New-Generation Cochlear and Auditory Brainstem Implants

Educational Objectives

The goal of this program is to improve use of new-generation cochlear and auditory brainstem implants (ABI). After hearing and assimilating this program, the clinician will be better able to:

1. Explain placement of ABIs.
2. List Food and Drug Administration criteria for ABIs.

Summary

Cochlear implant: electrode array is designed to approximate the tonotopic axis of the cochlea

Auditory brainstem implants (ABI): a surface array with multiple contacts placed on or near the auditory brainstem, specifically at the cochlear nucleus, which contains second order auditory neurons to provide meaningful sound awareness, and perception to the user; it resembles a cochlear device; the difference is in the end effect or the paddle array, which is a surface form factor with exposed electrical contacts; ABI was approved by the Food and Drug Administration (FDA) in 2000 for patients with neurofibromatosis type 2 (NF2)

Placement of ABI: accurate placement is challenging, and requires the use of anatomic landmarks to indirectly guide the surgical approach, eg, entry zone of the glossopharyngeal nerve, choroid plexus, and posterior fossa; ABIs are placed through a posterior fossa craniotomy approach; surgeons rely on electrophysiological measures for guidance, eg, electrical auditory brainstem response; in contrast, the placement of cochlear implants are guided by vision

FDA criteria for ABI: includes patients >12 yr of age with NF2; ABIs may be placed during the removal of the first or second tumor, or to provide hearing in a patient without addressing any tumor burden

Drawbacks: lack of meaningful speech perception abilities; electrical current spread because of overlapping electrical fields results in activation of nonauditory axonal passages, eg, facial, trigeminal, and vestibular nerves; there are fewer auditory channels that can be utilized to drive performance because many channels must be turned off

Spectral resolution: the perceptual ability of a listener to distinguish sounds of different pitches; the human inner ear leverages thousands of channels of auditory information through the hair cell system to convey spectral cues; modern prostheses have ≤22 channels; sound, speech awareness, and perception is limited because of the mismatch of auditory channels between number of those normally used and number prosthesis can provide; possible improvements to the electrode array's form factor may include redesigning it so it is placed closer to the neurons

Infrared neurostimulation: the direct application of a near infrared radiant energy pulse delivered through optical fiber into the cochlea to create sound perception in animal models

Optogenetics: involves the genetic modification of neurons to express proteins that are light sensitive; optical stimulation may improve spatial specificity; using optogenetics, one may accurately switch off and on neurons using different wavelengths of light with millisecond precision

Evidence: in a mouse model, a viral vector was employed to deliver genes that would express options on the surface of the auditory brainstem neuronal population; a collimator to reduce a laser beam to 40 or 50 μm in diameter; one can create 7 separate channels in the same surface area as one channel in a clinical device; the spread of excitation in the auditory system was evaluated by measuring responses in the downstream nucleus called the inferior colliculus (IC)

Limitations: gene therapy is required to photosensitize the neurons prior to delivery of laser light; this process is refractory to high stimulation rates; these devices require large amounts of power; hence, there is a need to create smaller batteries to make it more palatable to the user

Magnetic stimulation: a small copper microcoil is used to create an induced electrical field, and induced current to drive the activity of a smaller population of neurons within the region of the microcoil itself; theoretically, magnetic microcoil-based auditory prostheses enhance auditory resolution, improve auditory outcomes, and reduce associated adverse effects

Evidence: Dr. Fried (Lee et al [2016]) developed simple bent microwires that generate electrical fields that can exceed the threshold of cortical neurons; novel computational models were used to assay the efficacy of responses to microcoil stimulation in comparison with traditional approaches; time varying magnetic fields induced spatially confined electrical fields superior to electrical stimulation; a follow up study in 2020 showed selective neuronal stimulation in in-vivo models, eg, visual cortex of a mouse; microcoils were able to generate an electrical field ≤300 μm in diameter, whereas electrical stimulation produced fields ≤2 mm in diameter; there is also reduced foreign body response to microstimulation with magnetic microcoils in comparison with electrical stimulation

Limitations: device failures produce scarring around electrode array; scarring around electrode contacts increases impedance characteristics and power demands, and reduces spectral resolution

Lee et al (2022): another study conducted by the speaker and Dr. Fried's group employed a mouse model to demonstrate magnetic microcoils-enhanced spectral resolution; the speaker continues this work in a guinea pig model; focal activation to microcoil stimulation at the round window or at 1 to 2 mm depth of insertion was noted

Readings

Littlefield PD, Richter CP. Near-infrared stimulation of the auditory nerve: A decade of progress toward an optical cochlear implant. Laryngoscope Investig Otolaryngol. 2021 Mar 12;6(2):310-319. doi: 10.1002/lio2.541; Lee SW, Fallegger F, Casse BD, et al. Implantable microcoils for intracortical magnetic stimulation. Sci Adv. 2016 Dec 9;2(12):e1600889. doi: 10.1126/sciadv.1600889; Lee JI, Seist R, McInturff S, et al. Magnetic stimulation allows focal activation of the mouse cochlea. Elife. 2022 May 24;11:e76682. doi: 10.7554/eLife.76682; Ryu SB, Paulk AC, Yang JC, et al. Spatially confined responses of mouse visual cortex to intracortical magnetic stimulation from micro-coils. J Neural Eng. 2020 Oct 23;17(5):056036. doi: 10.1088/1741-2552/abbd22.

Disclosures

For this program, the following relevant financial relationships were disclosed and mitigated to ensure that no commercial bias has been inserted into this content: Dr. Lee is a Consultant for 3NT Medical, Frequency Therapeutics, and Skylark Bio. Members of the planning committee reported nothing relevant to disclose.

Acknowledgements

Dr. Lee was recorded at Pacific Rim Otolaryngology-Head and Neck Surgery Update, held February 18-21, 2023, on Honolulu, HI, and presented by the University of California, San Francisco School of Medicine. For information about upcoming CME activities from this presenter, please visit PacOto.ucsf.edu. Audio Digest thanks the speakers and the University of California, San Francisco School of Medicine for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 0.75 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 0.75 CE contact hours.

Lecture ID:

OT561601

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.