Fig. 1.1
Posterior fossa osseous and dural enclosure. (a) Superior view of the skull base. The posterior fossa is enclosed by the sphenoid, temporal, and occipital bones . The bony imprints of the major venous structures can be seen. (b) Medial view of the lateral bony wall of the posterior fossa. Sulci for venous sinuses encircling the medial face of the temporal bone are seen. The hypoglossal canal is bordered superiorly by the jugular tubercle, and the suprameatal tubercle is anterior and superior to the internal auditory canal (IAC). (c) Superior view of the posterior fossa dura. The dural lining of the posterior fossa invaginates into the neural foramina and forms venous channels between its layers. (d) Medial view of petrous temporal bone with internal structures exposed. The cochlea is located above the genu of the petrous carotid artery, and the semicircular canals are superior to the jugular bulb. The trigeminal nerve courses above the trigeminal depression of the petrous bone to enter Meckel’s cave. (e) Superior view of the tentorium attachments. The tentorium is anchored to the petrous ridge at the superior petrosal sinus and to the occipital bone at the transverse sinus and torcula. (f) Right superior oblique view of the tentorium. The midbrain is continuous with the thalamus through the tentorial incisura. Anteriorly, the tentorium extends as the anterior and posterior petroclinoid folds which insert on the anterior and posterior clinoid processes. The oculomotor nerve enters the roof of the cavernous sinus between these folds
Neural Foramina
The bones of the posterior fossa are lined with dura mater, which often contains venous lakes between its layers. Anteriorly and laterally, there are five pairs of neural foramina into which a sleeve of posterior fossa dura follows, forming small or large CSF-filled dural caves (Fig. 1.1c). The trigeminal nerve exits the posterior fossa through an ostium formed by a depression of the petrous temporal bone below and the superior petrosal sinus above (Fig. 1.1b, d). The dura of this foramen continues into the middle fossa to form Meckel’s cave – a CSF-filled space that also contains the trigeminal ganglion. More medially, the abducens nerve enters a narrow dural sleeve with CSF evagination (Dorello canal) that courses between the petrous apex and clivus before entering the cavernous sinus. The facial, cochlear, and vestibular nerves traverse the internal auditory canal, which is tapered toward its lateral fundus (Figs. 1.1b–d and 1.7e). Although the jugular foramen, which is formed by an opening between tfhe petrous and occipital bones, is a large neural foramen, it harbors relatively little CSF (Figs. 1.1b–d and 1.2a, b). The hypoglossal canal passes through the superior aspect of the occipital condyle and travels anterolaterally (Figs. 1.1a–c and 1.2a, d). Often there are duplicated inlets that converge to a single outlet which then emerges near the medial aspect of the jugular foramen.
Fig. 1.2
Jugular foramen and skull base venous sinuses. (a) Posterior view. Venous drainage from the cavernous sinus and basilar sinus empties in the superior and inferior petrosal sinuses. The inferior petrosal sinus ends at the petrosal part of the jugular foramen. The sigmoid, superior, and inferior petrosal sinuses form a venous ring around the medial face of the petrous bones, bridged by the basilar sinus anteriorly. (b) Enlarged view of the intracranial jugular foramen. The septum of the jugular foramen separates the glossopharyngeal meatus from the vagal meatus. (c) Posterior view of the left jugular foramen which has been opened posteriorly. The cranial nerves occupy the medial aspect of the jugular foramen, while the jugular bulb is lateral. The hypoglossal nerve exits the cranial cavity inferior and medial to the jugular foramen before joining the other lower cranial nerves. (d) Inferior view of the right jugular foramen region. The jugular foramen is bordered by the petrous carotid anteriorly, the styloid process and facial nerve laterally, the occipital condyle medially, and the jugular process of the occipital bone and the rectus capitis lateralis posteriorly
Tentorium
The “lid” covering the posterior fossa “bowl” is formed by the tentorium – an extension of dura that separates the cerebrum from the cerebellum with a central anterior opening (incisura) (Fig. 1.1f). The tentorium is anchored to the petrous ridge, where its layers separate to form the superior petrosal sinus, and to the occipital bone where it forms the transverse sinuses and torcula (Fig. 1.1e). It slopes downward from the incisura toward these lateral and posterior attachments. Additional venous channels often course between the layers of the tentorium . The falx cerebri joins the tentorium in the midline at the straight sinus, which communicates the vein of Galen at the incisura with the torcula (Figs. 1.1f and 1.5a). The tentorium extends anteriorly on either side of the incisura as the anterior and posterior petroclinoidal folds that attach to the anterior and posterior clinoidal processes, respectively. These two folds outline the posterior roof of the cavernous sinus, where the oculomotor nerve enters (Fig. 1.1f). The trochlear nerve is intimately related to the tentorial edge as it enters the posterior cavernous sinus, just medial to the anterior petroclinoidal fold (Fig. 1.4a).
