Abstract and Introduction
Abstract
Unfavorable outcomes such as facial paralysis and deafness were once unfortunate probable complications following resection of acoustic neuromas. However, the implementation of intraoperative neuromonitoring during acoustic neuroma surgery has demonstrated placing more emphasis on quality of life and preserving neurological function. A modern review demonstrates a great degree of recent success in this regard. In facial nerve monitoring, the use of modern electromyography along with improvements in microneurosurgery has significantly improved preservation. Recent studies have evaluated the use of video monitoring as an adjunctive tool to further improve outcomes for patients undergoing surgery. Vestibulocochlear nerve monitoring has also been extensively studied, with the most popular techniques including brainstem auditory evoked potential monitoring, electrocochleography, and direct compound nerve action potential monitoring. Among them, direct recording remains the most promising and preferred monitoring method for functional acoustic preservation. However, when compared with postoperative facial nerve function, the hearing preservation is only maintained at a lower rate. Here, the authors analyze the major intraoperative neuromonitoring techniques available for acoustic neuroma resection.
Introduction
Acoustic neuromas (vestibular schwannomas) are categorized as benign, extraaxial brain tumors (Fig. 1) developing near the internal auditory canal, typically with involvement of the cerebellopontine angle. Advances in treatment modalities have popularized the application of less invasive management methods such as radiotherapy and radiosurgery, which carry high efficacy and low morbidity. However, many acoustic neuromas, particularly those that are large in size, necessitate surgical intervention.
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Figure 1.
Illustration showing an acoustic neuroma with displaced facial and cochlear nerves, the nerves we are trying to preserve. Printed with permission from Dr. Nancy Huh, M.D., Illustrations.
The primary operative goals are gross tumor debulking while safeguarding the adjacent cranial nerves (Fig. 1). Neural preservation is particularly imperative in the contemporary management of acoustic neuromas. By virtue of their location, these tumors are close to the facial and vestibulocochlear cranial nerves (Fig. 1), and can thus severely impair the nerve function at the time of initial presentation. The neuroma can directly impinge, tightly adhere to, or overtly damage the nerves. These tumors often present as operative challenges, as resection may cause nerve irritation or injury leading to neurapraxia, axonotmesis, or neurotmesis.
The various options of surgical approaches (translabyrinthine vs middle fossa vs retrosigmoid) for acoustic neuromas and their respective patterns of postoperative cranial nerve preservation have been described. However, IONM may demonstrate improvements in structural and functional preservation of the cranial nerves during these operations. Several IONM techniques have been developed and evaluated with particular focus on CN VII and VIII preservation. Among these methods, the most frequently used are EMG for the facial nerve and BAEP monitoring for the vestibulocochlear nerve. Here, we assess the fundamental characteristics underlying the major techniques available in IONM, emphasizing specific advantages and limitations of their utilization for optimal patient management.