Role of electrode placement as a contributor to variability in cochlear implant outcomes

Abstract

Hypothesis: Suboptimal cochlear implant (CI) electrode array placement may reduce presentation of coded information to the central nervous system and, consequently, limit speech recognition.

Background: Generally, mean speech reception scores for CI recipients are similar across different CI systems, yet large outcome variation is observed among recipients implanted with the same device. These observations suggest significant recipient-dependent factors influence speech reception performance. This study examines electrode array insertion depth and scalar placement as recipient-dependent factors affecting outcome.

Methods: Scalar location and depth of insertion of intracochlear electrodes were measured in 14 patients implanted with Advanced Bionics electrode arrays and whose word recognition scores varied broadly. Electrode position was measured using computed tomographic images of the cochlea and correlated with stable monosyllabic word recognition scores.

Results: Electrode placement, primarily in terms of depth of insertion and scala tympani versus scala vestibuli location, varies widely across subjects. Lower outcome scores are associated with greater insertion depth and greater number of contacts being located in scala vestibuli. Three patterns of scalar placement are observed suggesting variability in insertion dynamics arising from surgical technique.

Conclusion: A significant portion of variability in word recognition scores across a broad range of performance levels of CI subjects is explained by variability in scalar location and insertion depth of the electrode array. We suggest that this variability in electrode placement can be reduced and average speech reception improved by better selection of cochleostomy sites, revised insertion approaches, and control of insertion depth during surgical placement of the array.

FIG. 1

Rank order presentation of post activation CNC word recognition scores for a selection of Nucleus and Advanced Bionics adult CI recipients at WU School of Medicine. Scores are interpolated to 6 months post activation based on longitudinally collected data, unless otherwise noted. Black bars indicate scores for the subset of individuals with the AB device who participated in this study. These subjects are numbered (S#) in descending rank order of each individual's CNC score at entry into study. At entry into the study all subjects had stable CNC performance and at least 4 months of experience post activation.

FIG. 2

CT-based views of the boundary between the scalar fluid space and bony wall in each CI subject as seen from the apex of the cochlea along the midmodiolar axis. Electrode array styles are HiFocus® I (S2, S5, S10, and S11), HiFocus® Ij (S1, S3, S6-S9, S12-S14), and HiFocus® Helix (S4). The position and insertion depth of electrode contacts are shown relative to the bony wall. White contacts (○) are in SV, and black contacts (•) are in ST. Boxed contacts (□) are non stimulating marker contacts. The apical-most contact is marked as E1. Subjects S2, S10 and S11 have electrode positioners in place. The radial white line is the 0° reference from which angular insertion depth is measured and extends from the midmodiolar axis through the beginning of the cochlear canal.

FIG. 3

CT-based views similar to Fig. 2 showing electrode position and insertion depth in recipients of Nucleus and MedEl cochlear implant systems. These recipients are not participants in the present study but are included with those in Fig. 2 to illustrate that variability of electrode placement is not unique to a single electrode design and is possible with any manufacturer's devices. Displayed arrays are Nucleus Contour (a-c), Nucleus Contour Soft-Tip (d-e), and MedEl Combi 40+ (f).

FIG. 4

Angular insertion depths, scalar locations and insertion patterns of individual electrode contacts for subjects ranked in order of CNC word recognition scores. Each line of 16 symbols is located along the ordinate to indicate the subject's CNC Word Recognition score and mark the angular insertion depth of individual electrode contacts (E16 to E1, left-to-right). Open diamonds (◇) indicate contacts located outside of the cochlea (e.g. basal-most contacts for S5). Black circles (•) indicate electrode contacts located in ST, whereas white squares (□) indicate contacts located in SV. The vertical line at 34° insertion depth indicates the expected position of E16 for an array insertion with which the marker contact is positioned at a RW cochleostomy site. Columns to the right indicate presence of an electrode array positioner (POS), subject designation, age at time of the study, and four metrics, Insertion Pattern(A,B or M), Scalar Position (1 = All contacts in ST; 1.5 = Contacts initially in ST followed by a section of the array mapping into SV space and returning to ST; 2.0 = Initially ST then entering SV for the remainder of the cochlea; 2.5 = contacts are initially in SV followed by contacts mapping into ST and returning to SV; and 3.0 = All contacts in SV), Total Number of Electrodes in SV, and the Occurrence of Apical Pitch Confusions [present (+); absent (−)], each described in the text. Zone B and Zone A represent basal and apical cochlear regions, respectively, in which electrode function may be altered due to deep insertions.