Root Exit Zone Research Articles

Pathogenesis, Surgical Treatment, and Cure for SUNCT Syndrome

Short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT) are characterized by attacks of moderate to severe stabbing pain, strictly unilateral, with periorbital or temporal distribution, associated with cranial autonomic symptoms, such as lacrimation and redness of the ipsilateral eye. To obtain mechanistic insights into the pathogenesis of SUNCT syndrome, more than 800 cases treated in our institution during the last 7 years were retrospectively reviewed. Two patients showed typical autonomic symptoms of SUNCT. Magnetic resonance imaging suggested potential compression of the trigeminal nerve by the intracranial artery in these cases and complete remission was achieved by microvascular decompression. Microvascular decompression provides an appropriate therapeutic choice if vascular compression of the trigeminal nerve is identified. From our 2 cases, we propose that, in some cases of SUNCT diagnosed previously, characteristic symptoms were induced by compression of the side surface of the first branch of the trigeminal nerve at the root exit zone by the intracranial artery.

2-P-F-16. Multimodality monitoring during microvascular decompression for hemifacial spasm

Objective The present study investigated whether free-running electromyography (frEMG) monitoring correlated with abnormal muscle responses (AMRs), or facial motor evoked potentials (FMEP) in 26 hemifacial spasm patients who underwent microvascular decompression (MVD). Methods frEMGs were recorded after injection of 20 ml of saline onto the root exit zone of the facial nerve. Using a software tool, data were rectified and measured for frEMG activity from the orbicularis oculi or mentalis (ME) muscle as an integral value 10 s after injection. frEMG activity was measured before and after MVD. With regard to AMRs, patients were divided into two groups based on disappearance or persistence of AMRs after MVD. Ratios of final to baseline FMEP amplitudes were used for analysis. Results Thirteen patients with decreased frEMG activity after surgery had significantly smaller FMEP amplitude ratios in the ME muscles compared with 13 patients without decreased activity (65% vs. 94%, p Conclusions/significance Decreases in frEMG activity elicited by a saline injection after MVD night reflect normalization of facial nerve excitability.

Residual hemifacial spasm after microvascular decompression: prognostic factors with emphasis on preoperative psychological state

Residual hemifacial spasm (HFS) after microvascular decompression (MVD) is common, and the factors associated with residual HFS are still controversial. In the present study, we analyzed the outcome of 212 patients with hemifacial spasm after a single microvascular decompression and evaluated the prognostic factors involved in residual hemifacial spasm. Based on our study, possible prognostic factors included indentation of the root exit zone (REZ), preoperative illness duration, and preoperative psychological state. We suggest that MVD should be performed as early as possible for it may decrease the rate of residual HFS. Preoperative assessment of psychological state in HFS patients is a timely intervention that should be implemented to minimize the residual HFS.

Placement of Teflon Sponges in Microvascular Decompression Procedure for Treatment of Hemifacial Spasm.

Background Hemifacial spasm (HFS) is generally treated by microvascular decompression (MVD). Inadequate separation of vessel and nerve or adhesive inflammation surrounding the nerve root may cause recurrence. Objective To explore a method to reduce the incidence of adhesions and to ensure sufficient separation of the offending vessel and nerve during MVD. Methods Fifty-one patients diagnosed with HFS were studied. During the MVD procedure, Teflon sponges were placed between the offending vessels and medulla oblongata to push compressing vessels away from the facial nerve without contacting the nerve. Results Our method of placement of the Teflon sponge effectively shifts the compressing artery and ensures that both the Teflon sponge and offending vessels do not contact the root exit zone. This method also ensures that the Teflon sponge is fixed in place. Conclusion The technique described for the treatment of HFS provides an effective, safe, and durable resolution to patient symptoms that minimizes surgical complications and may be useful in treating HFS.

