Does the intracochlear position of an electrode array impact performance?

BACKGROUND Cochlear implants (CI) restore hearing by stimulating spiral ganglion neurons via multichannel electrodes inserted into the cochlea. Many types of electrode arrays have been developed and fall into three categories based on intracochlear position: perimodiolar (PM), lateral wall (LW), and midscala (MS) electrodes. PM electrodes are designed to hug the medial wall to reduce the distance between the electrodes and the spiral ganglion neurons and typically do not coil around the apex. LW electrodes are designed to remain laterally in the scala tympani, with longer array options capable of reaching the apex. MS electrodes are a new type of electrode designed to remain in the middle of the scala tympani, thereby avoiding contact with either the medial or lateral walls to decrease trauma and fibrosis. It has been hypothesized that PM electrodes may have improved hearing performance and longer battery life given their closer proximity to the spiral ganglion. With less electrical current needed to stimulate the spiral ganglion neurons at each electrode position, PM electrodes theoretically should offer improved speech discrimination. When considering electrode type, however, the surgeon must also consider other factors that vary based on electrode type, such as insertional trauma, development of intracochlear fibrosis, transcalar positioning, battery life, and extrusion rates. This article summarizes the major advantages and disadvantages of these electrodes to help guide this decision. LITERATURE REVIEW Since the development of PM electrodes, many studies have compared their design and performance with LW electrodes. A recent study by O’Connell et al. retrospectively looked at 184 patients (220 implants; mean age at time of CI 60.2 years) and showed that electrode type was predictive of scalar position, with LW electrodes successfully placed in the scala tympani in 95.6% of patients compared to significantly lower rates for PM (48.7%) and MS electrodes (42.9%). Electrode placement completely within the scala tympani minimizes trauma to the surrounding structures because accidental translocation into the scala vestibuli damages the basilar and, potentially, Reissner’s membranes and the organ of Corti. Given the intracochlear trauma caused by interscalar translocation of the electrode arrays, the authors concluded that LW electrodes are likely the superior choice to PM. Their MS group was too small to draw conclusions. Other studies have specifically compared which electrode type is better at preserving low-frequency hearing. Wanna et al. performed a retrospective study of 196 patients (225 implants, mean age at time of CI 63.7 years) looking at hearing preservation, which they defined as a postoperative air-conduction threshold <= 80 dB hearing loss (HL) at 250 Hz. They found that LW electrodes were more likely to preserve functional hearing when compared to PM electrodes at both short-term (2–3 weeks postoperative) and long-term (>1 year postoperative) follow-up. Although MS electrodes had 3.4 times higher odds of hearing preservation when compared to PM arrays in the short term, there was no significant difference between these two in the long term. The authors suggested that at least part of the reason for improved hearing preservation was that LW electrodes are more likely to remain within the scala tympani, causing less trauma. Studies have also looked at outcomes in pediatric populations. Gordin et al. performed a prospective nonrandomized control trial involving 115 children to look for differences in performance between the Nucleus 24M straight array (LW), the PM Nucleus 24RC Contour, and the PM Nucleus 24RE with stylet insertion (Cochlear Ltd., Sydney, Australia). Children implanted with the 24RE PM From the Department of Otolaryngology–Head and Neck Surgery (R.K.T., M.R.H.); the Department of Neurosurgery (M.R.H.); and the Institute of Clinical and Translational Sciences (M.R.H.), the University of Iowa, Iowa City, Iowa, U.S.A. Editor’s Note: This Manuscript was accepted for publication on March 1, 2019. Marlan Hansen is a cofounder of IotaMotion, Inc., and holds stock in the company. This article does not discuss any of the technologies under development at IotaMotion. The authors have no other funding, financial relationships, or conflicts of interest to disclose. Send correspondence to Marlan R. Hansen, MD, Department of Otolaryngology–Head and Neck Surgery, The University of Iowa, 200 Hawkins Drive, Iowa City, IA 52242. E-mail: marlan-hansen@uiowa.edu