Structure and hearing preservation in cochlear implant surgery

Abstract

Preserving cochlear structures during cochlear implantation is considered one of the most important steps in improving the hearing performance of CI patients. Minimizing trauma to the cochlea, i.e. preserving the residual hearing, allows for better speech perception performance by the added benefits of acoustical stimulation to the electrical hearing with CI, and probably overall better electrical hearing due to deaccelerated degeneration of the auditory nerve. The differences between PM and LW arrays regarding structure preservation are substantial, and should be carefully considered before commencing cochlear implantation, especially in patients with (substantial) residual hearing. STL and TF of the array can cause severe intracochlear trauma by piercing the cochlear partition, and it impacts speech perception and the residual hearing of CI recipients. STL and TF were primarily associated with PM arrays. However, studies with CI recipients had a substantial risk of bias, mainly caused by lack of randomization, and lack of standardization of insertion approaches (i.e. RW and CO). Subsequent research with human cadaveric temporal bones showed that the interaction between array type and surgical approach is an important factor regarding insertion trauma. Research showed that scalar localization of LW arrays on CT scans is difficult to interpret, due to LW arrays translocating not only towards SV but also to SM. Therefore, LW array STL rates are probably underestimated in literature. Additionally, PM arrays have in general more efficient stimulation of the auditory nerve than LW arrays, potentially improving speech perception, and lowering battery consumptions. Currently it is not clear which aspect, the preserved cochlear structure or more efficient auditory nerve stimulation, is more important for CI recipients, and how this relates to the surgical approach. This notion should be tested in the clinical practice, particularly the consequences for preservation of residual hearing and hearing with CI in general including speech perception. Although the osseous spiral lamina and basilar membrane are not visible on CT scans, accurate assessment of scalar position is possible for both type of arrays if using an adaption of curved multiplanar reconstruction method (i.e. uncoiled cochlear duct). Lastly, ECochG during several stages of the cochlear implantation procedure showed that acute basal turn trauma in the form of cochleostomy and subsequent short array insertion can affect the apical region of the cochlea.