Venous Sinuses
A rich anastomosing system of venous sinuses is organized around the inner walls of the posterior fossa [3]. Superiorly, the superior petrosal and transverse sinuses course along the edges of the tentorium. The two transverse sinuses meet at the torcula, along with the straight sinus and the superior sagittal sinus which flank the falx cerebri. A variable occipital sinus projects inferiorly from the torcula along the falx cerebelli. The sigmoid sinus begins at the junction of the transverse and petrosal sinuses and then courses in a sulcus of the mastoid bone (Fig. 1.1b), before emptying into the jugular bulb inferior to the petrous bone (Figs. 1.1d and 1.2a). The sigmoid sinus is more often larger on the right side. The inferior petrosal sinus courses along the petroclival fissure (Fig. 1.1b, d), transmitting flow to the jugular bulb posteriorly from the junction of the superior petrosal, basilar, and cavernous sinuses anteriorly (Fig. 1.2a). Thus, the superior and inferior petrosal sinuses, along with the sigmoid sinus and jugular bulb, form a venous ring around the medial face of the petrous bone (Fig. 1.2a). The petrosal and sigmoid portions of the jugular foramen empty into the jugular bulb, which occupies the lateral pars vascularis of the jugular foramen (Fig. 1.2c). The basilar sinus extends across the clivus, completing the anterior venous anastomosis of the posterior fossa. The posterior cavernous sinus of the middle fossa communicates with the posterior fossa venous circulation through the superior and inferior petrosal sinuses as well as the basilar sinus. This confluence of sinuses is often referred to as the petroclival venous confluence , through which the abducens nerve travels as it passes below the petrosphenoidal ligament (Fig. 1.2a).
Obstacles to Surgery
Several structures within the posterior fossa enclosure can result in significant morbidity if injured; therefore, these obstacles must be considered when designing surgical approaches. Smaller venous sinuses, such as the superior petrosal sinus, are sacrificed with rare consequences, but larger sinuses such as the transverse or sigmoid pose greater risk. Therefore, as an obstacle, the sigmoid sinus is a significant boundary between approaches even though it can be mobilized and retracted to increase exposure (Fig. 1.6b). Division of the tentorium rarely causes morbidity if care is taken to preserve the trochlear nerve and adequate venous outflow. Traversal of neural foramina should be avoidedif preservation of nerve function is desired. In addition, the course of the facial nerve in the temporal bone must be appreciated as it passes through the tympanic cavity and then inferiorly into the anterior aspect of the mastoid (Fig. 1.8d). Manipulation of the greater superficial petrosal nerve or geniculate ganglion can also result in facial palsy when working through the middle fossa floor to access the posterior fossa (Fig. 1.11d). If hearing preservation is desired, then the labyrinth, which lies posterior to the internal auditory canal, should be preserved (Fig. 1.8c–d). The cochlea is immediately anterior to the fundus of the internal auditory canal (Fig. 1.9c). The internal carotid artery enters the petrous bone anterior to the jugular foramen and runs vertically for a short distance before turning anteriorly below the cochlea into a horizontal orientation until it exits at the petrous apex (Figs 1.1d, 1.2d, and 1.9h). This presents a significant obstacle during anterior petrosectomy. Finally, the cavernous segment of the internal carotid artery can block lateral access from transnasal approaches (Fig. 1.12b, c).
