Last Updated: August 23, 2020
The tentorial incisura provides the only communication between the supratentorial and infratentorial spaces (17) (Fig. 5.1). The area between the upper brainstem and the incisural edges is divided into the anterior, middle, and posterior incisural spaces (Fig. 5.2). The anterior incisural space is located anterior to the brainstem and extends upward around the optic chiasm to the subcallosal area; the middle incisural space is located lateral to the brainstem and is intimately related to the hippocampal formation in the medial part of the temporal lobe; and the posterior incisural space is located posterior to the midbrain and corresponds to the region of the pineal gland and vein of Galen. The arterial relationships in the anterior incisural space and the venous relationships in the posterior incisural space are extremely complex, since the anterior incisural space contains all of the components of the circle of Willis and the bifurcation of the internal carotid and basilar arteries, and the posterior incisural space contains the convergence of the internal cerebral and basal veins and many of their tributaries on the vein of Galen. The incisura is intimately related to the depths of the cerebrum and cerebellum, the first six cranial nerves, and the upper brainstem. Some part of the incisura is commonly exposed during the operations for aneurysms, deep tumors and arteriovenous malformations, trigeminal neuralgia, and epilepsy. Much attention has been focused on the distortions of this anatomy by herniation of the brain through the incisural space.
Anatomy of the Tentorium
The tentorium covers the cerebellum, supports the cerebrum, and forms a collar around the brainstem (Figs. 5.2 and 5.3). The tentorium slopes downward from its apex, located at the posterior edge of the incisura, to its attachment to the temporal, occipital, and sphenoid bones. All of the tentorial margins, except the free edges bordering the incisura, are rigidly attached to the cranium. The anterior border is attached to the petrous ridge and divides to enclose the superior petrosal sinus. The lateral and posterior borders, which divide to enclose the transverse sinus and the torcula, are attached to the inner surface of the occipital and temporal bones along the internal occipital protuberance and to the edges of the shallow osseous groove for the transverse sinus.
The anterior end of each free edge is attached to the petrous apex and the anterior and posterior clinoid processes (Figs. 5.1–5.3). The attachment to the petrous apex and the clinoid processes forms three dural folds: the anterior and posterior petroclinoid folds and the interclinoid fold. Between these folds is located the oculomotor trigone, a shallow depressed area over the posterior part of the roof of the cavernous sinus, through which the oculomotor and trochlear nerves enter the sinus. The posterior petroclinoid fold extends from the petrous apex to the posterior clinoid process; the anterior petroclinoid fold extends from the petrous apex to the anterior clinoid process; and the interclinoid fold covers the ligament extending from the anterior to the posterior clinoid process. The oculomotor nerve penetrates the dura in the central part of this triangle, the oculomotor triangle, and the trochlear nerve enters the dura at the posterolateral edge of this triangle. The petrosphenoid ligament passes between the leaves of the posterior petroclinoid fold from the petrous apex to the lateral border of the dorsum sellae, just below the posterior clinoid process. The abducens nerve passes below the petrosphenoid ligament to enter the cavernous sinus. The dura forming the roof of the oculomotor trigones extends medially across the sella to form the diaphragma sellae, which covers the pituitary gland and contains an opening for the infundibulum.
Anterolateral to the diaphragma are two orifices: a bone orifice, the optic canal (through which the optic nerve enters the orbit), and a dural orifice through which the internal carotid artery exits the cavernous sinus (Fig. 5.3). From the anterior part of the free edge, the dura mater slopes steeply downward to form the lateral wall of the cavernous sinus and to cover the middle cranial fossa. Plaut reported that the attachment of the anterior end of the free edge to the petrous apex may be situated as much as 10 mm lateral and 8 mm below the level of the clinoid processes and that the low position of the free edge may facilitate descending tentorial herniations (20).
The falx cerebri fuses into the dorsal surface of the tentorium in the midline behind the apex (Fig. 5.1). The straight sinus, which is enclosed in the falcotentorial junction, begins at the tentorial apex, where it receives the vein of Galen and the inferior sagittal sinus, and terminates in the torcular.
The incisura is roughly triangular and has its anterior edge or base on the dorsum sellae and its apex dorsal to the midbrain, just posterior to the pineal gland (Fig. 5.2). The incisura, when viewed from above after removal of the cerebral hemispheres, is filled by the midbrain, pons, and cerebellum, and the free edges skirt the cerebral peduncles, either touching or being separated from them by a variable distance (Fig. 5.2). The amount of cerebellar cortex visible between the midbrain and the free edge varies from none when the free edge hugs the tectum to a large amount when the incisura extends far posteriorly. When viewed from below after removal of the cerebellum, the incisura is filled by the midbrain and the uncus and parahippocampal gyrus (Fig. 5.4). The amount of parahippocampal gyrus visible from below varies from none when the free edge hugs the tectum to a large amount when the incisura is very wide. The width of the incisura varies from 26 to 35 mm (average, 29.6 mm) and the anteroposterior diameter varies from 46 to 75 mm (average, 52.0 mm) (17).
The area between the brainstem and the free edges is divided into: an anterior incisural space located in front of the brainstem; paired middle incisural spaces situated lateral to the brainstem; and a posterior incisural space located behind the brainstem (Figs. 5.1–5.4). The description of each incisural space is divided into sections on neural, cisternal, ventricular, cranial nerve, arterial, and venous relationships.
