Fourth Ventricle

The fourth ventricle is the most common location, followed by the lateral ventricles and the third ventricle.

From: Cytology (Third Edition) , 2009

Neuroanatomy

D. Gupta , in Essentials of Neuroanesthesia, 2017

Fourth Ventricle

The fourth ventricle is a cavity of hindbrain connected to the third ventricle by a narrow cerebral aqueduct. The fourth ventricle is a diamond-shaped cavity located dorsal to the pons and upper medulla oblongata and anterior to the cerebellum ( Fig. 1.13). Fourth ventricle connected to the third ventricle above and central canal below. Through medial aperture, foramen of Magendie, it communicates with subarachnoid space. Laterally on either side it communicated with subarachnoid space through foramen of Luschka.

The superior cerebellar peduncles and the anterior and posterior medullary vela form the roof of the fourth ventricle. The apex or fastigium is the extension of the ventricle up into the cerebellum. The floor of the fourth ventricle is named the rhomboid fossa. The lateral recess is an extension of the ventricle on the dorsal inferior cerebellar peduncle.

Inferiorly, it extends into the central canal of medulla. The fourth ventricle communicates with the subarachnoid space through the lateral foramen of Luschka, located near the flocculus of the cerebellum, and through the median foramen of Magendie, located in the roof of the ventricle. Most of the CSF outflow passes through the medial foramen. The cerebral aqueduct contains no choroid plexus. The tela choroidea of the fourth ventricle, which is supplied by branches of the posterior inferior cerebellar arteries, is located in the posterior medullary velum. 11,12

The lateral wall of fourth ventricle on the upper side is formed by superior cerebellar peduncle and lower part is formed by inferior cerebellar peduncle and gracile and cuneate tubercle. The roof is tent in shape and projected into cerebellum (Fig. 1.13). Roof is formed superiorly by superior cerebellar peduncle and superior medullary velum and inferiorly by membrane consisting of ependymal and double layer of pia meter which constitute tela choroidea of the fourth ventricle. Floor of the fourth ventricle is rhomboid in shape and thus called as rhomboid fossa. Upper triangular part is formed by pons and lower triangular part by medulla. Intermediate part prolonged laterally to form the lateral recess. The floor of the fourth ventricle is divided into two symmetrical halves. Each half contains facial colliculus, hypoglossal triangle, sulcus limitans, vestibular area, stria medullaris, and vagal triangle. The vital centers are situated in vagal triangle and injury during surgery into the fourth ventricle to these areas can be fatal.

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The Ventricles, Choroid Plexus, and Cerebrospinal Fluid

J.J. Corbett , D.E. Haines , in Fundamental Neuroscience for Basic and Clinical Applications (Fifth Edition), 2018

Fourth Ventricle

The fourth ventricle is a roughly pyramid-shaped space that forms the cavity of the metencephalon and myelencephalon (Figs. 6.4 and 6.8). The apex of this ventricle extends into the base of the cerebellum, and caudally it tapers to a narrow channel that continues into the cervical spinal cord as the central canal. Laterally the fourth ventricle extends over the surface of the medulla as the lateral recesses, eventually to open into the area of the pons-medulla-cerebellum junction, the cerebellopontine angle, through the foramina of Luschka (Figs. 6.4 and 6.9). The irregularly shaped foramen of Magendie is located in the caudal sloping roof of the ventricle (Figs. 6.4 and 6.10). Although the roof of the caudal part of the fourth ventricle and the lateral recesses is composed of tela choroidea, the rostral boundaries of this space are formed by brain structures. These include the cerebellum (covering about the middle third of the ventricle) and the superior cerebellar peduncles and anterior medullary velum (covering the rostral third of the ventricle). The floor of the fourth ventricle, the rhomboid fossa (see Fig. 10.3), is formed by the pons and medulla (Fig. 6.8). The only naturally occurring openings between the ventricles of the brain and the subarachnoid space surrounding the brain are the foramina of Luschka and Magendie in the fourth ventricle.

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Meningiomas

Federico Landriel , Peter Black , in Principles of Neurological Surgery (Third Edition), 2012

Midline Suboccipital Infratentorial Approach

The position and approach have already been described.

