Electrophysiological Properties of Cochlear Implantation in the Gerbil Using a Flexible Array

Abstract

Objectives Cochlear implants (CI) perform especially well if residual acoustic hearing is retained and combined with the CI in the same ear (also termed hybrid or electric-acoustic stimulation). However, in most CI patients, residual hearing is at least partially compromised during surgery, and in some it is lost completely. At present, clinicians have no feedback on the functional status of the cochlea during electrode insertion. Development of an intraoperative physiological recording algorithm during electrode insertion could serve to detect reversible cochlear trauma and optimal placement relative to surviving hair cells. In this report, an animal model was used to assist in determining physiological markers for these conditions using a flexible electrode similar to human surgery. Design The animal model was the normal-hearing gerbil. The flexible electrodes had 1 to 2 platinum-iridium contacts embedded in a 200 µm diameter silastic carrier. As control experiments some insertions were also made with much smaller (50 µm diameter) rigid electrodes. In either case, the electrode was positioned at or just inside the round window membrane and subsequently advanced into the scala tympani longitudinally in 50 to 100 µm increments. After each advancement, acoustic stimulation was used to elicit a cochlear microphonic (CM) and compound action potential (CAP). Stimuli were suprathreshold tone bursts of 1 to 16 kHz in octave steps with 2 msec rise and fall times and a 10 msec plateau. Anatomical integrity of the cochlea was subsequently assessed using a whole-mount preparation. Results In contrast with the CAP, which was relatively stable during insertion, the CM showed a variety of changes related to electrode movement. To tone bursts of 1 to 8 kHz the CM typically remained stable or increased during the insertion before contact with cochlear structures. After contact, the potentials often dropped dramatically. The CM to 16 kHz was the most variable; in some cases it increased but in other cases it decreased early in the insertion and later showed large and abrupt increases. In some instances, this pattern was seen to progressively lower frequencies as well. Histological analysis and the gerbil frequency map indicate that electrode travel was limited to the basal turn (~4 mm from the hook) and did not intrude into the characteristic frequency regions of most frequencies used. Conclusions First, the CM provides a more sensitive indication of cochlear trauma than does the CAP. Second, stable or steady increases in the CM are a physiological marker for unimpeded travel through the scala tympani as the electrode approaches responding hair cells. Third, abrupt reductions in the CM across frequency are a physiological marker of contact with cochlear structures. Fourth, abrupt increases after a decline, which occurred primarily to 16 kHz but to a lesser degree to other frequencies as well, are a physiological marker for a release from contact. The interpretation is that as the tip of the electrode bends the shaft can ...

Extra- and Intracochlear Electrocochleography in Cochlear Implant Recipients

“…Gross intracochlear trauma (e.g. rupture of the basilar membrane) should cause an immediate reduction or loss of the ECoG signal Ahmad et al, 2012;Campbell et al, 2010;Choudhury et al, 2011Choudhury et al, , 2014DeMason et al, 2012]. In our series, the largest decrease in the ECoG signal was 4.9 dB at 500 Hz.…”

“…We monitored the ongoing ECoG signal at suprathreshold intensities as a decrease in this signal has been the most sensitive marker for cochlear trauma in animal studies Ahmad et al, 2012;Campbell et al, 2010;Choudhury et al, 2011Choudhury et al, , 2014DeMason et al, 2012]. Gross intracochlear trauma (e.g.…”

Assessment of Cochlear Trauma During Cochlear Implantation Using Electrocochleography and Cone Beam Computed Tomography

“…Animal studies have demonstrated that a decrease of the ongoing ECoG at low frequencies is a reliable marker for intracochlear trauma during insertion of an electrode (19)(20)(21)(22)(23)(24)37). In cochlear implant recipients, findings thus far have suggested that a decrease of low-frequency ECoG signals at suprathreshold intensities during surgery results in hearing loss of >10 dB or complete hearing loss in a majority of patients (9,10).…”

“…Electrocochleography (ECoG) has shown great potential in this regard and has been conducted in animal studies (19)(20)(21)(22)(23)(24) as well as during and after cochlear implantation in humans (9)(10)(11)(25)(26)(27)(28)(29). To our knowledge, four reports have correlated intraoperative ECoG findings with postoperative hearing preservation rates (9)(10)(11)28).…”

Electrophysiologic Consequences of Flexible Electrode Insertions in Gerbils With Noise-Induced Hearing Loss

“…However, reversible damage was not observed. Insertions with a flexible electrode mimicking a clinic implant have been done in normal-hearing animals 17 . In that condition, the responses to different frequencies showed complex changes as the electrode longitudinally traversed the basal turn.…”