Vestibular Monitoring via Abducens EMG: Observations on a Single Case

Abstract

Advancement in skull base surgery has occurred due to the improved ability to remove larger and less accessible tumors while minimizing morbidity. Petroclival meningiomas represent a particular challenge due to their proximity to and involvement with multiple cranial nerves. Intraoperative neural monitoring has become routine in assisting the surgeon to avoid injury to these structures. We present a case of a large petroclival meningioma removed by a transpetrosal approach with intent of hearing preservation in which an interesting finding was obtained from neural monitoring. In addition to facial, trigeminal, SSEP, and BAER monitoring, continuous EMG monitoring of the abducens nerve was performed. During the transmastoid approach, an opening was inadvertently made in the lateral semicircular canal. With the membranous labyrinth intact, it was decided to pack the canal to prevent further injury. The cupular deflection caused by the packing resulted in a rhythmic repeated firing of the ipsilateral abducens, representing the correcting saccades of the induced nystagmus. This confirmed that the vestibular apparatus, local brainstem, and nerve were intact and functional, as ablative nystagmus would have been in the opposite direction. This was confirmed further by there being no change in the wave I or V of the BAER. For the remainder of the case, the rhythmic firing of the abducens nerve served as a monitor of the health of both the VI and VIII nerves. Tumor involvement at the brainstem insertion of the VIII nerve eventually resulted in the loss of this phenomenon, mirroring the findings on BAER. It is felt that this phenomenon may be exploited in future selected cases to provide real-time feedback on the health of the vestibular nerve during skull base surgery.