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Rectus Capitis Lateralis: An Important Landmark for Posterolateral Approaches to the Jugular Foramen Michael A. Cohen MD1,2, Brandon T. Pagan MD3, Alexander I. Evins PhD1, Gennaro Lapadula MD4, Philip E. Stieg, MD1, Antonio Bernardo, MD1 1Microneurosurgery Skull Base and Surgical Innovations Laboratory, Department of Neurological Surgery, Weill Cornell Medical College 2Department of Neurological Surgery, Rutgers New Jersey Medical School 3University of Puerto Rico Medical School 4Departement of Neurology and Psychiatry, Neurosurgery, “Sapienza” University of Rome, Rome, Italy A INTRODUCTION B The jugular foramen can be approached from posteriorly during a far lateral approach, from laterally during a trans-mastoid approach. The intricacy of surgical anatomy of the upper cervical region and jugular foramen requires the identification of anatomical landmarks to help in protecting the neurovascular structures. C The rectus capitis lateralis (RCL) muscle is a small deep muscle, located superolateral to the Figure 1. (A) The RCL is located just anterior to the superior oblique, suboccipital triangle, that connects the originates from the C1 transverse process and inserts on the jugular process, transverse process of C1 (C1TP) with the jugular medial to the mastoid tip. A triangle (yellow) is formed by the RCL, superior process of the occipital bone. The RCL is an oblique, and a line connecting these two muscles along the skull (red dotted line). The OA generally travels along this line in its intramuscular segment. excellent landmark aiding in localization of the (B) The OA passes between the posterior belly of the digastric muscle and the RCL and forms the roof of the condylar triangle. IJV within the carotid sheath, the contents of the jugular foramen as they exit the skull, and facial E nerve as it exits the stylomastoid foramen. METHODS A horseshoe shaped incision was made and the muscles of the suboccipital triangle were dissected on 3 preserved cadaveric specimens injected with colored latex into the arterial and venous systems. The incision was extended into the neck and a high cervical dissection of the facial nerve and contents of the carotid sheath was performed. The RCL was exposed anterior to the posterior belly of the digastric muscle. A mastoidectomy is performed, skeletonizing the sigmoid sinus, jugular bulb, and facial nerve within the fallopian canal. The mastoid tip is removed, exposing the jugular process and RCL. The jugular bulb is removed, exposing cranial nerves IX, X, and XI, which are dissected and exposed within the jugular foramen. The hypoglossal canal is exposed within the occipital condyle. RESULTS Musculoskeletal Relationships The RCL courses superiorly and the inferior oblique courses posteriorly from the C1TP, forming a right angle. The superior oblique bisects this angle as it courses toward its insertion site (Figure 1A). A second triangle is formed by the RCL, the superior oblique, and a line connecting these two muscles along the occipital bone (Figure 1B). The occipital artery coursed along this line connecting the RCL and superior oblique in all specimens studied. The RCL originates along the superior and anterior portions of the C1TP and inserts on the jugular process of the occipital bone (Figure 2). The jugular process is an average of 17.3 mm (range 17 – 18 mm) superior from the C1TP. The styloid process lies 6.5 mm (range 6 – 7 mm) anterior to the anterior aspect of the RCL insertion site on the jugular process. The jugular process forms the floor of the jugular bulb and the posterior aspect of the jugular foramen meatus. A G B D F H Figure 2. The RCL originates on the C1TP and inserts on the jugular process (shaded in pink). The RCL separates the far lateral approach (purple) and the transcondylar variant (green) from the jugular foramen (dark blue), anteriorly. Vascular Relationships The RCL lies directly posterior to the IJV, separated only by the carotid sheath (Figure 3). The IJV turns posteriorly as it ascends to form the jugular bulb, superior to the RCL and jugular process. Both the anterior and superior surfaces of the RCL are surrounded by the jugular