Intraoperative monitoring of auditory function: experimental observations and new applications

The three different methods of intraoperative monitoring, namely, auditory brainstem response evaluation (ABR), electrocochleography (ECoG), and direct eighth nerve compound action potentials monitoring are reviewed. Both ECoG and direct eighth nerve monitoring surpass the ABR in their ability to provide fast, almost instantaneous, large, reproducible potentials in response to click stimuli. Changes in the amplitude and the latency of these potentials alert the surgeon of an impending eighth nerve injury. Human clinical observations revealed instances of persistence of the ECoG despite a total eighth nerve section in the cerebellopontine angle. Animal and human experiments were conducted to evaluate the effectiveness of ECoG and direct eighth nerve monitoring in hearing preservation. Twelve cats underwent progressive sectioning of the eighth nerve with simultaneous monitoring of changes in their ECoG and direct eighth nerve potentials. ECoG thresholds were obtained intraoperatively in eight patients undergoing surgery for conductive hearing loss. The results of these experiments suggest that recording directly from the eighth nerve is a more accurate technique during cerebellopontine angle and eighth nerve surgery. ECoG, however, appears to have a promising role in the intraoperative monitoring of middle ear reconstruction procedures under general anesthesia.