Tentorium incisura

CHAPTER 5

Tentorial Incisura

Albert L. Rhoton, Jr., M.D.Department of Neurological Surgery, University of Florida, Gainesville, Florida

Key words: Anatomic study, Anatomy, Circle of Willis, Incisura, Midbrain, Neurovascular, Tentorium

The tentorial incisura provides the only communicationbetween the supratentorial and infratentorial spaces(17) (Fig. 5.1). The area between the upper brainstemand the incisural edges is divided into the anterior, middle,and posterior incisural spaces (Fig. 5.2). The anterior incisuralspace is located anterior to the brainstem and extends upwardaround the optic chiasm to the subcallosal area; the middleincisural space is located lateral to the brainstem and is inti-mately related to the hippocampal formation in the medialpart of the temporal lobe; and the posterior incisural space islocated posterior to the midbrain and corresponds to the regionof the pineal gland and vein of Galen. The arterial relationshipsin the anterior incisural space and the venous relationships in theposterior incisural space are extremely complex, since the ante-rior incisural space contains all of the components of the circle ofWillis and the bifurcation of the internal carotid and basilararteries, and the posterior incisural space contains the conver-gence of the internal cerebral and basal veins and many of theirtributaries on the vein of Galen. The incisura is intimately relatedto the depths of the cerebrum and cerebellum, the first six cranialnerves, and the upper brainstem. Some part of the incisura iscommonly exposed during the operations for aneurysms, deeptumors and arteriovenous malformations, trigeminal neuralgia,and epilepsy. Much attention has been focused on the distortionsof this anatomy by herniation of the brain through the incisuralspace.

ANATOMY OF THE TENTORIUM

The tentorium covers the cerebellum, supports the cere-brum, and forms a collar around the brainstem (Figs. 5.2 and5.3). The tentorium slopes downward from its apex, located atthe posterior edge of the incisura, to its attachment to thetemporal, occipital, and sphenoid bones. All of the tentorialmargins, except the free edges bordering the incisura, arerigidly attached to the cranium. The anterior border is at-tached to the petrous ridge and divides to enclose the superiorpetrosal sinus. The lateral and posterior borders, which divideto enclose the transverse sinus and the torcula, are attached tothe inner surface of the occipital and temporal bones along theinternal occipital protuberance and to the edges of the shallowosseous groove for the transverse sinus.

The anterior end of each free edge is attached to the petrousapex and the anterior and posterior clinoid processes (Figs.5.15.3). The attachment to the petrous apex and the clinoidprocesses forms three dural folds: the anterior and posteriorpetroclinoid folds and the interclinoid fold. Between thesefolds is located the oculomotor trigone, a shallow depressedarea over the posterior part of the roof of the cavernous sinus,through which the oculomotor and trochlear nerves enter thesinus. The posterior petroclinoid fold extends from the pe-trous apex to the posterior clinoid process; the anterior pet-roclinoid fold extends from the petrous apex to the anteriorclinoid process; and the interclinoid fold covers the ligamentextending from the anterior to the posterior clinoid process.The oculomotor nerve penetrates the dura in the central partof this triangle, the oculomotor triangle, and the trochlearnerve enters the dura at the posterolateral edge of this trian-gle. The petrosphenoid ligament passes between the leavesof the posterior petroclinoid fold from the petrous apex tothe lateral border of the dorsum sellae, just below theposterior clinoid process. The abducens nerve passes belowthe petrosphenoid ligament to enter the cavernous sinus.The dura forming the roof of the oculomotor trigones ex-tends medially across the sella to form the diaphragmasellae, which covers the pituitary gland and contains anopening for the infundibulum.

Anterolateral to the diaphragma are two orifices: a boneorifice, the optic canal (through which the optic nerve entersthe orbit), and a dural orifice through which the internalcarotid artery exits the cavernous sinus (Fig. 5.3). From theanterior part of the free edge, the dura mater slopes steeplydownward to form the lateral wall of the cavernous sinus andto cover the middle cranial fossa. Plaut reported that theattachment of the anterior end of the free edge to the petrousapex may be situated as much as 10 mm lateral and 8 mmbelow the level of the clinoid processes and that the lowposition of the free edge may facilitate descending tentorialherniations (20).

The falx cerebri fuses into the dorsal surface of the tento-rium in the midline behind the apex (Fig. 5.1). The straightsinus, which is enclosed in the falcotentorial junction, beginsat the tentorial apex, where it receives the vein of Galen andthe inferior sagittal sinus, and terminates in the torcular.

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TENTORIAL INCISURA

The incisura is roughly triangular and has its anterior edge orbase on the dorsum sellae and its apex dorsal to the midbrain,just posterior to the pineal gland (Fig. 5.2). The incisura, whenviewed from above after removal of the cerebral hemispheres, isfilled by the midbrain, pons, and cerebellum, and the free edgesskirt the cerebral peduncles, either touching or being separatedfrom them by a variable distance (Fig. 5.2). The amount ofcerebellar cortex visible between the midbrain and the free edgevaries from none when the free edge hugs the tectum to a largeamount when the incisura extends far posteriorly. When viewedfrom below after removal of the cerebellum, the incisura is filledby the midbrain and the uncus and parahippocampal gyrus (Fig.5.4). The amount of parahippocampal gyrus visible from belowvaries from none when the free edge hugs the tectum to a largeamount when the incisura is very wide. The width of the inci-sura varies from 26 to 35 mm (average, 29.6 mm) and theanteroposterior diameter varies from 46 to 75 mm (average, 52.0mm) (17).

The area between the brainstem and the free edges is di-vided into: an anterior incisural space located in front of the

brainstem; paired middle incisural spaces situated lateral tothe brainstem; and a posterior incisural space located be-hind the brainstem (Figs. 5.15.4). The description of eachincisural space is divided into sections on neural, cisternal,ventricular, cranial nerve, arterial, and venous relationships.

ANTERIOR INCISURAL SPACE

Neural relationships

The anterior incisural space is located anterior to the mid-brain and pons. It extends inferiorly between the brainstemand clivus and obliquely forward and upward around theoptic chiasm to the subcallosal area. It opens laterally intothe medial part of the Sylvian fissure, and posteriorly be-tween the uncus and the brainstem into the middle incisuralspace (Figs. 5.3 and 5.4).

The part of the anterior incisural space located below theoptic chiasm has posterolateral and posterior walls. The pos-terolateral wall is formed by the bulbous prominence of theanterior third of the uncus, which hangs over the anterior part

FIGURE 5.1. Tentorial incisura. A, the left cerebralhemisphere has been removed. The tentorial incisura islocated between the tentorial edges and is the only site ofcommunication behind the supra and infratentorial spaces.The tentorial apex is located at the junction of the vein ofGalen and the straight sinus. The tentorial edges slopedownward from the apex. The free edge passes along the sideof the brainstem and anteriorly blends into the dura coveringthe petrous apex and the anterior and posterior clinoidprocesses. The incisura, in relation to the midbrain, is dividedinto anterior, middle, and posterior spaces. The anteriorincisural space extends above the optic chiasm to the laminaterminalis and below the chiasm and third ventricular floor tothe interpeduncular fossa. The middle incisural space islocated between the midbrain and tentorial edge, opensupward into the ambient and crural cisterns, and extendsinferiorly into the anterior part of the cerebellomesencephalicfissure. The posterior incisural space, located between theposterior midbrain and the tentorial apex, encompasses thequadrigeminal cistern, which extends into thecerebellomesencephalic fissure and along the outer surface ofthe upper part of the fourth ventricular roof. The anteriorincisural space, located below the frontal horn, contains thebasilar bifurcation. The PCA and SCA arise in the anterior andpass around the brainstem to reach the middle and posteriorincisural spaces. The branches of the PCA and SCA passthrough the lateral part of the posterior incisural space, andthe large venous structures converging on the vein of Galen

course in the medial part of the posterior incisural space. B, part of the left central hemisphere and all of the left thalamushave been removed, while preserving the fornix and choroid plexus. The frontal horn and anterior part of the third ventricleis located above the anterior incisural space. The middle incisural space is located medial to the temporal horn, between thetemporal lobe and midbrain. The posterior incisural space is located between the tentorial apex and posterior midbrainsurface. A., artery; A.C.A., anterior cerebral artery; Ant., anterior; Bas., basilar; Car., carotid; Chor., choroid; CN, cranialnerve; Front., frontal; Gyr., gyrus; Incis., incisural; Mid., middle; Parahippo., parahippocampal; Ped., peduncle; Plex., plexus;Post., posterior; Temp., temporal; Tent., tentorial; V., vein; Vent., ventricle.

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of the free edge above the oculomotor trigone (Fig. 5.2). Theposterior wall is formed by the pons and cerebral peduncles. Theinfundibulum of the pituitary gland crosses the anterior incisuralspace to reach the opening in the diaphragma sellae. The part ofthe anterior incisural space situated above the optic chiasm islimited superiorly by the rostrum of the corpus callosum, pos-

teriorly by the lamina terminalis, and laterally by the part of themedial surfaces of the frontal lobes located below the rostrum.

