Operative approaches to the lateral and third ventricles are made challenging by their deep position near the center of intracranial space; variable shape and size in the different lobes; narrow communicating orifices making them susceptible to obstruction; expansile nature allowing them to act as mass lesions; and walls containing important motor, sensory, and visual fiber pathways, the basal ganglia, and vital autonomic and endocrine centers. The lateral ventricles provide deep cavities through which the third ventricle and basal cisterns may be approached.
4.2 Lateral Ventricle
Each lateral ventricle is a C-shaped cavity that wraps around the thalamus and is situated deep within the cerebrum.1 Each has five parts: the frontal, temporal, and occipital horns, the body, and the atrium. Each of these five parts has medial and lateral walls, a roof, and a floor. In addition, the frontal and temporal horns and atrium have anterior walls. These walls are formed predominantly by the thalamus, septum pellucidum, deep cerebral white matter, corpus callosum, and two C-shaped structures—the caudate nucleus and fornix that wrap around the thalamus (Fig. 4.1, Fig. 4.2, Fig. 4.3, Fig. 4.4, Fig. 4.5, Fig. 4.6, and Fig. 4.7, Video 4.1and Video 4.2, Animation 4.1).
Fig. 4.1 (a) Each lateral ventricle has five parts: the frontal, temporal, and occipital horns, the body, and the atrium. The head and body of the caudate nucleus forms the lateral wall of the frontal horn and body of the lateral ventricle, respectively. The tail of the caudate nucleus descends along the anterior part of the lateral wall of the atrium and into the medial part of the roof of the temporal horn to the level of the amygdala in the anterior wall of the temporal horn. The tapetum forms the lateral wall of the temporal and occipital horns, the posterior part of the atrium, and part of the roof of the atrium. (b) The frontal horn is situated anterior to the thalamus, the body is above the thalamus, the atrium and occipital horn are behind the thalamus, and the temporal horn is below and lateral to the thalamus. The septum pellucidum forms the medial wall of the frontal horn and body of the lateral ventricle, while two prominences form the medial wall of the atrium: the calcar avis and bulb of the corpus callosum. (c) Superior view. The roof of the left lateral ventricle has been removed. The rostrum of the corpus callosum forms the floor of the frontal horn; its genu forms the anterior wall and roof of the frontal horn; its body forms the roof of the body of the lateral ventricle; and its splenium forms a part of the roof of the atrium. The anterior wall of the atrium is formed by the pulvinar laterally and crus of the fornix medi-ally. The lateral wall of the atrium has an anterior part, formed by the tail of the caudate nucleus, and a posterior part, formed by the fibers of the tapetum. The roof of the atrium is formed by the body, splenium, and tapetum of the corpus callosum, while the floor is formed by the collateral trigone. The caudate nucleus and thalamus are separated by the striothalamic sulcus, the groove in which the stria terminalis runs. (d) The crura of the fornix, which wrap around the superomedial edge of the thalamus, meet in the midline to form the body of the fornix. The body of the fornix at the level of the foramen of Monro is divided into the two columns of the fornix, which form the posteroinferior part of the medial wall of the frontal horn. The columns of the fornix also form the superoanterior wall of the foramen of Monro, while the anterior nucleus of the thalamus forms its posterior wall. The splenium of the corpus callosum gives rise to the fibers making up the forceps major, which forms a part of the the medial wall of the atrium. The splenium also give rise to a fiber bundle called the tapetum, which sweeps downward to form the roof and lateral wall of the atrium and temporal and occipital horns. The genu of the corpus callosum, which forms the anterior wall of the frontal horn, gives rise to a fiber bundle called the forceps minor, which forms the anterior wall of the frontal horn. (e) Inferior view, the bilateral thalami and left hippocampus have been removed. The hippocampus forms the floor of the temporal horn of the lateral ventricle. The choroid plexus extends along the full length of the choroidal fissure from the foramen of Monro to the inferior choroidal