The choroidal fissure

Offprint from Advances and Technical Standards in Neurosurgery, Vol. 38 Edited by J.D. Pickard # Springer-Verlag/Wien 2011 – Printed in Austria – Not for Sale

The choroidal fissure: anatomy and surgical implications I. ZEMMOURA, S. VELUT, P. FRANC° OIS & With 5 Figures

Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Embryology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anatomy and surgical implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rostral portion (body portion). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Veins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surgical implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dorsal portion (atrial portion) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Veins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surgical implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caudal portion (temporal portion) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Veins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surgical implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract The choroidal fissure (CF) is an important landmark that allows approaches to the deepest aspects of the brain. It is the C-shaped site of attachment of the choroid plexus in the lateral ventricles, which runs between fornix and thalamus. The thinness and the absence of neural tissue between the epen-

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dyma and pia matter of this part of the medial wall of the lateral ventricles provides a surgical pathway to the third ventricle and perimesencephalic cisterns. A precise knowledge of the microsurgical anatomy of the region, particularly vascular relationships, is essential to consider surgery through the CF. We decided to present the anatomy of the CF in three distinct chapters, corresponding to three compartments of the C-shaped structure of the CF. In each compartment – rostral, dorsal and caudal – we developed the neurovascular anatomy and then discussed the clinical and surgical applications.

Keywords: Choroidal fissure; choroid plexus; third ventricle; neuroanatomy; surgery; surgical anatomy.

Embryology The choroid plexus are villous structures extending from the ependymal surface of the cerebral ventricles. They are the site of production of the cerebrospinal fluid. They first develop in the fourth ventricle between the 48th and 50th day (20th stage, 20 mm embryo). In the adult human brain, one can usually describe four different choroid plexuses; one in each lateral ventricle, another lying in the roof of the third ventricle, and the last one in the caudal roof of the fourth ventricle. In fact, the choroid plexuses of the lateral ventricles and of the third ventricle are in continuity via the CF. Each choroid plexus is formed by the apposition of two different tissues. The surface is covered by a cuboidal epithelium, which is in continuity with the ependymal cells of the ventricles and derives from the neural tube itself (Fig. 1). The core of the plexus choroid is formed with mesenchymal and vascular cells produced by the paraxial mesenchyme [3]. The CF, as the fornix that forms the outer limit of the CF, becomes a C shaped structure with the development of the telencephalon, which wraps around the thalamus, main structure of the diencephalon. Anatomy and surgical implications The CF is part of the medial wall of the lateral ventricles. It is a C shaped structure with a ventral concavity (Fig. 1). It begins at the interventricular foramen, continues through the body and atrium, and ends in the temporal horn. Its termination is called the inferior choroidal point [17], and is situated lateral to the uncus, beside the lateral geniculate body. The CF is the site of attachment of the choroid plexus in the lateral ventricles, between the thalamus and the fornix. As the fornix, the CF wraps

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Fig. 1A, B. (A) Superolateral view of a right lateral ventricle showing the C-shaped structure of the choroidal fissure. The choroidal plexus is seen from the interventricular foramen superoanteriorly to the temporal horn of the lateral ventricle inferoanteriorly. The posterior portion of the C is the choroidal glomus, a thickening of the choroidal plexus that correspond to the atrium of the lateral ventricle. The body of the corpus callosum has been removed so that the choroid plexus is seen in the left lateral ventricle. (B) High-magnification photograph (1000) of the surface of a choroid plexus observed with electron microscopy. Ant. Sept. V., anterior septal vein; Caud. Nucl., caudate nucleus; Chor. Glomus, choroid glomus; Corp. Call., corpus callosum; Int. Caps., internal capsule; IVF, interventricular foramen; LPChA, lateral posterior choroidal artery; Sept. Pell., septum pellucidum; Thal., thalamus

around the thalamus. Therefore, the fornix forms the outer circumference of the CF. The choroid plexuses are linked to the thalamus by the tenia choroidea, and to the fornix by the tenia fornicis (Fig. 2). In the temporal horn, the tenia fornicis is replaced by the tenia fimbriae [23]. The teniae are small ridges composed of two layers, one of pia mater and one of ependyma, then containing no nervous tissue. The choroid plexuses are surrounded with that ependymal layer, thus separating them from the ventricular fluid. Inside the CF run the choroidal arteries. They arise from the internal carotid arteries or the posterior cerebral arteries, run through the basal cisterns before entering the choroidal fissure. Along their course, they give branches to the surrounding neural structures. The anterior choroidal artery vascularizes the choroid plexus of the temporal horn and atrium. The lateral posterior choroidal arteries vascularize the choroid plexus of the atrium, body and posterior part of the temporal horn. The medial posterior choroidal arteries vascularize the choroid plexus of the roof of the third ventricle and the body of the lateral ventricle.