The anterior incisural space opens laterally into the part ofthe Sylvian fissure situated below the anterior perforatedsubstance (Fig. 5.4). The anterior limb of the internal capsule,the head of the caudate nucleus, and the anterior part of the

FIGURE 5.2. Tentorial incisura,superior views. A, the leftcerebrum, above the level of thecerebral peduncle, has beenremoved to expose the anterior,middle, and posterior incisuralspaces. The thalamus, which formsthe floor of the body of the lateralventricle, sits directly above thecentral part of the tentorialincisura. The right lateral ventricleand the lower wall of the sylvianfissure have been preserved. Theleft half of the tentorium, exceptthe edge, has been removed toexpose the tentorial cerebellarsurface. The frontal horn islocated above the anteriorincisural space. Structures locatedin the anterior incisural spacebelow the frontal horn include theoptic nerves and chiasm, internalcarotid arteries, and the upperpart of the basilar artery and itsbranches. The middle incisuralspace, located between themidbrain and tentorial edge,opens upward into the crural andambient cisterns and downwardinto the anterior part of thecerebellomesencephalic fissure.The posterior incisural space,located between the midbrain andthe tentorial apex, includes thearea of the quadrigeminal cisternand opens into the central part ofthe cerebellomesencephalicfissure. The atrium of the lateral

ventricle is situated lateral to the posterior incisural space. B, view of the tentorial incisura after removing the cerebrum. Thetentorial edges sweep along the lateral margin of the cerebral peduncle. The oculomotor nerve passes medial to the anterioredge of the tentorium and enters the cavernous sinus by passing through a triangular patch of dura called the oculomotortrigone. C, superior view of the tentorial incisura before removing the left temporal lobe. The crural cistern is locatedbetween the cerebral peduncle and uncus. The ambient cistern opens upward between the midbrain and the medial surfaceof the temporal lobe formed by the parahippocampal and dentate gyri. The thalamus and the genu of the internal capsule arelocated above the central part of the tentorial incisura. D, enlarged view after removing the temporal lobe. The internalcapsule and the lentiform nucleus are located above the middle incisural space. The genu of the internal capsule abuts on thelateral ventricular wall at the level of the foramen of Monro. A., artery; Ant., anterior; Cap., capsule; Car., carotid; Caud.,caudate; Cist., cistern; CN, cranial nerve; For., foramen; Front., frontal; Incis., incisural; Int., internal; Lent., lentiform; Mid.,middle; Nucl., nucleus; P.C.A., posterior cerebral artery; Ped., peduncle; Post., posterior; Quad., quadrigeminal; Tent.,tentorial; Trig., trigone.

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FIGURE 5.3. Stepwise dissection of the neural structures above the tentorial incisura. A, the coronal section of the righthemisphere crosses vertically through the thalamus and lateral geniculate body and the transverse section crosses the cere-bral peduncle. The right temporal horn has been opened to expose the hippocampus and amygdaloid nucleus. The floor ofthe third ventricle is exposed in the midline. The coronal section of the left hemisphere crosses anterior to the thalamus nearthe foramen of Monro and genu of the internal capsule. The anterior incisural space extends from the interpeduncular fossa,around the chiasm, and into the suprachiasmatic area. B, the right thalamus has been removed while preserving the fornix,which wraps around the thalamus to form the outer edge of the choroidal fissure situated between the thalamus and fornix.The middle incisural space extends upward into the ambient and crural cisterns. The crural cistern is located between theuncus and the cerebral peduncle. The ambient cistern in located between the parahippocampal and dentate gyri and the fim-bria of the fornix laterally and the midbrain medially. The posterior part of the tentorial edge is exposed. The quadrigeminalcistern is located in the posterior incisural space between the tentorial apex and the pineal. The atrium is located lateral tothe quadrigeminal cistern and posterior incisural space. C, enlarged view. The coronal section through the left hemispherehas been extended backward to the level of the thalamus and posterior limb of the internal capsule. The left temporal horn is

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lentiform nucleus are located above the anterior perforatedsubstance (Fig. 5.2).

Cisternal relationships

The interpeduncular cistern, which sits in the posterior partof the anterior incisural space between the cerebral pedunclesand the dorsum sellae, communicates laterally with the Syl-vian cistern below the anterior perforated substance and an-teriorly with the chiasmatic cistern located below the optic chi-asm. The interpeduncular and chiasmatic cisterns are separatedby Liliequists membrane, an arachnoidal sheet extending fromthe dorsum sellae to the anterior edge of the mammillary bodies(14, 35, 36). The chiasmatic cistern communicates around theoptic chiasm with the cisterna laminae terminalis, which liesanterior to the lamina terminalis.

Ventricular relationships

The anterior part of the third ventricle projects into theanterior incisural space in the medial plane, dividing it intosupra and infra chiasmatic portions. The frontal horns of thelateral ventricles are located above the anterior incisural space(Figs. 5.15.3). The tip of the temporal horn is separated fromthe uncal surface, forming the posterolateral wall of the ante-rior incisural space, by the amygdaloid nucleus.

Cranial nerves

The optic and oculomotor nerves and the posterior part ofthe olfactory tracts pass through the anterior incisural space.Each olfactory tract runs posteriorly, and splits just above theanterior clinoid process to form the medial and the lateralolfactory striae, which course along the anterior margin of theanterior perforated substance (Fig. 5.4).

The optic nerves and chiasm and the anterior part of theoptic tracts cross the anterior incisural space (Fig. 5.3). Theoptic nerves emerge from the optic canal medial to the attach-ment of the free edge to the anterior clinoid processes, and aredirected posteriorly, superiorly, and medially toward the op-tic chiasm. The optic chiasm is usually located above thediaphragma sellae, but it may be prefixed and lie over thetuberculum sellae or postfixed and lie over the dorsum sellae.

From the chiasm, the optic tract continues in a posterolateraldirection around the cerebral peduncle to enter the middleincisural space (Fig. 5.4). The oculomotor nerve emerges fromthe midbrain on the medial surface of the cerebral peduncle.It crosses the anterior incisural space between the posteriorcerebral artery (PCA) and the superior cerebellar artery (SCA)and passes inferomedial to the uncus to enter the roof of thecavernous sinus through the oculomotor trigone. The abdu-cens nerve ascends from deep within the infratentorial part ofthe anterior incisural space. It emerges from the pontomedul-lary sulcus, ascends in the prepontine cistern to pierce thedura covering the clivus, and passes below the petrosphenoidligament to enter the cavernous sinus.

Arterial relationships

The arterial relationships of the anterior incisural space arecomplex because it contains all of the components of the circleof Willis (4, 5, 7, 18, 19, 27, 37). The internal carotid arteryenters the anterior incisural space by passing along the medialsurface of the anterior clinoid process and bifurcates belowthe anterior perforated substance (Figs. 5.5 and 5.6). The pos-terior communicating artery arises from the posteromedialaspect of the carotid artery and courses superomedial to theoculomotor nerve to join the PCA in the anterior incisuralspace. The anterior choroidal artery originates from the pos-terior surface of the carotid artery 0.1 to 3.0 mm distal to theorigin of the posterior communicating artery and courses belowthe optic tract before passing between the uncus and the cerebralpeduncle to enter the middle incisural space (3, 24).

The proximal part of the anterior cerebral artery alsocourses in the anterior incisural space (Fig. 5.6). It arises belowthe anterior perforated substance and courses anteromediallyabove the optic chiasm, where it is joined to its mate from theopposite side by the anterior communicating artery. It thencourses upward in front of the lamina terminalis. The middlecerebral artery courses laterally from its origin below theanterior perforated substance. The major bifurcation of themiddle cerebral artery is usually located in the lateral part ofthe anterior incisural space.

The basilar artery ascends and gives rise to the PCA andSCA in the posterior part of the anterior incisural space be-

exposed below the basal ganglia. The optic nerves, chiasm, and tracts, and the oculomotor nerves cross the anterior incisuralspace. The middle incisural space extends into the ambient and crural cisterns, and the posterior incisural space, located infront of the tentorial apex, contains the quadrigeminal cistern. D, the upper parts of the anterior and middle incisural spaceshave been exposed by removing the thalami on both sides. The tentorial edges extend forward from the apex, located at theposterior margin of the pineal region, along the side of the midbrain to attach to the petrous ridge and clinoid processes. E,the temporal lobe has been sectioned in a coronal plane and the third ventricular floor has been removed. The lateral wall ofthe ambient cistern is formed by the parahippocampal and dentate gyri and the fimbria of the fornix. F, enlarged view. Therounded medial edge of the parahippocampal gyrus, the site of the subiculum, which blends into the hippocampus, joins thedentate gyri and fimbria to form the lateral wall of the ambient cistern. The fimbria arises on the hippocampal surface. A.,artery; Amyg., amygdaloid; Ant., anterior; Cap., capsule; Car., carotid; Chor., choroid; Cist., cistern; CN, cranial nerve; Coll.,colliculus; Dent., dentate; Front., frontal; Gen., geniculate; Gyr., gyrus; Incis., incisural; Int., internal; Lat., lateral; Lent., lenti-form; Nucl., nucleus; Parahippo., parahippocampal; Ped., peduncle; Plex., plexus; Quad., quadrigeminal; Sulc., sulcus; Temp.,temporal; Tent., tentorial; Vent., ventricle.



