How I do it: retrosigmoid intradural inframeatal petrosectomy

Background Lesions infiltrating the petrous temporal bone are some of the most complex to treat surgically. Many approaches have been developed in order to address these lesions, including endoscopic endonasal, anterior petrosectomy, posterior petrosectomy, and retrosigmoid. Method We describe in a stepwise fashion the surgical steps of the retrosigmoid intradural inframeatal petrosectomy. Conclusion The retrosigmoid intradural inframeatal petrosectomy may afford satisfactory exposure with limited drilling and minimal disruption of perilesional anatomical structures. It can provide excellent surgical results, especially for soft tumors, while minimizing surgical morbidity. Electronic supplementary material The online version of this article (10.1007/s00701-020-04587-0) contains supplementary material, which is available to authorized users.


Introduction
Lesions infiltrating the petrous temporal bone are certainly among the most difficult to treat in neurosurgery. Multiple approaches have been developed in order to access the petroclival space, and which approach is chosen should be assessed on a case-by-case basis [6,8]. Each of these approaches, such as the endoscopic endonasal [2,10], subtemporal [3], or anterior or combined petrous [5,7] routes comes with certain built-in surgical comorbidities that must be accepted and explained to the patient [1]. We present a case of a grade II chondrosarcoma of the petrous bone ( Fig. 1) operated (TRM) via the retrosigmoid intradural inframeatal petrosectomy (RESIP), and aim to discuss the advantages, but also the pitfalls and challenges involved.

Relevant surgical anatomy
Multiple key anatomic landmarks must be evaluated [4,9]. We rely on craniometric landmarks adjusted based on patient individual anatomy from preoperative imaging as well as neuronavigation and augmented reality. In planning the craniotomy, the transverse (TS) and sigmoid sinus (SS) must be identified. In charting the surgical trajectory, the pneumatization of the mastoid bone, the caudal cranial nerves (CNs), the jugular bulb, the superior and inferior petrosal sinuses (SPS, IPS), the acoustico-facial bundle, the internal acoustic canal (IAC), the petrous apex, the carotid canal, and Eustachian tube must be taken into account and outlined for the use of neuronavigation and augmented reality.

Patient positioning and preparation
The patient is positioned in a supine position, with elevation of the ipsilateral shoulder. The head is fixed in a Doro® headclamp and turned 100°contralaterally with the vertex turned This article is part of the Topical Collection on Neurosurgery general Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00701-020-04587-0) contains supplementary material, which is available to authorized users. slightly downwards. Brainlab® neuronavigation and augmented reality are installed, and the incision and craniotomy are planned. Markings are placed on the skin for the asterion, the trajectory of the TS and SS, and a curvilinear skin incision. Inomed® neuromonitoring is installed for auditory evoked potentials and caudal cranial nerves V-XII.

Skin incision and soft tissue dissection
After minimal shaving and standard draping, a curvilinear, retroauricular skin incision is made extending from the mastoid tip, posterior to the asterion, and superior to the TS. Using a periosteal elevator, the skin and occipital muscles are elevated, and a self-retaining retractor is placed.

Retrosigmoid craniotomy
The asterion is localized, and the TS and SS are verified with neuronavigation. A single burr hole is performed with a 50mm cutting burr just medial and inferior to the TS-SS junction. A 2.5 × 3.0 cm craniotomy is performed. The SS and TS are further deskeletonized with the cutting burr. Mastoid air cells are opened in order to allow sufficient inferior access. The air cells are ablated with bone wax before the dural opening.

Intradural dissection
The dura is opened following a C-shaped incision along the SS. The arachnoid membrane of the posterior wall of the cerebello-medullary cistern is opened. CSF is drained by aspiration until sufficient cerebellar relaxation is achieved. The jugular foramen, the hypoglossal canal, as well as the caudal cranial nerves and the CN VII/ VIII bundle, are identified (Fig. 2).

Inframeatal petrosectomy
The inframeatal dura is identified between the internal auditory meatus and the jugular bulb. The periosteum is incised with a knife and peeled off with a Rhoton dissector #3 (Fig. 3). A match stick 2-mm sharp burr (Medtronic® T9MH20 drill bit) is utilized to remove the bone overlying the tumor and then up to unfloor the IAC with visualization of the CN VII/VIII bundle (Fig. 4). The trajectory towards the petrous apex anteromedially is utilized.

Tumor resection
The surgical corridor begins inferiorly to the CN VII/VIII bundle with a trajectory medial to the semicircular canals and the fallopian canal. In addition, the greater superficial, lesser petrosal, and deep petrosal nerves are protected. The resection terminates on reaching the carotid canal. Fig. 2 The dura has been incised and the CN VII/VIII bundle is identified as well as the AICA In this case, the tumor was quite soft, wherefore resection was performed by aspiration and angled curettes (Fig. 5). The cavity can be inspected and the resection can be completed under visual guidance using an endoscope.
Important venous bleeding might be encountered from the IPS during tumor resection which is controlled by Sutter® bipolar electrocautery or Gelfoam®, compression, and irrigation.
The inframeatal approach affords a sufficient surgical corridor from a simple retrosigmoid craniotomy while precipitating less surgical comorbidity compared with anterior petrosectomy [1,10] or endoscopic endonasal approaches.

Closure
The dural defect over the jugular foramen is closed with TachoSeal®. Water-tight dural closure is performed with

Postoperative course
Postoperatively, our patient had an uneventful recovery with no CN deficits and no complications. She was mobilized the first postoperative day, and an MRI demonstrated a near-total resection of her chondrosarcoma (Fig. 6).

Indications
Smaller petroclival tumors or larger soft or cystic petroclival lesions.

Limitations
Upper petrous ridge tumors are limited by acoustic-facial bundle. Venous anatomy in case of high jugular bulb position precludes any drilling under and behind the acoustico-facial porus.    infection, and CSF leak should be discussed. & Given the particular anatomy, additional risks include damage to adjacent cranial nerves in the jugular foramen and the CN VII/VIII bundle, and the ICA and the vertebrobasilar arterial system, as well as risk of injury to the Eustachian tube. In specific, there is a risk of facial palsy, hearing loss, vertigo, dysphagia, cerebellar ataxia, stroke, and death. & Depending on the pathology, complementary treatment may be proposed, such as proton therapy.

How to avoid complications
Acknowledgments The authors would like to thank Philippe Bijlenga, MD, PhD, for the augmented reality data.
Funding Open access funding provided by University of Geneva.

Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Fig. 6 Postoperative imaging demonstrating near-total resection of the chondrosarcoma in the petrous apex left side on axial T1 Gd+ MRI subtraction series (a) and CT (b)