The hybrid or electric-acoustic stimulation
cochlear implant is indicated in patients with a residual hearing at low frequencies. It provides electric and acoustic stimulation for compensating for high- and low-frequency sounds, respectively. However, the implantation procedure damages the cochlea, resulting in loss of the residual-hearing and diminished effects of the acoustic-hearing in several patients. To prevent
hearing loss after implantation,
corticosteroids have been used clinically although their effects are limited. As an alternative to
corticosteroids,
insulin-like growth factor 1 (IGF1) has shown potent effects in various types of cochlear injury. In this study, the effects of IGF1 on hearing preservation were examined after
cochlear implantation to a normal-hearing guinea pig model. The
electrode was inserted in an atraumatic way through the round window membrane of guinea pigs with the application of a
gelatin-sponge soaked with IGF1 or saline. The auditory brainstem response (ABR) was recorded pre-operatively, immediately after
cochlear implantation, and 7, 14, 28, and 56 days after
electrode insertion. In comparison to the control group, the IGF1-treated group showed better hearing preservation at low frequencies, 7 days after surgery. IGF1 application was effective at low frequencies (2 and 4 kHz) throughout the period of examination. Histological studies revealed that outer hair cell numbers, in the IGF1-treated group, were maintained in the cochlear region responsible for low-frequency hearing (upper midbasal turn) and that there was less fibrous tissue formation around the
electrode. Both the outer hair cell counts and the extent of
fibrosis significantly correlated with the ABR threshold shifts at low frequencies. These results indicate that IGF1 might attenuate loss of low-frequency hearing after
cochlear implantation, suggesting its possible clinical use in soft surgeries involving
cochlear implants with electric-acoustic stimulation for hearing preservation.