Virtual surgical analysis of endoscopic microvascular decompression for hemifacial spasm

Objective: Endoscopic microvascular decompression (eMVD) for hemifacial spasm (HFS) clearly demonstrates the root entry zone (REZ) of the facial nerve with minimal invasiveness. The aim of this study was to identify the trajectory of the angled view under eMVD for HFS and to better understand the relevant anatomical landmarks using a virtual straight microscopic view on three-dimensional (3D) fusion images. Methods: Each 3D fusion image including arteries, veins, brainstem, cranial nerves, and skull was created using an image processing workstation. A trajectory of view under a 30° angled endoscope was defined as the angled view trajectory (AVT). To investigate the anatomy along the AVT, 3D fusion images were manually made consistent with the 30° endoscopic view, and a straight microscopic approach to obtain the same view as the endoscopic surgery was analyzed on 3D fusion images. Results: The 30° endoscopic view during eMVD visualized the lateral side of the pons, pontomedullary junction, REZ of the facial nerve, and offending arteries. The 30° endoscopic view correlated with 3D fusion images from the straight microscopic view obtained by the posterior petrosal approach after mastoidectomy with incision of the transition between the sigmoid sinus and jugular bulb. Conclusions: This 3D fusion image analysis revealed the AVT of eMVD for HFS. The present results explain how the angled endoscope can effectively visualize the REZ of the facial nerve without needing extensive skull base resection.