Angular Electrode Insertion Depth and Speech Perception in Adults With a Cochlear Implant: A Systematic Review

Abstract

Objective: By discussing the design, findings, strengths, and weaknesses of available studies investigating the influence of angular insertion depth on speech perception, we intend to summarize the current status of evidence; and using evidence based conclusions, possibly contribute to the determination of the optimal cochlear implant (CI) electrode position.

Data sources: Our search strategy yielded 10,877 papers. PubMed, Ovid EMBASE, Web of Science, and the Cochrane Library were searched up to June 1, 2018. Both keywords and free-text terms, related to patient population, predictive factor, and outcome measurements were used. There were no restrictions in languages or year of publication.

Study selection: Seven articles were included in this systematic review. Articles eligible for inclusion: (a) investigated cochlear implantation of any CI system in adults with post-lingual onset of deafness and normal cochlear anatomy; (b) investigated the relationship between angular insertion depth and speech perception; (c) measured angular insertion depth on imaging; and (d) measured speech perception at, or beyond 1-year post-activation.

Data extraction and synthesis: In included studies; quality was judged low-to-moderate and risk of bias, evaluated using a Quality-in-Prognostic-Studies-tool (QUIPS), was high. Included studies were too heterogeneous to perform meta-analyses, therefore, effect estimates of the individual studies are presented. Six out of seven included studies found no effect of angular insertion depth on speech perception.

Conclusion: All included studies are characterized by methodological flaws, and therefore, evidence-based conclusions regarding the influence of angular insertion depth cannot be drawn to date.

FIG. 1

Method for angular insertion depth measurement on Computed Tomography (CT)-scan of an implanted electrode array with 16 electrode contacts. With in a three-dimensional cylindrical coordinate system all spatial information of the cochlea and an implant is measurable. By consensus this cochlear framework is defined by a plane of rotation through the basal turn of the cochlea and a z-axis through the modiolus. This can be applied on CT of the temporal bone by making a multiplanar reconstruction along the basal turn of the cochlea (A–C), and placing the z-axis through the center of the cochlea; the modiolus (M). A, An angular measurement of the insertion depth can then be made by indicating the center of the round window (RW) and the tip of the electrode array (dark grey circle). B, A 0 degree reference line between the modiolus (M) and the middle of the round window (RW), and a perpendicular line from the modiolus on the 0 degree reference line is drawn (cross). C, An angle is drawn (in white) from the modiolus over the 0 degree reference line, and through the most apical point the tip of the electrode array (dark grey circle). In this example the angular insertion depth of the most apical electrode contact is 368.3 degrees; the sum of four quadrants equal to 360 degrees plus the measured white angle equal to 8.3 degrees.

FIG. 2

Flow diagram of the study selection.

FIG. 3

Risk of bias graph: review authors’ judgements about each risk of bias item presented as percentages across all included studies.

FIG. 4

Risk of bias summary: review authors’ judgements about each risk of bias item for each included study.