Case report: Significance of the large rhomboid lip in microvascular decompression: insights from two clinical cases

Abstract

The rhomboid lip (RL) is a layer of neural tissue that extends outside the fourth ventricle and is connected to the lateral recess of the fourth ventricle. Although this anatomical structure has been rigorously studied, it is often overlooked in microvascular decompression (MVD) surgery. In this report, we present two cases, one of hemifacial spasm (HFS) and one of glossopharyngeal neuralgia (GPN), in which a large RL was observed during surgery. We found that a large RL is easily confused with arachnoid cysts, and accurate identification and dissection are important to protect the lower cranial nerves.

Keywords: flocculus; glossopharyngeal neuralgia; hemifacial spasm; microvascular decompression; rhomboid lip.

Figure 1

Right hemifacial spasm. (A) The anterior inferior cerebellar artery loop (AICA) compresses the facial nerve REZ (red arrow). (B) A sac-like structure on the ventral side of the flocculus (yellow arrow) is observed on T2WI. This structure communicates inwardly with the fourth ventricle through the foramen of Luschka (blue arrow). (C, D) A large rhomboid lip (RL) wraps around the jugular foramen and combines with the choroid to form a large lacuna. CN IX and X are located ventral to it and stick tightly. (E–G) After the RL is released, the vertebral artery (VA) and AICA are lifted away, revealing the REZ of the facial nerve (FN). Teflon is then placed between them.

Figure 2

Glossopharyngeal neuralgia on the right side. (A) T2WI shows a sac-like structure (yellow arrow) located ventral to the flocculus that connects to the fourth ventricle through the foramen of Luschka (red arrow). (B) The large RL surrounds the jugular foramen and tightly adheres to the lower cranial nerves. (C–E) By releasing and resecting most of the RL, CN IX, CN X, and the posterior inferior cerebellar artery (PICA) were effectively exposed to compress the REZ. Teflon is then inserted between them.

Figure 3

(A) The RL, a thin membranous neural structure, extends laterally from the floor of the fourth ventricle to join with the choroid plexus (Tela), forming a lacuna (red arrow) outside the lateral recess and the foramen of Luschka. (B) Located posterior to CNs IX, X, and XI, the RL is ventral to the recess and extends outward with the choroid plexus. (C) A large RL overlays the surface of the lower cranial nerves, obstructing the view of the REZ of CN VII and IX from the rear (Note: A, B are reproduced from Cranial anatomy and surgical approaches by Rhoton AL; Lippincott Williams & Wilkins, 2008. C is reproduced from Matsushima T. (ed): Microsurgical Anatomy and Surgery of the Posterior Fossa; Springer, 2015.).