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

Abstract

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. 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. 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 T2-weighted images, it appeared as a high-intensity nonstructural area on the ventral side of the flocculus and continued into the fourth ventricle via the foramen of Luschka. 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.