Microvascular decompression surgery is an effective treatment for hemifacial spasm (HFS) and glossopharyngeal neuralgia (GPN). Using the lateral suboccipital infrafloccular approach, the culprit vessels can be identified and isolated, which results in satisfactory outcomes. In this approach, the choroid plexus protruding from the foramen of Luschka serves as a significant anatomical landmark to locate the root exit zone (REZ) (1). The choroid plexus and the lower cranial nerves can be adequately visualized by gently retracting the flocculus posterosuperiorly. The rhomboid lip (RL) is sometimes encountered during dissection; however, it is sometimes discovered under normal circumstances (2–5). In this study, we report two microvascular decompression (MVD) cases with a large RL.

Since the introduction of MVD by Jannetta for the treatment of cranial nerve disorders, it has become the mainstay of treatment for HFS and GPN (6–9). Although this technique is well-established, the literature reports an incidence of 0.5–3% for permanent cranial nerve dysfunction, such as facial palsy (10), hearing loss (11), and lower cranial nerve deficits (12, 13). These complications may be caused by intraoperative malpractice, as well as the complexity and variability of individual anatomy, which increases the difficulty of surgery. Furthermore, inadequate knowledge and mishandling of the RL can contribute to nerve injuries. The RL is occasionally mistaken for thickened arachnoid membranes, which increases the risk of cranial nerve injuries during separation. Therefore, it is vital to clearly understand the anatomical relationship between the arachnoid membrane, RL, and the lower CNs to ensure the safety of MVD.

As the fourth ventricle migrates into the lateral recess, the floor of the fourth ventricle and the RL form the ventral wall of the lateral recess (14). The RL continues to migrate ventrally. Along with the choroid plexus, the RL forms the most lateral end of the lateral recess, creating a trap-like structure (Figure 3A). At the lateral end, the RL forms the ventral margin of the foramen of Luschka, while the choroid and its accompanying choroid plexus form the dorsal margin. CN IX and CN X are located ventrally to the RL, with the CN VII and CN VIII and the choroid located anteriorly above it (Figures 3B, C). Akiyama et al. (15) classified RL into three types: non-extension type, lateral extension type, and jugular foramen type, based on the relationship of the RL to the choroid plexus. All types of RL surround CN IX and CN X and are in contact with or locally adherent to them. The jugular foramen type adheres extremely closely to CN IX.

Although researchers have provided detailed descriptions of the microanatomy of RL, the clinical significance of RL has not yet been recognized (16). This lack of recognition may be attributed to the inadequate understanding of RL anatomy and structure. In a study conducted by Nakahara et al. (4), only 9 out of 34 patients with HFS had visualized RL during surgery (26.5%), and only 3 cases (8.8%) could be demonstrated on preoperative MRI. On T2WI MRI, a larger RL exhibited high signal cystic manifestations ventral to the flocculus, which connected medially with the fourth ventricle through the foramen of Luschka. However, visualizing smaller RLs was challenging. A case study by Cho et al. (3) reported the misdiagnosis of a large RL as an arachnoid cyst, which highlights the importance of understanding RL anatomy and features to avoid such misdiagnoses. RL and arachnoid cysts are distinct tissue structures. The RL is visually thicker, translucent, and milky white compared to the arachnoid. Histologically, the RL has a two-layered structure with ciliated cells in the inner layer, similar to the ventricular cells at the base of the fourth ventricle, and an outer layer that consists of glial cells and neurons. In contrast, the arachnoid membrane lacks nerve cells. Anatomically, the outer arachnoid membrane accompanies the lower CNs into the jugular foramen, whereas the RL does not enter the jugular foramen.

In MVD surgery, it is important to release and separate the RL in a precise manner. Precision separation allows the surgeon to clearly view the REZ of CN VII, IX, and X while avoiding any unnecessary retraction of the lower CNs. By gently retracting the flocculus posterosuperiorly using the infrafloccular approach, the RL and the REZ of CN VII can be sufficiently visualized. However, if the RL is large or tightly adherent to CN IX and X, it becomes difficult to visualize the REZ of CN VIII, IX, and X. Improper retraction or failure to correctly identify the relationship between the RL and the lower nerves can also lead to postoperative cranial nerve injury. In patients with a large RL, considering its anatomical relationship with CN VII and IX is important. To avoid injury to the lower CNs, the RL should be patiently and carefully dissected from these cranial nerves or separated in the direction of the nerve alignment before retracting the flocculus. This allows for improved visualization of the periphery of the REZ of CN VII, IX, and X and the brainstem.

A thorough anatomical knowledge and meticulous dissection can help surgeons observe the structures of the foramen of Luschka region more clearly. These practices can ensure the protection of cranial nerves, thereby reducing the risk of cranial nerve injuries.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving humans were approved by Medical Ethics Committee of Nanjing Brain Hospital. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants' legal guardians/next of kin in accordance with the national legislation and institutional requirements. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

FD: Writing – original draft, Writing – review & editing. PL: Data curation, Writing – review & editing. XZ: Data curation, Writing – review & editing. WS: Data curation, Investigation, Writing – review & editing. YX: Investigation, Supervision, Writing – original draft. DW: Investigation, Supervision, Validation, Writing – review & editing. LG: Supervision, Validation, Writing – review & editing. XH: Supervision, Validation, Writing – review & editing. KY: Supervision, Validation, Writing – review & editing. YL: Supervision, Validation, Writing – review & editing. YZ: Funding acquisition, Supervision, Validation, Visualization, Writing – review & editing.