Advances in microvascular decompression for hemifacial spasm

Primary hemifacial spasm (HFS) is a disorder that causes frequent involuntary contractions in the muscles on one side of the face, due to a blood vessel compressing the nerve at its root exit zone (REZ) from the brainstem. Numerous prospective and retrospective case series have confirmed the efficacy of microvascular decompression (MVD) of the facial nerve in patients with HFS. However, while MVD is effective, there are still significant postoperative complications. In this paper, recent technological advances related to MVD (such as lateral spread response, brainstem auditory evokes potential, three dimensional time of flight magnetic resonance angiography, intraoperative neuroendoscopy) are reviewed for the purposes of improving MVD treatment efficacy and reducing postoperative complications.

Characteristic anatomical conformation of the vertebral artery causing vascular compression against the root exit zone of the facial nerve in patients with hemifacial spasm.

Hemifacial spasm (HFS) is caused by tortuous offending vessels near the facial nerve root exit zone. However, the definitive mechanism of offending vessel formation remains unclear. We hypothesized that vascular angulation and tortuosity, probably caused by uneven vertebral artery blood flow, result in vascular compression of the facial nerve root exit zone. The authors observed two anatomical characteristics of the vertebrobasilar arterial system in 120 subjects in the surgical group and 188 controls. The presence of the dominant vertebral artery (DVA) and laterality of the vertebrobasilar junction (VBJ) were observed. We also analyzed the morphological characteristics of the surgical group showing the presence of DVA. The morphological characteristics were classified into three types: type I had the VBJ and DVA on the same side, type II had the VBJ within 2 mm of the midline, and type III had the VBJ opposite the DVA. The DVA was more prevalent in the surgical group than in the control group (71 % versus 54 %, P < 0.05). The surgical group patients with HFS on the left were more likely to have a DVA on the left (P < 0.05) and with HFS on the right were more likely to have a DVA on the right (P < 0.01) compared with controls. The direction of the VBJ was more common on the same side as the DVA, which corresponds with the laterality of the HFS. In the surgical group with the DVA and HFS on the same side, type I was predominant, but in the surgical group with a contralateral DVA and HFS, type III was predominant. The presence of a DVA and shifting of the VBJ on the same side plays a role in the angulation and tortuosity of vessels in the perivertebrobasilar junction, resulting in neurovascular compression of the facial nerve root exit zone and thereby causing HFS.

Improved preservation of function during acoustic neuroma surgery.

Restoration of cranial nerve functions during acoustic neuroma (AN) surgery is crucial for good outcome. The effects of minimizing the injury period and maximizing the recuperation period were investigated in 89 patients who consecutively underwent retrosigmoid unilateral AN surgery. Cochlear nerve and facial nerve functions were evaluated during AN surgery by use of continuous auditory evoked dorsal cochlear nucleus action potential monitoring and facial nerve root exit zone-elicited compound muscle action potential monitoring, respectively. Factors affecting preservation of function at the same (preoperative) grade were analyzed. A total of 23 patients underwent standard treatment and investigation of the monitoring threshold for preservation of function; another 66 patients underwent extended recuperation treatment and assessment of its effect on recovery of nerve function. Both types of final action potential monitoring response and extended recuperation treatment were associated with preservation of function at the same grade. Preservation of function was significantly better for patients who received extended recuperation treatment.

Pre-surgical simulation of microvascular decompression for hemifacial spasm using 3D-models

We have been performing pre-surgical simulations using custom-built patient-specific 3D-models. Here we report the advantageous use of 3D-models for simulating microvascular decompression(MVD)for hemifacial spasms. Seven cases of MVD surgery were performed. Two types of 3D-printers were used to fabricate the 3D-models:one using plaster as the modeling material(Z Printer®450, 3D systems, Rock Hill, SC, USA)and the other using acrylonitrile butadiene styrene(ABS)(UP! Plus 3D printer®, Beijing Tiertime Technology, Beijing). We tested three types of models. Type 1 was a plaster model of the brainstem, cerebellum, facial nerve, and the artery compressing the root exit zone of the facial nerve. Part of the cerebellum was digitally trimmed off to observe "the compressing point" from the same angle as that used during actual surgery. Type 2 was a modified Type 1 in which part of the skull was opened digitally to mimic a craniectomy. Type 3 was a combined model in which the cerebellum and the artery of the Type 2 model were replaced by a soft retractable cerebellum and an elastic artery. The cerebellum was made from polyurethane and cast from a plaster prototype. To fabricate elastic arteries, liquid silicone was painted onto the surface of an ABS artery and the inner ABS model was dissolved away using solvent. In all cases, the 3D-models were very useful. Although each type has advantages, the Type-3 model was judged extremely useful for training junior surgeons in microsurgical approaches.

