The JF can be regarded as a hiatus between the temporal and occipital bones; its shape is irregular and complex (Fig. 20.1a–c) [13, 14, 31]. Its anterolateral wall is formed by the temporal bone and posteromedial wall by the occipital bone, particularly the jugular tubercle. The posterior wall, which is the inferior edge of the sigmoid sinus, is formed by the jugular process of the occipital bone (Fig. 20.1b, c). The JF is situated between the petroclival fissure and occipitomastoid suture. The petroclival fissure lies between the lateral edge of the clival part of the occipital bone and the posteromedial edge of the petrous part of the temporal bone, and it intersects the anteromedial edge of the JF. The occipitomastoid suture—the suture between the mastoid part of the temporal bone and condylar part of the occipital bone—intersects its posterolateral edge. When viewed from the intracranial side, the JF appears to be gourd-shaped. The JF is separated into two components, petrosal and sigmoid, by the intrajugular process of the temporal bone and, occasionally, smaller opposite processes of the occipital bone (Fig. 20.1a). The small anteromedial part is the petrosal part (pars nervosa), which connects with the groove of the inferior petrosal sinus, and the large posterolateral part is the sigmoid part (pars venosa), which connects with the sigmoid sulcus. The anterolateral wall of the petrosal part consists of the pyramidal fossa, and the cochlear canaliculus opens into it from the bony labyrinth. The sigmoid part is large and ovoid and is occupied with the jugular bulb, which receives blood from the sigmoid sinus and posterior condylar emissary vein. When viewed from the extracranial posterior side, the jugular process between the occipital condyle and mastoid process forms the posterior wall of the JF. Because the JF is located anterolateral to the condyle, it is possible to reach its posterior wall from the posterolateral side by following the posterior condylar canal anterolaterally (Fig. 20.1c, d). The transverse process of C1 and the extracranial VA are obstacles in this direction. The mastoid canaliculus on the lateral wall of the sigmoid part and the tympanic canaliculus on the carotid ridge—the ridge between the carotid canal and JF—can be observed. The mastoid canaliculus travels the tympanomastoid suture, opening on the lateral surface of the temporal bone posterior to the external auditory canal, and the auricular branch (Arnold’s nerve) of CN X courses through it (Fig. 20.1b). The tympanic canaliculus is connected to the tympanic cavity, and the tympanic branch (Jacobson’s nerve), arising from the inferior glossopharyngeal ganglion of CN IX, courses through it.

The lower CNs pass through the JF, and CN XII passes through the hypoglossal canal located just below the JF (Fig. 20.2a). The JF, when viewed from the intracranial side, is covered by the dura mater, and the lower CNs pass through the dura mater. The lower CN bundles are separated by a dural septum, and CN IX and the CNs X and XI bundles usually pass anterosuperior and posteroinferior to the septum, respectively (Fig. 20.2b). The jugular dural folds (glossopharyngeal and vagal dural folds) are lateral to the nerves. After removing the dura mater, the sigmoid and inferior petrosal sinuses are exposed, and the intrajugular processes of the temporal and occipital bones can be identified. The JF is separated into the petrosal and the sigmoid parts by the intrajugular processes and a fibrous bridge called the intrajugular septum. All lower CNs pass through the petrosal part at the intracranial orifice. The lower CN bundles running in the JF can be seen through the thin anterior wall of the jugular bulb, which is situated in the sigmoid part (Fig. 20.2c).

Removing the jugular bulb wall exposes the intraforaminal portions of the lower CNs, sympathetic trunk, ICA, and ascending pharyngeal arteries (Fig. 20.2d). CN IX descends apart from and anterior to CNs X and XI. After passing through the JF with CN XI, CN X descends with CN XII. CN XI diverges from CN X outside the JF. The lower CNs descend near the internal jugular vein, and CN XI often descends entwined around the internal jugular vein (Fig. 20.3a, b).

The jugular bulb in the JF communicates with several venous systems with both intracranial and extracranial sources, such as the inferior petrosal sinus, inferior petroclival vein, sigmoid sinus, venous plexus of the hypoglossal canal (anterior condylar emissary vein), and posterior condylar emissary vein, finally draining into the internal jugular vein. It is important to understand the venous system to select the appropriate surgical approach to the JF (Fig. 20.4). The inferior petrosal sinus courses on the intracranial side of the petroclival fissure. The sinus drains into the petrosal confluence in the petrosal part and connects with the jugular bulb through a main channel and several smaller channels. The venous plexus of the hypoglossal canal from the marginal sinus and the inferior petroclival vein on the extracranial surface of the petroclival fissure also drain into the jugular bulb after joining the petrosal confluence (Fig. 20.5). The main channel, which is 2–3 mm in diameter, is located between CN IX and the nerve bundles of CNs X and XI. The other channels are usually smaller than 1 mm. The sigmoid sinus is the largest vein draining into the jugular bulb, and the posterior condylar emissary vein is often connected with the junction between the sigmoid sinus and the jugular bulb (Figs. 20.3, 20.4, 20.5, and 20.6c, d).

When the posterior wall of the JF is approached from the posterolateral side or after the lateral foramen magnum approach, it is important to understand the anatomy of the region lateral to the lateral foramen magnum: the area between the condyle and condylar fossa and the mastoid process and digastric groove (Fig. 20.6). The JF lies lateral to the jugular tubercle and anterior to the jugular process. The posterior condylar canal, containing the posterior condylar emissary vein, courses from the condylar fossa to the posteromedial part of the junction between the sigmoid sulcus and JF. Removing the posterolateral part of the condyle and jugular process by following the posterior condylar canal opens the posterior wall of the JF (Figs. 20.5 and 20.6c, d) [21]. The digastric groove, transverse process of C1, and posterior condylar canal comprise useful intraoperative landmarks during procedures in this region. The rectus capitis lateralis muscle is attached from the transverse process of C1 to the jugular process and covers the origin of the internal jugular vein and lower CNs (Fig. 20.6c, d). The extracranial part of VA sometimes forms an obstacle and, consequently, working space between the suboccipital surface of the occipital bone and VA is too narrow. In such cases, transposition of the extracranial VA is required after opening the transverse foramen of C1.

Most tumors of the JF, e.g., schwannoma, meningioma, and glomus jugulare tumor, are often located both intra- and extracranially (Fig. 20.7a) [17, 25, 32, 40, 42]. Because neurosurgeons have greater opportunities to surgically treat schwannomas than other tumors of the JF, we will explain the surgical approaches to the JF using JF schwannomas as an example. Based on the three growth patterns of schwannomas, Kaye AH et al. classified schwannomas of the JF into three types:

Kaye’s classification logically considers the location of a schwannoma in each case, but many tumors cannot be simply classified as one of Kaye’s three types. Pellet et al. added another type:

From the viewpoint of preoperative planning of the optimal surgical approach, Bulsara et al. proposed another modified grading scale:

These classifications are useful when considering the appropriate surgical approach in each case. Because no surgical approach can expose all sides of the JF at once, the most appropriate approach or combination of approaches should be selected based on the tumor’s anatomical location in each case.

When we approach the JF, we should consider the following four basic approaches, based on an approaching direction and an opened area of the skull: (I) posterior approach; (II) posterolateral approach; (III) superolateral approach; and (IV) inferolateral approach (Table 20.1; Fig. 20.8) [23]. The surgical approaches previously reported can be classified as one or combinations of these basic approaches.

The posterior (I) approach includes the lateral suboccipital retrosigmoid approach, through which the intracranial orifice of the JF and the intraforaminal surface are exposed from the intracranial side. The far-lateral and transcondylar approaches also belong to this group [2, 11

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