Intraoperative Intracochlear Electrocochleography and Hearing Preservation Early after Cochlear Implantation: A Meta-Analysis

AIM: There are a range of approaches to intraoperative electrocochleography (ECochG) monitoring across different implant systems, and these show inconsistent results. Here, we synthesize by meta-analysis the evidence for real-time intracochlear ECochG recorded during cochlear implantation predicting residual hearing. METHODS: Inclusion criteria included original studies in which real-time ECochG was undertaken on a commercial cochlear implant (CI) and residual hearing outcomes at least 4 weeks after implantation were reported. Thirteen studies comprising 313 individual patient data sets met inclusion criteria. Full data sets were sought from the authors. Studies were rated for quality and bias. Details about study design (observational or interventional) and electrode characteristics (lateral wall or perimodiolar) were extracted. Relative hearing loss across the low frequencies (250-1,000 Hz) was the main outcome, with decibel loss and functional hearing as secondary outcomes. Effects were explored by forest plot, and then linear mixed modelling on the full data sets. Sensitivity analyses included recalculation of hearing preservation using thresholds across different frequencies. RESULTS: Thirteen studies, including 313 full patient data sets, were included, and meta-analysis was performed on lateral wall electrodes. A drop of ECochG amplitude of >30%, even if transient, predicted residual hearing across all metrics (decibel, relative, and functional), and data-synthesis methods (forest plots and linear modelling) for all systems where real-time measurements were available. A drop of amplitude of at least 60% across insertion predicted relative but not decibel hearing loss in linear mixed modelling. The "pattern" of ECochG drop, as described by Harris in 2017, did not predict residual hearing by any method. DISCUSSION: The only truly "real-time" measurement that predicted residual hearing was an instantaneous drop in ECochG amplitude, with or without recovery. Large drops over the insertion did too, but these can only be calculated after insertion, and after the cochlear trauma has been done. All CI systems now offer real-time monitoring - these results provide guidance on how best to use it to optimize clinical outcomes.