Anterior Incisural Space
The anterior incisural space is located anterior to the midbrain and pons. It extends inferiorly between the brainstem and clivus and obliquely forward and upward around the optic chiasm to the subcallosal area. It opens laterally into the medial part of the Sylvian fissure, and posteriorly between the uncus and the brainstem into the middle incisural space (Figs. 5.3 and 5.4).
The part of the anterior incisural space located below the optic chiasm has posterolateral and posterior walls. The posterolateral wall is formed by the bulbous prominence of the anterior third of the uncus, which hangs over the anterior part of the free edge above the oculomotor trigone (Fig. 5.2). The posterior wall is formed by the pons and cerebral peduncles. The infundibulum of the pituitary gland crosses the anterior incisural space to reach the opening in the diaphragma sellae. The part of the anterior incisural space situated above the optic chiasm is limited superiorly by the rostrum of the corpus callosum, posteriorly by the lamina terminalis, and laterally by the part of the medial surfaces of the frontal lobes located below the rostrum.
The anterior incisural space opens laterally into the part of the Sylvian fissure situated below the anterior perforated substance (Fig. 5.4). The anterior limb of the internal capsule, the head of the caudate nucleus, and the anterior part of the lentiform nucleus are located above the anterior perforated substance (Fig. 5.2).
The interpeduncular cistern, which sits in the posterior part of the anterior incisural space between the cerebral peduncles and the dorsum sellae, communicates laterally with the Sylvian cistern below the anterior perforated substance and anteriorly with the chiasmatic cistern located below the optic chiasm. The interpeduncular and chiasmatic cisterns are separated by Liliequist’s membrane, an arachnoidal sheet extending from the dorsum sellae to the anterior edge of the mammillary bodies (14, 35, 36). The chiasmatic cistern communicates around the optic chiasm with the cisterna laminae terminalis, which lies anterior to the lamina terminalis.
The anterior part of the third ventricle projects into the anterior incisural space in the medial plane, dividing it into supra and infra chiasmatic portions. The frontal horns of the lateral ventricles are located above the anterior incisural space (Figs. 5.1–5.3). The tip of the temporal horn is separated from the uncal surface, forming the posterolateral wall of the anterior incisural space, by the amygdaloid nucleus.
The optic and oculomotor nerves and the posterior part of the olfactory tracts pass through the anterior incisural space. Each olfactory tract runs posteriorly, and splits just above the anterior clinoid process to form the medial and the lateral olfactory striae, which course along the anterior margin of the anterior perforated substance (Fig. 5.4).
The optic nerves and chiasm and the anterior part of the optic tracts cross the anterior incisural space (Fig. 5.3). The optic nerves emerge from the optic canal medial to the attachment of the free edge to the anterior clinoid processes, and are directed posteriorly, superiorly, and medially toward the optic chiasm. The optic chiasm is usually located above the diaphragma sellae, but it may be prefixed and lie over the tuberculum sellae or postfixed and lie over the dorsum sellae.
From the chiasm, the optic tract continues in a posterolateral direction around the cerebral peduncle to enter the middle incisural space (Fig. 5.4). The oculomotor nerve emerges from the midbrain on the medial surface of the cerebral peduncle. It crosses the anterior incisural space between the posterior cerebral artery (PCA) and the superior cerebellar artery (SCA) and passes inferomedial to the uncus to enter the roof of the cavernous sinus through the oculomotor trigone. The abducens nerve ascends from deep within the infratentorial part of the anterior incisural space. It emerges from the pontomedullary sulcus, ascends in the prepontine cistern to pierce the dura covering the clivus, and passes below the petrosphenoid ligament to enter the cavernous sinus.
The arterial relationships of the anterior incisural space are complex because it contains all of the components of the circle of Willis (4, 5, 7, 18, 19, 27, 37). The internal carotid artery enters the anterior incisural space by passing along the medial surface of the anterior clinoid process and bifurcates below the anterior perforated substance (Figs. 5.5 and 5.6). The posterior communicating artery arises from the posteromedial aspect of the carotid artery and courses superomedial to the oculomotor nerve to join the PCA in the anterior incisural space. The anterior choroidal artery originates from the posterior surface of the carotid artery 0.1 to 3.0 mm distal to the origin of the posterior communicating artery and courses below the optic tract before passing between the uncus and the cerebral peduncle to enter the middle incisural space (3, 24).
The proximal part of the anterior cerebral artery also courses in the anterior incisural space (Fig. 5.6). It arises below the anterior perforated substance and courses anteromedially above the optic chiasm, where it is joined to its mate from the opposite side by the anterior communicating artery. It then courses upward in front of the lamina terminalis. The middle cerebral artery courses laterally from its origin below the anterior perforated substance. The major bifurcation of the middle cerebral artery is usually located in the lateral part of the anterior incisural space.
The basilar artery ascends and gives rise to the PCA and SCA in the posterior part of the anterior incisural space between the posterior perforated substance and the clivus (Fig. 5.7). The position of the basilar tip and bifurcation varies from as far caudal as 1.3 mm below the pontomesencephalic sulcus to as far rostral as the mammillary bodies (17). The PCA courses laterally around the cerebral peduncle, above the oculomotor nerve. It exits the anterior and enters the middle incisural space by coursing between the uncus and the cerebral peduncle. The SCA originates in the anterior incisural space below the PCA and courses laterally below the oculomotor nerve (Fig. 5.7). The origin is usually just rostral to the level of the free edge. It dips below the tentorium to reach the superior surface of the cerebellum at the junction of the anterior and middle incisural spaces. The structures in the walls of the anterior incisural space receive perforating branches from all of the above arteries.
