Iatrogenic Impairment of Hearing During Surgery for Acoustic Neuroma

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Abstract
Isolated or combined labyrinthine, neural, and vascular damage account for failure to preserve hearing during removal of acoustic neuromas. However, the specific mechanisms of auditory impairment remain unclear unless surgical maneuvers can be related to peri- and postoperative hearing on the basis of intraoperative monitoring of auditory function. Among the different auditory monitoring techniques, recording of cochlear nerve action potentials (CNAPs) from the intracranial portion of the nerve has proven particularly useful for identifying the mechanisms of iatrogenic auditory injury. The present investigation analyzes intra- and postoperative auditory impairment in relation to surgical steps in a group of 38 subjects with acoustic neuroma (size ranging from 5 to 24 mm) undergoing removal via a retrosigmoid approach. Coagulation close to the cochlear nerve, drilling of the internal auditory canal, and removal of the intrameatal portion of the acoustic neuroma have prove to be the most critical surgical steps in hearing preservation. Changes were correlated with intra- and extrameatal tumor size, the relationship between the internal auditory canal and vestibule, and internal auditory canal enlargement, anatomic involvement of the cochlear nerve, preoperative auditory level, and ABR and ENG test findings. Changes in CNAP morphology and latency are detailed, and mechanisms of injury are analyzed and discussed as a function of these variables.