Microsurgical Resection

The fourth ventricle is exposed through a vertical incision of the cerebellar vermis. The tumor is debulked and coagulation of the vascular plexus coming from the choroid supply is achieved. The IVM is detached from the lower choroid tela or the choroid plexus, avoiding traction on the floor of the fourth ventricle.

Postoperative Care

An intraventricular drain may be considered for draining out the blood in the ventricular system and thus avoiding postoperative arachnoiditis and hydrocephalus.

Operative Results

Liu and associates 79 reported a series of 24 IVMs with total tumor removal in 87.5% of patients. No deaths or postoperative hydrocephalus occurred in their group although the morbidity rate was approximately 25%. Tumor recurrence rate was 8.3% in a follow-up period from 6 months to 15 years.

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Essential Anatomy and Function of the Brain

Paul Rea , in Essential Clinical Anatomy of the Nervous System, 2015

2.2.7.3 Fourth Ventricle

The fourth ventricle is found in the posterior region of the pons and medulla and is rhomboid in shape. Superiorly, it narrows to become continuous with the aqueduct of the midbrain. Inferiorly, it narrows and leads into the central canal of the medulla. This in turn is continuous with the central canal of the spinal cord. The fourth ventricle is widened at the point called the lateral recess.

The anterior boundary, or floor is formed by the pons superiorly and medulla inferiorly. The nuclei of origin of the vestibulocochlear nerves are closely related to this. The median groove divides the floor into left and right halves. Each half is divided by the sulcus limitans into the medial, or basal portion and the lateral, or alar portion.

The lowermost portion of the floor of the fourth ventricle is called the calamus scriptorius, as it appears to resemble the tip of a pen. This region contains the cardiorespiratory, deglutition and vasomotor centers.

The posterior boundary or roof of the fourth ventricle is very thin and concealed by the cerebellum. It consists of white matter referred to as the superior and inferior medullary vela. This is lined by ependymal. There is a deficiency in the lower portion of the roof called the median aperture (Foramen of Magendie). At the median aperture, there is direct communication with the subarachnoid space. The ends of the lateral recess have openings called the lateral apertures (Foramen of Luschka). It is through the median and lateral apertures that the cerebrospinal fluid enters the subarachnoid space. The blood supply of the choroid plexus is from the cerebellar branches of the basilar and vertebral arteries.

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Ventricular Anatomy

Antonio Cesar de Melo Mussi , Evandro Pinto da Luz de Oliveira , in Comprehensive Overview of Modern Surgical Approaches to Intrinsic Brain Tumors, 2019

Fourth Ventricle (Fig. 5.7-5.8)

The fourth ventricle is a midline cavity located between the brainstem and the cerebellum ( Matsushima, Rhoton, & Lenkey, 1982; Rhoton Jr, 2000). It is connected to the third ventricle through the aqueduct, to the cisterna magna through the foramen of Magendie, and to the cerebellopontine angle through the foramina of Luschka. The fourth ventricle, when seen from a sagittal cut, resembles the form of a tent. The floor of the tent is formed by the pons and medulla. The roof points posteriorly and is divided in a superior and an inferior roof. The superior and the inferior roof meet at a point called the fastigium. Each part of the tent has a corresponding cerebellar part. The superior part of the roof is related to the tentorial surface of the cerebellum. The inferior part of the roof is related to the suboccipital surface of the cerebellum. The floor of the tent, formed by the pons and the medulla, is related to the petrosal surface of the cerebellum. Most of the surgical approaches to the fourth ventricle are performed through the inferior part of the roof and also involve exposure of the suboccipital surface of the cerebellum.

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Superior roof of the fourth ventricle: If we remove the pons and the medulla to see the superior and the inferior roofs of the fourth ventricle, we can notice a major difference between them. The superior roof of the fourth ventricle has thick neural structures, the superior cerebellar peduncles. The superior cerebellar peduncles are on the lateral wall of the superior part of the roof and are the continuation of the dentate nuclei. The dentate nuclei are located just above the superior pole of the tonsils. The superior medullary velum is located on the midline, between both superior cerebellar peduncles.