tween the posterior perforated substance and the clivus (Fig.5.7). The position of the basilar tip and bifurcation varies fromas far caudal as 1.3 mm below the pontomesencephalic sulcusto as far rostral as the mammillary bodies (17). The PCAcourses laterally around the cerebral peduncle, above theoculomotor nerve. It exits the anterior and enters the middle

incisural space by coursing between the uncus and the cere-bral peduncle. The SCA originates in the anterior incisuralspace below the PCA and courses laterally below the oculo-motor nerve (Fig. 5.7). The origin is usually just rostral to thelevel of the free edge. It dips below the tentorium to reach thesuperior surface of the cerebellum at the junction of the ante-

FIGURE 5.4. Neural relationshipsabove the tentorial incisura.Stepwise dissection viewed frombelow. A, the anterior incisuralspace extends forward from theinterpeduncular fossa below thefloor of the third ventricle andaround the optic chiasm to thelamina terminalis. The middleincisural space extends upwardinto the crural cistern locatedbetween the uncus and cerebralpeduncle and the ambient cisternslocated between the lateralmidbrain and the temporal lobe.The posterior incisural space islocated behind the midbrain andincludes the quadrigeminal cisternand pineal region. The anteriorpart of the tentorial edge hasgrooved the uncus. B, the medialedge of the parahippocampalgyrus has been removed to exposethe roof of the ambient cisternformed by the lower surface ofthe thalamus and the geniculatebodies. The optic tract extendsposteriorly in the roof of thecrural cistern and terminates inthe lateral geniculate bodylocated in the anterior part of theroof of the ambient cisterns. Thedentate gyrus and the fimbria ofthe fornix are located in thelateral margin of the ambientcistern above theparahippocampal gyrus. C, thepart of the parahippocampal gyrusbelow the temporal horn has beenremoved while preserving thefimbria of the fornix. The choroidplexus in the temporal horn isattached along the choroidalfissure located between thefimbria and the thalamus. D, all but the upper part of the left temporal lobe and fimbria has been removed. The optic tractextends posteriorly through the crural cistern to the anterior part of the ambient cistern where it terminates in the lateralgeniculate body. The posterior incisural space between the midbrain and the tentorial apex borders the atrium laterally.Amyg., amygdaloid; Ant., anterior; Chor., choroid, choroidal; Cist., cistern; CN, cranial nerve; Dent., dentate; Fiss., fissure; Gen.,geniculate; Gyr., gyrus; Interped., interpeduncular; Lat., lateral; Med., medial; Nucl., nucleus; Olf., olfactory; Parahippo.,parahippocampal; Ped., peduncle; Perf., perforated; Pit., pituitary; Plex., plexus; Quad., quadrigeminal; Subst., substance; Temp.,temporal; Tent., tentorial; Tr., trunk.

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rior and middle incisural spaces. The structures in the walls ofthe anterior incisural space receive perforating branches fromall of the above arteries.

Venous relationships

The main venous trunk related to the anterior incisuralspace is the basal vein (Figs. 5.5 and 5.6) (16). It coursesthrough the anterior, middle, and posterior incisural spaces toempty into the vein of Galen. It originates below the anteriorperforated substance, courses posterolaterally around the ce-rebral peduncle, below the optic tract and medial to the un-cus, to enter the middle incisural space.

MIDDLE INCISURAL SPACE


The middle incisural space is located lateral to the brain-stem (Figs. 5.3 and 5.4). This narrow space extends upwardbetween the temporal lobe and the midbrain and down-ward between the cerebellum and the upper brainstem. It hasmedial and lateral walls and a roof. The medial wall, formedby the lateral surface of the midbrain and upper pons, is dividedby the pontomesencephalic sulcus, which lies at the level of thefree edge. The surface of the midbrain facing the middle inci-sural space is divided into a larger anterior part formed by the

FIGURE 5.5. Tentorial incisura. A,view from below of the cisternsbordering the tentorial incisura.The middle incisural space opensupward into the crural cisternlocated between the uncus andpeduncle and the ambient cisternlocated between theparahippocampal gyrus and thelateral surface of the brainstem.The PCAs course through thecrural and ambient cisterns toreach the posterior incisuralspace, the site of thequadrigeminal cistern. The basalvein accompanies the PCA in theupper part of the crural andambient cisterns and empties intothe vein of Galen in thequadrigeminal cistern. The medialposterior choroidal arteries coursearound the brainstem on themedial side of the PCAs with thelong circumflex perforatingbranches. B, the medial part ofthe right temporal lobe has beenremoved to expose the temporalhorn. The fimbria of the fornix,which arises on the upper surfaceof the hippocampus and forms thelower margin of the choroidalfissure, has been preserved. Thethalamus, geniculate bodies, andoptic tract are in the roof of the

crural and ambient cisterns. C, the right PCA has been removed. The basal vein passes backward above the PCA and emptiesinto the vein of Galen with the internal cerebral and internal occipital veins. The lower surface of the thalamus, thegeniculate bodies, and the optic tract form the roof of the crural and ambient cisterns. The anterior choroidal artery passesposteriorly above the uncus and through the choroidal fissure to supply the choroid plexus in the temporal horn. D, bothPCAs have been removed to expose the roof of the middle incisural space on both sides and the basal veins, which drain theneural structures in the region. A., artery; Ant., anterior; Car., carotid; Cer., cerebral; Chor., choroid, choroidal; Cist., cistern;CN, cranial nerve; Fiss., fissure; Gen., geniculate; Gyr., gyrus; Incis., incisural; Int., internal; Lat., lateral; Med., medial; Mid.,middle; Occip., occipital; Parahippo., parahippocampal; P.C.A., posterior cerebral artery; Plex., plexus; Post., posterior;Quad., quadrigeminal; Temp., temporal; V., vein.



cerebral peduncle and a smaller posterior part formed by thetegmental surface. The optic tract forms a smooth white band atthe upper edge of the cerebral peduncle that stands in sharpcontrast to the vertically striated surface of the peduncle. Thepeduncular and tegmental surfaces are separated by the lateralmesencephalic sulcus, a vertical groove that extends from thepulvinar above to the pontomesencephalic sulcus below.

The roof of the middle incisural space has a narrow anteriorpart formed by the posterior part of the optic tract that isflattened between the cerebral peduncle and the uncus, and a

wider posterior part formed by the inferior surface of thethalamus (Fig. 5.4). The lateral geniculate body protrudesfrom the lower surface of the thalamus just behind the uncus.The medial geniculate body bulges into the roof posterome-dial to the lateral geniculate body just behind the lateralmesencephalic sulcus.

The lateral wall of the supratentorial part of the middleincisural space is composed of the hippocampal formation onthe medial surface of the temporal lobe (Figs. 5.3 and 5.4). Theuncus and parahippocampal gyri, the most inferior structures

FIGURE 5.6. Superior views ofthe anterior, middle, and posteriorincisural spaces. A and B are fromone specimen and C is fromanother. A, the basal cisterns inthe region of the tentorial incisurahave been exposed by removingthe thalamus and all of the leftcerebral hemisphere except theoccipital and temporal lobes. Theroof of the temporal horn hasbeen removed. The structuresrelated to the anterior incisuralspace, located between thetuberculum sellae anteriorly, themidbrain posteriorly, and theanterior tentorial edge laterally,includes the optic nerve andchiasm, and the internal carotid,basilar, superior cerebellar, andPCAs. The anterior incisural spaceopens posteriorly into the middleincisural space, which extendsinto the crural and ambient

cisterns. The crural cistern is located between the cerebral peduncle and theuncus, and the ambient cistern is located between the lateral midbrain andthe medial surface of the temporal lobe. The ambient cistern opens posteriorlyinto the posterior incisural space, which contains the quadrigeminal cistern.The basal vein and the PCA and SCA pass around the midbrain in the middleincisural space to reach the posterior incisural space and quadrigeminalcistern. B, enlarged view. The preserved tentorial edge is exposed between thebasal vein and trochlear nerve. C, superior view of the middle and posteriorincisural space in another specimen. The basal vein courses through the cruraland ambient cisterns. The upper lip of the calcarine sulcus has been removedbut the lower lip of the sulcus has been preserved. The calcarine branch ofthe PCA loops laterally into the calcarine sulcus, which extends so deeplyinto the medial part of the hemisphere that it forms a prominence, the calcaravis, in the lower part of the medial wall of the atrium. A., artery; A.C.A.,anterior cerebral artery; Ant., anterior; Car., carotid; Calc., calcarine; Cer.,cerebral; Chor., choroid, choroidal; Cist., cistern; CN, cranial nerve; Comm.,communicating; Gyr., gyrus; Incis., incisural; Int., internal; Interped.,interpeduncular; M.C.A., middle cerebral artery; Mid., middle; Parahippo.,parahippocampal; P.C.A., posterior cerebral artery; Plex., plexus; Post.,posterior; Quad., quadrigeminal; Sulc., sulcus; Temp., temporal; Tent.,tentorial; V., vein.

