The Vestibulocochlear Nerve: Anatomy and Pathology.

The vestibulocochlear nerve is the eighth cranial nerve, entering the brainstem in the medullopontine sulcus after crossing the internal auditory canal and cerebellopontine angle cistern. It is a purely sensitive nerve, originating from the Scarpa's and spiral ganglions, responsible for balance and hearing. It has 6 nuclei located in the lower pons. Magnetic resonance imaging (MRI) is useful for evaluating the vestibulocochlear nerve, although computed tomography may have a complementary role in assessing bone lesions. A heavily T2-weighted sequence, such as fast imaging employing steady-state acquisition (FIESTA) or constructive interference steady state (CISS), is crucial in imaging exams to depict the canalicular and cisternal segments of the vestibulocochlear nerve, as well as the fluid signal intensity in the membranous labyrinth. The vestibulocochlear nerve can be affected by several diseases, such as congenital malformations, trauma, inflammatory or infectious diseases, vascular disorders, and neoplasms. The purpose of this article is to review the vestibulocochlear nerve anatomy, discuss the best MRI techniques to evaluate this nerve and demonstrate the imaging aspect of the main diseases that affect it.

Is the cochleovestibular nerve function affected in patients with hemifacial spasm?

Hemifacial spasm (HFS) is a motor disorder caused by the vascular compression of the facial nerve in the posterior fossa. The cochleovestibular nerve is close to the facial nerve and shares the same entry to the periphery, also has disorders caused by vascular compression. We evaluated the cochleovestibular nerve function in patients with HFS based on the hypothesis that vascular compression, which causes HFS, can also affect the nearby cochleovestibular nerve function. The medical charts of 49 patients with surgically confirmed HFS were reviewed retrospectively. The results of the pure-tone threshold, auditory brainstem response (ABR), video head impulse test (vHIT), and magnetic resonance imaging were analyzed. In each patient, the HFS side and the unaffected side were compared in the paired manner. The anterior inferior cerebellar artery was the major offending vessel (69.4%). There were no significant differences in the pure-tone threshold, properties of ABR waves, and vHIT gain. There was no evidence of cochleovestibular nerve compression syndrome in all patients. The angulation of the nerve by the offending vessel was more frequently identified in the HFS side than in the unaffected side (p = 0.040). The effect of HFS on cochleovestibular nerve function is limited.

Can Imaging Predict Hearing Outcomes in Children With Cochleovestibular Nerve Abnormalities?

OBJECTIVES/HYPOTHESIS: To identify the imaging characteristics associated with better hearing outcomes found in cochleovestibular nerve (CVN) abnormalities treated with hearing aids and/or cochlear implantation (CI). STUDY DESIGN: Retrospective review. METHODS: A retrospective review was undertaken of 69 ears with CVN abnormalities seen on magnetic resonance imaging (MRI) treated at a tertiary referral academic center analyzing the clinical features, imaging characteristics, and hearing data. We searched for associations among the hearing and imaging data, hypothesizing that the imaging data was not a good indicator of hearing function. RESULTS: In univariable analysis of all those who underwent aided testing (hearing aid and CI), health status (P = .016), internal auditory canal (IAC) midpoint diameter (P < .001), and number of nerves in the IAC (P < .001) were predictors of positive hearing outcome. Modiolar abnormalities, cochlear aperture diameter, cochlear malformations, vestibular malformations, and nerves in the cerebellar cistern did not predict hearing outcome (P = .79, .18, .59, .09, .17, respectively). For patients who received CI, health status (P = .018), IAC midpoint (P = .024), and number of nerves in the IAC (P = .038) were significant. When controlling for health status, IAC midpoint diameter (P < .001) and number of nerves in the IAC (P < .001) remained significant. In our cohort, one out of the eight ears (13%) with Birman class 0 or 1 exhibited responses to sound compared to nine out of 13 ears (70%) with Birman class 2-4. CONCLUSIONS: Current imaging modalities cannot accurately depict the status of the cochleovestibular nerve or predict a child's benefit with a CI. Cochlear implantation should be considered in children with abnormal cochleovestibular nerves. LEVEL OF EVIDENCE: 3 Laryngoscope, 132:S1-S15, 2022.

