seven studies for 23 patients.

Conclusion

FNS after CI activation could be controlled and resolved with many advances that range from readjusting the fitting parameters to surgical intervention. However, further studies are required to validate the efficacy of each management strategy and its impact on patients' performance. Our findings demonstrate that CI recipients with FNS could still benefit from the CI devices and their FNS could be controlled.

Cochlear Implant Stimulation Parameters Play a Key Role in Reducing Facial Nerve Stimulation

A percentage (i.e., 5.6%) of Cochlear Implant (CI) users reportedly experience unwanted facial nerve stimulation (FNS). For some, the effort to control this problem results in changing stimulation parameters, thereby reducing their hearing performance. For others, the only viable solution is to deactivate the CI completely. A growing body of evidence in the form of case reports suggests that undesired FNS can be effectively addressed through re-implantation with an Oticon Medical (OM) Neuro-Zti implant. However, the root of this benefit is still unknown: is it due to surgical adjustments, such as varied array geometries and/or positioning, or does it stem from differences in stimulation parameters and/or grounding? The OM device exhibits two distinct features: (1) unique stimulation parameters, including anodic leading pulses and loudness controlled by pulse duration-not current-resulting in lower overall current amplitudes; and (2) unconventional grounding, including both passive (capacitive) discharge, which creates a pseudo-monophasic pulse shape, and a 'distributed-all-polar' (DAP) grounding scheme, which is thought to reduce current spread. Unfortunately, case reports alone cannot distinguish between surgical factors and these implant-related ones. In this paper, we present a novel follow-up study of two CI subjects who previously experienced FNS before re-implantation with Neuro-Zti implants. We used the Oticon Medical Research Platform (OMRP) to stimulate a single electrode in each subject in two ways: (1) with traditional monopolar biphasic cathodic-first pulses, and (2) with distinct OM clinical stimulation. We progressively increased the stimulation intensity until FNS occurred or the sound became excessively loud. Non-auditory/FNS sensations were observed with the traditional stimulation but not with the OM clinical one. This provides the first direct evidence demonstrating that stimulation parameters and/or grounding-not surgical factors-play a key role in mitigating FNS.

A review of neurophysiological effects and efficiency of waveform parameters in deep brain stimulation

Neurostimulation has diverse clinical applications and potential as a treatment for medically refractory movement disorders, epilepsy, and other neurological disorders. However, the parameters used to program electrodes-polarity, pulse width, amplitude, and frequency-and how they are adjusted have remained largely untouched since the 1970 s. This review summarizes the state-of-the-art in Deep Brain Stimulation (DBS) and highlights the need for further research to uncover the physiological mechanisms of neurostimulation. We focus on studies that reveal the potential for clinicians to use waveform parameters to selectively stimulate neural tissue for therapeutic benefit, while avoiding activating tissue associated with adverse effects. DBS uses cathodic monophasic rectangular pulses with passive recharging in clinical practice to treat neurological conditions such as Parkinson's Disease. However, research has shown that stimulation efficiency can be improved, and side effects reduced, through modulating parameters and adding novel waveform properties. These developments can prolong implantable pulse generator lifespan, reducing costs and surgery-associated risks. Waveform parameters can stimulate neurons based on axon orientation and intrinsic structural properties, providing clinicians with more precise targeting of neural pathways. These findings could expand the spectrum of diseases treatable with neuromodulation and improve patient outcomes.

Intra-cochlear differences in the spread of excitation between biphasic and triphasic pulse stimulation in cochlear implants: A modeling and experimental study

Triphasic pulse stimulation can prevent unpleasant facial nerve stimulation in cochlear implant users. Using electromyographic measurements on facial nerve effector muscles, previous studies have shown that biphasic and triphasic pulse stimulations produce different input-output functions. However, little is known about the intracochlear effects of triphasic stimulation and how these may contribute to the amelioration of facial nerve stimulation. The present study used a computational model of implanted human cochleae to investigate the effect of pulse shape on the intracochlear spread of excitation. Biphasic and triphasic pulse stimulations were simulated from three different cochlear implant electrode contact positions. To validate the model results, experimental spread of excitation measurements were conducted with biphasic and triphasic pulse stimulation from three different electrode contact positions in 13 cochlear implant users. The model results depict differences between biphasic and triphasic pulse stimulations depending on the position of the stimulating electrode contact. While biphasic and triphasic pulse stimulations from a medial or basal electrode contact caused similar extents of neural excitation, differences between the pulse shapes were observed when the stimulating contact was located in the cochlear apex. In contrast, the experimental results showed no difference between the biphasic and triphasic initiated spread of excitation for any of the tested contact positions. The model was also used to study responses of neurons without peripheral processes to mimic the effect of neural degeneration. For all three contact positions, simulated degeneration shifted the neural responses towards the apex. Biphasic pulse stimulation showed a stronger response with neural degeneration compared to without degeneration, while triphasic pulse stimulation showed no difference. As previous measurements have demonstrated an ameliorative effect of triphasic pulse stimulation on facial nerve stimulation from medial electrode contact positions, the results imply that a complementary effect located at the facial nerve level must be responsible for reducing facial nerve stimulation.