The main venous trunk related to the anterior incisural space is the basal vein (Figs. 5.5 and 5.6) (16). It courses through the anterior, middle, and posterior incisural spaces to empty into the vein of Galen. It originates below the anterior perforated substance, courses posterolaterally around the cerebral peduncle, below the optic tract and medial to the uncus, to enter the middle incisural space.
Middle Incisural Space
The middle incisural space is located lateral to the brainstem (Figs. 5.3 and 5.4). This narrow space extends upward between the temporal lobe and the midbrain and downward between the cerebellum and the upper brainstem. It has medial and lateral walls and a roof. The medial wall, formed by the lateral surface of the midbrain and upper pons, is divided by the pontomesencephalic sulcus, which lies at the level of the free edge. The surface of the midbrain facing the middle incisural space is divided into a larger anterior part formed by the cerebral peduncle and a smaller posterior part formed by the tegmental surface. The optic tract forms a smooth white band at the upper edge of the cerebral peduncle that stands in sharp contrast to the vertically striated surface of the peduncle. The peduncular and tegmental surfaces are separated by the lateral mesencephalic sulcus, a vertical groove that extends from the pulvinar above to the pontomesencephalic sulcus below.
The roof of the middle incisural space has a narrow anterior part formed by the posterior part of the optic tract that is flattened between the cerebral peduncle and the uncus, and a wider posterior part formed by the inferior surface of the thalamus (Fig. 5.4). The lateral geniculate body protrudes from the lower surface of the thalamus just behind the uncus. The medial geniculate body bulges into the roof posteromedial to the lateral geniculate body just behind the lateral mesencephalic sulcus.
The lateral wall of the supratentorial part of the middle incisural space is composed of the hippocampal formation on the medial surface of the temporal lobe (Figs. 5.3 and 5.4). The uncus and parahippocampal gyri, the most inferior structures in this part of the lateral wall, form a curved border around the middle incisural space. The uncus bulges medially at the anterior end of the parahippocampal gyrus. The amygdaloid nucleus is situated just lateral to the medial surface of the uncus and just anterior to the tip of the temporal horn.
The uncus commonly prolapses into the incisura anteriorly and has a groove along its undersurface marking the free edge (Fig. 5.4). This groove usually disappears at the lateral margin of the peduncle, because the free edge often hugs the peduncle at this site, but it may reappear posterior to the peduncle on the lower surface of the parahippocampal gyrus as the space between the brainstem and the free edge increases. In our specimens, these grooves were commonly present on the uncus and adjacent part of the parahippocampal gyrus without being observed on the posterior part of the parahippocampal gyrus, but they were only rarely present posteriorly, and not anteriorly (17). The distance from the most medial point of the uncus to this groove varied from 2 to 8.6 mm (average, 4.4 mm). Howell reported that these grooves may measure up to 15 mm in length and lie as far as 10 mm from the medial tip of the uncus (10). Klintworth (12, 13) noted unilateral uncal grooving in 88.4% of brains and bilateral grooving in 80%.
Posterior to the uncus, the surface of the temporal lobe facing the middle incisural space is formed by three longitudinal strips of neural tissue, one located above the other, which are interlocked with the hippocampal formation to make an important part of the limbic system (Figs. 5.3 and 5.4). The most inferior strip is formed by the rounded medial edge of the parahippocampal gyrus; the middle strip is formed by the dentate gyrus, a serrated or beaded strip of gray matter located on the medial surface of the hippocampal formation; and the superior strip is formed by the fimbria of the fornix, a white band formed by the fibers emanating from the hippocampal formation that are directed posteriorly into the crus of the fornix.
The middle incisural space extends below the tentorium to communicate with the anterior part of the cerebellomesencephalic fissure, located between the anterosuperior part of the cerebellum and the lateral surface of the tegmentum.
The supratentorial part of the middle incisural space contains the crural and ambient cisterns (Figs. 5.2–5.6). The crural cistern, located between the cerebral peduncle and the uncus, is a posterolateral extension of the interpeduncular cistern. The crural cistern opens posteriorly into the ambient cistern, demarcated medially by the midbrain, above by the pulvinar, and laterally by the parahippocampal and dentate gyri and fimbria of the fornix. The ambient cistern is continuous posteriorly with the quadrigeminal cistern, the major cistern in the posterior incisural space. The ambient cistern extends below the free edge into the part of the cerebellomesencephalic fissure located above the origin of the trigeminal nerve.
The temporal horn extends into the medial part of the temporal lobe lateral to the middle incisural space and ends approximately 3 cm from the temporal pole (Figs. 5.2–5.7). The choroidal fissure, located between the fimbria of the fornix and the lower surface of the thalamus, is the site of attachment of the choroid plexus in the temporal horn. The paired bodies of the lateral ventricles are located directly above the central part of the incisura. They sit on and are separated from the central part of the incisura by the thalamus.
The trochlear and trigeminal nerves are related to the middle incisural space (Fig. 5.8). The trochlear nerve has the longest course within the incisura of any nerve and is the cranial nerve most intimately related to the free edge. The trochlear nerve arises below the inferior colliculus in the posterior incisural space and passes forward through the middle incisural space between the PCA and SCA. Its initial course around the midbrain is medial to the free edge in the space between the tectum and cerebellum. It reaches the lower margin of the free edge at the posterior edge of the cerebral peduncle. It pierces the free edge in the posterior part of the oculomotor trigone and runs for a short distance in the anterior petroclinoid fold before entering the lateral wall of the cavernous sinus.