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Inferior roof of the fourth ventricle: In contrast to the superior roof, the inferior roof of the fourth ventricle is formed mainly by two thin membranes, the tela choroidea and the inferior medullary velum. The inferior medullary velum is all that remains of the connection between the nodule and flocculus. It extends laterally from the nodule above the superior pole of the tonsil and forms the peduncle of the flocculus at the level of the lateral recess. The space between the superior cerebellar peduncle and the inferior medullary velum is called the superolateral recess. The tela choroidea, when seen from anteriorly, resembles the letter T. It extends laterally to form the floor of the two lateral recesses and it extends inferiorly from the inferior medullary velum (the telovelar junction) to attach to the inferolateral edges of the floor of the fourth ventricle along the taenia, which are narrow white ridges that meet at the obex.

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Cerebellum: The inferior portion of the suboccipital surface of the cerebellum hides the inferior roof of the fourth ventricle. The tonsil and the biventral lobule cover the tela choroidea and the lateral recess. In the midline, the uvula covers the nodule. The cerebellomedullary fissure is a natural space between the cerebellum and the medulla. It is continuous with the vallecula, the space between both tonsils. The tonsil is the structure that blocks most of the view of the inferior roof of the fourth ventricle. If we remove the tonsils, we have a direct view of the tela choroidea and the inferior medullary velum. The tonsil is attached to the cerebellar hemisphere through the tonsilar peduncle, located at the superolateral aspect of the tonsil.

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Lateral recess: Extends laterally below the cerebellar peduncles to open into the cerebellopontine cistern through the foramina of Luschka. The lateral recess may be divided into a peduncular and a floccular part. The peduncular part is formed by the inferior cerebellar peduncle anteriorly and the peduncle of the flocculus posteriorly. The floccular part is formed by the rhomboid lip anteriorly and by the flocculus posteriorly. The rhomboid lip is a thin sheet of neural tissue located posteriorly to the glossopharyngeal and vagus nerves. The tela choroidea forms the floor of both parts of the lateral recess.

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Floor of the fourth ventricle: Has a rhomboid shape, and formed by the pons and the medulla. The pons forms the superior two-thirds of the floor and the medulla forms the inferior one-third of the floor. The pontine part of the floor has a triangular shape, with its apex continuous with the aqueduct and the base of the triangle is represented by an imaginary line connecting the lower margin of the cerebellar peduncles. The medullary part of the floor also has a triangular shape, but its apex points inferiorly at the obex. The base of the medullary part is an imaginary line along the site of attachment of the tela choroidea to the tenia just below the lateral recess. The intermediate part of the floor is formed by lateral recesses, between these two imaginary lines. The intermediate part of the floor corresponds to the transition between the pons and the medulla. The floor is divided longitudinally in two halves by the median sulcus. Parallel to the median sulcus is the sulcus limitans, which marks two longitudinal strips between both sulci, the two median eminences. The sulcus limitans does not mark the median, but it has two distinct depressions along its way. The superior depression is the superior fovea, located laterally to the facial colliculus. The inferior depression is the inferior fovea, located laterally to the hypoglossal triangle. The motor nuclei of the cranial nerves are located medially to the sulcus limitans, whereas the sensory nuclei are located laterally. The median eminence contains the facial colliculus and three triangular areas on its inferior end: hypoglossal triangle, vagal triangle, and area postrema. The inferior part of the fourth ventricle is called the calamus scriptorius, because these three triangular areas are grouped together near the median sulcus on the inferior part of the floor, giving a configuration of a feather or pen nib.

Figure 5.7. (A) View of the superior and inferior roof of the fourth ventricle after removal of the pons and medulla. 1. Superior medullary velum 2. Superior cerebellar peduncle 3. Nodule 4. Tela choroidea extending laterally to form the floor of the lateral recess and inferiorly 5. Tonsil. (B) Same view of A after removal of the tela choroidea. 1. Nodule 2. Inferior medullary velum 3. Tonsil 4. Uvula. (C) Suboccipital surface of the cerebellum and inferior roof of the fourth ventricle seen from below. 1. Inferior medullary velum 2. Uvula 3. Tela choroidea 4. Foramen of Magendie. (D) Same as D after removal of the tela choroidea. 1. Nodule 2. Inferior medullary velum 3. Superior cerebellar peduncle 4. Superior medullary velum 5. Lateral recess 6. Obex.

