LABORATORY INVESTIGATION J Neurosurg 134:831–842, 2021 The endoscopic endonasal eustachian tube anterolateral mobilization strategy: minimizing the cost of the extreme-medial approach Mohamed A. Labib, MDCM,1 Evgenii Belykh, MD, PhD,1 Claudio Cavallo, MD,1 Xiaochun Zhao, MD,1 Daniel M. Prevedello, MD,2 Ricardo L. Carrau, MD,3 Andrew S. Little, MD,1 Mauro A. T. Ferreira, MD,4 Mark C. Preul, MD,1 A. Samy Youssef, MD, PhD,5 and Peter Nakaji, MD1 1Department of Neurosurgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona; Departments of 2Neurosurgery and 3Otolaryngology–Head and Neck Surgery, Wexner Medical Center at The Ohio State University, Columbus, Ohio; 4Department of Anatomy and Radiology, University Hospital, Federal University of Minas Gerais, Belo Horizonte, Brazil; and 5Department of Neurosurgery, University of Colorado Medical Center, Aurora, Colorado OBJECTIVE The ventral jugular foramen and the infrapetrous region are difficult to access through conventional lateral and posterolateral approaches. Endoscopic endonasal approaches to this region are obstructed by the eustachian tube (ET). This study presents a novel strategy for mobilizing the ET while preserving its integrity. Qualitative and quantitative comparisons with previous ET management paradigms are also presented. METHODS Ten dry skulls were analyzed. Four ET management strategies were sequentially performed on a total of 6 sides of cadaveric head specimens. Four measurement groups were generated: in group A, the ET was intact and not mobilized; in group B, the ET was mobilized inferolaterally; in group C, the ET underwent anterolateral mobilization; and in group D, the ET was resected. ET range of mobilization, surgical exposure area, and surgical freedom were measured and compared among the groups. RESULTS Wide exposure of the infrapetrous region and jugular foramen was achieved by removing the pterygoid pro- cess, unroofing the cartilaginous ET up to the level of the posterior aspect of the foramen ovale, and detaching the ET from the skull base and soft palate. Anterolateral mobilization of the ET facilitated significantly more retraction (a 126% increase) of the ET than inferolateral mobilization (mean ± SD: 20.8 ± 11.2 mm vs 9.2 ± 3.6 mm [p = 0.02]). Compared with group A, groups C and D had enhanced surgical exposure (142.5% [1176.9 ± 935.7 mm2] and 155.9% [1242.0 ± 1096.2 mm2], respectively, vs 485.4 ± 377.6 mm2 for group A [both p = 0.02]). Furthermore, group C had a significantly larger surgical exposure area than group B (p = 0.02). No statistically significant difference was found between the area of exposure obtained by ET removal and anterolateral mobilization. Anterolateral mobilization of the ET resulted in a 39.5% increase in surgical freedom toward the exocranial jugular foramen compared with that obtained through inferolat- eral mobilization of the ET (67.2° ± 20.5° vs 48.1° ± 6.7° [p = 0.047]) and a 65.4% increase compared with that afforded by an intact ET position (67.2° ± 20.5° vs 40.6° ± 14.3° [p = 0.03]). CONCLUSIONS Anterolateral mobilization of the ET provides excellent access to the ventral jugular foramen and infra- petrous region. The surgical exposure obtained is superior to that achieved with other ET management strategies and is comparable to that obtained by ET resection. https://thejns.org/doi/abs/10.3171/2019.12.JNS192285 KEYWORDS clivus; eustachian tube; extreme-medial approach; hypoglossal canal; jugular foramen; petroclival fissure; skull base RADITIONAL access to the jugular foramen and ad- summarized the inherent challenges associated with these jacent area is afforded via postauricular transtem- approaches, stating, “The difficulties in exposing this fo- poral, retrosigmoid, extreme-lateral transcondylar, ramen are created by its deep location and its proximity andT preauricular subtemporal-infratemporal approaches to critical anatomic structures, such as the carotid artery or by combinations thereof.2,4–7,12,14 Rhoton13 eloquently anteriorly, the facial nerve laterally, the hypoglossal nerve ABBREVIATIONS CN = cranial nerve; ET = eustachian tube; ICA = internal carotid artery. SUBMITTED August 20, 2019. ACCEPTED December 16, 2019. INCLUDE WHEN CITING Published online March 13, 2020; DOI: 10.3171/2019.12.JNS192285. ©AANS 2021, except where prohibited by US copyright law J Neurosurg Volume 134 • March 2021 831 Unauthenticated | Downloaded 10/09/21 07:09 AM UTC Labib et al. medially, and the vertebral artery inferiorly, all of which constant position of one of the rays forming the angles was block access to the foramen and require careful manage- defined as a ray originating at the anatomical landmark ment.” (jugular foramen or hypoglossal canal) and pointing to the In the past decade, endoscopic endonasal approaches to parapharyngeal tubercle (i.e., midline). A second ray orig- the jugular foramen and infrapetrous region have evolved inated at the anatomical landmark (jugular foramen or hy- as alternatives for resecting select lesions that originate poglossal canal) and pointed