The trigeminal nerve courses in the infratentorial part of the middle incisural compartment. It arises on the anterolateral aspect of the mid pons and passes above the petrous apex to enter Meckel’s cave (the arachnoidal and dural cavern) where it separates into the three sensory divisions (6). The medial edge of the posterior trigeminal root is observed just medial to the tentorial edge if one looks from straight superior through the incisura with the cerebrum removed, but it is hidden below the free edge in the lateral view provided by the subtemporal operative exposure.
The major arteries in the middle incisural space, the anterior choroidal, PCA, and SCA, arise in the anterior incisural space and reach the middle incisural space by coursing around the brainstem parallel to the free edge (Figs. 5.5–5.8). The anterior choroidal artery enters the superior part of the middle incisural space below the optic tract and passes through the choroidal fissure near the inferior choroidal point to supply the choroid plexus in the temporal horn.
The PCA enters the middle incisural space between the cerebral peduncle and uncus and passes straight posteriorly between the tegmentum and subiculum (Figs. 5.6 and 5.8). It gives off several cortical branches, which cross the free edge to reach the inferior surface of the temporal and occipital lobes, and the lateral posterior choroidal and thalamogeniculate arteries, which course medial to the free edge. The lateral posterior choroidal arteries, arising in the middle incisural space, course superolaterally through the choroidal fissure and around the pulvinar to reach the choroid plexus in the temporal horn and atrium (Fig. 5.7). The medial posterior choroidal artery arises from the proximal part of the PCA in the anterior incisural space and courses parallel and medial to the PCA through the middle incisural space to reach the posterior incisural space (Fig. 5.5). The thalamogeniculate branches arise below the pulvinar and pass upward through the geniculate bodies to reach the thalamus and internal capsule.
The SCA usually passes below the level of the free edge and bifurcates into rostral and caudal trunks as it passes around the lateral margin of the cerebral peduncle to enter the middle incisural spaces (Figs. 5.7 and 5.8). It passes above the trigeminal nerve and enters the cerebellomesencephalic fissure in the anterior part of the middle incisural space. The walls of the supratentorial part of the middle incisural space are supplied by the perforating branches of the anterior choroidal and PCA, and the walls in the infratentorial part are supplied by the SCA.
The venous relationships in the middle incisural space are relatively simple (Figs. 5.5–5.7). The basal vein courses along the upper part of the cerebral peduncle and below the pulvinar to reach the posterior incisural space. It may infrequently terminate in a tentorial sinus in the free edge at this level.
Posterior Incisural Space
The posterior incisural space lies posterior to the midbrain and corresponds to the pineal region (Figs. 5.1–5.4) (33). It has a roof, floor, and anterior and lateral walls, and extends backward to the level of the tentorial apex. The quadrigeminal plate is located at the center of the anterior wall. The anterior wall rostral to the colliculi is formed by the pineal body. The habenular commissure forms the upper half and the posterior commissure forms the lower half of the attachment of the pineal body to the posterior part of the third ventricle. The part of the anterior wall below the colliculi is formed in the midline by the lingula of the vermis and laterally by the superior cerebellar peduncles as they ascend beside the lingula.
The roof of the posterior incisural space is formed by the lower surface of the splenium, the terminal part of the crura of the fornices, and the hippocampal commissure (Figs. 5.1 and 5.4). Each crus arises as a continuation of the fimbria, passes around the posterior margin of the pulvinar, and blends into the lower margin of the splenium. The hippocampal commissure is an oblique band of fibers that courses below the splenium between the medial margins of the crura. The floor of the posterior incisural space is formed by the anterosuperior part of the cerebellum and consists of the culmen of the vermis in the midline and the quadrangular lobules of the hemispheres laterally. The posterior incisural space extends inferiorly into the cerebellomesencephalic fissure.
Each lateral wall is formed by the pulvinar, crus of the fornix, and the medial surface of the cerebral hemisphere. The anterior part of the lateral wall is formed by the part of the pulvinar located just lateral to the pineal body. The lateral wall, posterior to the pulvinar, is formed by the segment of the crus of the fornix that wraps around the posterior margin of the pulvinar (Fig. 5.1). The posterior part of the lateral walls is formed by the cortical areas located below the splenium on the medial surface of the hemisphere. These areas include the posterior part of the parahippocampal and dentate gyri. The posterior part of the parahippocampal gyrus usually extends medially above the posterior part of the free edge and may have shallow grooves from the free edge on its lower surface.
The quadrigeminal cistern, situated posterior to the quadrigeminal plate, is the major cistern in the posterior incisural space (Figs. 5.1–5.4). The quadrigeminal cistern communicates above with the posterior pericallosal cistern; inferiorly into the cerebellomesencephalic fissure; inferolaterally into the posterior part of the ambient cistern located between the midbrain and the parahippocampal gyrus; and laterally into the retrothalamic areas medial to where the crus of the fornix wraps the posterior part of the pulvinar. The quadrigeminal cistern may communicate with the velum interpositum, a space that extends forward into the roof of the third ventricle between the splenium above and the pineal body below.
The posterior portion of the third ventricle and the cerebral aqueduct are anterior and the atria and occipital horns of the lateral ventricles are lateral to the posterior incisural space (Figs. 5.2–5.4). The aqueduct passes ventral to the anterior wall of the posterior incisural space. The atrium is separated from the posterior incisural space by the crus of the fornix as it passes posterior to the pulvinar and by the cortical gyri located in the lateral wall of the posterior incisural space.