96. Blink reflex intraoperative monitoring during microvascular decompression for hemifacial spasm: Our experience

Hemifacial spasm (HFS) is caused by vascular compression of the facial nerve at its root exit zone from the brainstem. Microvascular decompression (MVD) is a surgical treatment option that offers the prospect of a definitive cure for HFS. Intraoperative monitoring of abnormal muscle response has been used to predict MVD effectiveness. In patients with HFS Blink Reflex (BR) recorded in laboratory shows abnormal R1 diffusion in inferior facial muscles. BR has recently been introduced as a feasible technique during intraoperative monitoring (IOM) of facial nerve function. We evaluated intraoperative feasibility and modifications of BR during MVD for HFS in 8 patients. BR was recorded in 7/8 patients. R1 recorded from inferior facial muscles (orbicularis oris, mentalis muscles) disappeared in 3/7 patients, while it decreased in amplitude during surgery in 4/7 patients. HFS was completely resolved at early follow up evaluation (one week after surgery) in all patients with intraoperative loss of R1 from inferior facial muscles; in the other 4 patients HFS resolution was heterogeneous. Intraoperative BR monitoring can be a complementary technique during HFS surgery to optimize the MVD.

Endoscopic criteria of offending vessel in neurovascular compression syndrome

Abstract The aim of this work was to detect the anatomical relationship criteria of the offending vessel with respect to the cranial nerve in neurovascular compression syndrome using the endoscopic-assisted minimally invasive retrosigmoid approach for microvascular decompression (MVD); these criteria help the surgeon to define the actual conflicting vessel during surgery for better results. Between 1994 and 2009, we have performed 782 cases of MVD surgeries using endoscopic-assisted minimally invasive retrosigmoid approach (453 MVD surgeries for hemifacial spasm, 269 for trigeminal neuralgia, 56 for tinnitus and vertigo, and four decompressions for glossopharyngeal neuralgia). During these surgeries we recorded the anatomical relationships between the offending vessel and the affected cranial nerve to detect the endoscopic criteria of the offending vessel. There are certain criteria for the anatomical relationship between the offending vessel and affected cranial nerve; these criteria include common features such as a perpendicular contact between the vascular loop and the cranial nerve along two different perpendicular planes at its root exit zone, distortion of the nerve course, and distortion or compression of adjacent neural structures mainly brain stem. Other criteria include the vascular loop causing impression of the nerve or encircling the nerve causing reduction of its diameter. Certain criteria for the anatomical relationship between the offending vessel and the affected cranial nerve should be fulfilled to diagnose the actual conflicting vessel during MVD surgery in vascular compression syndrome using endoscopic-assisted minimally invasive retrosigmoid approach.

The midline suboccipital subtonsillar approach to the cerebellomedullary cistern and its structures: Anatomical considerations, surgical technique and clinical application

ObjectiveLesions of the cerebellomedullary cistern lateral to the lower brainstem in an area extending from the foraminae of Luschka to the foramen magnum are rare and can be caused by various sources. There is no consensus on an ideal surgical approach. We describe the anatomical features and the surgical technique of the midline suboccipital subtonsillar (STA) approach to the cerebellomedullary cistern and its pathologies. MethodsThe study was performed on three alcohol (ETOH)-fixed specimens (6 sides), and the technique of the approach was highlighted. The tonsillar retraction needed to view the important structures was measured. Additionally, the records of 31 patients who underwent the STA procedure were evaluated. We provide three clinical cases as examples. ResultsTonsillar retraction of 0.3cm (SD±0.1cm) exposed the PICA with its telo-velo-tonsillar and cortical branches. Retraction of 0.4cm (SD±0.2cm) exposed the spinal root of CN XI. Retraction of 0.9cm (SD±0.01cm) exposed the hypoglossal canal. Retraction of 1.3cm (SD±0.2cm) exposed the root exit zone of the glossopharyngeal nerve. Retraction of 1.6cm (SD±0.3cm) exposed the jugular foramen (JF), and retraction of 2.4cm (SD±0.2cm) exposed the inner auditory canal (IAC). In all of the selected cases, the pathology could be reached and exposed using the STA. ConclusionsWe recommend STA as a straightforward, easy-to-learn and therefore time-saving and safe procedure compared with other standard approaches to the cerebellomedullary cistern and its pathologies.