Anodic Polarity Minimizes Facial Nerve Stimulation as a Side Effect of Cochlear Implantation

One severe side effect of the use of cochlear implants (CI) is coincidental facial nerve stimulation (FNS). Clinical methods to alleviate FNS range from the reprogramming of processor settings to revision surgery. We systematically assessed different changes in CI stimulation modes that have been discussed in the literature as "rescue factors" from FNS: electrode configuration (broad to focused), pulse shape (symmetric biphasic to pseudo-monophasic), and pulse polarity (cathodic to anodic). An FNS was assessed, based on electrophysiological thresholds, in 204 electrically evoked compound action potential (eCAP) input/output functions recorded from 33 ears of 26 guinea pigs. The stimulation level difference between auditory nerve eCAP threshold and FNS threshold was expressed as the eCAP-to-FNS offset. Coincidental FNS occurred in all animals and in 45% of all recordings. A change from monopolar to focused (bipolar, tripolar) configurations minimized FNS. The Euclidean distance between the CI contacts and the facial nerve explained no more than 33% of the variance in FNS thresholds. For both the FNS threshold and the eCAP-to-FNS offset, the change from cathodic to anodic pulse polarity significantly reduced FNS and permitted a gain of 14-71% of the dynamic range of the eCAP response. This "anodic rescue effect" was stronger for pseudo-monophasic pulses as compared to the symmetric biphasic pulse shape. These results provide possible mechanisms underlying recent clinical interventions to alleviate FNS. The "anodic-rescue effect" may offer a non-invasive therapeutic option for FNS in human CI users that should be tested clinically, preferably in combination with current-focusing methods.

The Effect of Pulse Shape and Interphase Gaps on Speech Perception and Perceived Sound Quality in Electrical Hearing

BACKGROUND

Stimulation with triphasic pulses has been shown to reduce the occurrence of unwanted facial nerve stimulation (FNS) with cochlear implants (CIs). However, there is little data available on how different pulse shapes affect the hearing outcome with electrical hearing in general. The aim of the study was to evaluate the effects of different stimulation pulse shapes on speech perception in noise, as well as loudness perception and subjective sound quality.

METHODS

Twenty experienced cochlear-implant users not suffering from FNS participated in a prospective single-visit study. Based on the subjects' current clinical fitting, six fitting maps with different pulse shapes (biphasic and triphasic) and different interphase gap (IPG) durations (2.1 µs, 10 µs, and 20 µs) were created. First, the loudness was balanced for each configuration by adjusting the stimulation charge amount. Then, speech perception in noise was measured with a German matrix sentence test (Oldenburg Sentence test). The perception of particular sound attributes of speech and music, as well as overall preference, was evaluated with visual analog scales.

RESULTS

Similar levels of speech perception were obtained with triphasic stimulation ( P = 0.891) and longer IPGs ( P = 0.361) compared to the subjects' clinical map settings. The stimulation amplitudes for equal loudness were significantly higher with triphasic stimulation compared to biphasic stimulation when keeping the IPG constant. Increasing the IPG had a significantly larger effect on perceived loudness ( P < 0.0001) and charge reduction for equal loudness with triphasic pulses compared to biphasic pulses. Triphasic configuration showed lower overall subjective sound quality ratings than biphasic for speech intelligibility, clarity, naturalness, and overall preference, as well as for music naturalness, and overall preference in the acute setting without adaptation time. Post-hoc pairwise comparisons against the clinical map revealed significantly lower speech naturalness ratings for triphasic with 2.1 µs IPG and for triphasic with 20 µs IPG only.