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in this part of the lateral wall, form a curved border aroundthe middle incisural space. The uncus bulges medially at theanterior end of the parahippocampal gyrus. The amygdaloidnucleus is situated just lateral to the medial surface of theuncus and just anterior to the tip of the temporal horn.

The uncus commonly prolapses into the incisura anteriorlyand has a groove along its undersurface marking the free edge(Fig. 5.4). This groove usually disappears at the lateral marginof the peduncle, because the free edge often hugs the pedun-cle at this site, but it may reappear posterior to the peduncleon the lower surface of the parahippocampal gyrus as thespace between the brainstem and the free edge increases. In

our specimens, these grooves were commonly present on theuncus and adjacent part of the parahippocampal gyrus withoutbeing observed on the posterior part of the parahippocampalgyrus, but they were only rarely present posteriorly, and notanteriorly (17). The distance from the most medial point of theuncus to this groove varied from 2 to 8.6 mm (average, 4.4 mm).Howell reported that these grooves may measure up to 15 mmin 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 lobefacing the middle incisural space is formed by three longitu-

FIGURE 5.7. AD. Anterior and middle incisural space. A, the right temporal lobe has been elevated. The middle incisuralspace, located between the lateral surface of the midbrain and the tentorial edge, opens upward into the ambient cisternwhere the PCA and basal vein course. The internal carotid artery is exposed in front of the midbrain in the anterior incisural space.B, enlarged view of the junction of the anterior and middle incisural space. The internal carotid artery, optic nerves, and basilarbifurcation are located in the anterior incisural space. The oculomotor nerve passes forward between the PCA and SCA.C, the inferior temporal and fusiform gyri have been removed to expose the lateral edge of the parahippocampal gyrusabove the middle incisural space. The opening into the temporal horn exposes the choroid plexus attached along thechoroidal fissure. The veins draining the roof of the temporal horn empty into the basal vein. D, the choroidal fissurehas been opened by detaching the choroid plexus from the fimbria of the fornix. Opening the fissure exposes the upperpart of the ambient cistern and the branches of the PCA and basal vein. A., artery; Ant., anterior; Bas., basilar; Br.,branch; Car., carotid; Chor., choroid, choroidal; Cist., cistern; CN, cranial nerve; Comm., communicating; Coll., collicu-lus; Fiss., fissure; Gen., geniculate; Gyr., gyrus; Inf., inferior; Lat., lateral; Med., medial; Mes., mesencephalic; Para-hippo., parahippocampal; P.C.A., posterior cerebral artery; Ped., peduncle; Plex., plexus; Post., posterior; S.C.A., supe-rior cerebellar artery; Sup., superior; Temp., temporal; Tent., tentorial; V., vein; Vent., ventricular.



dinal strips of neural tissue, one located above the other,which are interlocked with the hippocampal formation tomake an important part of the limbic system (Figs. 5.3 and5.4). The most inferior strip is formed by the rounded medialedge of the parahippocampal gyrus; the middle strip isformed by the dentate gyrus, a serrated or beaded strip ofgray matter located on the medial surface of the hippocampalformation; and the superior strip is formed by the fimbriaof the fornix, a white band formed by the fibers emanatingfrom the hippocampal formation that are directed posteriorlyinto the crus of the fornix.

The middle incisural space extends below the tentorium tocommunicate with the anterior part of the cerebellomesence-

phalic fissure, located between the anterosuperior part of thecerebellum and the lateral surface of the tegmentum.


The supratentorial part of the middle incisural space con-tains the crural and ambient cisterns (Figs. 5.25.6). The cruralcistern, located between the cerebral peduncle and the uncus,is a posterolateral extension of the interpeduncular cistern. Thecrural cistern opens posteriorly into the ambient cistern, demar-cated medially by the midbrain, above by the pulvinar, andlaterally by the parahippocampal and dentate gyri and fim-bria of the fornix. The ambient cistern is continuous pos-

FIGURE 5.7. EH. E, anterior and middle incisural space, enlarged view. The opening through the choroidal fissure exposesthe basal vein and branches of the PCA in the upper part of the ambient cistern. The PCA gives off numerous branches to thechoroid plexus, including a large lateral posterior choroidal artery. F, the hippocampus and the medial part of the temporallobe, including the parahippocampal gyrus, have been removed to expose the upper part of the middle incisural space. ThePCA and basal vein course through the middle incisural space on the medial side of the parahippocampal gyrus, which hasbeen removed. The choroid plexus remains attached along the choroidal fissure located between the fimbria and the lowersurface of the thalamus. The inferior ventricular veins drain the roof of the temporal horn and empty into the basal vein. G,the branches of the PCA have been removed to expose the basal vein, which originates below the anterior perforated sub-stance and courses posteriorly through the middle incisural space to gain access to the posterior incisural space and thequadrigeminal cistern. The pulvinar and lower surface of the thalamus, including the geniculate bodies, are in the upper mar-gin of the exposure. H, the basal vein has been removed. This exposes the lateral aspect of the cerebral peduncle and the teg-mental part of the midbrain, which are separated by the lateral mesencephalic sulcus. The medial and lateral geniculate bod-ies protrude downward from the lower surface of the thalamus.

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teriorly with the quadrigeminal cistern, the major cistern inthe posterior incisural space. The ambient cistern extendsbelow the free edge into the part of the cerebellomesence-phalic fissure located above the origin of the trigeminal nerve.


The temporal horn extends into the medial part of thetemporal lobe lateral to the middle incisural space and endsapproximately 3 cm from the temporal pole (Figs. 5.25.7). Thechoroidal fissure, located between the fimbria of the fornixand the lower surface of the thalamus, is the site of attach-ment of the choroid plexus in the temporal horn. The pairedbodies of the lateral ventricles are located directly above the

central part of the incisura. They sit on and are separated fromthe central part of the incisura by the thalamus.

Cranial nerves

The trochlear and trigeminal nerves are related to the middleincisural space (Fig. 5.8). The trochlear nerve has the longestcourse within the incisura of any nerve and is the cranial nervemost intimately related to the free edge. The trochlear nerve arisesbelow the inferior colliculus in the posterior incisural spaceand passes forward through the middle incisural space be-tween the PCA and SCA. Its initial course around the mid-brain is medial to the free edge in the space between thetectum and cerebellum. It reaches the lower margin of the free

FIGURE 5.8. Anterior and middle subtemporal exposure of the anterior and adjacent part of the middle incisural space. A,the craniotomy flap and dural opening exposes the temporal lobe and the floor of the middle cranial fossa. The insert showsthe site of the scalp incision. B, the temporal lobe has been elevated to expose the PCA and SCA in the anterior and middleincisural space. The PCA passes above and the SCA below the oculomotor nerve. The SCA branches course with the trochlearnerve around the side of the brainstem. C, the PCA has been depressed to expose the basilar artery. The anterior choroidalartery arises in the anterior incisural space and passes between the cerebral peduncle and uncus to enter the crural cistern inthe middle incisural space. D, the tentorium has been divided behind the petrous ridge to expose the SCA and the trigeminaland trochlear nerves in the region of the middle incisural space. The SCA sends branches above the trigeminal nerve and intothe anterior part of the cerebellomesencephalic fissure. The medial posterior choroidal artery also passes around the lateralside of the brainstem. A., artery; Ant., anterior; Bas., basilar; Br., branch; Car., carotid; Chor., choroidal, CN, cranial nerve;Comm., communicating; Fiss., fissure; M.C.A., middle cerebral artery; Med., medial; P.C.A., posterior cerebral artery; Ped.,peduncle; Post., posterior; S.C.A., superior cerebellar artery; Temp., temporal; Tent., tentorial.



edge at the posterior edge of the cerebral peduncle. It piercesthe free edge in the posterior part of the oculomotor trigoneand runs for a short distance in the anterior petroclinoid foldbefore entering the lateral wall of the cavernous sinus.

The trigeminal nerve courses in the infratentorial part of themiddle incisural compartment. It arises on the anterolateralaspect of the mid pons and passes above the petrous apex toenter Meckels cave (the arachnoidal and dural cavern) whereit separates into the three sensory divisions (6). The medialedge of the posterior trigeminal root is observed just medial tothe tentorial edge if one looks from straight superior through theincisura with the cerebrum removed, but it is hidden belowthe free edge in the lateral view provided by the subtemporaloperative exposure.


The major arteries in the middle incisural space, the ante-rior choroidal, PCA, and SCA, arise in the anterior incisuralspace and reach the middle incisural space by coursingaround the brainstem parallel to the free edge (Figs. 5.55.8).The anterior choroidal artery enters the superior part of themiddle incisural space below the optic tract and passesthrough the choroidal fissure near the inferior choroidal pointto supply the choroid plexus in the temporal horn.

