Purpose <p>The causes of residual hearing loss during cochlear implantation are generally poorly understood. This study sought to understand whether the complexity of this surgical approach related to a loss of residual hearing.</p> Method <p>Thirty-four adults underwent implantation with a sheathed perimodiolar electrode, via an extended round window approach. During implantation, there was simultaneous video fluoroscopy, electrocochleography (ECochG) and video from the operating microscope. Three investigators reviewed the data simultaneously. Surgery was classified as straightforward or complex, defined as difficulty with either introduction of the electrode/sheath, advancement of the electrode, or withdrawal of the sheath. ECochG signal amplitude was correlated to intracochlear electrode movements, as determined by video fluoroscopy. The primary outcome was relative hearing loss, at least six weeks after surgery. The impact of surgical complexity on relative hearing loss was assessed. A secondary outcome was the impact of surgical complexity or electrode movement on ECochG signal amplitude.</p> Results <p>Complex surgery was associated with significantly worse relative hearing preservation (<i>p</i>=0.011), as well as lower maximum ECochG amplitudes (Kruskal-Wallis, chi2 = 4.35, <i>p</i>= 0.037). After the electrode had been advanced out of the sheath, sudden fluctuations in ECochG amplitude were related to sudden changes in insertion depth. In non-complex surgeries, the residual hearing was independent of such fluctuations (Kruskal-Wallis, chi2=1.12,<i>p</i>=0.289).</p> Conclusion <p>These data suggest residual hearing is lost early during the implant procedure, following complex surgical events. The low ECochG amplitudes encountered in complex surgeries support this interpretation. ECochG amplitude fluctuations were not associated with poorer residual hearing.</p>