Figure 5.8. (A) Telovelar approach to the fourth ventricle. 1. inferior medullary velum 2. Tela choroidea 3. Foramen of Magendie. (B) Exposure of the fourth ventricle after opening the tela choroidea. 1. Inferior medullary velum 2. Nodule 3. Superior cerebellar peduncle 4. Superior medullary velum 5. Median sulcus 6. Obex. (C) Floor of the fourth ventricle. 1. Median sulcus 2. Superior cerebellar peduncle 3. Sulcus limitans and superior fovea 4. Facial colliculus 5. Inferior cerebellar peduncle 6. Lateral recess 7. Hypoglossal triangle 8. Inferior fovea 9. Obex.

Surgical approaches to the fourth ventricles are performed by splitting the vermis, removing a tonsil and part of the biventral lobule, or by dissecting the cerebellomedullary fissure (Fig. 5.8) (Kellog & Piatt Jr, 1997; Matsushima et al., 1992; Mussi & Rhoton Jr, 2000; Tomasello, Conti, Cardali, Torre, & Angileri, 2015; de Melo Mussi et al., 2015). Splitting of the inferior part of the vermis is a classical approach to the fourth ventricle and is especially useful in cases where tumor invasion of the vermis is observed. Dissection of the cerebellomedullary fissure involves retraction of the tonsils laterally, exposing the tela choroidea. Opening the tela choroidea and the inferior medullary velum (telovelar approach) exposes the floor of the fourth ventricle from the obex to the aqueduct. Opening the tela choroidea laterally exposes the lateral recess. Exposure of the superolateral recess and the area around the confluence of the cerebellar peduncles may be difficult to achieve by dissecting the cerebellomedullary fissure alone. This is because the tonsil may block the view to this area. In this case, removal of the ipsilateral tonsil and biventral lobule will provide the greatest exposure of the fourth ventricle.

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Volume 1

Jonathan Miller , ... Alan Cohen , in Schmidek and Sweet Operative Neurosurgical Techniques (Sixth Edition), 2012

Anatomy

The fourth ventricle is a broad tent-shaped cerebrospinal fluid (CSF) cavity located behind the brain stem and in front of the cerebellum in the center of the posterior fossa ( Fig. 31-1). CSF enters through the cerebral aqueduct, which opens into the fourth ventricle at its rostral end. The ventricle widens caudally until its maximum width at the level of the lateral recesses, from which CSF exits through the two foramina of Luschka into the cerebellopontine cisterns on either side. The ventricle narrows again to its caudal terminus at the obliterated central canal of the spinal cord, called the obex from the Latin for "barrier." The foramen of Magendie is just posterior to the obex and allows CSF to exit into the cerebellomedullary cistern, which is continuous with the cisterna magna. There are no arteries or veins within the cavity of the fourth ventricle. All of the vessels associated with this region are in the fissures located just outside the fourth ventricular roof.

The glistening white floor of the fourth ventricle is the posterior surface of the brain stem (Fig. 31-2). The border between the pons and medulla occurs approximately at the level of the foramina of Luschka. The superior (pontine) part of the floor begins at the aqueduct and expands to the lower margin of the cerebellar peduncles. The inferior (medullary) part of the floor begins just below the lateral recesses at the attachment of the tela choroidea to the taenia choroidea and extends to the obex, limited laterally be the taeniae, which mark the inferolateral margins of the floor. Between these is the intermediate part, which extends into the lateral recesses on either side. There is a longitudinal midline sulcus in the fourth ventricular floor called the median sulcus. On either side of the median sulcus is the sulcus limitans, which also runs longitudinally parallel to the median sulcus. The sulcus limitans is an important landmark for functional anatomy of nuclei beneath the ventricular floor, as motor nuclei are medial and sensory nuclei lateral to the sulcus limitans. Medial to the sulcus limitans on either side of the median sulcus is the median eminence, a collection of four paired elevations in the fourth ventricular floor that are collectively referred to as the calamus scriptorius since they resemble the head of a fountain pen (Fig. 31-1). Rostral to caudal, the median eminence consists of the facial colliculus, which overlies the facial nucleus; the hypoglossal triangle, which overlies the hypoglossal nucleus; the vagal triangle, which overlies the dorsal nucleus of the vagus; and the area postrema, a tongue-shaped structure that is part of the brain-stem emetic center. Lateral to the sulcus limitans is the vestibular area, so named because is overlies the vestibular nuclei. This area is widest in the neighborhood of the lateral recess, where the striae medullaris cross transversely across the inferior cerebellar peduncles to disappear into the median sulcus. The auditory tubercle in the lateral part of the vestibular area overlies the dorsal cochlear nucleus and cochlear nerve.

