Several postoperative complications of MVD have been reported [3, 10]. One of the most frequent complications of MVD for HFS is postoperative hearing disturbance due to injury of CN VIII caused by retraction of the petrosal (lateral) cerebellar surface. During cerebellar retraction through the conventional LSA, the flocculus is the biggest obstacle to the view of REZ of CN VII [10, 14, 15, 17]. In the infrafloccular approach, however, the inferior portion of the flocculus and the choroid plexus is gently retracted superiorly and the lateral portion of the pontomedullary sulcus, especially the REZ of CN VII, can be observed through the space between the origin of CN IX and the flocculus. Because the flocculus is retracted in a direction perpendicular to CN VIII, damage to CN VIII can be reduced [10, 14].

Important anatomical landmarks on the brainstem for cerebellopontine angle (CPA) surgeries include the flocculus and the choroid plexus, which protrudes from the foramen of Luschka (Fig. 12.1a). CNs VII and VIII are located superomedial to the flocculus and the lower CNs are inferior to it. The foramen of Luschka is formed by a membrane of neural tissue called the rhomboid lip (Fig. 12.2a). The choroid plexus and rhomboid lip are situated on the origins of CNs IX and X (Fig. 12.1a). Occasionally, the flocculus and/or rhomboid lip is large and becomes an obstacle (Fig 12.2b, c). REZ of CN VII is situated immediately medial to the origin of CN VIII and in the supraolivary fossette, which consists of the pons, medulla oblongata, and flocculus (Fig. 12.1a) [12, 14, 17]. When the conventional LSA is used for exposing REZ of CN VII, CN VIII and the flocculus are obstacles to the surgeon’s view (Fig. 12.1b). In patients with a deep supraolivary fossette, it becomes more difficult to observe REZ of CN VII. REZ of CN VII exists near the pontomedullary sulcus anteromedial to the entry zone of CN VIII (Fig. 12.1a) and can be seen easily through the space between the origin of CN IX and the inferior portion of the flocculus without strong retraction of the cerebellar surface (Fig. 12.1c, d). Therefore, the infrafloccular approach is preferable for MVD for HFS [4, 10, 14, 15]. The rhomboid lip and the choroid plexus are sometimes adherent to the lower CNs (Fig. 12.2) [2, 21]. The choroid plexus and/or rhomboid lip should be dissected and separated from the lower CNs through the infrafloccular approach. It is important to note that intraoperative monitoring of auditory brainstem evoked potential should be performed even when the infrafloccular approach is used (regarding the anatomy of the supraolivary fossette, refer to Chap. 10: “Anatomy for Microvascular Decompression Procedures: Relationships between Cranial Nerves and Vessels, Preoperative Images, and Anatomy for the Stitched Sling Retraction Technique”).