Electrophysiological properties of cochlear implantation in the gerbil using a flexible array

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.

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.

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.

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 move in the mediolateral dimension, sometimes contacting the basilar membrane and sometimes not. Overall, the results indicate that recordings during cochlear implantations can provide valuable feedback to the surgeon regarding electrode position and the integrity of surviving hair cells.