The roof of the fourth ventricle is tent-shaped, rising to an apex called the fastigium that divides the superior roof from the inferior roof. The median part of the superior roof, called the superior medullary velum, consists of a thin lamina of white matter between the cerebellar peduncles. Just behind its outer surface is the lingula, the uppermost division of the vermis. The lateral walls of the superior roof are formed by the superior and inferior cerebellar peduncles, which lie between the fourth ventricle and the middle cerebellar peduncle. The rostral midline of the inferior roof is formed by the nodule, which lies directly in front of the uvula, the lower part of the vermis that hangs down between the tonsils (mimicking the appearance of the pharynx). Lateral to the nodule is the inferior medullary velum, a thin sheet of neural tissue that stretches over the fourth ventricle to connect the nodule to the flocculi on either side just superior to the outer extremity of the lateral recess. The inferior medullary velum is thus part of the primitive flocculonodular lobe of the cerebellum. The caudal inferior roof consists of the tela choroidea, two thin arachnoid-like membranes sandwiching a vascular layer of choroidal vessels to which the choroid plexus is attached. The junction between the tela choroidea and the nodule/inferior medullary velum (telovelar junction) is at the level of the lateral recess. The tela choroidea is attached to the ventricular floor at narrow white ridges called taeniae choroidea, which meet at the obex and extend upward to turn laterally over the inferior cerebellar peduncles into each lateral recess, forming its lower border. As a result, the choroid plexus (extending from the ventricular surface of the tela) forms an upside-down L shape on either side of midline. There is a medial segment of choroid plexus that extends longitudinally from the foramen of Magendie up to the nodule and a lateral segment that extends transversely from the rostral ends of the medial segments out to the foramen of Luschka. The three fourth ventricular outlet foramina (Magendie and Luschka) are located in the tela choroidea itself, and frequently choroid plexus protrudes from these foramina.

External to the fourth ventricle are three deep V-shaped fissures between the cerebellum and brain stem that enclose subarachnoid cisterns and through which course the principle arteries and veins of the posterior fossa. These three fissures are intimately related to the structures of the posterior fossa. Located between the midbrain and cerebellum, the cerebellomesencephalic fissure (also called the precentral cerebellar fissure) is the most rostral of the three and is intimately associated with the superior part of the fourth ventricular roof. This fissure is shaped like a V in the axial plain with the point facing posteriorly. The brain stem and fourth ventricle line the inner surface along with the lingula of the vermis, dorsal superior cerebellar peduncles, and rostral middle cerebellar peduncles. The outer surface of the V consists of the cerebellum, specifically the culmen and wings of the central lobule. The trochlear nerves run through the cerebellomesencephalic fissure, as do the superior cerebellar arteries (SCA). The SCAs leave the brain stem between cranial nerves IV and V to enter the fissure, and then after several sharp hairpin turns give rise to the precerebellar arteries that pass along the superior cerebellar peduncle to reach the superior fourth ventricle and dentate nucleus. Upon leaving the fissure the arteries supply end branches to the tentorial surface of the cerebellum. Venous drainage from the superior fourth ventricle occurs primarily through the vein of Galen. The vein of the cerebellomesencephalic fissure (also called the precentral cerebellar vein) is formed by the union of the paired veins of the superior cerebellar peduncle and ascends through the quadrigeminal cistern to drain into the vein of Galen either directly or through the superior vermian vein.

