Intraoperative quantification of floating mass transducer coupling quality in active middle ear implants: a multicenter study

Abstract

Purpose Evaluating the effectiveness of intraoperative auditory brainstem responses (ABRs) to stimulation by the Vibrant Soundbridge (VSB) active middle ear implant for quantifying the implant's floating mass transducer (FMT) coupling quality. Methods In a diagnostic multicentric study, patients (> 18 years) who received a VSB with different coupling modalities were included. Pre-and postoperative bone conduction thresholds, intraoperative VSB-evoked ABR thresholds (VSB-ABR) using a modified audio processor programmed to preoperative bone conduction thresholds, postoperative vibrogram thresholds, and postoperative VSB-ABR thresholds were measured. Coupling quality was calculated from the difference between the pure tone average at 1000, 2000, and 4000 Hz (3PTA) vibrogram and postoperative 3PTA bone conduction thresholds. Results Twenty-three patients (13 males, 10 females, mean age 56.6 (± 12.5) years) were included in the study. Intraoperative VSB-ABR response thresholds could be obtained in all except one patient where the threshold was > 30 dB nHL. Postoperatively, an insufficient coupling of 36.7 dB was confirmed in this patient. In a Bland-Altman analysis of the intraoperative VSB-ABRs and coupling quality, the limits of agreement exceeded ± 10 dB, i.e., the maximum allowed difference considered as not clinically important but the variation was within the general precision of auditory brainstem responses to predict behavioral thresholds. Five outliers were identified. In two patients, the postoperative VSB-ABR thresholds were in agreement with the coupling quality, indicating a change of coupling before the postoperative testing. Conclusion The response thresholds recorded in this setup have the potential to predict the VSB coupling quality and optimize postoperative audiological results.

Active Middle Ear Implant Evoked Auditory Brainstem Response Intensity-Latency Characteristics

“…Saturation in VSB evoked ABR wave-V latencies most likely occurred to due limited dynamic range of the audio processor used for signal transmission. It was already reported in the study investigating the relation between L ABR and coupling efficiency that the audio processor output saturated between 35 and 40 dB nHL stimulation level depending on gain settings [see Figure 1 in ( 27 )]. Stimulation levels at threshold level L ABR were perceived by the patient as very soft sounds.…”

“…The total offset between VSB evoked wave-V latencies and normative data for acoustic stimulation was most likely caused by a signal processing delay in the audio processor. Reference data for acoustic click stimulation were used here due to technical measurements which showed that the CE-chirp used for stimulation emerged as a click-like stimulus after transmission through the audio processor and VSB ( 24 , 27 ).…”

“…VSB evoked ABRs were recorded intraoperatively after positioning of the FMT as described in Froehlich et al ( 27 ). Broadband CE-Chirps were generated by an Eclipse EP25 (Interacoustics A/S, Middelfart, Denmark) clinical auditory evoked potential stimulation and recording system.…”

“…Custom-made experimental set-ups were used in all studies for providing stimulus transmission by the FMT. While most studies only aimed on relative measurements, it has recently been reported that coupling efficiency could also be determined quantitatively by measuring VSB evoked ABRs ( 26 , 27 ). A modified AP404 audio processor (MED-EL, Innsbruck, Austria) programmed to preoperative BC thresholds and fitted with an insert earphone sound tube attached to the microphone aperture was used for providing stimulation by the implanted FMT.…”

“…Small differences indicate good coupling, i.e., efficient energy transmission to the inner ear without loss of energy. To determine the coupling efficiency objectively, auditory evoked potentials like auditory steady state responses (ASSRs) (19,20), compound action potentials (CAPs) (21)(22)(23), and auditory brainstem responses (ABRs) (24)(25)(26)(27) have been recorded in patients treated with a VSB. Custom-made experimental setups were used in all studies for providing stimulus transmission by the FMT.…”