point. In the atrium, the choroid plexus forms a prominent triangular tuft called the glomus. The fornix passes through the medial part of the temporal horn, atrium, and body of the corpus callosum. The fimbria of the fornix arises from the hippocampal formation in the temporal horn and blends into the crus of the fornix at the level of the splenium. The crura of the fornix pass through the anterior wall of the atrium, and meet to form the body of the fornix in the lower part of the medial wall of the body of the corpus callosum. The body of the fornix splits into two columns at the level of the foramen of Monro, which descend to the mammillary bodies. (f) Inferior view. The left thalamus and bilateral hippo-campi have been removed to expose the roof of the temporal horn and body of the lateral ventricle. The temporal horn extends forward from the atrium below the pulvinar into the medial part of the temporal lobe. The anterior wall of the temporal horn is made up of the amygdala, while the roof is formed by the tail of the caudate nucleus medially, and the tapetum laterally. The foramen of Monro is a passage connecting the lateral and third ventricles at the same axial level as the genu of the internal capsule. The third ventricle is a narrow, funnel-shaped, unilocular cavity situated between the cerebral hemispheres, the two halves of the thalamus, and the two halves of the hypothalamus. (g) The third ventricle has a roof, a floor, and anterior, posterior, and two lateral walls. The roof of the third ventricle forms a gentle upward arch, extending from the foramen of Monro anteriorly to the suprapineal recess posteriorly. The upper layer of the anterior part of the roof of the third ventricle is formed by the body of the fornix. The floor extends from the optic chiasm anteriorly to the orifice of the aqueduct of Sylvius posteriorly. The anterior wall extends from the foramen of Monro above to the optic chiasm below. The posterior wall extends from the suprapineal recess above to the aqueduct of Sylvius below. The lateral wall is formed by the thalamus superiorly and hypothalamus inferiorly, separated by the hypothalamic sulcus (green dotted line). The superior limit of the thalamic surfaces of the third ventricle is marked by the striae medullaris thalami. These striae extend forward from the habenulae along the superomedial surface of the thalamus near the attachment of the inferior layer of the tela choroidea. The massa intermedia projects into the upper half of the third ventricle and often connects the opposing surfaces of the thalamus. (Abbreviations: Ant., anterior; Amygd., amygdala; Calc., calcar; Call., callosum; Caud., caudate; Chor., choroid, choroidal; Comm., commissure; Coll., collateral; Corp., corpus; Fiss., fissure; Front., frontal; For., foramen; Forc., forceps; Gl., gland; Hippo., hippocampus; Hypothal., hypothalamus; Inf., inferior; Int., intermedia; Lat., lateral; Mam., mammillary; Med., medullaris; Nucl., nucleus; Pell., pellucidum; Plex., plexus; Post., posterior; Quad., quadrigeminal; Rec., recess; Sag., sagittal; Sept., septum; Str., stria; Strat., stratum; Striothal., striothalamic; Sulc., sulcus; Occip., occipital; Thal., thalami, thalamic; Temp., temporal; Term., terminalis; Vent.,ventricle.)
4.2.1 Lateral Ventricular Walls
Frontal horn. The frontal horn, the part of the lateral ventricle located anterior to the foramen of Monro, has a medial wall formed by the septum pellucidum, an anterior wall and roof formed by the genu of the corpus callosum, a lateral wall composed of the head of the caudate nucleus, and a narrow floor formed by the rostrum of the corpus callosum. The columns of the fornix, as they pass anterior to the foramen of Monro, are in the posteroinferior part of the medial wall (Fig. 4.1a–dand Fig. 4.2a).
Body. The body of the lateral ventricle extends from the posterior edge of the foramen of Monro to the point where the body of the caudate nucleus turns downward to blend into the tail of the caudate nucleus. The roof is formed by the body of the corpus callosum, the medial wall by the septum pellucidum above and the body of the fornix below, the lateral wall by the body of the caudate nucleus, and the floor by the thalamus. The caudate nucleus and thalamus are separated by the striothalamic sulcus, the groove in which the stria terminalis and thalamostriate vein course (Fig. 4.1a–f, Fig. 4.2b, and Fig. 4.9a,b).