The trunks and branches of the PCA and SCA enter the posterior incisural space from anteriorly (Figs. 5.5 and 5.6). The PCA courses through the lateral part of the posterior incisural space and bifurcates into the calcarine and parietooccipital arteries near where it crosses above the free edge. The medial posterior choroidal arteries enter the posterior incisural space from anteriorly, turn forward beside the pineal body, and enter the velum interpositum to supply the choroid plexus in the roof of the third ventricle and the body of the lateral ventricle. The lateral posterior choroidal arteries that arise in the posterior incisural space pass around the posteromedial surface of the pulvinar and through the choroidal fissure to supply the choroid plexus in the atrium, giving branches to the thalamus along the way.
The SCA is coursing within the cerebellomesencephalic fissure when it reaches the posterior incisural space. These branches, upon exiting the cerebellomesencephalic fissure, are anterior to the free edge, but they pass below the free edge to supply the tentorial surface of the cerebellum (Fig. 5.2).
The perforating branches of the PCA and SCA, and the medial posterior choroidal arteries supply the walls of the posterior incisural space. The PCAs supply the structures above the level of the lower margin of the superior colliculi and the SCAs supply the structures below the upper margin of the inferior colliculus.
The posterior incisural space has the most complex venous relationships in the cranium, because the internal cerebral and basal veins and many of their tributaries converge on the vein of Galen within this area (Figs. 5.1, 5.5, and 5.6). The internal cerebral veins exit the velum interpositum and the basal veins exit the ambient cistern to reach the posterior incisural space, where they join to form the vein of Galen. The vein of Galen passes below the splenium to enter the straight sinus at the tentorial apex. The junction of the vein of Galen with the straight sinus varies from being nearly flat if the tentorial apex is located below the splenium to forming a sharp angle if the apex is located above the splenium, so that the vein of Galen must turn sharply upward to reach the straight sinus at the apex. The largest vein from the infratentorial part of the posterior incisural space, the vein of the cerebellomesencephalic fissure, originates from the union of the paired veins of the superior cerebellar peduncle.
The tentorial arteries arise from three sources (8). The first source, the cavernous segment of the carotid artery, provides two arteries: the basal tentorial artery (the artery of Bernasconi-Cassinari) from the meningohypophyseal trunk, and the marginal tentorial artery from the artery from the inferolateral trunk (also called the artery of the inferior cavernous sinus). The basal tentorial artery arises from the meningohypophyseal trunk and courses posterolaterally along the medial part of the tentorial attachment to the petrous ridge. The marginal tentorial artery arises from the inferolateral trunk, passes laterally over the abducens nerve, then superoposteriorly near the trochlear nerve to enter the tentorial edge. If this artery is absent, a branch from the meningohypophyseal artery may replace it (8, 28, 32).
The second source of tentorial arteries is from the SCA. The meningeal branch originates from the main or rostral trunk near where the artery passes under the tentorium, and it enters the free edge in the middle incisural space. In our specimens, 28% of the SCAs gave rise to a tentorial branch, and such a vessel may be encountered when the tentorium is divided through a subtemporal approach (17).
The third source is the proximal part of the PCA. The tentorial branch of the PCA arises as a long circumflex artery that courses around the brainstem and below the free edge to enter the tentorium near the apex (17, 37). This artery may also give branches to the superior vermis and inferior colliculi.
Tentorial herniation is the most common and most important form of brain herniation (10, 12, 15). In descending herniation caused by supratentorial mass lesions, the uncus and parahippocampal gyri herniate downward through the incisura, and in ascending herniation resulting from infratentorial masses, the superior part of the cerebellum may herniate upward through the incisura. These brain herniations may cause combinations of direct effects caused by neural compression and indirect effects caused by vascular compromise. Symptoms may result from displacement, compression, and stretching of the brainstem and cranial nerves, hemorrhage and infarction caused by compression and tearing of arteries and veins, increasing edema and intracranial pressure caused by venous obstruction, hydrocephalus caused by obstruction of the aqueduct and subarachnoid space at the incisura, and strangulation of the prolapsed tissue.
The type of the tentorial herniation in each case depends on the position and rate of expansion of the lesion and the size and shape of the incisura. The signs appear early when structures are deformed rapidly, whereas advanced distortion may occur before the appearance of signs if the herniation develops slowly. A wide space between the free edge and brainstem facilitates cerebral herniation since more tissue can herniate into the space (20). A low position of the anterior portion of the free edge also facilitates descending herniation (20).
Descending herniations are divided into anterior, posterior, and complete types. In the anterior type, the uncus herniates into the interpeduncular and crural cisterns. This shift carries the brainstem to the opposite side, thus increasing the space between the free edge and the brainstem, and facilitating a further shift of tissue through the aperture. Eventually, the parahippocampal gyrus, from the splenium to the uncus, may be forced through the opening and the incisura becomes plugged with herniated temporal lobe, deformed hypothalamus, and compressed midbrain. The amygdaloid nucleus is involved with the uncus in the herniated mass. Distortion and compression of the midbrain reticular activating pathways causes a decreased level of consciousness. Compression of the ipsilateral cerebral peduncle causes contralateral pyramidal signs and, if the lateral displacement of brainstem is severe, the contralateral cerebral peduncle may be forced against the free edge, thus producing a groove on the peduncle called a Kernohan’s notch, with ipsilateral pyramidal signs (30). In the terminal stage, deformation of the midbrain causes decerebrate rigidity. Distortion and compression of the posterior hypothalamus may cause cardiovascular, respiratory, and thermoregulatory disturbances. The pituitary stalk may be stretched and compressed against the dorsum sellae, causing diabetes insipidus. The oculomotor nerve courses between the medial border of the uncus and the posterior petroclinoidal fold, and may be kinked or compressed here or between the PCA and SCA, or it may be stretched as the hernia displaces the midbrain posteriorly. Initially, the pupilloconstrictor fibers, which are concentrated on the superior surface of the nerve, are compressed. Later, somatic fibers to the extraocular muscles are disturbed. In the early stages, irritation of the pupilloconstrictor fibers may cause pupillary constriction, but this usually gives way to a paralytic effect with pupillary dilation as the hernia enlarges. The optic tract is displaced medially and downward, but the resulting visual loss is often masked by deepening coma. Compression of the uncus, amygdaloid nucleus, parahippocampal gyrus, and hippocampal formation against the free edge may cause memory, behavior, and personality changes. Residual scarring of the hippocampal formation may cause seizures. The trochlear nerve usually escapes involvement in such herniations, but caudal displacement of the brainstem may result in a palsy of the abducens nerve by stretching it in the subarachnoid space or by strangling it in its course around the AICA.