The cerebellopontine fissures are intimately related to the lateral recesses of the fourth ventricle. They are produced by the folding of the cerebellum laterally around the sides of the pons and middle cerebellar peduncles. Each cerebellopontine fissure is shaped like a V in the coronal plain with the point facing laterally. The outer surface of the V is made up of the petrosal surfaces of the cerebellum, and the inner surface is made up of the middle cerebellar peduncles. The lateral recess and foramen of Luschka open into the medial part of the inferior limb of the V near the flocculus. Several cranial nerves run through the cerebellopontine fissure, including the trigeminal (through the superior limb) and the facial, glossopharyngeal, and vagus (through the inferior limb). The anterior inferior cerebellar arteries (AICA) also run through these fissures. Each AICA courses posteriorly around the pons then sends branches to nerves of the acoustic meatus and choroid plexus protruding from the foramen of Luschka before passing around the flocculus on the middle cerebellar peduncle to supply the petrosal surface of the cerebellum. Venous blood from the cerebellopontine fissure and lateral recess primarily drains into the superior petrosal sinus. The vein of the cerebellopontine fissure is formed by the convergence of several veins on the apex of the fissure, including the vein of the middle cerebellar peduncle into which the vein of the inferior cerebellar peduncle drains. This vein courses near the superior limb of the fissure to drain into the superior petrosal sinus rostral to the facial and glossopharyngeal nerves.

The cerebellomedullary fissure is directly behind the inferior roof of the fourth ventricle. It is the most caudal of the three fissures and extends between the cerebellum and medulla. Like the cerebellomesencephalic fissure, it is shaped like a V in the axial plain with the point facing posteriorly. The ventral wall consists of the inferior roof of the fourth ventricle (inferior medullary velum and tela choroidea) and the posterior medulla. The dorsal wall consists of the uvula in the midline and the tonsils (paired ovoid structures attached to the cerebellar hemispheres along their superolateral borders) and biventral lobules laterally. The fissure communicates with the cisterna magna around the superior poles of the tonsils through the telovelotonsillar cleft (tonsils to tela/velum) and "supratonsillar cleft" (superior extension of this cleft over superior pole of tonsil). The posterior inferior cerebellar arteries (PICA) course around the medulla to reach the cerebellar tonsil and lower half of the floor of the fourth ventricle. They then loop superiorly at the caudal pole of the tonsil (caudal loop) to ascend in the fissure as far as the upper pole of the tonsil, and then loop again inferiorly over the inferior medullary velum (cranial loop). Branches of the artery radiate outward from the borders of the tonsils to supply the suboccipital surface of the cerebellum. Most of the venous blood from this region drains anteriorly into the superior petrosal sinus through the vein of the cerebellopontine fissure, although some drains posteriorly into the tentorial sinuses converging on the torcular Herophili. The vein of the cerebellomedullary fissure originates on the lateral edge of the nodule and uvula and courses laterally near the telovelar junction to reach the cerebellopontine angle.

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Vestibular System

Pierre-Paul Vidal , Alain Sans , in The Rat Nervous System (Third Edition), 2004

Medial Vestibular Nucleus (Mve)

The fourth ventricle limits the dorsomedial border of the Mve over its entire length. The rostral part extends up to the posterior ventral zone of the superior vestibular nucleus and the caudal part ends at the level of the prepositus hypoglossal nucleus bounded ventrally by the nucleus of the solitary tract. Laterally the Mve is limited by the LVe and then by the spinal vestibular nucleus. The cytostructure of this nucleus is relatively homogeneous with small and medium cells densely packed together, giving a dark staining aspect to the Mve, which contrasts with the neighboring nuclei. Small cells are more numerous in the rostral part and thin fibers cross the nucleus in all directions.

