Computed-tomography estimates of interaural mismatch in insertion depth and scalar location in bilateral cochlear-implant users

Abstract

ABSTRACTHypothesisWe hypothesized that the bilateral cochlear-implant (BI-CI) users would have a range of interaural insertion-depth mismatch because of different physical placements or characteristics of the arrays, but less than half of electrodes would have less than 75° or 3 mm of interaural insertion-depth mismatch. We also hypothesized that interaural insertion- depth mismatch would be more prevalent nearer the apex, when electrodes were located outside of scala tympani (i.e., possible interaural scalar mismatch), and when the arrays were a mix of pre-curved and straight types.BackgroundBrainstem neurons in the superior olivary complex are exquisitely sensitive to interaural differences, the cues to sound localization. These binaurally sensitive neurons rely on interaurally place-of-stimulation-matched inputs at the periphery. BI-CI users may have interaural differences in insertion depth and scalar location, causing interaural place- of-stimulation mismatch that impairs binaural abilities.MethodsInsertion depths and scalar locations were calculated from temporal-bone computed-tomography (CT) scans of 107 BI-CI users (27 Advanced Bionics, 62 Cochlear, and 18 Med-El). Each subject had either both pre-curved, both straight, or one of each type of array (mixed).ResultsThe median interaural insertion-depth mismatch was 23.4° or 1.3 mm. Relatively large interaural insertion-depth mismatch sufficient to disrupt binaural processing occurred for about 15% of electrode pairs [defined as >75° (13.0% of electrode pairs) or >3 mm (19.0% of electrode pairs)]. There was a significant three-way interaction of insertion depth, scalar location, and array type. Interaural insertion-depth mismatch was most prevalent when electrode pairs were more apically located, electrode pairs had interaural scalar mismatch (i.e., one in Scala Tympani, one in Scala Vestibuli), and when the arrays were both pre-curved.ConclusionLarge interaural insertion-depth mismatch can occur in BI-CI users. For new BI-CI users, improved surgical techniques to avoid interaural insertion-depth and scalar mismatch is recommended. For existing BI-CI users with interaural insertion-depth mismatch, interaural alignment of clinical frequency allocation tables by an audiologist might remediate any negative consequences to spatial-hearing abilities.