Vestibular paroxysmia: Clinical features and imaging findings; a literature review.

According to the definition of neurovascular compression syndromes (NVCS), a vascular structure in direct contact with a cranial nerve is causing mechanical irritation of the neural tissue producing correlating symptoms. Vestibular paroxysmia is an example of a neurovascular compression which is caused by neurovascular contact between the eighth cranial nerve and a vessel. It is crucial to understand the unique anatomy of the vestibulocochlear nerve in order to study the syndrome which is the result of its compression. More specifically, the long transitional zone between central and peripheral myelin plays a central role in clinical significance, as the transitional zone is the structure most prone to mechanical injury. Imaging techniques of the eighth cranial nerve and the surrounding structures are substantial for the demonstration of clinically significant cases and potential surgical decompression. The goal of the current review is to present and study the existing literature on vestibular paroxysmia and to search for the most appropriate imaging technique for the syndrome. An extensive literature search of PubMed database was performed, and the studies were ranked based on evidence-based criteria, followed by descriptive statistics of the data. The present analysis indicates that 3D CISS MRI sequence is superior to any other sequence, in the most studies reviewed, regarding the imaging of neurovascular compression of the eighth cranial nerve.

Correlation between Cranial Nerve Microstructural Characteristics and Vestibular Schwannoma Tumor Volume.

BACKGROUND AND PURPOSE: Vestibular schwannomas are common cerebellopontine angle tumors arising from the vestibulocochlear nerve and can result in cranial nerve dysfunction. Conventional MR imaging does not provide information that could correlate with cranial nerve compression symptoms of hearing loss or imbalance. We used multitensor tractography to evaluate the relationship between the WM microstructural properties of cranial nerves and tumor volume in a cohort of patients with vestibular schwannomas. MATERIALS AND METHODS: A retrospective study was performed in 258 patients with vestibular schwannomas treated at the Gamma Knife clinic at Toronto Western Hospital between 2014 and 2018. 3T MR images were analyzed in 160 surgically naïve patients with unilateral vestibular schwannomas. Multitensor tractography was used to extract DTI-derived metrics (fractional anisotropy and radial, axial, and mean diffusivities of the bilateral facial and vestibulocochlear nerves [cranial nerves VII/VIII]). ROIs were placed in the transition between cisternal and intracanalicular segments, and images were analyzed using the eXtended Streamline Tractography reconstruction method. Diffusion metrics were correlated with 3D tumor volume derived from the Gamma Knife clinic. RESULTS: DTI analyses revealed significantly higher fractional anisotropy values and a reduction in axial diffusivity, radial diffusivity, and mean diffusivity (all P < .001) within the affected cranial nerves VII and VIII compared with unaffected side. All specific diffusivities (axial, radial, and mean diffusivity) demonstrated an inverse correlation with tumor volume (axial, radial, and mean diffusivity, P < .01). CONCLUSIONS: Multitensor tractography allows the quantification of cranial nerve VII and VIII WM microstructural alterations in patients with vestibular schwannomas. Our findings support the hypothesis that tumor volume may cause microstructural alterations of the affected cranial nerves VII and VIII. This type of advanced imaging may represent a possible avenue to correlate diffusivities with cranial nerve function.

Automated facial-vestibulocochlear nerve complex identification based on data-driven tractography clustering.

Small size and intricate anatomical environment are the main difficulties facing tractography of the facial-vestibulocochlear nerve complex (FVN), and lead to challenges in fiber orientation distribution (FOD) modeling, fiber tracking, region-of-interest selection, and fiber filtering. Experts need rich experience in anatomy and tractography, as well as substantial labor costs, to identify the FVN. Thus, we present a pipeline to identify the FVN automatically, in what we believe is the first study of the automated identification of the FVN. First, we created an FVN template. Forty high-resolution multishell data were used to perform data-driven fiber clustering based on the multishell multitissue constraint spherical deconvolution FOD model and deterministic tractography. We selected the brainstem and cerebellum (BS-CB) region as the seed region and removed the fibers that reach other brain regions. We then performed spectral fiber clustering twice. The first clustering was to create a BS-CB atlas and separate the fibers that pass through the cerebellopontine angle, and the other one was to extract the FVN. Second, we registered the subject-specific fibers in the space of the FVN template and assigned each fiber to the closest cluster to identify the FVN automatically by spectral embedding. We applied the proposed method to different acquirement sites, including two different healthy datasets and two tumor patient datasets. Experimental results showed that our automatic identification results have ideal colocalization with expert manual identification in terms of spatial overlap and visualization. Importantly, we successfully applied our method to tumor patient data. The FVNs identified by the proposed method were in agreement with intraoperative findings.

