“Medial” Vestibular Schwannoma: Excision using a Retrosigmoid Approach

Key Message: • A typical vestibular schwannoma has an intrameatal extension, even if it is small. In the absence of this meatal extension, an alternate intraoperative diagnosis must be considered. • Some authors have hypothesized a nervus intermedius origin of these schwannomas. • It is possible to completely resect these tumors with facial nerve preservation.Vestibular schwannoma (VS), the commonest tumor of the cerebellopontine angle (CPA), accounts for nearly 95% of all CPA lesions.[1,2] This tumor typically arises from the vestibular nerve, commonly the superior division from the point where central myelin gives rise to the peripheral myelin. This zone, known as the Obersteiner Reidlich zone, lie approximately 9.75 mm from the brainstem, at or near the medial end of Internal Auditory Meatus (IAM).[3] Therefore, some form of intrameatal extension is always seen with VS, so much so that, this radiological feature is considered a sine qua non for VS diagnosis. In 1992, Tos et al.[4] coined the term “medial VS” as those rare variants that have no meatal extension. Subsequent reports provided better characterization of these tumors.[5,6] Recently, a possible origin from the intermediate nerve (IN)/nerve of Wrisberg nerve has been postulated.[7] Considering the rarity of the condition and the absence of any published surgical video as a teaching material, the authors think this would be a useful educational resource. Objective: To draw attention to the unique “medial” vestibular schwannomas, illustrate the operative findings and revisit the basic principles of CPA tumor excision. Surgical technique/procedure An illustrative case is shown with pre and postoperative imaging and intraoperative photographs [Figures 1F2-3]. The patient was taken up for a retrosigmoid craniotomy (3 * 3 cm) in a park bench position. Dura was opened in a standard C-shape. The tumor was cystic, moderately vascular and confined to the CPA cistern with normal meatal anatomy. Arachnoid over the tumor was peeled off followed by an intra-tumoral decompression. Using a combination of microsurgical dissection and facial nerve triggered electromyogram (Tr EMG) response, the facial nerve was identified. A small pedicle connected the tumor with the arachnoid covered facial nerve that allowed a gross total tumor resection and a functional facial nerve preservation. There was no intrameatal tumor extension.Figure 1: Magnetic Resonance Imaging (MRI) Brain (a) T2 axial section, (b) T2 Coronal section, (c) ADC showing T2 hyperintense, predominantly cystic lesion of around 3.5 to 4 cm occupying right Cerebellopontine angle region without extension into the internal auditory meatus. (d), (e) Non-Contrast Computed Tomography (NCCT) head bone window showing no meatal dilatation or destructionFigure 2: Postoperative Magnetic Resonance Imaging (MRI) brain at 2 months post-surgery (a) T2 Axial, (b) Post-contrast Axial image showing complete tumor resection with small resolving hemorrhage in right superior cerebellar peduncle. (c) Clinically preserved facial nerve function at 2 months follow-upFigure 3: Intraoperative image showing (a) Tumor and Petrous Dura following CSF release and cerebellar retraction, (b) 7TH-8TH Nerve complex entering the Internal Acoustic Meatus (IAM), loop of Anterior Inferior Cerebellar Artery (AICA) with no intrameatal tumor extension, (c) Stimulation of 7th-8th nerve complex along with the Nerve of Origin of tumor and (d) Arachnoid covered 7th-8th nerve complex with small remnant of tumor left attached to Nerve of OriginVideo hyperlink https://youtu.be/6yBAeaL3qEY QR Code: Video audio transcript 0.00–0.38: In this video, the authors present the technical nuances of medial vestibular schwannoma, excision by using a retrosigmoid approach. The patient was a 30-year-old gentleman with painless progressive right sided hearing loss with, loss of taste sensation right side, numbness on the right-side face and imbalance, while walking for 2 months. Neurological examination showed a complete loss of taste sensation on the right side, anterior two third of the tongue with 30% sensory loss on the right hemi face. He had cerebellar signs and no evidence of facial motor weakness. On pure tone audiometry patient had profound right hearing loss. On neuroimaging he had right side T2 hyperintense, predominantly cystic lesion occupying right CP angle region, without extension into the internal auditory meatus. On CT scan we did not see any dilatation of the internal auditory meatus on any side. 0.39–1.20: We prefer a park bench position and a lazy s incision, as shown here. Following dural opening, the tumor was exposed. The arachnoid was swept towards the cerebellum. The dorsal capsule of the tumor is being mapped. On the upper pole, we can see the 5th cranial nerve. It is dissected and lifted away. At the lower pole, we can see the lower cranial nerves. We can see the 7th cranial nerve which has come into our view. Coming back to the upper pole again, the 5th cranial nerve was draped on the tumor. Using the combination of sharp and blunt dissection, this was lifted. 1.22–2:05: Toward the internal auditory meatus, we can see the anterior inferior cerebellar artery (AICA) loop, the 7th/8th complex, and a lack of any extension of this tumor into the internal auditory meatus. Using a sharp dissection, the AICA loop was freed. We can see the 7th/8th complex. The tumor is rolled from lateral toward the medial direction. This is the attachment of the tumor to the 7th complex with an arachnoid cover over it. On stimulation, this nerve did not show any stimulus. possibly indicating a sensory division of the 7th nerve. You can see the 5th cranial nerve totally covered with arachnoid. 2.06–2.39: Coming back to the tumor again, the tumor has been debulked now and we can very well see the point of attachment to, possibly the intermediate nerve here. Therefore, to preserve the intermediate nerve, the tumor was divided leaving a very small stump onto the nerve of origin as you see here. Finally, the tumor was completely removed. We can see the arachnoid covered 7/8th complex and a small remnant of the tumor that was kept over there. 2.40–2.58: The patient had an excellent radiological outcome. Clinically, the patient recovered without any facial nerve deficits. Although he reported subjective improvement in hearing, but audiometry showed there were no improvement and possibly the hearing was lost irreversibly in the preoperative period only. So, these are the references and thank you for listening Conclusion Medial VSs are CPA tumors with interesting and unique characteristics. Irrespective of the speculations regarding the point of origin, a lack of intrameatal extension must raise a clinical suspicion. Intraoperatively, as we showed, absolute adherence to the arachnoid layer dissection is mandatory to reach the point of tumor origin. A complete excision is possible with preservation of the facial nerve. Pearls and pitfalls The key is to clinically evaluate a CPA mass. Loss of taste sensation clinically in a patient with CPA mass that does not extend to the IAM, is a typical medial VS. As our findings show, many of these patients could arise from the nervus intermedius. The retrosigmoid craniotomy must expose the transverse sigmoid junction and must be enough lateral to allow a straight line of sight to the CPA. Once the arachnoid over the tumor is peeled off to free it from the cerebellum, a generous tumor debulking is recommended. Once the tumor bulk has reduced, it is possible to verify the meatal bony anatomy and tumor extent. In a case like the one shown here, there will be no meatal extension of the tumor. Traction on the tumor must be minimal to avoid an indirect damage to the neurovascular pedicle it relates to. Before handling the tumor origin, it is important to reduce the tumor size to the maximum amount as possible. Facial nerve monitoring is invaluable with a combination of triggered facial electromyogram and cortico-bulbar motor evoked potential working best in our hands. Informed consent A full and detailed consent from the patient/guardian has been taken. The patient’s identity has been adequately anonymized. If anything related to the patient’s identity is shown, adequate consent has been taken from the patient/relative/guardian. The journal will not be responsible for any medicolegal issues arising out of issues related to patient’s identity or any other issues arising from the public display of the video. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.