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Fig. 2. Coronal T2 weighted MRI of the roof of the third ventricle. The choroid plexus is attached to the body of the fornix (asterisk) by the tenia fornicis (A) and to the thalamus by the tenia choroidea (B). Both internal cerebral veins (arrow heads) are seen. They are part of the roof of the third ventricle, inside the superior tela choroidea

The ventricular veins join the deep venous system passing through the CF. The lateral group of ventricular veins course through the thalamic (or inner) side of the CF, while the medial group veins course through the forniceal (or outer) side of the CF. Veins draining the frontal horn and body of the lateral ventricle empty into the internal cerebral veins (ICVs). Those draining the temporal horn empty into the basal veins. And veins from the atrium empty into the segments of the basal, internal cerebral or great veins that are medial to the atrium. The transventricular transchoroidal approaches allow access to the deepest cerebral structures, by minimizing cerebral retraction. Most of the time, opening through the CF along the tenia fornicis or tenia fimbria must be prefered because fewer difficulties are encountered than along the tenia choroidea, through which pass many important veins and arteries.


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Rostral portion (body portion) The rostral portion of the CF is situated in the body of the lateral ventricle, between the body of the fornix and the superior surface of the thalamus. Its anterior termination is the posterior part of the interventricular foramen. In this portion, the choroid plexus of the lateral ventricle and those of the third ventricle are in continuity (Fig. 3A, B). Opening through the CF exposes the roof of the third ventricle and the velum interpositum, a close space from the interventricular foramen to the area above the pineal body, through which course the ICVs. Infrequently, the two membranous layers of tela choroidea in the roof of the third ventricle may separate to form the cisterna velum interpositum that communicates with the quadrigeminal cistern. The upper wall of the cisterna velum interpositum is the pial layer attached to the lower surface of the fornix. The lower wall is the pial layer attached to the teniae choroidea, the striae medullaris thalami, and the pineal body. Veins

All the veins passing through that portion of the CF join the homolateral Internal Cerebral Vein. This large vein originates from behind the interventricular foramen, courses posteriorly with a gentle upward convexity in the velum interpositum (or the cisterna velum interpositum), passes superolaterally to the pineal body and then continues with an upward concavity at the lower surface of the splenium of the corpus callosum, in the quadrigeminal cistern. It joins the contralateral ICV to form the great vein (Fig. 3B). The medial group of ventricular veins, passing through the tenia fornicis, is composed of the anterior and posterior septal veins (Fig. 3A). The anterior septal vein courses medially from the tip of the frontal horn to reach the septum pellucidum, passes around the column of the fornix, through the tenia fornicis, and terminates into the ICV. The posterior septal vein courses medially from the roof of the body of the lateral ventricle to reach the septum pellucidum. It joins the ICV passing through the junction of the fornix and septum pellucidum. The lateral group of ventricular veins, passing through the tenia choroidea, is larger (Fig. 3A). The anterior caudate vein courses along the surface of the head of the caudate nucleus to reach the thalamostriate vein. The posterior caudate vein courses across the body of the caudate nucleus and terminates either in the thalamostriate or in the thalamocaudate vein. The thalamostriate vein is large. It courses anteriorly in the small sulcus between the caudate nucleus and the thalamus, covered by the stria terminalis in this part. It then turns medially and posteriorly at the superior surface of the

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thalamus to enter the CF and reach the ICV. The junction of the thalamostriate vein and the ICV is called the ‘‘venous angle’’ and is usually located at the posterior margin of the interventricular foramen [12]. When that junction is behind the interventricular foramen, it is called a ‘‘false venous angle’’ [22]. The thalamocaudate vein is directed medially across the surface of the body portion of the caudate nucleus and the superior surface of the thalamus. It joins the ICV well behind the thalamostriate vein (Fig. 3B). Its size is inversely proportional to the size of the thalamostriate vein. The superior choroidal vein, the largest of the choroidal veins, runs forward, inside the choroid plexus of the body of the lateral ventricle, and terminates near the interventricular foramen, into the homolateral ICV or thalamostriate vein.


