A State-of-the-Art Method for Preserving Residual Hearing During Cochlear Implant Surgery

Summary

The preservation of cochlear structures and possible residual hearing is one important factor to consider during cochlear implant surgery. Here, we present a state-of-the-art method for preserving residual hearing during cochlear implant surgery under local anesthesia.

Abstract

The developments in surgical techniques and cochlear implant (CI) electrode design have expanded the indications for CI treatment. Currently, patients with high-frequency hearing loss may benefit from CIs when low-frequency residual hearing can be preserved, as this enables combined electric-acoustic stimulation (EAS). The possible benefits of EAS include, for example, improved sound quality, music perception, and speech intelligibility in noise.

The risks of inner ear trauma and a deterioration or even complete loss of residual hearing vary according to the surgical technique and the type of electrode array used. Short, lateral-wall electrodes with shallower angular insertion depths have demonstrated higher rates of hearing preservation than longer electrodes. The very slow insertion of the electrode array through the round window of the cochlea contributes to insertion atraumaticity and, thus, may lead to favorable hearing preservation results. However, residual hearing can be lost even after an atraumatic insertion.

Electrocochleography (ECochG) can be used to monitor inner ear hair cell function during the insertion of the electrode. Several investigators have demonstrated that the ECochG responses during surgery may predict postoperative hearing preservation results.

In a recent study, we correlated the patients’ subjective hearing perception with simultaneously recorded intracochlear ECochG responses during the insertion. This is the first report evaluating the association between intraoperative ECochG responses and hearing perception in a subject undergoing cochlear implantation under local anesthesia without sedation. The combination of intraoperative ECochG responses with the patient’s real-time feedback to sound stimuli has excellent sensitivity for the intraoperative monitoring of cochlear function. This paper presents a state-of-the-art method for the preservation of residual hearing during CI surgery. We describe this treatment procedure with the special consideration of performing the surgery under local anesthesia, which makes it feasible for monitoring the patient’s hearing during the insertion of the electrode array.

Introduction

Cochlear implantation is the only treatment in clinical use that restores the function of a sensory organ. Currently, cochlear implantation is applied to treat severe-to-profound sensorineural hearing loss in children and adults. The cochlear implant (CI) system consists of an implantable internal device that is combined with an external sound processor. The internal device is inserted during surgery through the mastoid cavity. The facial recess is opened to gain access to the middle ear and cochlea. The electrode array is inserted into the cochlea through the posterior tympanotomy and round window of the cochlea. The electrode array provides the electric stimulation by bypassing defective outer and inner hair cells and directly stimulating the auditory nerve's ganglion cells.

It has been demonstrated that the preservation of the integrity of the inner ear structures after CI surgery may contribute to more favorable hearing outcomes in comparison to electrode insertions, which can cause structural damage to the inner ear^1,2. This has led to the development of more delicate surgical techniques and thinner, more flexible, and thus, less traumatic electrode arrays. These developments have contributed to improved hearing outcomes, leading to an expansion of the indications for CI rehabilitation. Patients with high-frequency (>1.5 kHz) severe hearing loss may also benefit from cochlear implantation, especially when their natural hearing in the low frequencies can be preserved (<1 kHz). Low-frequency hearing after surgery makes it possible for the patient to benefit from electro-acoustic stimulation (EAS)^3,4. The EAS is a combination of electric stimulation to high-frequency areas of the cochlea, which are located in the basal parts of the cochlea, and acoustic amplification for the preserved low-frequency areas (typically 125-500 Hz) in the apical parts of the cochlea.

The best results in hearing preservation (HP) surgery have been achieved with shorter electrode arrays, specifically designed for EAS candidates^4,5,6. There may be one major drawback associated with the short electrode arrays-the loss of residual hearing (either after surgery or subsequently). The patient may ultimately benefit from a longer electrode array that covers a broader range of frequencies, especially in cases where there has been an unfortunate loss of residual hearing^5,7. Recently, the introduction of the partial insertion of the longer lateral-wall electrode in patients with moderate hearing loss at higher frequencies and normal or mild hearing loss at lower frequencies has created the option for initial shallow insertion in EAS candidates, but it also facilitates deeper insertion with the same array if the residual hearing is lost⁸.