The PCA enters the middle incisural space between thecerebral peduncle and uncus and passes straight posteriorlybetween the tegmentum and subiculum (Figs. 5.6 and 5.8). Itgives off several cortical branches, which cross the free edge toreach the inferior surface of the temporal and occipital lobes,and the lateral posterior choroidal and thalamogeniculate ar-teries, which course medial to the free edge. The lateral pos-terior choroidal arteries, arising in the middle incisural space,course superolaterally through the choroidal fissure andaround the pulvinar to reach the choroid plexus in the tem-poral horn and atrium (Fig. 5.7). The medial posterior choroi-dal artery arises from the proximal part of the PCA in theanterior incisural space and courses parallel and medial tothe PCA through the middle incisural space to reach theposterior incisural space (Fig. 5.5). The thalamogeniculatebranches arise below the pulvinar and pass upward throughthe geniculate bodies to reach the thalamus and internalcapsule.

The SCA usually passes below the level of the free edge andbifurcates into rostral and caudal trunks as it passes aroundthe lateral margin of the cerebral peduncle to enter the middleincisural spaces (Figs. 5.7 and 5.8). It passes above the trigem-inal nerve and enters the cerebellomesencephalic fissure inthe anterior part of the middle incisural space. The wallsof the supratentorial part of the middle incisural space aresupplied by the perforating branches of the anterior choroidaland PCA, and the walls in the infratentorial part are suppliedby the SCA.


The venous relationships in the middle incisural space arerelatively simple (Figs. 5.55.7). The basal vein courses alongthe upper part of the cerebral peduncle and below the pulv-

inar to reach the posterior incisural space. It may infrequentlyterminate in a tentorial sinus in the free edge at this level.

POSTERIOR INCISURAL SPACE


The posterior incisural space lies posterior to the midbrainand corresponds to the pineal region (Figs. 5.15.4) (33). It hasa roof, floor, and anterior and lateral walls, and extendsbackward to the level of the tentorial apex. The quadrigeminalplate is located at the center of the anterior wall. The anteriorwall rostral to the colliculi is formed by the pineal body. Thehabenular commissure forms the upper half and the posteriorcommissure forms the lower half of the attachment of the pinealbody to the posterior part of the third ventricle. The part of theanterior wall below the colliculi is formed in the midline by thelingula of the vermis and laterally by the superior cerebellarpeduncles as they ascend beside the lingula.

The roof of the posterior incisural space is formed by thelower surface of the splenium, the terminal part of the crura ofthe fornices, and the hippocampal commissure (Figs. 5.1 and5.4). Each crus arises as a continuation of the fimbria, passesaround the posterior margin of the pulvinar, and blends intothe lower margin of the splenium. The hippocampal commis-sure is an oblique band of fibers that courses below thesplenium between the medial margins of the crura. The floorof the posterior incisural space is formed by the anterosuperiorpart of the cerebellum and consists of the culmen of the vermisin the midline and the quadrangular lobules of the hemisphereslaterally. The posterior incisural space extends inferiorly into thecerebellomesencephalic fissure.

Each lateral wall is formed by the pulvinar, crus of thefornix, and the medial surface of the cerebral hemisphere.The anterior part of the lateral wall is formed by the partof the pulvinar located just lateral to the pineal body. Thelateral wall, posterior to the pulvinar, is formed by the seg-ment of the crus of the fornix that wraps around the posteriormargin of the pulvinar (Fig. 5.1). The posterior part of thelateral walls is formed by the cortical areas located belowthe splenium on the medial surface of the hemisphere. Theseareas include the posterior part of the parahippocampal anddentate gyri. The posterior part of the parahippocampal gyrususually extends medially above the posterior part of the freeedge and may have shallow grooves from the free edge on itslower surface.


The quadrigeminal cistern, situated posterior to the quad-rigeminal plate, is the major cistern in the posterior incisuralspace (Figs. 5.15.4). The quadrigeminal cistern communicatesabove with the posterior pericallosal cistern; inferiorly intothe cerebellomesencephalic fissure; inferolaterally into theposterior part of the ambient cistern located between themidbrain and the parahippocampal gyrus; and laterally intothe retrothalamic areas medial to where the crus of the fornixwraps the posterior part of the pulvinar. The quadrigeminalcistern may communicate with the velum interpositum, a

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space that extends forward into the roof of the third ventriclebetween the splenium above and the pineal body below.


The posterior portion of the third ventricle and the cerebralaqueduct are anterior and the atria and occipital horns of thelateral ventricles are lateral to the posterior incisural space(Figs. 5.25.4). The aqueduct passes ventral to the anteriorwall of the posterior incisural space. The atrium is separatedfrom the posterior incisural space by the crus of the fornix asit passes posterior to the pulvinar and by the cortical gyrilocated in the lateral wall of the posterior incisural space.


The trunks and branches of the PCA and SCA enter theposterior incisural space from anteriorly (Figs. 5.5 and 5.6).The PCA courses through the lateral part of the posteriorincisural space and bifurcates into the calcarine and parieto-occipital arteries near where it crosses above the free edge.The medial posterior choroidal arteries enter the posteriorincisural space from anteriorly, turn forward beside the pinealbody, and enter the velum interpositum to supply the choroidplexus in the roof of the third ventricle and the body of thelateral ventricle. The lateral posterior choroidal arteries thatarise in the posterior incisural space pass around the postero-medial surface of the pulvinar and through the choroidalfissure to supply the choroid plexus in the atrium, givingbranches to the thalamus along the way.

The SCA is coursing within the cerebellomesencephalicfissure when it reaches the posterior incisural space. Thesebranches, upon exiting the cerebellomesencephalic fissure, areanterior to the free edge, but they pass below the free edge tosupply the tentorial surface of the cerebellum (Fig. 5.2).

The perforating branches of the PCA and SCA, and themedial posterior choroidal arteries supply the walls of theposterior incisural space. The PCAs supply the structuresabove the level of the lower margin of the superior colliculiand the SCAs supply the structures below the upper marginof the inferior colliculus.


The posterior incisural space has the most complex venousrelationships in the cranium, because the internal cerebral andbasal veins and many of their tributaries converge on the veinof Galen within this area (Figs. 5.1, 5.5, and 5.6). The internalcerebral veins exit the velum interpositum and the basal veinsexit the ambient cistern to reach the posterior incisural space,where they join to form the vein of Galen. The vein of Galenpasses below the splenium to enter the straight sinus at thetentorial apex. The junction of the vein of Galen with thestraight sinus varies from being nearly flat if the tentorial apexis located below the splenium to forming a sharp angle if theapex is located above the splenium, so that the vein of Galenmust turn sharply upward to reach the straight sinus at theapex. The largest vein from the infratentorial part of theposterior incisural space, the vein of the cerebellomesence-

phalic fissure, originates from the union of the paired veins ofthe superior cerebellar peduncle.

Tentorial arteries

The tentorial arteries arise from three sources (8). Thefirst source, the cavernous segment of the carotid artery,provides two arteries: the basal tentorial artery (the artery ofBernasconi-Cassinari) from the meningohypophyseal trunk,and the marginal tentorial artery from the artery from theinferolateral trunk (also called the artery of the inferior cav-ernous sinus). The basal tentorial artery arises from the me-ningohypophyseal trunk and courses posterolaterally alongthe medial part of the tentorial attachment to the petrousridge. The marginal tentorial artery arises from the inferolat-eral trunk, passes laterally over the abducens nerve, thensuperoposteriorly near the trochlear nerve to enter the tento-rial edge. If this artery is absent, a branch from the meningo-hypophyseal artery may replace it (8, 28, 32).

The second source of tentorial arteries is from the SCA. Themeningeal branch originates from the main or rostral trunknear where the artery passes under the tentorium, and itenters the free edge in the middle incisural space. In ourspecimens, 28% of the SCAs gave rise to a tentorial branch,and such a vessel may be encountered when the tentorium isdivided through a subtemporal approach (17).

The third source is the proximal part of the PCA. Thetentorial branch of the PCA arises as a long circumflex arterythat courses around the brainstem and below the free edge toenter the tentorium near the apex (17, 37). This artery mayalso give branches to the superior vermis and inferiorcolliculi.

DISCUSSION

Tentorial herniation

Tentorial herniation is the most common and most impor-tant form of brain herniation (10, 12, 15). In descending her-niation caused by supratentorial mass lesions, the uncus andparahippocampal gyri herniate downward through the inci-sura, and in ascending herniation resulting from infratentorialmasses, the superior part of the cerebellum may herniateupward through the incisura. These brain herniations maycause combinations of direct effects caused by neural com-pression and indirect effects caused by vascular compromise.Symptoms may result from displacement, compression, andstretching of the brainstem and cranial nerves, hemorrhageand infarction caused by compression and tearing of arteriesand veins, increasing edema and intracranial pressure causedby venous obstruction, hydrocephalus caused by obstructionof the aqueduct and subarachnoid space at the incisura, andstrangulation of the prolapsed tissue.

The type of the tentorial herniation in each case depends onthe position and rate of expansion of the lesion and the sizeand shape of the incisura. The signs appear early when struc-tures are deformed rapidly, whereas advanced distortion mayoccur before the appearance of signs if the herniation devel-ops slowly. A wide space between the free edge and brain-



stem facilitates cerebral herniation since more tissue can her-niate into the space (20). A low position of the anterior portionof the free edge also facilitates descending herniation (20).