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Arteries

The rostral portion of the CF is mainly related to the terminal branches of the medial posterior choroidal arteries (MPChA). The MPChA is a one to three branches artery arising from the proximal part of the posterior cerebral artery (P1 segment). Their course is parallel and medial to the posterior cerebral artery through the ambient and quadrigeminal cisterns. They reach the pineal region and then turn forward to enter the velum interpositum, course between the two layers of the superior tela choroidea, medial to the ICVs. 1

Fig. 3A, B, C, D. (A) Superior view of the lateral ventricles. The corpus callosum has been removed. The septum pellucidum and fornix are exposed. The body portion and atrium portion of the choroid plexus are seen in both lateral ventricles. In the left lateral ventricle, anterior and posterior caudate veins join the thalamostriate vein, which courses in a sulcus between caudate nucleus and thalamus, covered by the lamina terminalis. The thalamostriate vein turns medially at the posterior edge of the interventricular foramen. Anterior septal veins course from the tip of the frontal horns of the lateral ventricles and join the choroidal fissure after crossing the columns of the fornix. (B) Superior view after the corpus callosum, septum pellucidum, body and crus of the fornix, and superior tela choroidea have been removed. The components of the roof of the third ventricle – medial posterior choroidal arteries, internal cerebral veins and their tributaries – are exposed. (C) Lateral view of the left lateral ventricle. The chroroid plexus of the temporal horn and the pulvinar have been removed, exposing the ambient cistern and the anterior part of the quadrigeminal cistern. The medial and inferior surfaces of the ventricular atrium are formed by the bulb of the corpus callosum, the calcar avis and the collateral trigone, that correspond to the forceps major of the corpus callosum, the calcarine sulcus and the collateral sulcus. (D) Inferior view showing the course of the anterior choroidal artery from its rise in the internal carotid artery, distal to the posterior communicating artery, to the crural cistern and the anterior end of the choroid plexus of the temporal horn of the lateral ventricle at the so called ‘‘inferior choroidal point’’. Ambient Cist., ambient cistern; Amygd. Body, amygdaloid body; Ant. Ch. A., anterior choroidal artery; Ant. Caud. V., anterior caudate vein; Ant. Sept. V., anterior septal vein; Basil. A., basilar artery; Bulb Corp. Call., bulb of the corpus callosum; Chor. Glomus, choroid glomus; Chor. Pl., choroid plexus; Coll. Trig., collateral trigone; Corp. Call., corpus callosum; Great V., great vein; Int. Caps., internal capsule; ICA, internal carotid artery; Int. Cer. V., internal cerebal vein; LPChA, lateral posterior choroidal artery; MCA, middle cerebral artery; MPChA, medial posterior choroidal artery; Optic N., optic nerve; P. Com. A., posterior communicating artery; PCA, posterior cerebral artery; Post. Caud. V., posterior caudate vein; Post. Sept. V., posterior septal vein; Sept. Pell., septum pellucidum; Sup. Cereb. A., superior cerebellar artery; Sup. Chor. V., superior choroidal artery; Thal. Caud. V., thalamocaudate vein; Thal. Str. V., thalamostriate vein

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They supply the roof of the third ventricle and the homolateral choroid plexus of the lateral ventricle with small branches passing through the tenia choroidea of the CF. The most frequent arteries passing through the choroid plexus of the body portion of the lateral ventricle are the branches of the lateral posterior choroidal artery (LPChA). It is a one to six branches artery arising from the posterior cerebral artery, distally to the origin of the MPChA (P2 segment). Its branches penetrate the choroid plexus of the lateral ventricle directly through the atrial or temporal portion of the tenia choroidea of the CF, or later after a course through the velum interpositum, just medial to the CF. They supply the atrial and body portion of the choroid plexus, and give branches to the thalamus. Both MPChA and LPChA give branches to adjacent neural structures along their course, and can sometimes supply the choroid plexus of the contralateral lateral ventricle.