Atrium and occipital horn. The atrium and occipital horn together form a roughly triangular cavity, with the apex posteriorly in the occipital lobe and the base anteriorly on the pulvinar. The roof of the atrium is formed by the body, splenium, and tapetum of the corpus callosum. The medial wall is formed by two roughly horizontal prominences that are located one above the other. The upper prominence, called the bulb of the corpus callosum, overlies and is formed by the large bundle of fibers called the forceps major, and the lower prominence, called the calcar avis, overlies the deepest part of the calcarine sulcus. The lateral wall has an anterior part, formed by the tail of the caudate nucleus, and a posterior part, formed by the fibers of the tapetum. The anterior wall has a medial part composed of the crus of the fornix as it wraps around the posterior part of the pulvinar, and a lateral part, formed by the pulvinar of the thalamus. The floor is formed by the collateral trigone, a triangular area that bulges upward over the posterior end of the collateral sulcus. The occipital horn extends posteriorly into the occipital lobe from the atrium. It varies in size from being absent to extending far posteriorly in the occipital lobe, and it may vary in size from side to side (Fig. 4.1a–f, Fig. 4.2c, and Fig. 4.9a–d, Animation 4.1).
Temporal horn. The temporal horn extends forward from the atrium below the pulvinar into the medial part of the temporal lobe and ends blindly in an anterior wall that is situated immediately behind the amygdala. The floor of the temporal horn is formed medially by the hippocampus and laterally by the collateral eminence, the prominence overlying the collateral sulcus that separates the parahippocampal and occipitotemporal gyri on the inferior surface of the temporal lobe. The medial part of the roof is formed by the tail of the caudate nucleus. The lateral part of the roof is formed by the tapetum of the corpus callosum, which also sweeps inferiorly to form the lateral wall of the temporal horn. The tapetum separates the temporal horn from the optic radiations. The only structure in the medial wall is a narrow cleft, the choroidal fissure, situated between the inferolateral part of the thalamus and the fimbria of the fornix (Fig. 4.1e,f, Fig. 4.2d, and Fig. 4.9e,f, Video 4.2).
4.2.2 Choroid Plexus and Choroidal Fissure
The choroid plexus from each lateral ventricle extends along the full length of the choroidal fissure and through the foramen of Monro. In the atrium, the choroid plexus forms a prominent triangular tuft called the glomus (Fig. 4.1e). Notably, the thalamostriate vein is located on the right side of the choroid plexus in the right lateral ventricle, and it is located on the left side of the choroid plexus in the left lateral ventricle (Fig. 4.6and Fig. 4.7). The choroidal fissure is the narrow cleft between the fornix and thalamus along which the choroid plexus in the lateral ventricle is attached.2 The choroidal fissure extends in a C-shaped arc from the foramen of Monro around the superior, posterior, and inferior surfaces of the thalamus to its inferior termination, called the inferior choroidal point, which is located just behind the head of the hippocampus and lateral to the lateral geniculate body. The choroidal fissure is divided into body, atrial, and temporal parts. The fissure is positioned in the body of the ventricle between the body of the fornix and the thalamus, in the atrium between the crus of the fornix and pulvinar, and in the temporal horn between the fimbria of the fornix and thalamus. The fornix forms the outer margin of the fissure, and the thalamus forms the inner margin. The edges of the thalamus and fornix bordering this choroidal fissure have small ridges, called the teniae, along which the tela choroidea, the membrane in which the choroid plexus arises, is attached (Fig. 4.2d). The tenia on the thalamic side is called the tenia thalami or tenia choroidea. The tenia on the forniceal side of the fissure is called the tenia fornicis except in the temporal horn where it is referred to as the tenia fimbriae.