Epidural Anterior Petrosectomy with Subdural Visualization of Sphenobasal Vein via the Anterior Transpetrosal Approach

The drainage of the superficial middle cerebral vein (SMCV) is classified into four subtypes. The sphenobasal vein (SBV) drains from the SMCV to the pterygoid venous plexus at the temporal skull base. Epidural procedures in the standard anterior transpetrosal approach (ATPA) may damage the route of the SBV. We report a case in which modified surgical procedures via the ATPA were used to preserve the SBV. A 45-year-old male complained of right facial pain. Magnetic resonance (MR) images revealed a right cerebellopontine tumor suggestive of an epidermoid cyst. Right carotid angiography revealed that the SMCV drained into the pterygoid venous plexus via the SBV. The convexity dura mater of the temporal lobe was cut and the anterior part of the temporal lobe was retracted subdurally. The basal dura of the temporal lobe posterior to the SBV vein was visualized and cut subdurally. The posterior part of the temporal lobe was retracted epidurally. After dissecting, the dura mater medial to the greater petrosal nerve and to the edge of the petrous apex, the petrous apex was exposed and drilled out without injuring the SBV. The superior petrous sinus and the tentorium were cut. The tumor was compressed at the root exit zone of the trigeminal nerve. The tumor was removed grossly and totally. The modified ATPA (epidural anterior petrosectomy with subdural visualization of sphenobasal vein) is effective to preserve the SBV.

Midline Suboccipital Subtonsillar Approach to the Cerebellomedullary Cistern and the Craniocervical Junction and Its Structures: Anatomical Considerations and Clinical Application

Objective: Lesions lateral to the lower brainstem are rare and harbor different pathologies. No consensus exists with regard to the surgical approach of choice for the treatment of these lesions. We describe the anatomy of the suboccipital-midline-subtonsillar-approach (STA). The STA provides a straight and wide angle view to various types of pathologies in the cerebellomedullary cistern and the craniocervical junction. Methods: The microsurgical features of the STA were examined in three ETOH fixed specimens in semisitting position. In addition 31 patients who underwent surgery using the STA from 2006 to 2012 were examined and evaluated with respect to the type of pathology, morbidity of surgery, and postoperative period. Results: The anatomical studies showed a distance between occipital protuberance and foramen magnum of 5 cm, between the occipital condyles of 3.4 cm. After retracting the tonsils 0.3 cm we gain vision of PICA. Retraction to 0.35 cm exposed spinal root of CN XI. Hypoglossal canal got visible after 0.9 cm, the root exit zone of glossopharyngeal nerve after 1.3 cm, the jugular foramen after 1.5 cm, the IAC after 2.4 cm of tonsil retraction. In 31 cases using the STA pathologies contained plexus-papillomas, hypoglossal-schwannomas, aneurysms, hemangioblastomas, and vascular conflicts. The STA approach from skin incision to dural opening was 40 minutes. The mean duration of surgery was 295 minutes. Conclusions: The STA provides a straight and wide angle view to various types of pathologies in the cerebellomedullary and lateral-premedullary cistern with moderate degree of tonsil retraction. The pathology itself helps to avoid spatula pressure by displacing the tonsils and giving space for surgery.