Retro-labyrinthine Lesion Site Detected by Galvanic Vestibular Stimulation Elicited Vestibular-evoked Myogenic Potentials in Patients with Auditory Neuropathy.

OBJECTIVE: Auditory neuropathy (AN) is a unique pattern of hearing loss with preservation of hair cell function. The condition is characterized by the presence of otoacoustic emissions (OAE) or cochlear microphonic (CM) responses with severe abnormalities of the auditory brainstem response (ABR). The vestibular branches of the VIII cranial nerve and the structures innervated by it can also be affected. However, the precise lesion sites in the vestibular system are not well characterized in patients with AN. METHODS: The air-conducted sound (ACS) vestibular-evoked myogenic potentials (VEMPs) and galvanic vestibular stimuli (GVS)-VEMPs were examined in 14 patients with AN. RESULTS: On examination of VEMPs (n=14, 28 ears), the absent rates of ACS-cervical VEMP (cVEMP), ACS-ocular VEMP (oVEMP), GVS-cVEMP, GVS-oVEMP and caloric test were 92.9% (26/28), 85.7% (24/28), 67.9% (19/28), 53.6% (15/28), and 61.5% (8/13), respectively. Impaired functions of the saccule, inferior vestibular nerve, utricle, superior vestibular nerve, and horizontal semicircular canal were found in 25.0% (7/28), 67.9% (19/28), 32.1% (9/28), 53.6% (15/28) and 61.5% (8/13) patients, respectively. On comparing the elicited VEMPs parameters of AN patients with those of normal controls, both ACS-VEMPs and GVS-VEMPs showed abnormal results in AN patients (such as, lower presence rates, elevated thresholds, prolonged latencies, and decreased amplitudes). CONCLUSION: The study suggested that patients with AN often have concomitant vestibular disorders. Retro-labyrinthine lesions were more frequently observed in this study. GVS-VEMPs combined with ACS-VEMPs may help identify the lesion sites and facilitate detection of areas of vestibular dysfunction in these patients.

The use of magnetic resonance imaging in the investigation of patients with unilateral non-pulsatile tinnitus without asymmetrical hearing loss.

BACKGROUND: Tinnitus is a common condition presenting to the ENT out-patient clinic. Vestibular schwannomas are benign cerebellopontine angle tumours that usually present with unilateral sensorineural hearing loss. Magnetic resonance imaging of the internal auditory meatus is the definitive investigation in their detection. The current recommendation is for unilateral tinnitus patients to undergo magnetic resonance imaging of the internal auditory meatus to exclude vestibular schwannoma. OBJECTIVE: To evaluate magnetic resonance imaging in the investigation of patients with unilateral non-pulsatile tinnitus without asymmetrical hearing loss. METHOD: A retrospective case series was conducted of all patients who underwent magnetic resonance imaging of the internal auditory meatus to investigate unilateral non-pulsatile tinnitus without asymmetrical hearing loss, from 1 January 2014 to 1 January 2019. RESULTS: Of 2066 scans, 566 (27 per cent) were performed to investigate patients (335 female, 231 male) with unilateral non-pulsatile tinnitus without asymmetrical hearing loss. Three vestibular schwannomas were detected on imaging, and 134 incidental findings were discovered. CONCLUSION: The detection rate of vestibular schwannoma in this group was just 0.3 per cent. This paper questions the utility of magnetic resonance imaging evaluation in these patients.

Applying artificial intelligence to longitudinal imaging analysis of vestibular schwannoma following radiosurgery.