The choroidal fissure is the one of the most important landmarks in surgery involving the body and temporal horn of the lateral ventricle and third ventricle (Fig. 4.6and Fig. 4.7). When opening the choroidal fissure, it is better to open through the tenia fornicis than through the tenia thalami because the large veins draining the internal capsule, optic radiations, and central core of the hemisphere pass through and could be damaged by opening the tenia thalami. The arterial supply of the choroid plexus is from the anterior choroidal artery, which arises from the internal carotid artery, and the medial and lateral posterior choroidal arteries, which arise from the posterior cerebral artery (Fig. 4.3).3,4 The choroidal arteries enter the ventricles through the choroidal fissure. In addition, the veins coursing in the walls of the ventricles exit the ventricles by passing through the margin of the choroidal fissure in a subependymal location to reach the internal cerebral, basal, or great veins. The velum interpositum, through which the internal cerebral veins course, is located on the medial side of the body portion of the choroidal fissure in the roof of the third ventricle. Opening through the choroidal fissure from the body of the ventricle will expose the velum interpositum and the roof of the third ventricle. Opening the fissure in the atrium will expose the quadrigeminal cistern, the pineal region, and the posterior portion of the ambient cistern. Opening the fissure in the temporal horn will expose the structures in the ambient and posterior part of the crural cisterns.
Fig. 4.2 Structures forming the surfaces of the lateral ventricles. The central diagram shows the level of the cross sections through the (a) frontal horn, (b) body, (c) atrium, and (d) temporal horn. (a) Frontal horn of the lateral ventricle. The genu of the corpus callosum is in the roof, the caudate nucleus is in the lateral wall, the rostrum of the corpus callosum is in the floor, and the septum pellucidum is in the medial wall of the lateral ventricle. (b) Body of the lateral ventricle. The body of the corpus callosum is in the roof, the caudate nucleus is in the lateral wall, the thalamus is in the floor, and the septum pellucidum and fornix are in the medial wall. The choroidal fissure, the site of the attachment of the choroid plexus in the lateral ventricle, is situated between the fornix and the thalamus. The striothalamic sulcus separates the caudate nucleus and thalamus. (c) Atrium of the lateral ventricle. The lateral wall and roof of the lateral ventricle are formed by the tapetum of the corpus callosum, and the floor is formed by the collateral trigone, which overlies the collateral sulcus. The inferior part of the medial wall is formed by the calcar avis, the prominence that overlies the deep end of the calcarine sulcus, and the superior part of the medial wall is formed by the bulb of the corpus callosum. (d) The right temporal horn of the lateral ventricle. The medial part of the floor of the temporal horn is formed by the prominence overlying the hippocampal formation, and the lateral part of the floor is formed by the prominence called the collateral eminence, which overlies the deep end of the collateral sulcus. The roof is formed by the caudate nucleus and the tapetum, the lateral wall is formed by the tapetum, and the medial wall of the temporal horn is little more than the cleft between the fimbria of the fornix and the inferolateral aspect of the thalamus. (Abbreviations: Calc., calcarine; Call., callosum; Caps., capsule; Caud., caudate; Chor., choroid; Coll., collateral; Corp., corpus; Emin., eminence; Gen., geniculate; Hippo., hippocampus; Int., intermedia, internal; Lat., lateral; Lent., lentiform; Nucl., nucleus; Pell., pellucidum; Plex., plexus; Sept., septum; Str., stria; Sulc., sulcus; Striothal., striathalamic; Term., terminalis; Trig., trigone.)Fig. 4.3 (a) Lateral, superior, and anterior views of the arterial relationships of the lateral ventricles. The internal carotid artery and its branches are shown in orange, and the basilar artery and its branches are shown in red. The internal carotid, basilar, anterior cerebral, middle cerebral, posterior cerebral, anterior choriodal, lateral posterior choriodal, and medial posterior choroidal arteries all have important relationships to the frontal, temporal, and occipital horns and the atria and bodies of the lateral ventricles. The carotid arteries bifurcate into their anterior and middle cerebral branches in the area