venous system. The RCL origin covers the C1 transverse foramen, where the vertebral artery (VA) makes its posterior turn (Figure 3). The average distance from the medial turn of VA, just before piercing the dura, to the posterior border of the RCL is 20.3 mm (range 19 – 22 mm). The ICA ascends anteromedial to the RCL and IJV to enter the carotid canal. The ICA is an average of 6.2 mm (range 5.5 – 7 mm) anterior to the top of the RCL. Neural Relationships The anterior aspect of the RCL insertion on the jugular process is an average of 1.8 mm (range 1.5 – 2 mm) from the facial nerve as it exits the stylomastoid foramen (Figure 3A and 3B). The glossopharyngeal nerve travels anteriorly in isolation through the jugular foramen medial to the intrajugular septum. After crossing the plane of the RCL, it turns inferiorly to exit the jugular foramen medial to the styloid process and lateral to the ICA (Figure 3D). The glossopharyngeal nerve is the furthest of the cranial nerves within the carotid sheath from the RCL, with an average distance of 9.2 mm (range 8.5 – 10 mm) at the top of the RCL. The vagus and accessory nerves enter the jugular foramen together, several millimeters inferior to the glossopharyngeal nerve. They wrap around the jugular process and RCL (Figure 3F), where the two nerves split. The accessory nerve descends in close proximity the RCL, just medial to the IJV, whereas the vagus nerve crosses first lateral to and then medial to the hypoglossal nerve, as it descends into the neck. The hypoglossal nerve enters a bony canal through the occipital condyle, coursing anteriorly and superiorly, posteromedial to the RCL, and joins the cranial nerves of the jugular foramen as they enter the carotid sheath together (Figure 3G). Figure 3. (A) Occipital bone and condyle, jugular process, C1 posterior arch and transverse process (C1TP), and VA are exposed. The RCL originates from the C1TP and inserts on the jugular process. A sub-labyrinthine mastoidectomy is performed with exposure of the mastoid segment of the facial nerve and the sigmoid sinus. High cervical dissection is performed, isolating the IJV and ICA within the carotid sheath. The extracranial facial, glossopharyngeal, and accessory nerves are identified. (B) The mastoid tip is removed and the stylomastoid foramen opened to show the facial nerve and the jugular process. (C) The IJV is retracted anteriorly to show the vagus, accessory, and hypoglossal nerves in the carotid sheath. (D) The IJV and jugular bulb are removed and the facial nerve is transposed to improve exposure of the jugular foramen. (E) Occipital condylectomy is performed to expose the hypoglossal canal. The hypoglossal nerve is seen on both sides of the RCL. (F) All cranial nerves within the jugular foramen travel anteriorly and inferiorly through the jugular foramen, forming a 120-degree angle with the RCL. After passing anterior to the RCL, the nerves turn inferiorly to exit the jugular foramen parallel to the RCL. (G) The hypoglossal nerve ascends through the hypoglossal canal at a sharp 60-degree angle, relative to the RCL, climbing above the RCL and making a sharp turn inferiorly after entering the carotid sheath. A wire is passed through the distal aspect of the hypoglossal canal to show the peak of the turn that the hypoglossal nerve makes above the RCL to enter to the carotid sheath, medial to the vagus and accessory nerves. (H) The intrajugular septum has been drilled to expose the course of the glossopharyngeal nerve through the jugular foramen. CONCLUSION The RCL muscle is an important anatomic landmark, separating the far lateral approach from the jugular foramen and facial nerve. A thorough understanding of the microsurgical anatomy of the RCL and its musculoskeletal, vascular, and neural relationships is necessary to safely perform combined approaches to the foramen magnum and jugular foramen. REFERENCES 1. Dallan I, Bignami M, Battaglia P, Castelnuovo P, Tschabitscher M. Fully endoscopic transnasal approach to the jugular foramen: anatomic study and clinical considerations. Neurosurgery. 2010;67:ons1–8. 2. Komune N, Matsushima K, Matsushima T, Komune S, Rhoton AL. 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