Surgical implications

Opening of the rostral part of the CF exposes the third ventricle. The transcortical transventricular approach to the third ventricle was first described in 1933 by Dandy [5] to remove a colloid cyst, partially resecting the frontal lobe. Later, this approach was used via a cortical incision into the right middle frontal gyrus, preferentially when patients presented ventriculomegaly. It offers good visualization of the homolateral interventricular foramen. The disadvantages of this approach are the need for a brain excision, for a brain retraction, the difficulty of access when there is no ventriculomegaly, the poor visualization of the contralateral interventricular foramen, and the high incidence of postoperative seizures (5 to 10%). The anterior transcallosal approach was first described in 1949 by Greenwood [8]. This approach is most of the time preferred as it offers a natural anatomic plane for dissection. It can be proceeded without ventriculomegaly. A cortical incision is not required, thus reducing the risk of postoperative motor deficit and seizures. Moreover, it has been shown that a callosotomy less than 22 mm in lengh does not result in persistent cognitive signs of interhemispheric disconnection [27], and that cognitive function is better preserved following transcallosal than transcortical exploration [7]. Once in the lateral ventricle, there are different pathways to reach the third ventricle [26]. If the lesion is in the anterior part of the third ventricle, or for a ventriculocisternostomy, the interventricular foramen is easily identified with an endoscope, at the anterior end of the choroid plexus of the lateral ventricle (Fig. 4). Thus the third ventricle is reached without opening of the CF.


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Fig. 4. Endoscopic view of the interventricular foramen. Anatomical landmarks are easily recognized to reach the interventricular foramen from the frontal horn of the lateral ventricle. The interventricular foramen is limited by the column of the fornix medially, the choroid plexus and thalamus posterolaterally. The anterior septal vein and thalamostriate vein are constant surgical landmarks for this approach. When the endoscope is oriented posteriorly, the septum pellucidum and choroid plexus as far as the atrium are exposed. Ant. Sept. V., anterior septal vein; Chor. Pl., choroid plexus; IVF, interventricular foramen; Sept. Pell., septum pellucidum; Thal., thalamus; Thal. Str. V., thalamostriate vein

If the lesion is more posterior in the third ventricle, or if it closes the interventricular foramen, a transforaminal exposure can be performed. This approach can be adapted according to the aspect of the patient’s venous anatomy. For T€ure [22], a preoperative MR venography can reveal the location of the junction between the anterior septal vein and the ICV, thus indicating which interventricular foramen can be enlarged to provide the widest access to the third ventricle without venous sacrifice. Although transforaminal exposure with unilateral sacrifice of the thalamostriate vein has been described without postoperative deficit [6, 11], it may cause drowsiness, hemiplegia, mutism, hemorrhagic infarction of the basal ganglia, or death [14, 15, 17, 19, 25]. The sacrifice of the anterior septal vein, which is not recommended, may be preferable to sacrificing the thalamostriate vein or injuring the column of the fornix. Transforaminal exposure with unilateral incision of the column of the fornix was described as a useful and acceptable technique by Dandy [5] and other authors. Fornix is an efferent

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pathway of the hippocampus that plays a major role in memory. Persistent and temporary memory deficits have been reported after unilateral lesion of the column of the fornix [13]. Thus, this technique, which does not solve the problem of the posterior limitation of the transforaminal exposure, should not be recommended anymore. To reach the midsuperior portion of the third ventricle, three approaches have been described. The interforniceal approach, first described in 1944 by Busch [2] can be performed after a transcortical or transcallosal approach. The interforniceal division must be made exactly in the midline to avoid damage of the fornix. Anteriorly, the incision does not extend beyond the anterior commissure, posteriorly, the extent should be no more than 2 cm from the interventricular foramen to avoid injury of the hippocampal commissure [20]. Postoperative memory loss is the main complication, result of a bilateral damage to the fornices. The transchoroidal approach consists of opening the CF along

SUP

ANT LEFT

Right Hem.

LAT

Left Hem. Ant. Caud. V.

Callosotomy Sept. Pell.

Ant. Sept. V.

Thal. Str. V. Thal.

Fornix Corp. Call. Caud. Nucl. Thal.

Chor. Pl.