below the posterior part of the frontal horns. The origins of the middle cerebral arteries are situated below the frontal horns. The anterior cerebral arteries pass anteromedially below the frontal horns and give rise to the pericallosal and callosomarginal branches, which curve around the anterior wall and roof of the frontal horn. The anterior choroidal arteries enter the anterior part of the temporal horns. The posterior communicating arteries are situated below the thalami and bodies of the lateral ventricles. The basilar artery bifurcates below the bodies of the lateral ventricles into the posterior cerebral arteries, which course below the thalami near the medial aspect of the temporal horns and atria. The medial posterior choroidal arteries arise from the proximal part of the posterior cerebral arteries, encircle the brainstem below the thalami, and pass forward in the roof of the third ventricle, where they give branches to the choroid plexus in the roof of the third ventricle and the bodies of the lateral ventricles. The lateral posterior choroidal branches of the posterior cerebral arteries pass laterally through the choroidal fissures to enter the temporal horns and atria of the lateral ventricles. The middle cerebral arteries course on the insulae in the area above the temporal horns and lateral to the bodies of the lateral ventricles. The posterior cerebral arteries bifurcate into the calcarine and parieto-occipital arteries in the area medial to the atria. (b) Lateral, superior, and anterior views of the venous relationships of the lateral ventricles. The ventricular veins are divided into medial (green) and lateral (blue) groups. The ventricular veins drain into the internal cerebral, basal, and great veins. The lateral group consists of the anterior caudate vein in the frontal horn; the thalamostriate, posterior caudate, and thalamocaudate veins in the body; the lateral atrial veins in the atrium and occipital horn; and the inferior ventricular and amygdalar veins in the temporal horn. The medial group is formed by the anterior septal vein in the frontal horn; the posterior septal veins in the body; the medial atrial veins in the atrium; and the transverse hippocampal veins in the temporal horn. The transverse hippocampal veins drain into the anterior and posterior longitudinal hippocampal veins. The superior choroidal veins drain into the thalamostriate and internal cerebral veins, and the inferior choroidal vein drains into the inferior ventricular vein. The great vein drains into the straight sinus. (Abbreviations: A., artery; Amygd., amygdalar; A.C.A., anterior cerebral artery; Ant., anterior; Atr., atrial; Bas., basilar; Cal. Marg., Callosomarginal; Calc., calcarine; Car.,carotid; Caud., caudate; Cer., cerebral; Chor., choroidal; Comm., communicating; Front., frontal; Hippo., hippocampal; Inf., inferior; Int., internal; Lat., lateral; Long., longus; M.C.A., middle cerebral artery; Med., medial; Occip., occipital; Par. Occip., parieto-occipital; P.C.A., posterior cerebral artery; Post., posterior; Sept., septal; Sup., superior; Temp., temporal; Thal. Caud., thalamocaudate; Thal. Str., thalamostriate; Trans., transverse; V., vein; Vent., ventricle.) (Reproduced with permission from Rhoton A Jr. The lateral and third ventricles. Neurosurgery 2002;51(4 Suppl):S207–S271.)Video 4.1 Lateral ventricle anatomy. This video demonstrates an endoscopic exploration of the right lateral ventricle with a flexible neuroendoscope. The first anatomical reference identified is the right foramen of Monro. The transparent structure corresponds to the septum pellucidum. As the endoscope is navigated into the anterior portion of the lateral ventricle, the head of the caudate nucleus can be observed. It is also possible to observe the interior of the third ventricle through the foramen of Monro. Following the choroid plexus posteriorly, the endoscope enters the occipital horn. Once in the occipital horn, the endoscope is advanced into the trigone, atrium, and temporal horn. Within the temporal horn, the continuation of the choroid plexus and hippocampus can be observed. The hippocampus is a pearly white structure visible at the upper right portion of the endoscopic field. The flexible endoscope is then withdrawn back into the occipital and anterior horns of the lateral ventricle.Video 4.2 Lateral ventricle anatomy 2. This video shows an endoscopic inspection of the right lateral ventricle with a flexible neuroendoscope. The right foramen of Monro and related structures are