Artificial intelligence (AI) has been applied with considerable success in the fields of radiology, pathology, and neurosurgery. It is expected that AI will soon be used to optimize strategies for the clinical management of patients based on intensive imaging follow-up. Our objective in this study was to establish an algorithm by which to automate the volumetric measurement of vestibular schwannoma (VS) using a series of parametric MR images following radiosurgery. Based on a sample of 861 consecutive patients who underwent Gamma Knife radiosurgery (GKRS) between 1993 and 2008, the proposed end-to-end deep-learning scheme with automated pre-processing pipeline was applied to a series of 1290 MR examinations (T1W+C, and T2W parametric MR images). All of which were performed under consistent imaging acquisition protocols. The relative volume difference (RVD) between AI-based volumetric measurements and clinical measurements performed by expert radiologists were + 1.74%, - 0.31%, - 0.44%, - 0.19%, - 0.01%, and + 0.26% at each follow-up time point, regardless of the state of the tumor (progressed, pseudo-progressed, or regressed). This study outlines an approach to the evaluation of treatment responses via novel volumetric measurement algorithm, and can be used longitudinally following GKRS for VS. The proposed deep learning AI scheme is applicable to longitudinal follow-up assessments following a variety of therapeutic interventions.

Tracking Spontaneous Vestibular Schwannoma Regression with Volumetric Measurements.

OBJECTIVE: To characterize a series of patients with MRI evidence of spontaneous vestibular schwannoma (VS) regression. STUDY DESIGN: Retrospective case series. METHODS: Retrospective review between 2012 and 2020 from a single, tertiary-care center of all patients with an untreated, sporadic VS and spontaneous regression in volumetric tumor size over the course of observation. The main outcome measures included VS size and location, presenting symptoms, medication use, changes in pure-tone averages and word recognition scores. RESULTS: The 13 treatment-naïve patients (62% female, mean age 67.1 years) with spontaneous VS regression represented 3.9% of all patients undergoing observation with serial imaging during the study period. Median tumor size from initial MRI was 529.0 mm3 (range: 108 mm3 -13,180 mm3 ). The mean interval between MRI measurements was 5.5 years (SD 4.4 years). The average percent decrease in tumor size was 36.1% (SD 21.9%) and the average rate of volume decrease was 15.8 mm3 /yr (SD 25.4 mm3 /yr). Five patients were classified as having major regression, defined by a relative decrease in volume of >40%, while eight patients had minor regression (<40% relative volume reduction). No significant differences in initial tumor size, rate of regression, or audiometric changes were observed between the major and minor regression cohorts. CONCLUSIONS: Patients with evidence of a spontaneously shrinking VS have a heterogeneous presentation. Due to the scarcity of this phenomenon, predicting which tumors will eventually undergo regression remains unclear. Employing volumetric measurements to compare serial MRI scans may improve the accuracy of detecting shrinking tumors. LEVEL OF EVIDENCE: 4 Laryngoscope, 131:E1647-E1652, 2021.

MRI surveillance following concurrent cochlear implantation in cases of vestibular schwannoma resection.

OBJECTIVE: Cochlear nerve preserving translabyrinthine vestibular schwannoma (VS) resection enables concurrent cochlear implantation. Implantation in patients with VS raises important concerns including the ability to undergo postoperative magnetic resonance imaging (MRI) monitoring of residual tumor growth or tumor recurrence, specifically with a retained magnet. We aim to assess the feasibility of MRI monitoring and the impact on image quality with retained cochlear implant (CI) magnets. METHODS: Retrospective review of post-operative head MRI scans in CI recipients with a retained CI magnet, after cochlear nerve preserving translabyrinthine excision of VS. The ability to visualize the ipsilateral and contralateral internal auditory canal (IAC) and cerebellopontine angle (CPA) were assessed. RESULTS: A total of eight surveillance head MRI were performed in six patients. In one case, in which the receiver was positioned lower, the view of the ipsilateral IAC and CPA was distorted. In all other cases, the views of both the ipsilateral and contralateral IAC and CPA were overall unimpaired. DISCUSSION: Imaging artifact only very rarely impedes adequate visualization of the ipsilateral IAC or CPA in CI recipients. In anticipation of the need for further IAC and CPA imaging, it would be advisable to place the receiver in an exaggerated superior-posterior position to further decrease obscuring artifact. Thus, serial monitoring of VS tumors can be performed safely with preservation of image quality with a retained receiver magnet. CONCLUSIONS: When placing the CI receiver-stimulator farther posterior-superiorly, excellent visualization of the IAC and CPA can be accomplished without significantly impairing the image quality.