Descending herniations are divided into anterior, posterior,and complete types. In the anterior type, the uncus herniatesinto the interpeduncular and crural cisterns. This shift carriesthe brainstem to the opposite side, thus increasing the spacebetween the free edge and the brainstem, and facilitating afurther shift of tissue through the aperture. Eventually, theparahippocampal gyrus, from the splenium to the uncus, maybe forced through the opening and the incisura becomesplugged with herniated temporal lobe, deformed hypothala-mus, and compressed midbrain. The amygdaloid nucleus isinvolved with the uncus in the herniated mass. Distortion andcompression of the midbrain reticular activating pathwayscauses a decreased level of consciousness. Compression of theipsilateral cerebral peduncle causes contralateral pyramidalsigns and, if the lateral displacement of brainstem is severe,the contralateral cerebral peduncle may be forced against thefree edge, thus producing a groove on the peduncle called aKernohans notch, with ipsilateral pyramidal signs (30). In theterminal stage, deformation of the midbrain causes decere-brate rigidity. Distortion and compression of the posteriorhypothalamus may cause cardiovascular, respiratory, andthermoregulatory disturbances. The pituitary stalk may bestretched and compressed against the dorsum sellae, causingdiabetes insipidus. The oculomotor nerve courses between themedial border of the uncus and the posterior petroclinoidalfold, and may be kinked or compressed here or between thePCA and SCA, or it may be stretched as the hernia displacesthe midbrain posteriorly. Initially, the pupilloconstrictor fi-bers, which are concentrated on the superior surface of thenerve, are compressed. Later, somatic fibers to the extraocularmuscles are disturbed. In the early stages, irritation of thepupilloconstrictor fibers may cause pupillary constriction, butthis usually gives way to a paralytic effect with pupillarydilation as the hernia enlarges. The optic tract is displacedmedially and downward, but the resulting visual loss is oftenmasked by deepening coma. Compression of the uncus,amygdaloid nucleus, parahippocampal gyrus, and hippocam-pal formation against the free edge may cause memory, be-havior, and personality changes. Residual scarring of the hip-pocampal formation may cause seizures. The trochlear nerveusually escapes involvement in such herniations, but caudaldisplacement of the brainstem may result in a palsy of theabducens nerve by stretching it in the subarachnoid space orby strangling it in its course around the AICA.

Stretching or compression of the anterior choroidal andPCA between the temporal lobe and the peduncle or obstruc-tion of the PCA as it crosses the free edge may cause visualfield loss caused by ischemia of the optic tract, optic radiation,or the lateral geniculate body; contralateral hemiplegia causedby involvement of the cerebral peduncle and midbrain; orchanges in personality and behavior caused by damage to theamygdaloid nucleus or hippocampal formation; unconscious-ness and decerebrate rigidity caused by midbrain ischemia;and contralateral sensory loss caused by ischemia of the ven-

tral thalamic nuclei. Brainstem hemorrhage frequently accom-panies tentorial herniation.

In the posterior type of tentorial herniation, the posteriorportion of the parahippocampal and lingual gyri and theisthmus of the cingular gyrus may shift through the incisurainto the quadrigeminal cistern and compress and displace thedorsal half of the midbrain. Tectal compression may causevertical gaze disturbances. Compression and obstruction ofthe aqueduct causes hydrocephalus and raises the intracranialpressure. In the posterior type of herniation, the PCA or itscalcarine branch is pressed against the free edge and may beobstructed, causing infarction of the occipital cortex andhemianopsia. The basal vein may be compressed between themidbrain and herniated temporal lobe, and the vein of Galenmay be obstructed as it curves around the splenium, thusaggravating the venous congestion, edema, and intracranialtension. The complete type of herniation yields a combinationof signs and symptoms observed with anterior and posteriorherniations.

Hemorrhage into the brainstem as a result of tearing ofarteries and veins without cerebral herniation may occur if theincisura hugs the brainstem so tightly that it prevents cerebralherniation while allowing axial displacement of the brainstem.

In ascending herniation attributable to a posterior fossamass lesion, the superior part of the cerebellar vermis andhemispheres herniate upward through the incisura into thequadrigeminal cistern. Cerebellar infarction may result fromcompression of the branches of the SCA where they passunder the free edge. The hernia may compress the greatcerebral vein against the splenium, which is fixed above bythe falx, thus increasing the venous congestion, edema, andintracranial pressure.

Pathology and operative approaches

Most aneurysms, many pineal, sellar, parasellar, and thirdventricular tumors, and some anteriovenous malformationsare approached through the incisural spaces. The arteries inthe incisura have been subject to bypass procedures, andmany operations for trigeminal neuralgia are directedthrough this area. In addition, structures bordering the areahave been ablated either at craniotomy or stereotactically forthe control of epilepsy. The selection of the best operativeapproach for a given lesion of the incisura depends on thespace involved.

Anterior incisural spaceNearly 95% of saccular arterial aneurysms arise within the

anterior incisural space. The basic anatomy of the commonaneurysms has been reviewed elsewhere by Rhoton (23).The aneurysms arising from the part of the circle of Willislocated anterior to Liliequists membrane, and from the inter-nal carotid and middle cerebral artery are most commonlyapproached through a frontotemporal (pterional) craniotomy(35) (Fig. 5.9). Aneurysms located behind Liliequists mem-brane at the basilar apex in the interpeduncular fossa may beexposed through either a frontotemporal or subtemporal cra-niotomy if they are located above the dorsum sellae (35, 36)

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FIGURE 5.9. AF. Exposure of the anterior incisural space through a frontotemporal craniotomy. A, the insert shows the site of thecraniotomy. The frontal and temporal lobes have been retracted to expose the optic and oculomotor nerves and the anterior andmiddle cerebral and posterior communicating arteries. B, the opticocarotid triangle, located between the optic nerve and thecarotid and anterior cerebral arteries, has been opened with gentle retraction to expose the basilar apex and the ipsilateral oculo-motor nerve passing forward between the PCA and SCA. C, the exposure has been directed medially above the optic chiasm toexpose the region of the anterior communicating artery. D, the frontal lobe has been elevated to expose the contralateral carotidand anterior and middle cerebral arteries. E, the carotid artery has been elevated to expose the basilar artery apex through theinterval between the carotid artery and oculomotor nerve. The posterior clinoid process blocks access to the basilar artery. F, theanterior clinoid process and the roof of the cavernous sinus have been removed to provide access to the posterior clinoid process.The upper dural ring is located at the level of the upper margin of the anterior clinoid process. A., artery; A.C.A., anterior cerebralartery; Ant., anterior; Bas., basilar; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Comm., communicating; Con-tra., contralateral; Ipsi., ipsilateral; Lam., lamina; M.C.A., middle cerebral artery; P.C.A., posterior cerebral artery; Post., posterior;S.C.A., superior cerebellar artery; Term., terminalis; V1., first ophthalmic branch, trigeminal nerve.



(Figs. 5.8 and 5.9). Those located below the dorsum or in theprepontine cistern may require a pretemporal, anterior, ormid subtemporal craniotomy with incision or retraction of thetentorium (Fig. 5.7).

Incision and retraction of the tentorium are commonly re-quired to gain access to lesions around the incisura. Theincision in the tentorium to expose the interpeduncular andprepontine cisterns is usually located just posterior to thepoint where the trochlear nerve enters the free edge. The freeedge may be retracted by means of sutures placed near to it,but special care is required to avoid stretching and damagingthe trochlear nerve in its course inferomedial to and entering thefree edge near the posterior margin of the oculomotor trigone.The tentorial arteries and venous sinuses may be encountered insectioning the tentorium (16). Sectioning of the tentorium hasbeen used to alleviate pressure on the brainstem caused by largeincisural lesions that cannot be removed (2).

Perforating arteries to the brainstem are at greatest risk inapproaches to the anterior incisural space, because they are

commonly stretched around lesions in this area. Hypoplasticarterial segments in the circle of Willis should not be sacri-ficed during the exposure because hypoplastic segments havebeen found to have the same number and size of perforatingbranches as arteries of a normal diameter (23).

Tumors arising in or extending into the anterior incisuralspace include pituitary adenomas, craniopharyngiomas, clivalchordomas, meningiomas arising from the tuberculum sellae,clivus, and medial part of the sphenoid ridge, gliomas of theoptic nerve and hypothalamus, some dermoid cysts and ter-atomas, and neuromas of the oculomotor nerve. Tumors inthe anterior incisural space may be approached by the bifron-tal, subfrontal, frontal-interhemispheric, frontotemporal, sub-temporal, and transsphenoidal routes. Tumors located ante-rior to Liliequists membrane between the optic chiasm andthe sellar floor are commonly operated on by the transsphe-noidal or subfrontal route. The transsphenoidal approach ispreferred if the tumor extends upward out of an enlargedsella turcica and is located above a pneumatized sphenoid

FIGURE 5.9. GJ. Exposure of the anterior incisural space through a frontotemporal craniotomy. G, the posterior clinoidprocess has been removed to increase access to the upper portion of the basilar artery. H, the anterior part of the tentorialedge has been removed to expose the upper margin of the posterior trigeminal root in Meckels cave and to provideincreased access to the upper anterior part of the posterior fossa. The trochlear nerve was preserved in opening the anteriorpart of the tentorial edge. I, another dissection in which the anterior clinoid process and roof of the cavernous sinus wereremoved to expose the posterior clinoid process in the interval between the carotid anteriorly and the oculomotor posteri-orly. J, the posterior clinoid was removed to provide increased access to the upper part of the basilar artery.