Stretching or compression of the anterior choroidal and PCA between the temporal lobe and the peduncle or obstruction of the PCA as it crosses the free edge may cause visual field loss caused by ischemia of the optic tract, optic radiation, or the lateral geniculate body; contralateral hemiplegia caused by involvement of the cerebral peduncle and midbrain; or changes in personality and behavior caused by damage to the amygdaloid nucleus or hippocampal formation; unconsciousness and decerebrate rigidity caused by midbrain ischemia; and contralateral sensory loss caused by ischemia of the ventral thalamic nuclei. Brainstem hemorrhage frequently accompanies tentorial herniation.
In the posterior type of tentorial herniation, the posterior portion of the parahippocampal and lingual gyri and the isthmus of the cingular gyrus may shift through the incisura into the quadrigeminal cistern and compress and displace the dorsal half of the midbrain. Tectal compression may cause vertical gaze disturbances. Compression and obstruction of the aqueduct causes hydrocephalus and raises the intracranial pressure. In the posterior type of herniation, the PCA or its calcarine branch is pressed against the free edge and may be obstructed, causing infarction of the occipital cortex and hemianopsia. The basal vein may be compressed between the midbrain and herniated temporal lobe, and the vein of Galen may be obstructed as it curves around the splenium, thus aggravating the venous congestion, edema, and intracranial tension. The complete type of herniation yields a combination of signs and symptoms observed with anterior and posterior herniations.
Hemorrhage into the brainstem as a result of tearing of arteries and veins without cerebral herniation may occur if the incisura hugs the brainstem so tightly that it prevents cerebral herniation while allowing axial displacement of the brainstem.
In ascending herniation attributable to a posterior fossa mass lesion, the superior part of the cerebellar vermis and hemispheres herniate upward through the incisura into the quadrigeminal cistern. Cerebellar infarction may result from compression of the branches of the SCA where they pass under the free edge. The hernia may compress the great cerebral vein against the splenium, which is fixed above by the falx, thus increasing the venous congestion, edema, and intracranial pressure.
Pathology and Operative Approaches
Most aneurysms, many pineal, sellar, parasellar, and third ventricular tumors, and some anteriovenous malformations are approached through the incisural spaces. The arteries in the incisura have been subject to bypass procedures, and many operations for trigeminal neuralgia are directed through this area. In addition, structures bordering the area have been ablated either at craniotomy or stereotactically for the control of epilepsy. The selection of the best operative approach for a given lesion of the incisura depends on the space involved.
Anterior Incisural Space
Nearly 95% of saccular arterial aneurysms arise within the anterior incisural space. The basic anatomy of the common aneurysms has been reviewed elsewhere by Rhoton (23). The aneurysms arising from the part of the circle of Willis located anterior to Liliequist’s membrane, and from the internal carotid and middle cerebral artery are most commonly approached through a frontotemporal (pterional) craniotomy (35) (Fig. 5.9). Aneurysms located behind Liliequist’s membrane at the basilar apex in the interpeduncular fossa may be exposed through either a frontotemporal or subtemporal craniotomy if they are located above the dorsum sellae (35, 36) (Figs. 5.8 and 5.9). Those located below the dorsum or in the prepontine cistern may require a pretemporal, anterior, or mid subtemporal craniotomy with incision or retraction of the tentorium (Fig. 5.7).
Incision and retraction of the tentorium are commonly required to gain access to lesions around the incisura. The incision in the tentorium to expose the interpeduncular and prepontine cisterns is usually located just posterior to the point where the trochlear nerve enters the free edge. The free edge may be retracted by means of sutures placed near to it, but special care is required to avoid stretching and damaging the trochlear nerve in its course inferomedial to and entering the free edge near the posterior margin of the oculomotor trigone. The tentorial arteries and venous sinuses may be encountered in sectioning the tentorium (16). Sectioning of the tentorium has been used to alleviate pressure on the brainstem caused by large incisural lesions that cannot be removed (2).
Perforating arteries to the brainstem are at greatest risk in approaches to the anterior incisural space, because they are commonly stretched around lesions in this area. Hypoplastic arterial segments in the circle of Willis should not be sacrificed during the exposure because hypoplastic segments have been found to have the same number and size of perforating branches as arteries of a normal diameter (23).