Ma. Interm.

Sept. Pell. ICV Sup. Tela Chor. V3

Sept. Pell.

A

INF LAT

POST Coll. Emin.

Sept. Pell.

C

Chor. Pl.

B SUP

ICV LPChA Pineal Body Bulb Corp. Call. Calcar Avis

IVF

Pulvinar Caud. Nucl. Chor. Pl. Lat. Atr. V. PCA Crus Fornix Coll. Trig.

Hippo. Amygd. Nucl. Basal V. Tail Caud. Nucl.

D

Fimbria PCA Mesenceph. Thal. Chor. Pl.


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the tenia fornicis [18, 20]. The advantage is to respect the ventricular veins and the thalamic branches of the choroidal arteries. With this technique, the body of the fornix is directly retracted, and the superior tela choroidea is opened between the two ICVs. Care should be taken when retracting the thalamus, even if it is partially protected by the choroid plexus with that approach. A lesion of the dorsomedial nucleus of the thalamus, which receives afferent fibers from the amygdaloid body and sends efferent fibers to the prefrontal cortex, may produce emotional disturbances [3]. Finally, the preferred approach is the subchoroidal approach (Fig. 5A, B), which consists of opening the CF along the tenia choroidea and retracting the fornix and the choroid plexus to expose the velum interpositum between ipsilateral ICV medially and thalamus laterally [4, 24, 25]. The advantage is the protection of the fornix and the ICVs given by the choroidal plexus. We also believe that preservation of the tenia fornicis preserves the microvascularisation of the ipsilateral fornix. The main disadvantages are the opening of the tenia choroidea through which pass major ventricular veins and the choroidal arteries, and the risk of damage to the thalamus. 1

Fig. 5A, B, C, D. (A) Coronal view of a right transcallosal subchoroidal approach to the third ventricle. After the callosotomy, the right lateral ventricle is reached. The tenia choroidea is then opened, the choroid plexus retracted medially. The third ventricle is reached without opening the superior tela choroidea. (B) Operating view of a right subchoroidal approach to the third ventricle. The choroid plexus is retracted to the left, after the tenia choroidea has been opened. The thalamus is gently retracted to the right. Care is taken to preserve the thalamostriate vein at the venous angle. The third ventricle is reached posteriorly to the interventricular foramen. (C) Operating view of a right transcortical transchoroidal approach to the pineal region. The right ventricular atrium is reached through the superior parietal gyrus. After entering atrium, the tenia fornicis is opened. The crus of the fornix is gently retracted to the left, and the choroid glomus retracted anteriorly and to the right, giving access to the quadrigeminal cistern. (D) Operating view of a right transchoroidal approach to the mesencephalon. The temporal horn of the right lateral ventricle is reach by a temporal transcortical approach. The choroidal fissure is opened through the tenia fimbria. The choroid plexus is then retracted upward to expose the perimesencephalic cisterns and mesencephalon. Amygd. Nucl., amygdaloid nucleus; Ant. Caud. V., anterior caudate vein; Ant. Sept. V., anterior septal vein; Bulb Corp.Call., bulb of corpus callosum; Caud. Nucl., caudate nucleus; Chor. Pl. choroid plexus; Coll. Emin., collateral eminencia; Coll. Trig., collateral trigone; Corp. Call., corpus callosum; Hippo., hippocampus; ICV, internal cerebral vein; IVF, interventricular foramen; Lat. Atr. V., lateral atrial vein; Left Hem., left hemisphere; LPChA, lateral posterior choroid artery; Ma. Interm., massa intermedia; Mesenceph., mesencephalon; PCA, posterior cerebral artery; Right Hem., right hemisphere; Sept. Pell., septum pellucidum; Sup. Tela Chor., superior tela choroidea; Thal., thalamus; Thal. Str. V., thalamostriate vein; V3, third ventricle

