Importance of awareness of the rhomboid lip in microvascular decompression surgery for hemifacial spasm

Abstract

Object. The authors adopted the infrafloccular approach for microvascular decompression (MVD) surgery to treat hemifacial spasm (HFS). The inferior portion of the flocculus is retracted to observe the root exit zone of cranial nerve (CN) VII between CN IX and the flocculus. During the procedure, the rhomboid lip, a sheetlike layer of neural tissue forming the lateral recess of the fourth ventricle, is sometimes encountered. The existence of the rhomboid lip in cases of HFS was reviewed to determine the importance of the structure during MVD surgery.Methods. Preoperative imaging and intraoperative observations in 34 consecutive cases of HFS treated in the period from October 2008 through September 2011 were used to assess the frequency of encountering the rhomboid lip.Results. The rhomboid lip was observed during MVD surgery in 9 (26.5%) of the 34 cases but had been demonstrated on preoperative MR images in only 3 cases (8.8%). On T2weighted images, it appeared as a highintensity nonstructural area on the ventral side of the flocculus and continued into the fourth ventricle via the foramen of Luschka.Conclusions. A large rhomboid lip presents an impediment to MVD surgery in a significant minority of patients with HFS. It is seldom observed on preoperative MR images. Proper dissection of the rhomboid lip away from the arachnoid membrane and/or the lower CNs during MVD surgery provides good visualization of the root exit zone of CN VII and reduces injury of CNs IX and X, avoiding postoperative deficits like dysphagia.

Lower cranial nerve palsy after the infrafloccular approach in microvascular decompression for hemifacial spasm

“…The rhomboid lip was observed and correctly incised in 29.1% of cases, which is comparable to previous findings. [7] Including separation of CN IX from the flocculus or choroid plexus, dissection around the lower CNs was necessary in almost half of the cases (45.6%). However, we found no significant relationship with postoperative lower CN palsy.…”

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

“…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.…”

Fully Endoscopic Microvascular Decompression for Hemifacial Spasm Using Improved Retrosigmoid Infrafloccular Approach: Clinical Analysis of 81 Cases

“…Matsushima et al 19 first reported the concept of MVD for HFS using an infrafloccular approach. They extended the incision in MVD for HFS downward to expose the REZ of facial nerve by dissecting the dorsal arachnoid of the posterior cranial nerve and distracting the flocculus from the side.…”

“…This surgical approach for exploring the REZ of the facial nerve from the side is called the infrafloccular approach. 6,19 However, the incision design and the bone window they described are more similar to those in the far lateral approach, which increases the difficulty of exposure and prolongs the operation. Based on the modern concepts of minimally invasive neurosurgery, we gradually improved the operation by combining the surgical concept of the retrosigmoid infrafloccular approach with the advantages of neuroendoscopy.…”