Tumors arising in or extending into the anterior incisural space include pituitary adenomas, craniopharyngiomas, clival chordomas, meningiomas arising from the tuberculum sellae, clivus, and medial part of the sphenoid ridge, gliomas of the optic nerve and hypothalamus, some dermoid cysts and teratomas, and neuromas of the oculomotor nerve. Tumors in the anterior incisural space may be approached by the bifrontal, subfrontal, frontal-interhemispheric, frontotemporal, subtemporal, and transsphenoidal routes. Tumors located anterior to Liliequist’s membrane between the optic chiasm and the sellar floor are commonly operated on by the transsphenoidal or subfrontal route. The transsphenoidal approach is preferred if the tumor extends upward out of an enlarged sella turcica and is located above a pneumatized sphenoid sinus. The subfrontal intracranial approach is reserved for those tumors in the chiasmatic cistern that are not accessible by the transsphenoidal route because they are located entirely above the diaphragma sellae, or extend upward out of a normal or small sella, or are located above a nonpneumatized (conchal) type of sphenoid sinus. The subfrontal approach permits exposure of the tumor within the anterior incisural space by four routes: 1) the subchiasmatic approach between the optic nerves and below the optic chiasm; 2) the opticocarotid route directed between the optic nerve and carotid artery; 3) the lamina terminalis approach directed above the optic chiasm through a thinned lamina terminalis; and 4) the transfrontal-transsphenoidal approach obtained by entering the sphenoid sinus and sella through the transfrontal craniotomy (22, 25, 26). The subchiasmatic approach is used if the subchiasmatic opening is enlarged by the tumor. The opticocarotid route is selected if parasellar extension of the tumor widens the space between the carotid artery and the optic nerve and the tumor cannot be reached by the subchiasmatic approach. The lamina terminalis approach is selected if the tumor has pushed the chiasm into a prefixed position and extends into the third ventricle to stretch the lamina terminalis so that the tumor is visible through it. The transfrontal-transsphenoidal approach is selected if the tumor grows upward out of the sella, the sphenoid sinus is pneumatized and the tumor does not stretch the lamina terminalis or widen the opticocarotid space, and a prefixed chiasm blocks the subchiasmatic exposure. A bifrontal craniotomy may be used if the tumor extends forward in both anterior cranial fossae and cannot be reached by a unilateral subfrontal exposure. A frontal interhemispheric approach directed along the anterior part of the falx is used for lesions restricted to the part of the anterior interhemispheric space located just below the rostrum, especially if the tumor arises in the genu or rostrum of and grows into the anterior incisural space.
The frontotemporal approach is used for a tumor arising from the sphenoid ridge or anterior clinoid process, or if it arises above the diaphragma and extends along the sphenoid ridge or into the middle cranial fossa, or if the lesion is accessible through the spaces between the optic nerve and carotid artery or between the carotid artery and the oculomotor nerve (Fig. 5.9). Some lesions may require that the above approach be combined with resection of the cranial base if the lesion involves the paranasal sinuses, nasal cavity, pharynx, orbit, or cavernous sinus, and for those extending from the anterior incisural space into the area behind the dorsum sella or petrous apex, and those in which the lower opening provided by cranial base resection will yield a better angle of exposure or reduce the need for brain retraction. These approaches include the transcranial-transbasal, extended frontal, fronto-orbital, orbitozygomatic, transcavernous, preauricular-infratemporal, and subtemporal anterior petrousectomy, some of which are discussed more fully in the chapters on the foramen magnum and temporal bone.
Middle Incisural Space
Lesions in the middle incisural space include meningiomas arising from Meckel’s cave, the anterior part of the free edge and the petrous apex, gliomas of the temporal lobe and thalamus, anteriovenous malformations of the medial temporal lobe, and neuromas of the trochlear and trigeminal nerves. The infrequent aneurysms arising in the middle incisural space are usually located on the PCA at the origin of its first major cortical branch or on the SCA at its bifurcation into rostral and caudal trunks. Bypass operations using vein and arterial grafts have been applied to the trunks and branches of the posterior cerebral and superior cerebellar branches in the middle incisural space bordering the incisura. The middle incisural space is exposed in performing amygdalohippocampectomy and temporal lobectomy for epilepsy since both the amygdalae and hippocampus extend medial to the free edge. The trigeminal nerve is also frequently exposed in the middle incisural space in the course of operations for trigeminal neuralgia.
Approaches to the middle incisural space include the posterior frontotemporal, subtemporal, temporal-transventricular, and the lateral suboccipital routes (Figs. 5.7 and 5.8). The subtemporal approach with elevation of the temporal lobe is commonly used to expose lesions in the cisterns around the incisura. Hemorrhage, venous infarction, and edema following retraction of the temporal lobe during this approach are minimized by placing the lower margin of the craniotomy and dural exposure at the cranial base so as to reduce the need for retraction, and by avoiding occlusion of the bridging veins, especially the vein of Labbe. The tentorium is frequently divided to increase the exposure or to decompress the brainstem when mass lesions are impacted in the incisura (2). Resection of part of the parahippocampal gyrus may facilitate exposure of the upper part of the middle incisural space (1). A transventricular approach using a cortical incision in the non-dominant inferior or middle temporal gyrus may be used if the lesion involves the temporal horn, choroidal fissure, hippocampal formation, or the upper part of the middle incisural space (9). A cortical incision in the medial occipitotemporal gyrus on the inferior surface of the temporal lobe has been used to minimize visual and speech deficits in exposing the temporal horn of the dominant hemisphere. After entering the temporal horn, the choroidal fissure is opened to expose the middle incisural space. The subtemporal craniectomy may be combined with a suboccipital craniectomy with section of the tentorium and transverse sinus to remove lesions in the prepontine or cerebellopontine cisterns. The trochlear nerve is the cranial nerve most frequently injured in the middle incisural space. It can be injured in dividing the free edge and is so thin and friable that it may rupture from gentle retraction on the leaves formed by dividing the tentorium. The above approaches may be combined with cranial base approaches involving resection or mobilization of the orbital rim, zygomatic arch, floor of the middle fossa, or a portion of the temporal bone as are accomplished in the orbitozygomatic craniotomy, and the preauricular infratemporal or anterior petrousectomy approaches.
