Response Changes During Insertion of a Cochlear Implant Using Extracochlear Electrocochleography

Abstract

Objectives: Electrocochleography is increasingly being utilized as an intraoperative monitor of cochlear function during cochlear implantation (CI). Intracochlear recordings from the advancing electrode can be obtained through the device by on-board capabilities. However, such recordings may not be ideal as a monitor because the recording electrode moves in relation to the neural and hair cell generators producing the responses. The purposes of this study were to compare two extracochlear recording locations in terms of signal strength and feasibility as intraoperative monitoring sites and to characterize changes in cochlear physiology during CI insertion.

Design: In 83 human subjects, responses to 90 dB nHL tone bursts were recorded both at the round window (RW) and then at an extracochlear position-either adjacent to the stapes or on the promontory just superior to the RW. Recording from the fixed, extracochlear position continued during insertion of the CI in 63 cases.

Results: Before CI insertion, responses to low-frequency tones at the RW were roughly 6 dB larger than when recording at either extracochlear site, but the two extracochlear sites did not differ from one another. During CI insertion, response losses from the promontory or adjacent to the stapes stayed within 5 dB in ≈61% (38/63) of cases, presumably indicating atraumatic insertions. Among responses which dropped more than 5 dB at any time during CI insertion, 12 subjects showed no response recovery, while in 13, the drop was followed by partial or complete response recovery by the end of CI insertion. In cases with recovery, the drop in response occurred relatively early (<15 mm insertion) compared to those where there was no recovery. Changes in response phase during the insertion occurred in some cases; these may indicate a change in the distributions of generators contributing to the response.

Conclusions: Monitoring the electrocochleography during CI insertion from an extracochlear site reveals insertions that are potentially atraumatic, show interaction with cochlear structures followed by response recovery, or show interactions such that response losses persist to the end of recording.

Figure 1

Surgical anatomy and extracochlear recordings sites. (A) Mastoidectomy cavity reveals the attic (a), incus buttress (ib), and facial recess (fr). (B) Recording sites for extracochlear recordings are adjacent the stapes (S) and on the promontory (P) just superior and anterior to the round window (RW). (C) Stapes electrode threads through the attic and under the incus buttress and is held in place with a piece of bone wax (bw). (D) Promontory electrode extends from a custom rigid electrode mount (arrows) which is fixed to the retractors.

Figure 2

Distribution of Total Response (ECochG-TR, see methods) from round Window electrocochleography just prior to insertion. The distribution of TRs across subjects in this study (red) was significantly higher than the magnitude of all subjects in our database (blue). Noise at the extracochlear recording site precluding inclusion was more likely in subjects with smaller RW responses (beige) but there was no consistent cutoff.

Figure 3

Magnitude of Extracochlear Responses compared to Round Window responses. Recordings at the RW could be of similar morphology to those at the extracochlear site (A), smaller than extracochlear responses (B), or larger than the extracochlear site (C). The smallest response measured was just above the noise floor (D). Across all subjects, extracochlear magnitudes were typically smaller than those at the RW but changes included increases as well as decreases (E). Moving to an extracochlear site was roughly 7 dB smaller than those at the RW but did not differ between stapes and promontory locations. Additionally, these changes in response were not explained by opening the RW (red).

Figure 4

Examples of response waveforms obtained during CI insertion. Three patterns emerged were seen (A) steady responses throughout the insertion, (B) a drop in response which persisted to the end of insertion, and (C) responses which dropped mid-insertion but recovered at deeper insertion depths.

Figure 5

Incremental response drops from one recording to the next, cumulative across all subjects and depths. This plot was used to judge test/retest reliability and estimate when a response was rare enough that it was likely not due to simple variability in measurement. The dotted line at 5 dB is on the knee of the distribution and encompasses 83% of all increments, so a criterion of 5 dB was used to indicate a significant change response.

Figure 6

Phenotypes of response changes across all subjects. (A) In the majority of subjects, response magnitude changed by < 5 dB throughout insertion depth, and were in the “No Change” group. (B) In 12 subjects, the response magnitude dropped below 5 dB and did not recover, and were assigned to the “Permanent Change” group. (C) In the “Reversible Change” group, 13 subjects showed a response change below 5 dB which at least partially recovered by the end of insertion. The case at the arrow dropped by more than 5 dB at 12 mm insertion and by a total of 25 dB overall, but was nearly fully recovered by the end of insertion.

Figure 7

Insertion depth when response dropped by 5 dB. In the Reversible Change group, initial response drops usually occurred within the first 15 mm. In the Permanent Change group, primary response drops occurred at deeper insertion depths. *The difference in the distributions was significant (t-test, t=4.7, df=161, p<0.001).

Figure 8

Inversion of response phase throughout CI insertion. (A) Left column represents waveform response while the right column represents the average cycle of the ongoing segment throughout 5 stages of CI insertion. An inversion of phase (arrow) during the latter stage occurred despite no change in response magnitude. (B) Phase change across all subjects. The majority of phase changes were small, with a median change of 0 degrees, but there was complete distribution across the full range.