Optimization and Validation of Electric Acoustic Stimulation (EAS) Mapping as a Function of Residual Hearing After CI-Implantation

Optimization and Validation of Electric Acoustic Stimulation (EAS) Mapping as a Function of Residual Hearing After CI-Implantation

The aim of this clinical study is to investigate how the fitting of an EAS (electrical acoustic stimulation) based on electrocochleography (ECochG) thresholds compares to fitting based on pure tone audiogram thresholds in terms of speech intelligibility in quiet and noise. The personal feedback from the study participants regarding the question of better fitting and the resulting sound quality should also be included in the analysis.

As the indications for Cochlear Implant (CI) Surgery expand to also include hearing impaired patients with some residual hearing in the ipsilateral (to be implanted) ear, residual hearing preservation during the surgical intervention becomes even more important. It is also known from the literature that patients will benefit from having good residual acoustic hearing after CI-Implantation in terms of the sound quality. This is particularly so for speech comprehension in noise as well as for music perception with the CI.

Present-day clinical Sound Processor programming software provides Electric Acoustic Stimulation (EAS) to account for preserved residual hearing. For an optimized EAS fitting, it is postulated here that optimal conditions are achieved when the electric and acoustic stimulation do not overlap. To achieve this, the acoustic stimulation ought to be limited to only regions where there is still useful acoustic hearing, while the electric stimulation should also be restricted to regions where there is no longer significant useful acoustic hearing. In the Maestro clinical software, the threshold for significant useful acoustic hearing is set to 65 dB Hearing Level (HL). This then determines the cut-off frequency between the acoustic and electric stimulation regions.

The standard clinical Maestro software computes this cut-off based on a regular pure tone audiogram (PTA) which has been manually stored in the software. However, it is arguable that this does not always represent an accurate cut-off frequency required for optimal fitting because the audiogram thresholds themselves are subjective in nature and can often vary in a test-retest situation, depending on the attention and reaction times of the patient. It has been reported that the unaided thresholds do not play a significant role in the EAS fitting. In this respect, it may be more accurate and beneficial to use objective measures such as electrocochleography (ECochG) to determine the thresholds of the residual hearing.

The current family of Med-El Cochlear Implants are equipped with the capability to perform ECochG measurements using the implant's own intracochlear electrodes within the scala tympani. The measurements can also be conducted intraoperatively or postoperatively. This intracochlear ECochG feature of the implants therefore allows us to objectively determine the residual hearing thresholds.

The aim of this study is to discuss the advantages of objective fitting of the EAS compared to subjective fitting using a PTA with respect to speech intelligibility in quiet and in noise. The personal feedback from the study participants regarding the question of better fitting and the resulting sound quality should also be included in the analysis.

From the existing literature, it is not expected that there should be any significant gender effects. This also does not belong to the primary aims of this study and we will not be investigating this aspect.

Inclusion Criteria:

1. Patients aged between 18-85 years.
2. Cochlear Implant (CI) candidates with measurable post-operative low-frequency residual hearing (125Hz <65dB, 250Hz <65dB, 500Hz <65dB).
3. Only patients with MED-EL EAS.
4. Written informed consent must be obtained prior to study start.

Exclusion Criteria:

1. Patients with cognitive impairments who, as a result, lack the capacity to make decisions.
2. Uncooperative patients regarding pure tone audiometry thresholds measurements.
3. Morbus Menière or intracochlear/ vestibular schwannoma