The posterior trigeminal root is frequently exposed through a lateral suboccipital craniectomy in the infratentorial part of the middle incisural space for rhizotomy or microvascular decompression operations. The exposure is directed along the angle formed by the insertion of the tentorium to the petrous ridge. The posterior root proximal to Meckel’s cave has also been exposed through a subtemporal craniectomy combined with incision of the tentorium (11). The posterior root may also be exposed for rhizotomy within Meckel’s cave through a subtemporal extradural approach.
Posterior Incisural Space
Lesions in the posterior incisural space include pineal tumors; meningiomas arising at the falcotentorial junction and from the tela choroidea of the velum interpositum and atrium; gliomas of the splenium, pulvinar, quadrigeminal plate, and cerebellum; aneurysms of the vein of Galen; and anterio-venous malformations involving the medial occipital lobe and upper cerebellum.
Lesions in the posterior incisural space may be approached from above the tentorium along the medial surface of the occipital lobe using an occipital transtentorial approach, through the posterior part of the lateral ventricle using a posterior transventricular approach, and through the corpus callosum using a posterior interhemispheric transcallosal approach, or from below the tentorium through the supracerebellar space using an infratentorial supracerebellar approach (Figs. 5.10 and 5.11). The infratentorial supracerebellar and occipital transtentorial approaches, which are most commonly selected for pineal region tumors, may be combined with incision of the tentorium lateral to the straight sinus and less commonly with division of the tentorium and transverse sinus. A tentorial branch of the PCA or SCA may enter the dura lateral to the straight sinus. Venous sinuses are more commonly encountered in the posterior than in the anterior parts of the tentorium. Part of the tentorium may be removed in resecting tumors that arise from or invade it.
The infratentorial supracerebellar approach may be selected for lesions in the pineal region located below the vein of Galen and its major tributaries (29). The approach is best suited to tumors in the midline that grow into the lower half of the posterior incisural space, displacing the quadrigeminal plate and apex of the tentorial cerebellar surface. The occipital transtentorial approach is preferred for lesions centered at or above the tentorial edge, especially if they are located above the vein of Galen. The latter approach may also provide a better angle of access for some lesions involving the ipsilateral half of the cerebellomesencephalic fissure and posterior part of the ambient cistern, although they may be located below the level of the vein of Galen (21, 34) The posterior transcallosal approach, in which the splenium is divided, would be used only if the lesion appears to arise in the splenium above the vein of Galen and extends into the posterior incisural space. The posterior transventricular approach provides adequate exposure of the atrium and posterior portion of the body of the lateral ventricle and would be the preferred approach to a tumor involving the posterior incisural space if the tumor extends into the pulvinar or involves the atrium or the glomus of the choroid plexus. The preferable approach to the ventricle is through the superior parietal lobule, although on approach to the pineal region using a cortical incision in the superior temporal gyrus and directed through the atrium has been advocated (31).
Comparison of Occipital Transtentorial and Infratentorial Supracerebellar Approaches
In examining the posterior incisural space, we compared the midline and paramedian variants of the infratentorial supracerebellar approach and the occipital transtentorial approach (Figs. 5.10 and 5.11). The midline infratentorial supracerebellar approach is directed steeply upward over the apex of the vermis where the large complex of veins emptying into the vein of Galen, and especially the vein of the cerebellomesencephalic fissure, blocks access to the pineal region. The venous complex could be gently displaced to expose the lower part of the splenium, the pineal, and the superior colliculus, but the prominent vermian apex forming the posterior lip of the cerebellomesencephalic fissure limits exposure below the level of the superior colliculus. In the paramedian variant of the infratentorial supracerebellar approach, the retraction was advanced above the hemisphere lateral to the vermis. This approach was not as upwardly steep as the approach above the vermian apex and provided access to the pineal region, the lower part of the splenium, and gave greater access to the ipsilateral half of the cerebellomesencephalic fissure. In addition, the approach could be advanced along the lateral part of the cerebellar surface to expose the posterior part of the ambient cistern. In the occipital transtentorial approach, the occipital lobe was retracted and the tentorium divided along the edge of the straight sinus. This provided access to the splenium above the vein of Galen and, with gentle retraction of the venous complex in the posterior incisural space, the pineal and the upper part of the cerebellomesencephalic fissure could be visualized. The approach provided wider access to the midline and ipsilateral half of the cerebellomesencephalic fissure than did the midline infratentorial supracerebellar approach. In addition, it provided an excellent route for reaching the posterior part of the ambient cistern and even the lateral surface of the cerebral peduncle in the crural cistern. The exposure of the lateral part of the contralateral half of the quadrigeminal cistern was more limited than could be achieved with the midline infratentorial supracerebellar approach. The supra and infratentorial approaches can be converted into a combined approach by dividing the transverse sinus in addition to the tentorium, if the sinus is small and is well collateralized through the opposite side (Fig. 5.11).
Contributor: Albert L. Rhoton, Jr., MD
Content from Rhoton AL. The Posterior Cranial Fossa: Microsurgical Anatomy and Surgical Approaches. Neurosurgery 47(3), 2000, 10.1097/00006123-200105000-00065. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons.
The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton Jr., MD
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