Effect of Cochlear Implant Electrode Insertion Depth on Speech Perception Outcomes: A Systematic Review
Authors: Breitsprecher Tabita M. , Baumgartner Wolf-Dieter , Brown Kevin , Dazert Stefan , Doyle Una , Dhanasingh Anandhan , Großmann Wilma , Hagen Rudolf , Van de Heyning Paul , Mlynski Robert , Neudert Marcus , Rajan Gunesh , Rak Kristen , Van Rompaey Vincent , Schmutzhard Joachim , Volkenstein Stefan , Völter Christiane , Wimmer Wilhelm , Zernotti Mario , Weiss Nora M. 
Published: December 2023

Objective:
The suitable electrode array choice is broadly discussed in cochlear implantation surgery. Whether to use a shorter electrode length under the aim of structure preservation versus choosing a longer array to achieve a greater cochlear coverage is a matter of debate. The aim of this review is to identify the impact of the insertion depth of a cochlear implant (CI) electrode array on CI users’ speech perception outcomes.

Databases Reviewed:
PubMed was searched for English-language articles that were published in a peer-reviewed journal from 1997 to 2022.

Methods:
A systematic electronic search of the literature was carried out using PubMed to find relevant literature on the impact of insertion depth on speech perception. The review was conducted according to the preferred reporting items for systematic reviews and meta-analyses guidelines of reporting. Studies in both, children and adults with pre- or postlingual hearing loss, implanted with a CI were included in this study. Articles written in languages other than English, literature reviews, meta-analyses, animal studies, histopathological studies, or studies pertaining exclusively to imaging modalities without reporting correlations between insertion depth and speech outcomes were excluded. The risk of bias was determined using the “Risk of Bias in Nonrandomized Studies of Interventions” tool. Articles were extracted by 2 authors independently using predefined search terms. The titles and abstracts were screened manually to identify studies that potentially meet the inclusion criteria. The extracted information included: the study population, type of hearing loss, outcomes reported, devices used, speech perception outcomes, insertion depth (linear insertion depth and/or the angular insertion depth), and correlation between insertion depth and the speech perception outcomes.

Results:
A total of 215 relevant studies were assessed for eligibility. Twenty-three studies met the inclusion criteria and were analyzed further. Seven studies found no significant correlation between insertion depth and speech perception outcomes. Fifteen found either a significant positive correlation or a positive effect between insertion depth and speech perception. Only 1 study found a significant negative correlation between insertion depth and speech perception outcomes.

Conclusion:
Although most studies reported a positive effect of insertion depth on speech perception outcomes, one-third of the identified studies reported no correlation. Thus, the insertion depth must be considered as a contributing factor to speech perception rather than as a major decisive criterion.

Registration:
This review has been registered in PROSPERO, the international prospective register of systematic reviews (CRD42021257547), available at https://www.crd.york.ac.uk/PROSPERO/.