Contents of the Posterior Fossa
The cerebellum and brainstem occupy most of the posterior fossa volume. The brainstem has three morphologically distinct regions in the posterior fossa: the midbrain, pons, and medulla oblongata. Each of these is bordered by arachnoid cisterns [4]. The mesencephalon (midbrain) is a transition from the functionally reflexive spinal cord and medulla to the correlative and associative diencephalon and forebrain. It extends from the thalamus to the pontomesencephalic sulcus. Anteriorly, the cerebral peduncles represent the array of corticospinal, corticobulbar, and corticopontine fibers descending from the internal capsule (Figs. 1.3a and 1.4a). These peduncles are separated by a gap, called the interpeduncular fossa, which contains a cistern of the same name that is bounded anteriorly by Liliequist’s membrane. The lateral walls of this cistern give rise to the oculomotor nerves, and the posterior part contains the basilar apex and posterior cerebral artery perforators entering the posterior perforated substance (Fig. 1.3c). Relative to the dorsum sellae, the basilar apex can be high or low riding, which can influence the choice of surgical approach. The lateral midbrain is bordered by the crural cistern around the cerebral peduncle and the ambient cistern posterior to the peduncle (Fig. 1.4a). Posteriorly, the superior and inferior colliculi are bordered by the quadrigeminal cistern.
Fig. 1.3
Neurovascular relationships of the brainstem and cerebellum. (a) Anterior view of the brainstem and cerebellum. Following Rhoton’s rule of three, the upper group of structures includes the mesencephalon, superior cerebellar artery (SCA), oculomotor nerve, and trigeminal nerve. The middle group includes the pons, anterior inferior cerebellar artery (AICA), cranial nerves VI–VIII, and the middle cerebellar peduncle. The inferior group holds the posterior inferior cerebellar artery, lower cranial nerves, and medulla. (b) Lateral view of the brainstem and cerebellum. Branches of the SCA are seen coursing around the midbrain and the superior cerebellar peduncle before supplying the superior surface of the cerebellum. The SCA passes below the oculomotor nerve, which exits from the lateral aspect of the interpeduncular fossa. A caudal loop of the SCA may impinge upon the trigeminal nerve. The AICA supplies the middle cerebellar peduncle and the petrosal surface of the cerebellum. (c) Superior view of the midbrain and cerebellum. The SCA supplies the superior surface of the cerebellum, which conforms to the shape and slope of the posterior tentorium. The interpeduncular fossa is crowded by perforating branches of the basilar bifurcation. (d) Inferior view of the medulla and cerebellum. The suboccipital surface of the cerebellum and its tonsils are supplied by branches of PICA. The PICA forms a caudal loop near the cerebellar tonsil
Fig. 1.4
Posterior fossa cisterns. Axial slices, though the brainstem and surrounding structures, superior view. (a) Section through the mesencephalon showing the surrounding interpeduncular, crural, ambient, and quadrigeminal cistern. (b) Upper pons surrounded by the prepontine and cerebellopontine cisterns. The superior cerebellar branches (SCA) course above the trigeminal nerves. (c) Midlevel pons. The prepontine cistern faces the clivus, and cerebellopontine cistern faces the petrous bone. The abducens nerve is seen piercing the clival dural to enter Dorello canal as the anterior inferior cerebellar artery (AICA) courses below it. (d) Lower pons. Cranial nerves VII, VIII, the flocculus, and the AICA are seen within the cerebellopontine cistern. (e) Section through the medulla showing the premedullary and cerebellomedullary cisterns. The hypoglossal nerve originates at the preolivary sulcus lateral to the pyramid, while cranial nerves IX–XI exit from the postolivary sulcus