MRI findings in vestibular paroxysmia - An observational study.

BACKGROUND/OBJECTIVES: Vestibular paroxysmia (VP) presents as episodic vertigo believed to be caused by neurovascular cross-compression (NVCC) of the vestibulocochlear nerve. We investigated whether NVCC occurred at a higher rate in VP, compared with controls and whether angulation of the nerve, the vessel involved and location of the point of contact were significant features. METHODS: Retrospective analysis was conducted of MR imaging performed in patients with VP and also patients with unilateral tinnitus (in whom the asymptomatic side was used as a control). Two independent, blinded reviewers assessed each case. RESULTS: Nine patients with VP and 20 patients with unilateral tinnitus were included. NVCC was demonstrated in all 9 VP patients (100%), compared with 9 of the controls (45%), p = 0.0049. NVCC was mostly caused by a branch of the anterior inferior cerebellar artery (AICA). Nerve angulation at the point of contact occurred in 5 of the cases (44%), but in none of the controls (specificity = 100%), p = 0.0053. There was no correlation between site of contact and VP. CONCLUSION: Our study supports the concept of NVCC in VP and additionally suggests that nerve angulation may be a specific feature. Neurovascular contact remains a common phenomenon in asymptomatic patients and therefore correlation with neuro-otology assessment remains essential.

3 T - MRI improves intrameatal cranial nerve detection in CI-candidates.

BACKGROUND: Cochlear implantation meanwhile is a well established standard therapy in different forms of hearing loss in most ages. Clear depiction of the cochleovestibular anatomy and the cochlear nerve are very important for indication and effectiveness of cochlear implantation. Aim/objectives: Therefore the question raises whether Highfield MRI improves precision of preoperative diagnostic imaging especially along the the intrameatal portion of the vestibulocochlear nerve. METHODS: We analyzed the MRI scans of 216 temporal bones from 2007 until 2017. In all patients, CI was planned. We compared examinations using 1.5 and 3.0 T MR systems under the aspect of intracanalicular detection of the facial nerve, cochlear nerve, and the vestibular branches. RESULTS: 3 T-MRI was able to detect the cochlear nerve in all cases, a very important criterion for CI-indication. 3 T-MRI was also superior in the detection of facial nerve and especially the vestibular branches. CONCLUSION AND SIGNIFICANCE: The most effective preoperative examination of CI-candidates is the combination of 3 T MRI and multislice HR-CT of the temporal bone.

Sensorineural hearing loss in patients with vestibular schwannoma correlates with the presence of utricular hydrops as diagnosed on heavily T2-weighted MRI.

PURPOSE: The purpose of this study was to assess whether the volume of the vestibular endolymphatic space correlates with the degree of hearing loss using heavily T2-weighted fast imaging employing steady-state acquisition with cycle phase (FIESTA-C) MRI. MATERIALS AND METHODS: A total of 23 patients with vestibular schwannoma, as diagnosed on typical image findings, who underwent FIESTA-C MRI were included. There were 13 women and 10 men with a mean age of 63.5±9.3 (SD) years (range: 49-88years). Two radiologists independently evaluated the volume of the utricle and saccule. Correlation between tumor volume, vestibular endolymphatic space volume and degree of hearing loss - as evaluated with the levels of pure-tone average and speech recognition threshold - were searched for. RESULTS: The mean saccular, utricular and tumor volumes were 3.17±1.1 (SD) mm3 (range: 1.45-5.7mm3), 14.55±5 (SD) mm3; (range: 6.6-23.9mm3) and 17.4±5.5 (SD) mm3; (range: 8.3-25.4mm3), respectively. There was a moderate correlation between the volume of the utricle and the degree of hearing loss as evaluated with the levels of pure-tone average (rho=0.5; P=0.015) and speech recognition threshold (rho=0.58; P=0.004). There were no significant correlations between saccular and tumor volumes and the degree of hearing loss. CONCLUSION: The volume of the utricle in patients with obstructive vestibular schwannoma moderately correlates with the degree of hearing loss.