to the center of the orifice of at the petroclival fissure and displace the lower cranial the ET in each of the 3 evaluated positions, thus reflecting nerves (CNs) dorsally or laterally.10,11,16 Although some of the degree of surgical freedom with each paradigm. the anatomical constraints described previously by Rho- ton may not compromise the endoscopic approaches, these Results approaches are significantly restricted by the anatomical Relevant Osteological Landmarks position of the eustachian tube (ET). Two strategies to manage the ET have been complete resection, which risks Several anatomical relationships are of interest (Fig. chronic serous otitis and conductive hearing loss, and in- 2). First, the foramen ovale is located posterolateral to the ferolateral mobilization, which does not provide full expo- posterior aspect of the lateral pterygoid plate. The fora- sure of the jugular foramen. men has two lips that converge at its anterior and posterior We propose a novel strategy—anterolateral mobiliza- edges. We labeled the lateral and medial lips the “lateral tion—to mobilize the ET laterally and create direct access ovale process” and the “medial ovale process,” respective- to the jugular foramen, hypoglossal orifice, and parapha- ly. The anterior third of the medial ovale process, which is ryngeal internal carotid artery (ICA), while obviating the relevant to the anterolateral mobilization technique, veers need to use an angled endoscope. Detailed quantitative anteriorly toward the posterior edge of the lateral ptery- comparisons of this new strategy with inferolateral mobi- goid plate. lization or complete resection of the ET were performed Second, the medial pterygoid plate diverges at the level in cadaveric specimens. of the posterior opening of the vidian canal into two tuber- cles. The medial tubercle is the pterygoid tubercle and is inferomedial to the vidian canal. The lateral tubercle is a Methods posterior continuation of the scaphoid fossa, and it merges The study was conducted at the surgical neuroanatomy with the medial ovale process. Because of the intimate re- laboratory of Barrow Neurological Institute (St. Joseph’s lationship of the lateral tubercle to the scaphoid fossa, we Hospital and Medical Center). Osteological relationships termed it the “scaphoid tubercle” (Fig. 2). In the 10 dry were analyzed in 10 dry skulls. Six sides of silicone-in- skulls (20 sides), the mean (± SD) widths of the scaphoid jected cadaveric specimens were dissected. Endonasal and pterygoid tubercles were 6.43 ± 1.37 mm and 5.70 ± dissections were performed using standard endoscopes (4- 1.14 mm, respectively. The mean length of the pterygoid mm × 18-cm Hopkins II; Karl Storz & Co. KG) attached tubercle was 3.38 ± 1.36 mm (Table 1). to a high-definition camera. An illustrative case where the Third, the exocranial caudal aspect of the foramen lac- anterolateral mobilization strategy would have been use- erum is triangular in shape and has 3 borders: the anterior ful is included. border is formed by the pterygoid process of the sphenoid bone, the medial border by the clival segment of the oc- Quantitative Analysis cipital bone, and the posterior border by the petrous seg- A navigation system (StealthStation; Medtronic) was ment of the temporal bone. At the anterior margin of fo- used to perform measurements. Different ET manage- ramen lacerum is the pterygoid tubercle; this tubercle is ment strategies were sequentially performed on each of slightly inferomedial to the posterior orifice of the vidian the 6 sides of the cadaveric specimens. Four measurement canal. Staying in the axial plane of the pterygoid tubercle groups were generated: in group A, the ET was not mobi- while dividing the fibrocartilage attaching the ET to the lized; in group B, the ET was mobilized inferolaterally; foramen lacerum avoids violating the overlying ICA. We in group C, the ET underwent anterolateral mobilization; therefore consider the pterygoid tubercle to be an impor- and in group D, the ET was resected. Figure 1 depicts the tant landmark for defining the superior limit of any inci- relevant areas and angles that were measured. sion intended to detach the ET from the fibrocartilaginous The range of ET mobilization was defined as the dis- tissue of the foramen lacerum. tance between the center of the ET opening in its natural Finally, the sphenopetrosal fissure where the fibrocar- position and its opening after mobilization. The surgical tilaginous attachment of the ET inserts along the cranial area of exposure was defined as a surface of the polygon base extends medially to laterally in the coronal plane and limited by points 1 through 5 (Fig. 1). Surgical freedom anteriorly to posteriorly in the sagittal plane. Thus, the line was assessed as a quantitative value of the narrowness of of the incision for detaching the ET from the skull base the corridor created by the approach. For this assessment, should follow the exact trajectory of the sphenopetrosal angles of exposure within the boundaries of the approach fissure.