The pons (“bridge”) is developmentally a part of the medulla and is only distinguishable in mammals due to the pontine nuclei and the fibers of corticopontocerebellar and corticospinal tracts. It is a convex structure extending from the pontomesencephalic sulcus superiorly to the pontomedullary sulcus inferiorly. The descending corticospinal and corticobulbar fibers located in its anterior half are interrupted by transverse fibers coursing between the middle cerebellar peduncles. Intertwined between these are pontine nuclei involved in the corticopontocerebellar circuit. At the midlevel of the pons, the trigeminal nerve emerges anterior to the middle cerebellar peduncle (Figs. 1.3a and 1.4c). The posterior pons contains several cranial nerve nuclei, ascending tracts, and the floor of the fourth ventricle, where the abducens nucleus makes an impression called the facial colliculus (Fig. 1.5g). The pons is bordered anteriorly and laterally by the prepontine and cerebellopontine cisterns (Fig. 1.4b–d).
Fig. 1.5
Surgical views from posterior approaches. (a) The occipital transtentorial approaches exploit the corridor between the occipital lobe and falx-tentorium to access the posterior aspect of the tentorial incisura, dorsal mesencephalon, and pineal region. The vein of Galen complex is visualized deep in the surgical field. (b) Enlarged view of the occipital transtentorial approach. The pineal region and Galenian venous complex are exposed. (c) Posterior view of the suboccipital craniotomy. The suboccipital cerebellar surface is bordered by the transverse and sigmoid sinuses. (d) Posterior view of the supracerebellar infratentorial approach. The pineal gland and superior colliculi are exposed along with tributaries to the vein of Galen. The basal vein of Rosenthal, the internal cerebral veins, and the precentral veins are seen draining into the vein of Galen, which in turn empties in the straight sinus at the midline of the tentorium. (e) Posterior inferior view of the telovelar approach. The cerebellar tonsil is retracted to expose the inferior roof of the fourth ventricle formed by the inferior medullary velum and tela choroidea, which anchors the fourth ventricle choroid plexus. (f) View of the fourth ventricle through the telovelar approach. Division of the tela choroidea and velum exposes the floor of the fourth ventricle, lateral recesses and the aqueduct of Sylvius. (g) Posterior view of the floor of the fourth ventricle. The protuberances identified in the floor of the fourth ventricle include the facial colliculus (abducens nucleus), the hypoglossal trigone, and the vagal trigone. The lateral recess communicates with the cerebellopontine angle through the foramen of Luschka. The superior and inferior cerebellar peduncles contribute to the lateral walls of the fourth ventricle
The medulla oblongata is the inferior segment of the brainstem . The pyramids represent its most anterior prominences and contain the corticospinal tract. At this level, 85% of the corticospinal fibers decussate. Lateral to the pyramids, the inferior olive is bounded by the preolivary sulcus, where the hypoglossal rootlets emerge. Cranial nerves IX, X, and XI exit from the postolivary sulcus. The floor of the fourth ventricle at this level contains from medial to lateral: the hypoglossal nucleus, the dorsal nucleus of the vagus nerve, and the vestibular nuclei (Fig. 1.5g). The medulla is bounded anteriorly by the premedullary cistern and laterally by the cerebellomedullary cistern (Fig. 1.4e).