Combined intraoperative monitoring of abnormal muscle response and Z-L response for hemifacial spasm with tandem compression type

Multiple neurovascular compression is present in about 38 % cases of hemifacial spasm (HFS). In these cases, the vertebral artery (VA) compresses another vessel, which in turn compresses the nerve. This type was named as "the tandem type". In the tandem type, the real offending vessel is often concealed by the VA. It is sometimes neglected by the surgeons. In this study, we report our experience in using abnormal muscle response (AMR) and ZL-Response (ZLR) simultaneously as intraoperative monitoring for MVD surgery of HFS with "the tandem type" vascular compression involving VA. Fourteen "tandem type" patients treated with microvascular decompression surgery (MVD) surgery were included. ZLR and AMR were recorded simultaneously to identify the offending vessels in operation. After MVD surgery, 13 patients achieved excellent resolution of spasm. In one case, the patient failed to attain resolution in the first operation, underwent early reoperation and had good resolution. There were no operative deaths or serious operative complications. In all 14 cases, we found that VA compressed the anterior inferior cerebellar artery (AICA) or posterior inferior cerebellar artery (PICA), which in turn compressed the root exit zone (REZ). A typical ZLR was identified from the AICA or PICA but not from the VA. AMR was absent in one case and persisted in one case. After the VA was transposed, the typical AMR was unchanged, unstable or disappeared, and ZLR from the AICA/PICA also existed. AMR and ZLR did not disappear until AICA/PICA was sufficiently decompressed. The combination of AMR and ZLR provides more useful information than does the AMR alone, and ZLR may be the only useful intraoperative monitoring for MVD surgery in times when AMR is absent or persists. ZLR played a crucial role in finding the real offending vessel, which was often concealed by the VA in tandem type.

Significance of Arachnoid Dissection to Obtain Optimal Exposure of Lower Cranial Nerves and the Facial Nerve Root Exit Zone during Microvascular Decompression Surgery

This letter discusses the article2) entitled Delayed unilateral soft palate palsy without vocal cord involvement after microvascular decompression for hemifacial spasm recently published in J Korean Neurosurg Soc 53 : 364-367, 2013. In order to prevent cranial nerve palsies during microvascular decompression for hemifacial spasm, it is important to avoid retracting the cerebellum from the internal to the external side. Instead, it should be retracted from the lower to the upper side to obtain the optimal exposure of lower cranial nerves and the facial nerve root exit zone (FNREZ). The author of this manuscript devised an arachnoid dissection technique to expose the lower cranial nerves and the FNREZ. The retromastoid approach is most commonly conducted in the lateral decubitus position. The dura mater is incised along the sigmoid sinus, and the Cisterna magna is opened to drain the cerebrospinal fluid after careful retraction of the cerebellum from the lower to the upper side. The cerebellar cortical branches of the anterior inferior cerebellar artery (AICA), which is covered with an arachnoid membrane exterior to the lower cranial nerves, are exposed. Under the microscope, the outside of the arachnoid membrane is attached to the pia mater of the cerebellar cortical braches of AICA, which is dissected using an arachnoid knife and microscissors. At this point, the inner side of the arachnoid membrane of the cerebellar cortical braches of AICA should be preserved (Fig. 1). Next, the space between the cerebellar cortical braches of AICA and the cerebellar cortex is separated, and the sharp continuous arachnoid membrane dissection is extended towards the anterior and inferior end of lower cranial nerves modified by author's technique (Fig. 2)1). The cerebellar cortical branches of AICA are then retracted naturally towards the tentorium, and the choroid plexus, located in the lateral recess of the 4th ventricle, can be visualized. The offending artery and entire set of lower cranial nerves, facial nerves, including the FNREZ, can then be widely exposed (Fig. 3). Fig. 1 The outside of cortical anterior inferior cerebellar artery arachnoid membrane is dissected using microscissors. Fig. 2 The sharp coninuous arachnoid dissection is extended toward anterior and inferior end of lower cranial nerves. Fig. 3 The offending vessels and entire set of lower cranial nerves, facial nerve root exit zone are widely exposed. A : Left. B : Right. I think the significance of this arachnoid dissection procedures can prevent the lower cranial nerve palsies and also helpful to avoid cerebellar ischemia resulting from cerebellar cortical arterial compression/retraction during the operation.

Microanatomy and histological features of central myelin in the root exit zone of facial nerve.