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sinus. The subfrontal intracranial approach is reserved forthose tumors in the chiasmatic cistern that are not accessibleby the transsphenoidal route because they are located entirelyabove the diaphragma sellae, or extend upward out of anormal or small sella, or are located above a nonpneumatized(conchal) type of sphenoid sinus. The subfrontal approachpermits exposure of the tumor within the anterior incisuralspace by four routes: 1) the subchiasmatic approach betweenthe optic nerves and below the optic chiasm; 2) the opticoca-rotid route directed between the optic nerve and carotid ar-tery; 3) the lamina terminalis approach directed above theoptic chiasm through a thinned lamina terminalis; and 4) thetransfrontal-transsphenoidal approach obtained by enteringthe sphenoid sinus and sella through the transfrontal craniot-omy (22, 25, 26). The subchiasmatic approach is used if thesubchiasmatic opening is enlarged by the tumor. The optico-carotid route is selected if parasellar extension of the tumorwidens the space between the carotid artery and the opticnerve and the tumor cannot be reached by the subchiasmaticapproach. The lamina terminalis approach is selected if thetumor has pushed the chiasm into a prefixed position andextends into the third ventricle to stretch the lamina terminalisso that the tumor is visible through it. The transfrontal-transsphenoidal approach is selected if the tumor grows up-ward out of the sella, the sphenoid sinus is pneumatized andthe tumor does not stretch the lamina terminalis or widen theopticocarotid space, and a prefixed chiasm blocks the subchi-asmatic exposure. A bifrontal craniotomy may be used if thetumor extends forward in both anterior cranial fossae andcannot be reached by a unilateral subfrontal exposure. Afrontal interhemispheric approach directed along the anteriorpart of the falx is used for lesions restricted to the part of theanterior interhemispheric space located just below the ros-trum, especially if the tumor arises in the genu or rostrum ofand grows into the anterior incisural space.

The frontotemporal approach is used for a tumor arisingfrom the sphenoid ridge or anterior clinoid process, or if itarises above the diaphragma and extends along the sphenoidridge or into the middle cranial fossa, or if the lesion isaccessible through the spaces between the optic nerve andcarotid artery or between the carotid artery and the oculomo-tor nerve (Fig. 5.9). Some lesions may require that the aboveapproach be combined with resection of the cranial base if thelesion involves the paranasal sinuses, nasal cavity, pharynx,orbit, or cavernous sinus, and for those extending from theanterior incisural space into the area behind the dorsum sellaor petrous apex, and those in which the lower opening pro-vided by cranial base resection will yield a better angle of exposureor reduce the need for brain retraction. These approaches includethe transcranial-transbasal, extended frontal, fronto-orbital, or-bitozygomatic, transcavernous, preauricular-infratemporal, andsubtemporal anterior petrousectomy, some of which are dis-cussed more fully in the chapters on the foramen magnum andtemporal bone.

Middle incisural spaceLesions in the middle incisural space include meningiomas

arising from Meckels cave, the anterior part of the free edge

and the petrous apex, gliomas of the temporal lobe and thal-amus, anteriovenous malformations of the medial temporallobe, and neuromas of the trochlear and trigeminal nerves.The infrequent aneurysms arising in the middle incisuralspace are usually located on the PCA at the origin of its firstmajor cortical branch or on the SCA at its bifurcation intorostral and caudal trunks. Bypass operations using vein andarterial grafts have been applied to the trunks and branches ofthe posterior cerebral and superior cerebellar branches in themiddle incisural space bordering the incisura. The middleincisural space is exposed in performing amygdalohip-pocampectomy and temporal lobectomy for epilepsy sinceboth the amygdalae and hippocampus extend medial to thefree edge. The trigeminal nerve is also frequently exposed inthe middle incisural space in the course of operations fortrigeminal neuralgia.

Approaches to the middle incisural space include the poste-rior frontotemporal, subtemporal, temporal-transventricular,and the lateral suboccipital routes (Figs. 5.7 and 5.8). Thesubtemporal approach with elevation of the temporal lobe iscommonly used to expose lesions in the cisterns around theincisura. Hemorrhage, venous infarction, and edema follow-ing retraction of the temporal lobe during this approach areminimized by placing the lower margin of the craniotomy anddural exposure at the cranial base so as to reduce the need forretraction, and by avoiding occlusion of the bridging veins,especially the vein of Labbe. The tentorium is frequentlydivided to increase the exposure or to decompress the brain-stem when mass lesions are impacted in the incisura (2).Resection of part of the parahippocampal gyrus may facilitateexposure of the upper part of the middle incisural space (1). Atransventricular approach using a cortical incision in the non-dominant inferior or middle temporal gyrus may be used ifthe lesion involves the temporal horn, choroidal fissure, hip-pocampal formation, or the upper part of the middle incisuralspace (9). A cortical incision in the medial occipitotemporalgyrus on the inferior surface of the temporal lobe has beenused to minimize visual and speech deficits in exposing thetemporal horn of the dominant hemisphere. After enteringthe temporal horn, the choroidal fissure is opened to exposethe middle incisural space. The subtemporal craniectomy maybe combined with a suboccipital craniectomy with section ofthe tentorium and transverse sinus to remove lesions in theprepontine or cerebellopontine cisterns. The trochlear nerve isthe cranial nerve most frequently injured in the middle inci-sural space. It can be injured in dividing the free edge and isso thin and friable that it may rupture from gentle retractionon the leaves formed by dividing the tentorium. The aboveapproaches may be combined with cranial base approachesinvolving resection or mobilization of the orbital rim, zygo-matic arch, floor of the middle fossa, or a portion of thetemporal bone as are accomplished in the orbitozygomaticcraniotomy, and the preauricular infratemporal or anteriorpetrousectomy approaches.

The posterior trigeminal root is frequently exposed througha lateral suboccipital craniectomy in the infratentorial part ofthe middle incisural space for rhizotomy or microvasculardecompression operations. The exposure is directed along the



FIGURE 5.10. AF. Comparison of the midline and paramedian infratentorial supracerebellar and the occipital transtentorialapproaches to the quadrigeminal cistern and the posterior third ventricle. AD, views of the third ventricle and quadrigemi-nal cistern. A, third ventricle from above. The body of the fornix separates the body of the lateral ventricle from the roof ofthe third ventricle. The body of the fornix blends posteriorly into the crus of the fornix, which is situated above the posteriorpart of the third ventricle. The choroidal fissure, the site of attachment of the choroid plexus, is situated between the fornixand thalamus. B, the fornix was divided at the level of the columns, just behind the foramen of Monro, and reflected

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angle formed by the insertion of the tentorium to the petrousridge. The posterior root proximal to Meckels cave has alsobeen exposed through a subtemporal craniectomy combinedwith incision of the tentorium (11). The posterior root mayalso be exposed for rhizotomy within Meckels cave througha subtemporal extradural approach.

Posterior incisural spaceLesions in the posterior incisural space include pineal tu-

mors; meningiomas arising at the falcotentorial junction andfrom the tela choroidea of the velum interpositum and atrium;gliomas of the splenium, pulvinar, quadrigeminal plate, andcerebellum; aneurysms of the vein of Galen; and anterio-venous malformations involving the medial occipital lobe andupper cerebellum.

Lesions in the posterior incisural space may be approachedfrom above the tentorium along the medial surface of theoccipital lobe using an occipital transtentorial approach,through the posterior part of the lateral ventricle using aposterior transventricular approach, and through the corpuscallosum using a posterior interhemispheric transcallosal ap-proach, or from below the tentorium through the supracer-ebellar space using an infratentorial supracerebellar approach(Figs. 5.10 and 5.11). The infratentorial supracerebellar andoccipital transtentorial approaches, which are most com-monly selected for pineal region tumors, may be combinedwith incision of the tentorium lateral to the straight sinus andless commonly with division of the tentorium and transversesinus. A tentorial branch of the PCA or SCA may enter thedura lateral to the straight sinus. Venous sinuses are morecommonly encountered in the posterior than in the anteriorparts of the tentorium. Part of the tentorium may be removedin resecting tumors that arise from or invade it.

The infratentorial supracerebellar approach may be selectedfor lesions in the pineal region located below the vein of Galenand its major tributaries (29). The approach is best suited totumors in the midline that grow into the lower half of theposterior incisural space, displacing the quadrigeminal plateand apex of the tentorial cerebellar surface. The occipitaltranstentorial approach is preferred for lesions centered at orabove the tentorial edge, especially if they are located abovethe vein of Galen. The latter approach may also provide a

better angle of access for some lesions involving the ipsilateralhalf of the cerebellomesencephalic fissure and posterior partof the ambient cistern, although they may be located belowthe level of the vein of Galen (21, 34) The posterior transcal-losal approach, in which the splenium is divided, would beused only if the lesion appears to arise in the splenium abovethe vein of Galen and extends into the posterior incisuralspace. The posterior transventricular approach provides ade-quate exposure of the atrium and posterior portion of thebody of the lateral ventricle and would be the preferredapproach to a tumor involving the posterior incisural space ifthe tumor extends into the pulvinar or involves the atriumor the glomus of the choroid plexus. The preferable approachto the ventricle is through the superior parietal lobule, al-though on approach to the pineal region using a corticalincision in the superior temporal gyrus and directed throughthe atrium has been advocated (31).