Arteries and Rule of Three
Dr. Albert Rhoton Jr. divided the neurovascular contents of the posterior fossa into three groups of structures, each organized around a major posterior fossa artery. These zones are stacked vertically, defining the main longitudinal axis of the posterior fossa. A separate important organizing scheme divides the CSF-filled spaces surrounding the brainstem into cisterns (Fig. 1.4). The brainstem and cerebellum are supplied by branches of the vertebrobasilar system (Fig. 1.3). The vertebral arteries pierce the posterior fossa dura medial to the occipital condyles and pass inferior and then anterior to the lower cranial nerves before converging at the vertebrobasilar junction, near the pontomedullary sulcus (Figs. 1.2a and 1.3a). The basilar artery sends short and long circumflex arteries to the pons before it ultimately bifurcates into the posterior cerebral arteries at the level of the midbrain. Three pairs of cerebellar arteries arise from the vertebrobasilar system. The course of the cerebellar arteries often includes loops and turns that are not confined to a single plane. Nonetheless, it can be useful to conceptualize three zones along the vertical axis of the posterior fossa defined by these three arteries (Fig. 1.3).
The upper zone follows the superior cerebellar artery (SCA) and includes the midbrain, upper pons, and superior cerebellum. After arising near the basilar apex, the SCA travels immediately inferior to the oculomotor nerve before curving around the brainstem close to the pontomesencephalic junction. It typically bifurcates into rostral and caudal branches. Laterally, these branches enter the cerebellomesencephalic fissure where they travel with the trochlear nerve. Before entering this fissure, one of its branches may loop inferiorly to contact the trigeminal nerve, possibly causing trigeminal neuralgia. After coursing posteriorly around the superior cerebellar peduncle (Fig. 1.4b), the branches emerge from the cerebellomesencephalic fissure to supply the vermis, superior cerebellar hemispheres, and dentate nucleus.
The middle zone follows the anterior inferior cerebellar artery (AICA) and includes the middle pons and cerebellopontine angle. The AICA usually originates from the lower half of the basilar artery (Figs. 1.3a–b). As it courses around the brainstem , it may pass near any of the cranial nerves emerging from the pontomedullary sulcus – the abducens, facial, cochlear, and vestibular nerves. Compression of the root entry zone of the facial nerve may cause hemifacial spasm. AICA enters the cerebellopontine angle to supply the middle cerebellar peduncle and the portion of the cerebellum facing the petrous temporal bone (Fig. 1.3a). Often, a loop of AICA can extend into the internal auditory canal (IAC) and impinge upon the nervus intermedius to cause geniculate neuralgia (Fig. 1.7e). AICA may also pass between the nerves of the 7/8 complex before they enter the IAC.
The lower zone follows the posterior inferior cerebellar artery (PICA) and includes the medulla and inferior cerebellum. PICA typically arises from the distal vertebral artery along the anterolateral medulla (Fig. 1.3a, b) but can arise from any part of the vertebral artery. Its origin can also be extradural. The PICA courses posteriorly from the anterior medulla above or below the hypoglossal nerve rootlets. As it reaches the lateral medulla, it turns inferiorly before encountering the cerebellar tonsil. It then passes anterior or posterior to the glossopharyngeal, vagus, and spinal accessory nerves in a variable manner (Fig. 1.3a). After forming its caudal loop, the PICA ascends along the inferior cerebellar peduncle, deep to the cerebellar tonsil, before bifurcating into branches supplying the inferior vermis and inferior hemisphere (Fig. 1.5c, e, f).
Veins
The veins of the posterior fossa are more variable than the arteries [3]. Deep veins include the vein of the cerebellomesencephalic fissure, the vein of the cerebellopontine angle, and the vein of the cerebellomedullary fissure. Also, one can commonly identify the veins of the superior, middle, and inferior cerebellar peduncles. The superficial cerebellar veins drain the tentorial, petrosal, suboccipital surfaces. The brainstem has a variable network of veins which are longitudinal or transverse. There are three major groups of bridging veins. The galenic group drains the internal cerebral vein and the basal vein of Rosenthal. The petrosal group includes the inferior petrosal sinus and jugular foramen, as well as the superior petrosal sinus that drains Dandy’s vein. The tentorial group drains into the transverse sinus or directly into the tentorium.
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