identified. The translucent structure at the right side corresponds to the septum pellucidum. As the endoscope following the choroid plexus posteriorly, the occipital horn and the extent of the septum pellucidum can be observed.Video 4.3 Third ventricle anatomy. This video demonstrates an endoscopic exploration of the third ventricle with a flexible neuroendoscope. The endoscope is navigated through the right foramen of Monro into the third ventricle. First, the massa intermedia and mammillary bodies are identified. The anterior portion of the third ventricle houses the mammillary bodies, tuber cinereum, and infundibulum. The basilar artery can be visualized through the thinned floor of the third ventricle. Anteriorly, the optic chiasm, suprachiasmatic recess, lamina terminalis, and anterior commissure can be observed. In this particular case, a lamina terminalis fenestration was performed previously, so the anterior communicating artery complex can be observed. The endoscope is then withdrawn from the third ventricle back into the lateral ventricle.Fig. 4.4 (a) The lower two-thirds of the anterior surface of the third ventricle is seen on the external surface of the brain; the upper one-third is hidden posterior to the rostrum of the corpus callosum. The boundaries of the anterior wall of the third ventricle, superior to inferior, are formed by the columns of the fornix, foramen of Monro, anterior commissure, lamina terminalis, optic recess, and optic chiasm. The foramen of Monro on each side is located at the junction of the roof and the anterior wall of the third ventricle. The structures forming the floor from anterior to posterior include the optic chiasm, the infundibulum of the hypothalamus, the tuber cinereum, the mammillary bodies, the posterior perforated substance, and (most posteriorly) the part of the tegmentum of the midbrain located above the medial aspect of the cerebral peduncles. The optic chiasm is located at the junction of the floor and the anterior wall of the third ventricle. (b) The area between the midbrain and the free edges is divided into: an anterior incisural space located in front of the brainstem and anterior wall of the third ventricle; paired middle incisural spaces situated lateral to the midbrain; and a posterior incisural space located behind the midbrain. The frontal horns are located above the anterior incisural space; the bodies of the lateral ventricles are located directly above the central part of the incisura, where they sit on and are separated from the central part of the incisura by the thalamus; the atria are located above the posterior incisural space; and the temporal horns are situated superolateral to the middle incisural space. The middle incisural space is the site of the crural and ambient cisterns; and the posterior incisural space is the site of the quadrigeminal cistern. (c) Superior view of the frontal horn and body of the lateral ventricle. The right thalamostriate vein passes through the posterior margin of the foramen of Monro and the left thalamostriate vein passes through the choroidal fissure a few millimeters behind the foramen. The anterior septal and anterior caudate veins cross the wall of the frontal horn, while the posterior caudate veins cross the wall of the body of the lateral ventricle. The thalamus sits in the floor of the body. The septum pellucidum attaches to the body of the fornix inferiorly in the medial wall of the body of the lateral ventricle, and it is attached to the column of the fornix in the medial wall of the frontal horn. (d) Enlarged view of the right foramen of Monro. The columns of the fornix form the anterior and superior margins of the foramen. An anterior septal vein passes backward along the septum pellucidum and crosses the column of the fornix. The thalamostriate vein passes forward between the caudate nucleus and thalamus and turns medially to pass through the posterior margin of the foramen of Monro to empty into the internal cerebral vein. The right thalamostriate vein is located on the right side of the choroid plexus. (e) Superior view of the roof of the third ventricle. The body of the fornix and hippocampal commissure have been split longitudinally in the midline and retracted laterally to expose the third ventricle. This procedure is also performed for the interfornical approach to the third ventricle. The roof of the third ventricle has four layers: one neural layer formed by the fornix, two thin membranous layers of tela choroidea, and a layer of blood vessels, which