Management of Vestibular Schwannomas for the Radiologist.

Vestibular schwannomas are the most common tumor of the cerebellopontine angle. The history of their management has driven advances in imaging, lateral skull base surgery, as well as radiosurgery. With these advances, a shift has occurred from life-saving treatment for late-stage disease to quality of life focused management of smaller tumors. The complicated treatment paradigms involving observation, stereotactic radiosurgery and surgery require close communication between the treatment and neuroradiology teams.

Comparison of Clinical, Histopathological, and Genomic Features Between Malignant Peripheral Nerve Sheath Tumors and Cellular Schwannomas of the Eighth Cranial Nerve: A Case Series.

BACKGROUND: Malignant peripheral nerve sheath tumors (MPNSTs) and cellular schwannomas (CSs) of the eighth cranial nerve are exceedingly rare. The purpose of the present study was to evaluate clinical and genetic characterization of these rare tumors. METHODS: The clinical and radiological features were analyzed retrospectively. The histopathological characteristics were assessed by hematoxylin and eosin staining and immunohistochemistry. Genomic abnormalities were evaluated using array comparative genomic hybridization. RESULTS: Of the 1287 surgeries for vestibular schwannomas from 2014 to 2017, 2 were for MPNSTs and 5 were for CSs. The mean age at diagnosis was older for patients with MPNSTs (57.0 ± 4.2 years) than that of patients with CS (35.8 ± 9.4 years; P = 0.03). Two patients with MPNST died of tumor recurrence. None of the patients with CS died. The 2-year overall and progression-free survival of patients with MPNSTs were worse than those for patients with CSs (overall survival, 50.0% ± 35.4% vs. 100%, P = 0.027; progression-free survival, 0% vs. 100%; P = 0.012). The Ki-67 index for the MPNSTs (29.0% ± 3.5%) was greater than that for the CSs (10.3% ± 3.1%; P = 0.001). The common alterations in MPNSTs mainly included gains of chromosomes 7p, 8p, 9q, 12, and 17 and loss of heterozygosity of 1p, 6 and 9p. The common alterations in CSs included gain of 4p16.3, loss of heterozygosity of 2p15-14, and 22q11.1-13.3. CONCLUSIONS: To the best of our knowledge, the present study is the first high-resolution genomic analysis of MPNSTs and CSs of the eighth cranial nerve and has shown a significant difference that might be more accurate to distinguish between these 2 types of rare tumors.

Glioneuronal Heterotopia Presenting as Cerebellopontine Angle Tumor of Cranial Nerve VIII.

BACKGROUND: Vestibular schwannomas and meningiomas account for the great majority of lesions arising in the cerebellopontine angle. In this report, we present a case of glioneuronal heterotopia, also known as glioneuronal hamartoma, arising from cranial nerve VIII, which is an extremely uncommon lesion. Important radiologic and surgical aspects are reviewed, which may help in early recognition and intraoperative decision making when these lesions are encountered. CASE DESCRIPTION: A healthy 29-year-old female presented with intermittent right facial numbness. Magnetic resonance imaging showed an incidental, minimally enhancing cerebellopontine angle lesion on the right cranial nerve VII-VIII complex. The patient declined serial observation and opted for operative intervention for resection. Intraoperatively, the lesion resembled neural tissue and was continuous with the VIII cranial nerve. Pathologic analysis demonstrated mature glioneuronal tissue consistent with hamartomatous brain tissue. The patient maintained normal hearing and facial nerve function after surgery. Radiologic, surgical, and pathologic characteristics are described. CONCLUSIONS: Ectopic glioneuronal tissue of cranial nerve VIII is a rare non-neoplastic lesion and should be considered in the differential diagnosis of unusual-appearing intracanalicular and cerebellopontine angle lesions. The congenital and benign nature of this entity makes observation a valid option for these cases, although they are so infrequent that they are often presumptively managed as vestibular schwannomas. Attempts to radically resect these lesions may result in higher rates of hearing loss or facial palsy due to their continuity with cranial nerves.