[Monitoring of the Floor of the 4th Ventricle for Brainstem Surgery]

The brainstem is densely aggregated with important cranial nerve nuclei and nerve tracts. Surgery in this area is, therefore, risky. Not only anatomical knowledge but also electrophysiological monitoring is essential for brainstem surgery. The facial colliculus, obex, striae medullares, and medial sulcus are important visual anatomical landmarks at the floor of the 4th ventricle. As cranial nerve nuclei and nerve tracts deviate by lesion, it is important to have a firm image of the cranial nerve nuclei and nerve tracts before making an incision in the brainstem. The entry zone into the brainstem is selected where the parenchyma is the thinnest due to the lesions. The suprafacial or infrafacial triangle is often used as an incision site for the floor of the 4th ventricle. In this article, we introduce the electromyographic method of observing the external rectus muscle; orbicularis oculi muscle; orbicularis oris muscle; and tongue; and two cases in which monitoring was used(the pons and medulla cavernoma cases). By examining surgical indications in this way it may be possible to improve the safety of such operations.