CONCLUSION

Although some sound quality attributes were rated lower compared to the clinical map in the acute test setting, stimulation with triphasic pulses does not affect speech perception in noise and can be considered as a valuable option in CI fitting.

Comparison of response properties of the electrically stimulated auditory nerve reported in human listeners and in animal models

Cochlear implants (CIs) provide acoustic information to implanted patients by electrically stimulating nearby auditory nerve fibers (ANFs) which then transmit the information to higher-level neural structures for further processing and interpretation. Computational models that simulate ANF responses to CI stimuli enable the exploration of the mechanisms underlying CI performance beyond the capacity of in vivo experimentation alone. However, all ANF models developed to date utilize to some extent anatomical/morphometric data, biophysical properties and/or physiological data measured in non-human animal models. This review compares response properties of the electrically stimulated auditory nerve (AN) in human listeners and different mammalian models. Properties of AN responses to single pulse stimulation, paired-pulse stimulation, and pulse-train stimulation are presented. While some AN response properties are similar between human listeners and animal models (e.g., increased AN sensitivity to single pulse stimuli with long interphase gaps), there are some significant differences. For example, the AN of most animal models is typically more sensitive to cathodic stimulation while the AN of human listeners is generally more sensitive to anodic stimulation. Additionally, there are substantial differences in the speed of recovery from neural adaptation between animal models and human listeners. Therefore, results from animal models cannot be simply translated to human listeners. Recognizing the differences in responses of the AN to electrical stimulation between humans and other mammals is an important step for creating ANF models that are more applicable to various human CI patient populations.

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.

Electrostatic Discharge (ESD) as a Cause of Facial Nerve Stimulation After Cochlear Implantation: A Case Report

OBJECTIVE

To discuss persistent facial nerve stimulation (FNS) related to repeated electrostatic discharge (ESD) shock following cochlear implantation.

METHODS

Single case report with literature review.

RESULTS

FNS is a feared complication after cochlear implantation, occurring in approximately 7% of cases, with most patients having anatomic abnormalities. The presented case has no anatomical abnormalities but reported frequent environmental static shock. FNS during the first 1 to 3 seconds of processor attachment caused a significant decrease in the patient's quality of life, requiring subsequent re-implantation with full resolution.

CONCLUSIONS

FNS is a complication of cochlear implantation that can cause a great deal of distress and discomfort. Frequent electrostatic discharge (ESD) contributed to device malfunctioning and FNS in a patient with otherwise normal anatomy and should be avoided if possible.

Signal processing & audio processors

Signal processing algorithms are the hidden components in the audio processor that converts the received acoustic signal into electrical impulses while maintaining as much relevant information as possible. Signal processing algorithms should be smart enough to mimic the functionality of external, middle and the inner-ear to provide the cochlear implant (CI) user with a hearing experience as natural as possible. Modern sound processing strategies are based on the continuous interleaved sampling (CIS) strategy proposed by B. Wilson in 1991, which provided envelope information over several intracochlear electrodes. The CIS strategy brought significant gains in speech perception. Translational research activities of MED-EL resulted in further improvements in speech understanding in noisy environments as well as enjoyment of music by not only coding CIS-based envelope information, but by also representing temporal fine structure information in the stimulation patterns of the apical channels. Further developments include "complete cochlear coverage" made possible by deep insertion of the intracochlear electrode, elaborate front end processing, anatomy based fitting (ABF), triphasic pulse stimulation instrumental in the suppression of facial nerve stimulation, and bimodal delay compensation allowing unilateral CI users to experience hearing with hearing aids on the contralateral ear. The large number of hardware developments might be exemplified by the RONDO, the world's first single unit audio processor in 2013. This article covers the milestones of translational research around the signal processing and audio processor topic that took place in association with MED-EL.

Evaluation of computed tomography parameters in patients with facial nerve stimulation post-cochlear implantation

PURPOSE

To compare the preoperative computed tomography (CT) parameters, including the thickness and density of the bone separating the upper basal turn of the cochlea (UBTC) and the labyrinthine segment of the facial nerve (LSFN), in patients with and without facial nerve stimulation (FNS) in post-cochlear implants (CI).

METHODS

A retrospective case review of 1700 CI recipients in a tertiary referral center between January 2010 and January 2020 was performed; out of the 35 recipients who were found to have FNS, 29 were included in the study. The control group comprised the same number of randomly selected patients. CT parameters of the patients were measured independently by three fellowship-trained neuro-otologists blinded to the postoperative status of the patients. Thickness in axial and coronal views and density of the bone separating the UBTC and the LSFN were measured.