Comparison of occipital transtentorial andinfratentorial supracerebellar approaches

In examining the posterior incisural space, we comparedthe midline and paramedian variants of the infratentorialsupracerebellar approach and the occipital transtentorial ap-proach (Figs. 5.10 and 5.11). The midline infratentorial suprac-erebellar approach is directed steeply upward over the apexof the vermis where the large complex of veins emptying intothe vein of Galen, and especially the vein of the cerebellomes-encephalic fissure, blocks access to the pineal region. The venouscomplex could be gently displaced to expose the lower part ofthe splenium, the pineal, and the superior colliculus, but theprominent vermian apex forming the posterior lip of the cerebel-lomesencephalic fissure limits exposure below the level of thesuperior colliculus. In the paramedian variant of the infratento-rial supracerebellar approach, the retraction was advancedabove the hemisphere lateral to the vermis. This approach wasnot as upwardly steep as the approach above the vermian apexand provided access to the pineal region, the lower part of thesplenium, and gave greater access to the ipsilateral half of thecerebellomesencephalic fissure. In addition, the approach couldbe advanced along the lateral part of the cerebellar surface toexpose the posterior part of the ambient cistern. In the occipital

posteriorly to expose the posterior commissure, pineal, and adjacent part of the quadrigeminal cistern. C, the quadrigeminalcistern is located behind the pineal and the colliculi and between the pulvinars. It extends into the cerebellomesencephalicfissure. The trochlear nerves arise below the inferior colliculi. D, view similar to C, except that the vessels have been pre-served. The internal cerebral and basal veins join the vein of Galen behind the pineal. The PCA and SCA exit the ambient cis-tern to enter the lateral part of the quadrigeminal cistern. Both the infratentorial supracerebellar and occipital transtentorialapproaches are directed to this area. E and F, midline infratentorial supracerebellar approach. E, the venous complex empty-ing into the vein of Galen blocks access to the pineal region. This complex includes the internal occipital, basal and internalcerebral veins, and the vein of the cerebellomesencephalic fissure. A tentorial branch of the SCA crosses the exposure. F, thevein of Galen has been retracted to expose the splenium. The vein of the cerebellomesencephalic fissure has been retractedto expose the pineal. A., artery; Bridg., bridging; Cer., cerebral; Cer. Mes., cerebellomesencephalic; Chor., choroidal; Cist.,cistern; CN, cranial nerve; Coll., colliculus; Comm., communicating; Fiss., fissure; For., foramen; Inf., inferior; Int., internal;Lat., lateral; Med., medial; Occip., occipital; P.C.A., posterior cerebral artery; Ped., peduncle; Plex., plexus; Post., posterior;Quad., quadrigeminal; Sag., sagittal; S.C.A., superior cerebellar artery; Str., straight; Sup., superior; Temp., temporal; Tent.,tentorial; Trans., transverse; V., vein; Ve., vermian; Vent., ventricle.



FIGURE 5.10. GL. G and H, midline infratentorial supracerebellar approach. G, the left basal and internal cerebral veins havebeen elevated and the vein of the cerebellomesencephalic fissure, which is joined by a superior vermian vein, has been retracted tothe right to expose the superior colliculus, pineal, and splenium. H, the tela choroidea attached to the upper surface of the pinealhas been opened to expose the posterior part of the third ventricle. IL, paramedian variant of the infratentorial supracerebellarapproach. In this variant, the retraction of the tentorial surface is shifted off the vermis and tentorial apex to the paramedian partof the hemisphere. This paramedian variant of the approach accesses the lateral part of the quadrigeminal cistern and the posteriorpart of the ambient cistern and, in addition, provides a better view into the central and ipsilateral half of the cerebellomesence-phalic fissure than the approach directed in the midline above the vermian apex. I, the retraction for the paramedian approach hasbeen shifted to the left of the vermis. J, the left internal cerebral and internal occipital veins have been retracted to expose the pos-terior part of the splenium, the pineal and the superior and inferior colliculi, and the branches of the PCA and SCA exiting theambient cistern. K, enlarged view. The exposure has been shifted to where the PCA exits the ambient cistern. L, the paramedianapproach provides easier access to the superior and inferior colliculi and requires less retraction than is needed to expose thesestructures in the approach directed in the midline above the apex of the tentorial cerebellar surface.

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FIGURE 5.10. MR. Occipital transtentorial approach. M, the occipital transtentorial is directed along the medial surface ofthe occipital lobe below the lambdoid suture. This occipital lobe below the lambdoid suture is commonly free of bridgingveins to the superior sagittal sinus, making it a reasonable route for the occipital transtentorial approach. N, there are nolarge bridging veins between the posterior 6 cm of the occipital lobe and superior sagittal sinus. The first vein encountered isthe internal occipital vein that passes from the anterior part of the medial occipital lobe to the vein of Galen. O, the vein ofGalen has been retracted to expose the splenium and pineal from above. P, the tentorium has been opened lateral to thestraight sinus, and the vein of Galen has been displaced to the left side to expose the pineal and the superior and inferior col-liculi. Q, elevating the branches of the vein of Galen provides a satisfactory view into the quadrigeminal cistern, with a betterview into the cerebellomesencephalic fissure than can be achieved with the infratentorial supracerebellar approach directedover the apex of the tentorial cerebellar surface. R, the exposure has been directed laterally along the side of the brainstemto the ambient cistern where the lateral margin of the cerebral peduncle is exposed.



transtentorial approach, the occipital lobe was retracted and thetentorium divided along the edge of the straight sinus. Thisprovided access to the splenium above the vein of Galen and,with gentle retraction of the venous complex in the posteriorincisural space, the pineal and the upper part of the cerebel-lomesencephalic fissure could be visualized. The approach pro-

vided wider access to the midline and ipsilateral half of thecerebellomesencephalic fissure than did the midline infratento-rial supracerebellar approach. In addition, it provided an excel-lent route for reaching the posterior part of the ambient cisternand even the lateral surface of the cerebral peduncle in the cruralcistern. The exposure of the lateral part of the contralateral half

FIGURE 5.11. Comparison of infratentorial supracerebellar, the occipital transtentorial, and the combined supra- and infrat-entorial approaches. A, infratentorial supracerebellar approach. The approach has been directed between the lower surfaceof the tentorium and the tentorial cerebellar surface. The large venous complex draining into the vein of Galen is in the cen-tral part of the exposure and the PCA and SCA are exposed laterally. A large vein of the cerebellomesencephalic fissureblocks access to the pineal and limits access to the cerebellomesencephalic fissure. This approach is selected for lesionslocated in the midline below the vein of Galen and not extending deeply into the cerebellopontine fissure. The SCAbranches looping around the lip of the cerebellomesencephalic fissure may extend upward and limit access to the pinealregion. B, the vein of the cerebellomesencephalic fissure has to be divided to expose the pineal. The medial posteriorchoroidal arteries are intertwined with the veins in the region. C, the occipital transtentorial approach has beendirected along the medial side of the right occipital lobe. The tentorium behind the quadrigeminal cistern has beendivided. The approach provides access to the splenium and the upper part of the cerebellomesencephalic fissure and hasbeen extended forward to the lateral surface of the cerebral peduncles. Both the superior and inferior colliculi can beexposed and the arteries can be followed forward into the ipsilateral ambient cistern. In addition, the veins joining thevein of Galen can be elevated to expose the pineal. The trochlear nerve is exposed just distal to its brainstem exit belowthe inferior colliculus. D, combined supra and infratentorial exposure with the division of the transverse sinus and ten-torium. Division of the transverse sinus, if it is small and well collateralized, provides an exposure that combines boththe supra- and infratentorial approaches. A., artery; Cer., cerebral; Cer. Mes., cerebellomesencephalic; Chor., choroidal;CN, cranial nerve; Coll., colliculus; Fiss., fissure; Inf., inferior; Int., internal; Med., medial; Occip., occipital; P.C.A., pos-terior cerebral artery; Ped., peduncle; Post., posterior; S.C.A., superior cerebellar artery; Sup., superior; Temp., tempo-ral; V., vein.

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of the quadrigeminal cistern was more limited than could beachieved with the midline infratentorial supracerebellar ap-proach. The supra and infratentorial approaches can be con-verted into a combined approach by dividing the transversesinus in addition to the tentorium, if the sinus is small and is wellcollateralized through the opposite side (Fig. 5.11).

Reprint requests: Albert L. Rhoton, Jr., M.D., Department of Neuro-logical Surgery, University of Florida Brain Institute, P.O. Box 100265,100 S. Newell Drive, Building 59, L2100, Gainesville, FL 32610-0265.

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