ObjectiveThe aim of this study was to evaluate the microanatomy and histological features of the central myelin in the root exit zone of facial nerve.MethodsForty facial nerves with brain stem were obtained from 20 formalin fixed cadavers. Among them 17 facial nerves were ruined during preparation and 23 root entry zone (REZ) of facial nerves could be examined. The length of medial REZ, from detach point of facial nerve at the brain stem to transitional area, and the thickness of glial membrane of central myelin was measured. We cut brain stem along the facial nerve and made a tissue block of facial nerve REZ. Each tissue block was embedded with paraffin and serially sectioned. Slices were stained with hematoxylin and eosin (H&E), periodic acid-Schiff, and glial fibrillary acid protein. Microscopy was used to measure the extent of central myelin and thickness of outer glial membrane of central myelin. Thickness of glial membrane was examined at two different points, the thickest area of proximal and distal REZ.ResultsSpecial stain with PAS and GFAP could be differentiated the central and peripheral myelin of facial nerve. The length of medial REZ was mean 2.6 mm (1.6-3.5 mm). The glial limiting membrane of brain stem is continued to the end of central myelin. We called it glial sheath of REZ. The thickness of glial sheath was mean 66.5 µm (40-110 µm) at proximal REZ and 7.4 µm (5-10 µm) at distal REZ.ConclusionMedial REZ of facial nerve is mean 2.6 mm in length and covered by glial sheath continued from glial limiting membrane of brain stem. Glial sheath of central myelin tends to become thin toward transitional zone.

Transtemporal approach to the removal of a lateral pontine tumor

This video demonstrates a very useful trajectory to a pontine lesion. A 68-year-old man presented with tongue numbness and weakness. The approach used was a transtemporal presigmoid retrolabyrinthine approach to enable an orthogonal trajectory to the lateral pons. Following the transtemporal opening, the root entry zone of the trigeminal nerve and the root exit zone of the facial nerve are identified. The lateral pons is incised to access the tumor, which upon histological analysis was found to be a metastasis. Excellent visualization of the lateral pons is achieved. The opening, relevant anatomy, and closure are illustrated. The video can be found here: http://youtu.be/vS5fCOY6vp8 .

Preoperative assessment of hemifacial spasm by the coronal heavily T2-weighted MR cisternography

Microvascular decompression (MVD) has become a well-established surgical procedure for hemifacial spasm (HFS). Before surgery, it is essential to evaluate any possible deformity of the brainstem and establish the precise location of the offending vessels. In the present study of HFS patients we examined coronal sections taken by heavily T2-weighted MR cisternography in addition to routine axial sections, and assessed the usefulness of these images through comparison with intraoperative findings. Eighty patients with HFS underwent preoperative coronal heavily T2-weighted MR cisternography before microvascular decompression surgery. Three neurosurgeons examined the preoperative axial and coronal MR images and evaluated vessel invagination into the brainstem. The usefulness of coronal sections was assessed statistically by the Mann-Whitney U test. Invagination of the offending vessel into the brainstem was observed in 24 cases (30.0%). In 19 patients, it was predicted preoperatively that compression of the flocculus and brainstem would be required in order to approach the offending vessels. Coronal MR cisternography was significantly more useful in cases with vessel invagination into the brainstem than in cases without invagination. Coronal sections obtained by MR cisternography are able to demonstrate the severity of vessel invagination into the brainstem as well as revealing the presence of the offending vessel. This information is helpful for planning a suitable approach to the root exit zone.

Active Role of Cerebellar Flocculus in Hemifacial Spasm: Case Report

Hemifacial spasm (HFs) is an uncommon disorder characterized by involuntary paroxysmal movement of one-half of the face, accounting for a yearly incidence of about 1 person per 10,000 people [1]. It is generally known that essential HFs is caused by vascular compression of the root exit zone (reZ) of the facial nerve [1]. Magnetic resonance imaging (MrI) is established as the main diagnostic modality to examine facial anatomy and to evaluate anatomical relationships between the nerve and blood vessels. The role of the flocculus as a cause of facial nerve compression in case of HFs has not been described yet in the literature. Authors reported the case of a 56-year-old woman presenting with left HFs grade II (sMC grading system) [1] due to facial nerve compression by a hypertrophied flocculus, which was evident in preoperative Mr images and later confirmed by intraoperative findings.

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

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.