Microanatomy Around the Facial Nerve Pathway for Microvascular Decompression Surgery Investigated with Correlative Light Microscopy and Block-Face Imaging

Abstract

Microvascular decompression for hemifacial spasm is performed at the root exit zone. More proximal segments of the facial nerve, defined as the root emerging zone (REmZ), may also be susceptible to neurovascular compression. Consequently, detailed knowledge of the microanatomy around facial nerve fibers at the pontomedullary junction is essential for consistent success of microvascular decompression. Five human brainstems obtained from cadavers were investigated using correlative light microscopy and block-face imaging, which obtains arbitrary two-dimensional light microscopic and three-dimensional volume data from a single specimen. The entire facial nerve pathway, including the myelin transition, was evaluated inside and outside the brainstem. Correlative light microscopy and block-face imaging showed the intra-brainstem facial nerve fibers emerging at the brainstem surface deep at the pontomedullary sulcus (REmZ) and coursing along the pontine surface before detaching from the pons (root exit zone). An acute emerging angle significantly increased the surface area with central myelin. The facial nerve bundle formed 1 fasciculus in the portion covered by central myelin but divided into 2 fasciculi in the myelin transitional portion and then into multiple fasciculi more distally. Arteries around the REmZ were often anchored by perforating branches entangled with lower cranial nerves. Facial nerve fibers are susceptible to vascular compression on emerging onto the deep brainstem surface at the pontomedullary sulcus. The key procedure in microvascular decompression is full dissection of the lower cranial nerves down to the brainstem origin to explore both the root exit zone and the REmZ.