RESULT

There was satisfactory agreement between the readings of the three reviewers. The distances (in mm) between the UBTC and LSFN obtained from the coronal (0.43 ± 0.24 vs. 0.63 ± 0.2) and axial (0.42 ± 0.25 vs. 0.6 ± 0.18) views were statistically lower in the FNS group (p = 0.001 and 0.005, respectively). The density (in HU) of the bony partition was also statistically lower in the FNS group (1038 ± 821 vs. 1409 ± 519; p = 0.029).

CONCLUSION

Patients who experienced FNS postoperatively had significantly lower distance and bone density between the UBTC and the LSFN. This finding can help surgeons in preoperative planning in an attempt to decrease the occurrence of FNS.

Performance of cochlear implant recipients fitted with triphasic pulse patterns

PURPOSE

To assess the effect of triphasic pulse pattern stimulation strategy on the audiological performance of cochlear implant recipients with unintended facial nerve stimulation (FNS), and to compare the audiological and speech outcomes before and after switching to triphasic stimulation.

METHODS

A retrospective study of patients who have changed their fitting maps from biphasic to triphasic pulse pattern stimulation because of FNS after cochlear implantation (CI). All identified patients with FNS after CI from 2017 to 2019 were included in this study. The medical records of 11 patients (16 ears) were queried for demographic and radiological data, pure tone audiometry, speech reception thresholds, speech discrimination score at 65 dB, maximum comfortable levels, thresholds, and dynamic range. Then, these parameters were compared in the two conditions, biphasic and triphasic.

RESULTS

Using triphasic pulse stimulation only or combined with switch-off of few channels, complete resolution of FNS was achieved. Triphasic pulse pattern stimulation was associated with better speech discrimination scores (75.25 ± 26.13%) compared to the biphasic pulse (58.25 ± 26.13%). This triphasic strategy also showed higher maximum comfortable levels (36.62 ± 1.63 qu) than biphasic strategy (31.58 ± 2.5 qu). Moreover, the dynamic range was wider using triphasic pulse strategy. In general, the triphasic pulse pattern resulted in successful suppression of facial nerve stimulation with suitable maximum comfortable levels and better speech discrimination.

CONCLUSION

Triphasic pulse pattern stimulation is an appropriate tool in controlling FNS following cochlear implantation with wider dynamic range. We recommend that all patients with facial nerve stimulation after CI surgery be switched to a triphasic pulse program prior to considering further surgery.

Investigating Facial Nerve Stimulation After Cochlear Implantation in Adult and Pediatric Recipients

OBJECTIVES/HYPOTHESIS

Facial nerve stimulation (FNS) can occur after cochlear implantation for a small number of recipients. This study aimed to investigate if a correlation exists between the variables involved in FNS.

STUDY DESIGN

Retrospective cohort review.

METHODS

There were 32 out of 1,100 cochlear implant recipients who experienced FNS in our clinic between 2010 and 2019. The following variables were recorded from a retrospective chart review: grade of FNS, onset of FNS, the number of channels stimulating FNS, and radiological findings of abnormalities in the inner ear. Statistical analyses were performed to identify a correlation between any of the variables involved. The techniques used to reduce FNS were analyzed.

RESULTS

Eleven adult ears had progressive hearing loss, three had idiopathic sudden sensorineural hearing loss (SNHL), and one congenital SNHL. All pediatric ears were diagnosed with congenital SNHL, except for one ear with idiopathic sudden SNHL. The grade of FNS ranged from mild stimulation or slight motion in the eye, mouth, nasolabial, or forehead regions (n = 8) to total severe stimulation of the facial musculature and/or severe pain (n = 3). The onset of FNS occurred immediately after activation for nine ears, and up to 16 months later for the other subjects. A significant correlation was observed between the number of channels stimulating FNS, the grade of FNS, and the radiological findings of the inner ear. FNS was completely resolved for 30 ears and partially resolved for two ears.

CONCLUSIONS

FNS can occur any time after cochlear implantation and can affect both adult and pediatric. However, it can be effectively resolved using specific fitting techniques.

LEVEL OF EVIDENCE

2c Laryngoscope, 131:374-379, 2021.