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Dorsal portion (atrial portion) The dorsal portion of the CF is situated between the crus of the fornix and the pulvinar (Fig. 3A, C). The ventricular side of that portion of the CF is related to the floor, the medial and anterior walls of the atrium of the lateral ventricle. The floor is the collateral trigone, formed by the posterior end of the collateral sulcus, between lingual and fusiform gyri (Fig. 3C). The medial wall is formed by two prominences. The upper one is the bulb of the corpus callosum, formed by the forceps major, a large and compact white fiber bundle passing through the splenium of the corpus callosum. The lower prominence is the calcar avis, formed by the deepest part of the calcarine sulcus. The pulvinar forms the medial part of the anterior wall of the atrium, the caudate nucleus the lateral part. In the ventricular atrium, the choroid plexus forms a bulky triangle, often calcified, which is called glomus. The cisternal side of that portion of the CF is related to the quadrigeminal cistern. A roof, an anterior and a lateral wall delimit that cistern. The roof is formed by the lower surface of the splenium of the corpus callosum and the arachnoid layer surrounding the great vein. The lateral wall is formed by the crus of the fornix anteriorly, and the medial surface of the occipital lobe posteriorly. The anterior wall is formed by the tectal plate and pineal body medially, and the surface of the pulvinar laterally. The quadrigeminal cistern communicates with the ambient cistern below the pulvinar, and with the velum interpositum above. If the hippocampal commissure does not fuse with the lower surface of the splenium, the quadrigeminal cistern can open in another cavity just above the velum interpositum, called the cavum vergae. Veins

The medial atrial veins course forward and downward on the bulb of the corpus callosum and the calcar avis, pass through the tenia fornicis of the CF and terminate either in the ICV, basal or great vein. The lateral atrial veins drain the anterior and lateral walls of the atrium and occipital horn. They course forward on the lateral wall, turn medially to the cross the surface of the pulvinar, pass through the tenia choroidea of the CF and terminate as the medial atrial veins in the ICV, basal or great vein. Unfrequently, a common atrial vein drains the medial and lateral atrial veins. Arteries

The atrial part of the CF is in close relationship with the posterior cerebral artery, the MPChA and the LPChA, all of which course through the quadrigeminal cistern. The superior cerebellar artery also has a portion going through the inferior margin of the quadrigeminal cistern, but it cannot be reached by


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operative approach through the CF. The choroidal glomus is supplied by the LPChA. Surgical implications

Opening of the dorsal portion of the CF exposes the quadrigeminal cistern. The ventricular atrium is reached by a transcortical approach, with cortical incision of the superior parietal gyrus or the non-dominant hemisphere’s middle temporal gyrus. The prefered approach is through the superior parietal gyrus, as it avoids injury of the optic radiations that form the inferolateral wall of the atrium. Opening through the temporoparietal area should not be recommended as it may cause interruption of the optic radiations, visuospacial disturbance in the nondominant hemisphere, and aphasia in the dominant hemisphere. After entering the atrium, the CF is opened along the tenia fornicis, the choroidal glomus is retracted anteriorly to give access to the quadrigeminal cistern and pineal region (Fig. 5C). Care must be taken when retracting, not to injury the crus of the fornix posteriorly, and the pulvinar anteriorly. Injury of the fornix may cause recent memory deficit, while injury of the pulvinar may cause langage and speech disturbance [1]. The narrowness of that exposure makes it difficult to approach pineal tumors by this route, so that occipital transtentorial or infratentorial supracerebellar approaches are usually prefered in that indication [19]. Surgical treatment of arteriovenous malformations or vascular tumors of the quadrigeminal cistern is exceptional since the development of endovascular procedures and radiosurgery. Caudal portion (temporal portion) The atrial portion of the CF continues caudally with the temporal portion. This space is situated between the ventral surface of the thalamus and the fimbria of the fornix, so that the tenia fornicis becomes the tenia fimbria. The anterior end of the CF called the ‘‘inferior choroidal point’’ (Fig. 3D) is situated laterally to the uncus and caudally to the amygdaloid nucleus [17]. It is the entry point of the anterior choroidal artery (AChA) into the CF [16]. The caudal portion of the CF forms the medial wall of the temporal horn of the lateral ventricle. The floor of the temporal horn is made of the hippocampus medially and the collateral eminence, heightening caused by the collateral sulcus, laterally. The roof of the temporal horn is formed by the inferior surface of the thalamus medially, the tail of the caudate nucleus, and the tapetum laterally. The tapetum also forms the lateral wall of the temporal horn, and separates the optic radiations from the ventricle.