contains the internal cerebral veins and medial posterior choroidal arteries, between the sheets of tela choroidea. The upper layer of the anterior part of the roof of the third ventricle is formed by the body of the fornix, and the posterior part of the roof is formed by the crura and the hippocampal commissure. The tela choroidea consists of two thin, semiopaque membranes (superior and inferior layers) derived from the pia mater in the roof below the neural layer formed by fornix. The final layer in the roof is a vascular layer located between the two layers of tela choroidea. (f) The inferior tela has been opened to expose the floor of the third ventricle and the massa intermedia. The view has been directed posteriorly toward the aqueduct and the posterior and habenular commissures. The suprapineal recess extends into the base of the pineal gland between the habenular and posterior commissures. The pineal gland extends backward. (g) The view has been directed anteriorly to expose the anterior part of the floor of the third ventricle. The mammillary bodies are situated in the midportion of the floor. The tuber cinereum, the site commonly opened in a third ventriculostomy, is located between the mammillary bodies posteriorly and infundibular recess anteriorly. The chiasmatic recess extends forward above the posterior edge of the optic chiasm and below the anterior commissure. The upper end of the midbrain forms the posterior part of the floor of the third ventricle. (Abbreviations: Ant., anterior; Call., callosum; Caud., caudate; Cer., cerebral; Chor., choroid, choroidal; Chiasm., chiasmatic; Cin., cinereum; Cist., cistern; Comm., commissure; Corp., corpus; Front., frontal; For., foramen; Gl., gland; Haben., habenular; Inf., inferior; Infund., infundibulum; Int., intermedia, internal; Lam., lamina; Mam., mammillary; M. P. Ch. A., medial posterior choroidal artery; Mid., middle; Nucl., nucleus; Pell., pellucidum; Plex., plexus; Post., posterior; Quad., quadrigeminal; Rec., recess; Sept., septal, septum; Str., striate; Sup., superior; Thal., thalamo; Term., terminalis; V., vein; Vent., ventricle.)Animation 4.1 Animation of the lateral and third ventricle anatomy. This animation demonstrates the endoscopic exploration of the lateral and third ventricle. The right lateral ventricle is approached through a standard coronal burr hole. We identify the foramen of Monro, the anterior horn, and the head of the caudate nucleus. Following the choroid plexus posteriorly, we can observe the atrium, the occipital horn and the entrance of the temporal horn. Back into the anterior lateral ventricle, we observe the septum pellucidum medially. A fenestration of the septum allows us to navigate into the contralateral lateral ventricle. The endoscope is withdrawn back into the ipsilateral lateral ventricle and inserted into the third ventricle. We then observe the mammillary bodies, tuber cinerum, optic chiasm, and lamina terminalis. Turning the endoscope posteriorly, we observe the entrance of the cerebral aqueduct, the pineal gland, the posterior and habenular commissures.
Several methods of increasing the exposure of the roof of the third ventricle, in addition to opening the choroidal fissure, have been utilized (Fig. 4.6). One is to section a column of the fornix anterosuperior to the foramen on one side, but this will permit the exposure of no more than a small part of the anterior third ventricle. To prevent the complications associated with sectioning the fornix, Hirsch et al.5 sectioned the thalamostriate vein at the posterior margin of the foramen of Monro to enlarge the opening in the roof of the third ventricle. However, some of their patients developed drowsiness, hemiplegia, and mutism, and occlusion of the veins at the foramen of Monro may cause hemorrhagic infarction of the basal ganglia.2,6,7 Other routes to the anterior part of the third ventricle are by the interforniceal approach, in which the body of the fornix is split longitudinally in the midline, or by the transchoroidal approach, mentioned above, in which the choroidal fissure is opened between the fornix and thalamus, thus allowing the fornix to be pushed to the opposite side to expose the structures in the roof of the third ventricle (Fig. 4.4e, Fig. 4.6, and Fig. 4.7).6,8,9,10 The transchoroidal and interfornical approaches have the advantage of giving access to the central portion of the third ventricle by displacing, rather than dividing, the fibers in the fornix.
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