In the cat, the cristae ampullaris project in the dorsorostral part and the otolith organs in the Mve ventromedial part (Gacek, 1969). The selective projection of these sensors has so far not been documented in the rat. The rat Mve receives a contingent of cerebellar fibers from the caudal vermis (Bernard, 1987; Umetami, 1992; Xiong and Matsushita, 2000) and bilaterally from the medial nucleus (Rubertone et al., 1995). Medial nucleus GABAergic neurons project via axon collaterals to both the vestibular nuclei and the inferior olive (Diagne et al., 2001). A zonal organization of flocculovestibular connections has been established in rats with a specific pattern for dorsal and ventral surfaces of the flocculus. This pattern organization in relation with climbing fiber zones could be a specialization for controlling the activity of the extraocular muscles (Balaban et al., 2000). Other afferents concern a spinovestibular projection through the medial reticular formation in the caudal part of the nucleus with inputs originating from nerve endings in the ligaments and capsule of the upper cervical joints. Afferents from rostral cervical segments (C2-C3) mainly project in the caudal part of the Mve (Bankoul and Neuhuber, 1990). A more extensive projection of cervical afferents in the rat has also been described in the spinal vestibular nucleus and throughout the LVe (Matsushita et al., 1995, Xiong and Matsushita, 2001). Each Mve also receives an important projection from the controlateral medial nucleus through commissural fibers. The caudal part of Mve also receives (as spinal vestibular nucleus) inputs from the cerebral cortex. Cortical projections are bilateral predominantly and concern somatosensory areas and the frontal cortex. The rat corticofugal projections to the caudal vestibular nuclei could participate in modulation of the vestibuloocular and vestibulospinal reflex during locomotion (Nishiike et al., 2000).

Efferent connections of the Mve concern ascending and descending projections in the medial longitudinal fasciculus. Ascending connections are bilateral and mainly project onto the oculomotor nuclei. The ventral Mve sends fibers in the thalamus, especially in the lateral parafascicular nucleus and the dorsal part of the ventrobasal thalamus nucleus, with an ipsilateral predominance (Shiroyama et al., 1999). The descending connections form the medial vestibulospinal tract with the majority of fibers ending on the cervical anterior cord, essentially in laminae III to V, with some scattered fibers in laminae I and VI (Bankoul and Neuhuler, 1992). Efferent bilateral connections to the cerebellum with caudal vermis and medial nucleus corresponding to reciprocal projections between medial vestibular nucleus and cerebellum have also been demonstrated in the rat (Rubertone and Mehler, 1981; Haroian, 1984; Barmak et al., 1992; Voogd et al., 1996).

The convergence of inputs coming from vestibular receptors and neck in the Mve and the major output, which reach the oculomotor nuclei and upper cervical cord, indicate that the Mve is in stabilizing gaze and posture in the horizontal plane.

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Ventricles and the Cerebrospinal Fluid

David L. Felten MD, PhD , ... Mary Summo Maida PhD , in Netter's Atlas of Neuroscience (Third Edition), 2016

6.3 Anatomy of the Fourth Ventricle: Posterior View with Cerebellum Removed

The rhomboid-shaped fourth ventricle extends through the pons and medulla. The foramina of Magendie and Luschka must remain patent for proper flow of the CSF into the cisterns. Bilaterally symmetrical protrusions, depressions, and sulci on the floor of the fourth ventricle define the underlying anatomy of brain stem regions, such as the hypoglossal, vagal, and vestibular areas. Vital brain stem centers for cardiovascular, respiratory, and metabolic functions just below the floor of the fourth ventricle can be damaged by tumors in the region. The lateral margins of the fourth ventricle are embraced by the huge cerebellar peduncles interconnecting the cerebellum with the brain stem and diencephalon. These anatomical relationships are important when interpreting imaging studies in the compact brain stem regions where the diagnosis of tumors and vascular lesions is challenging.

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Gliomas

Riccardo Soffietti , ... David Reardon , in Handbook of Clinical Neurology, 2016

Pathology and molecular characteristics

RGNT of the fourth ventricle is composed of two distinct cellular components, one with uniform neurocytes forming rosettes and/or perivascular pseudorosettes, the other being astrocytic and resembling pilocytic astrocytoma. Mitoses and necrosis are absent, and Ki-67-labeling index is less than 3%. RGNT of the fourth ventricle corresponds to WHO grade I.

Genetic analysis of RGNT has revealed a mutation in PIK3CA gene (Ellezam et al., 2012), while IDH1/2 mutation and 1p/19q co-deletion are absent. Recent immunohistochemical studies have reported co-expression of neuronal markers (synaptophysin), glial markers (GFAP, OLIG2, CyclinD1, and PDGFRα), and stem cell markers (CD133) (Matsumura et al., 2014), thus confirming the hypothesis of a biphenotypic differentiation from a pluripotential progenitor cell in the subependymal plateau.