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The inner side of that portion of the CF is related with the superolateral edge of the ambient cistern, just above the tentorial incisure. That cistern is delimited medially by the midbrain, above by the pulvinar, and laterally by the medial surface of the temporal lobe (formed by the subiculum inferiorly, the dentate gyrus above, and the fimbria of the fornix superiorly). The ambient cistern opens anteriorly into the crural cistern, posteriorly into the quadrigeminal cistern. Veins

The basal vein is in close relationship with the caudal portion of the CF. It arises below the anterior perforated substance, courses through the crural cistern, enters the ambient cistern where it continues superomedial to the posterior cerebral artery, to reach the quadrigeminal cistern where it terminates into the great vein more inferiorly than the ICV. The main ventricular vein of the temporal horn of the lateral ventricle is the inferior choroidal vein. It arises in the posterolateral part of the roof of the temporal horn, courses near the tail of the caudate nucleus and then passes through the CF penetrating the tenia choroidea just behind the inferior choroidal point and drains into the basal vein in the ambient cistern. It frequently anastomoses with the superior choroidal vein on the choroid plexus of the atrium. The transverse hippocampal veins, which go directly medially through the tenia fimbria to reach the basal vein in the ambient cistern, form the lateral group of veins of the temporal horn. Arteries

The midportion of the posterior cerebral artery courses just medial to the temporal portion of the CF, into the ambient cistern, and may be exposed by surgical opening through the CF in the temporal horn. In that area, the MPChA course beside the posterior cerebral artery and may also be exposed. Most of the LPChA, which are multiple, arise from the posterior cerebral artery in the ambient cistern and quadrigeminal cistern and can therefore be exposed when opening the posterior portion of the temporal part of the CF. The thalamogeniculate branches of the posterior cerebral artery also arise in the ambient cistern. They are directed upward to the geniculate bodies, reach the thalamus and internal capsule, and can be exposed by the same approach. The anterior choroidal artery (AChA) arises from the postero-medial edge of the internal carotid in the crural cistern, courses posteriorly below the optic tract to reach the anterior margin of the ambient cistern before


15

entering laterally the CF by penetrating the tenia choroidea at the point called the ‘‘inferior choroidal point’’, which correspond to the anterior end of the CF in the temporal horn of the lateral ventricle. The AChA supplies the choroid plexus of the temporal horn and the atrium. In its cisternal segment, it gives superior branches to the globus pallidus, the genu and posterior limb of the internal capsule, and lateral and inferior branches to the amygdaloid nucleus, the hippocampus, the uncus, the tail of the caudate nucleus. It also gives branches to the midbrain, the optic tract and the thalamus. Surgical implications

Opening of the caudal portion of the CF exposes the perimesencephalic cisterns (crural, ambient and quadrigeminal cisterns), the veins and arteries coursing along them [18], and the midbrain [9] at the tentorial incisure level (lemniscal trigone). The subtemporal approaches expose that region but the extensive retraction of the temporal lobe needed with these approaches may cause venous infarction due to injury of the vein of Labbe. This complication is avoided with the transchoroidal approach [10]. The temporal horn of the lateral ventricle is reached by a transcortical approach, usually with cortical incision in the anterior part of the middle of inferior temporal gyrus. To minimize the risk of injury of the anterior part of the optic radiations (known as the Meyer’s loop), the temporal horn may be exposed using a cortical incision in the inferior temporal, the occipitotemporal gyrus, or the collateral sulcus [21]. The CF is opened between the tenia fimbria and the choroid plexus, which is retracted upward (Fig. 5D). Retraction of that part of the CF risks damaging the fimbria and hippocampus. However, unilateral damage of the hippocampus, stria terminalis or amygdaloid nucleus usually produces neither memory deficit nor emotional disturbance. Conclusion Surgical approach is a specific problematic of neurosurgical management of every kind of cerebral or skull base lesion. The third ventricle, mesencephalon, and perimesencephalic cisterns are the deepest aspects of the brain. To reach a lesion involving such a region is a real surgical challenge. The perfect knowledge of the anatomy of neural and vascular structures encountered on the surgical pathway to that anatomic area allows the neurosurgeon to treat patients with a considerable reduced risk of complication. That anatomical background, combined with the mastery of new techniques such as endoscopy, nowadays provides the best results for patients.

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I. ZEMMOURA et al.

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