Electrocochleography (ECochG) is a method that can be used to monitor cochlear function during cochlear implantation, and it has been increasingly exploited in clinical use. Currently, the three leading CI manufacturers provide a clinically approved tool, or at least a research tool, for intracochlear intraoperative ECochG measurements. One provides an all-in-one solution with the stimulation and response recording integrated into one tool, while the tools provided by the other manufacturers require a separate stimulator. ECochG measures the electrophysiological response from the inner ear and the auditory nerve to an acoustic stimulus. It seems that the amplitudes of the ECochG responses measured during the electrode insertion may predict postoperative hearing preservation. Thus, intraoperative ECochG seems to represent a promising tool to provide information that enables the prevention of trauma^9,10,11,12. Active research and the increased clinical interest in assessing intraoperative ECochG measurements have raised the question of how to analyze the ECochG responses objectively and how to react to the changes in the amplitude of the response during insertion.

The feasibility and benefits of CI surgery under local anesthesia have been reported in several studies^13,14,15,16. CI candidates with significant comorbidities can be operated on under local anesthesia, thus avoiding the risks related to general anesthesia. General anesthesia is associated with an increased risk of cognitive deterioration, especially in elderly patients, and it has also been associated with greater mortality in these groups¹⁷. When CI surgery is performed under local anesthesia, the recovery from the operation is faster, and, thus, the patient spends less time in the hospital as compared to with a similar operation performed under general anesthesia^14,15,18,19. It has also been proposed that patients with residual hearing could be able to monitor their hearing during the insertion, which may guide the surgeon during the insertion of the CI electrode and avoid intracochlear trauma¹⁴.

We recently compared ECochG and subjective sound perception as indicators of hearing preservation in CI surgery under local anesthesia²⁰. The results indicated a good prediction of hearing preservation with both methods, and similar patterns were observed between subjective reporting and ECochG monitoring. The patients' subjective feedback on sound perception, which was possible since only local anesthesia was in use, seemed to help minimize the risk of trauma to the inner ear during the insertion, even in cases when the ECochG responses could not be measured during the insertion.

Here, we present a state-of-the-art method for preserving residual hearing during CI surgery. We describe the treatment procedures, including the special considerations associated with performing the surgery under local anesthesia (e.g., ECochG) and subjective hearing monitoring.

Protocol

The protocol was approved by the Institutional Review Board (5551877) and the Research Ethics Committee of the Northern Savo Hospital District (1690/2019) and was carried out according to the guidelines of the Declaration of Helsinki. Informed consent was taken from all of the patients who volunteered for the study. 1. Preoperative considerations Assessment of the residual hearing threshold Ensure that the preoperative pure-tone threshold averages (PTA)…

Representative Results

Both subjective monitoring and ECochG may help prevent the occurrence of insertion trauma and, thus, provide better hearing preservation results postoperatively. In the audiogram, a decrease in the PTA(125-500 Hz) within 15 dB of the preoperative hearing levels is considered to represent preserved residual hearing and, thus, a positive result after surgery. A negative result is the loss of residual hearing: a PTA(125-500 Hz) change of over 30 dB from the p…

Discussion

The optimal utilization of cochlear implantation is important for the effective management of hearing loss. The expansion of the indications for CI rehabilitation has created a grey zone where individualized decisions must be made regarding the rehabilitation modalities used. Nowadays, for patients with high-frequency hearing loss, there is a trend toward the provision of CIs. However, residual hearing in the low frequencies cannot be preserved in every patient. Cochlear monitoring via ECochG has been proposed a…

Materials

EarPhone and sound tube	AB/Cochlear/Medel		Usually provided by the company in sterile packages, can be inserted in ear without sterility issues
Ecocgh program	AB/Cochlear/Medel		AB and Medel provides software for clinical use. Also Cochlear has software for ECochG, atleas for research purposes.
Equipments for Cochlear implantation			Basic setup and instrumentation for Cochlear implantation, not spesific to the ECoGh or Subjective hearing monitoring
Laptop/tablet	AB/Cochlear/Medel		AB has tablet consept as "AIM" for intraoperative ECochG measuring. It is provided by the company. Cochlear and Medelare operated with laptop
Sound Processor	AB/Cochlear/Medel		Also company-specific, need for the connection to the electrode and measuring during the insertion