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Neuroimaging Part II

Harold L. Rekate , Ari M. Blitz , in Handbook of Clinical Neurology, 2016

Outlet foramina of the fourth ventricle

Large tumors of the fourth ventricle can obstruct the outlet foramina of the fourth ventricle, but these cases frequently result from a mass in the fourth ventricle and dilatation of the supratentorial ventricular system. The most common cause of obstruction to outflow from the fourth ventricle causing quadriventricular hydrocephalus is meningitis and in particular "chronic meningitis" ( Fig. 64.4). This condition may be due to chronic conditions such as tuberculosis, fungal meningitis, sarcoid, and tumors that have spread throughout the meninges. It is also found in patients following severe subarachnoid hemorrhage. When faced with all four ventricles being enlarged, it is often necessary to obtain contrast-enhanced studies to visualize the pathology and occasionally a meningeal biopsy is necessary for the diagnosis. Patients who develop fourth-ventricular outflow obstruction in infancy or at a young age are subject to secondary distension of the central canal, producing "communicating syringomyelia," causing what is called by Oi et al. (1991) "hydromyelic hydrocephalus." If the condition begins later in life, the central canal of the spinal cord has normally been obliterated and the fourth ventricle itself distends (Milhorat et al., 1994).

Fig. 64.4. Sagittal T2-weighted magnetic resonance imaging from an outside institution demonstrating dilatation of the lateral third and fourth ventricles with apparent webs obstructing the foramen of Magendie (arrow). The foramina of Luschke (not shown) also appeared to be obstructed.

Hydrocephalus can be seen in the context of both Chiari I and II malformations. Is the hindbrain hernia the cause of the hydrocephalus or is it the result of the hydrocephalus? In the case of Chiari I malformation, the answer is almost certainly that the hydrocephalus causes the hindbrain herniation. In order to cause hydrocephalus, all three of the openings of the fourth ventricle must be closed. It is not infrequent to see the foramen of Magendie closed in the case of Chiari I, but it is nearly certain that without a confounding associated condition the foramina of Luschka will be open. This does not mean that it is necessary to treat the hydrocephalus and not the Chiari. Clinical judgment here is essential and sometimes difficult. In the case of the Chiari II malformation it is not unusual for hydrocephalus to develop as a result of the closure of the outlet foramina of the fourth ventricle. In utero there is mixing of CSF and amniotic fluid, leading to an intense scarring within the subarachnoid space, and the herniation involves not only the cerebellar vermis but also the medulla itself and there is often, or characteristically, dense scarring over these outflow channels. The pathophysiology of hydrocephalus in the context of Chiari II malformation will be discussed in detail below.

Finally there is the condition known as the isolated fourth ventricle in which, after shunting, in a patient with an outflow obstruction of the fourth ventricle there is a secondary closure of the aqueduct of Sylvius, making the fourth ventricle a closed system. The fourth ventricle dilates, causing severe cranial nerve dysfunction and balance problems. While an inflammatory condition is at the root cause of this condition, the secondary closure of the aqueduct can be caused by overdrainage of the spinal fluid by a shunt. Frequently this condition can be reversed by shunt revision and increasing the resistance of the valve (Foltz and DeFeo, 1980).

The treatment of obstruction of the outlet foramina of the fourth ventricle is often difficult and sometimes dangerous. Generally lateral ventricle shunts work well with care to make certain that the opening pressure is set high enough to keep the aqueduct open. As long as the aqueduct remains open, ETV will provide relief. In patients with enlarged third ventricle, cannulation of the aqueduct either from the third or the fourth ventricle has been used successfully, as discussed above. Incorporating ventricular catheters directly in the fourth ventricle carries substantial risks and requires careful planning. Placing a long catheter with extra holes from the third ventricle has proved successful but will often result in transient significant diplopia. Drilling a hole in the posterior fossa and inserting a ventricular catheter into the fourth ventricle directly should be done only using frameless stereotaxis. It is not rare to see good initial results but, as the fourth ventricle decreases in size, the catheter may come to lie within the parenchyma of the brainstem with new cranial nerve dysfunction. There are two ways to avoid this. One is to use a soft lumbar catheter instead of a firm ventricular catheter that will not penetrate the brainstem. The second technique is to formally open the foramen of Magendie either with an open craniectomy or using an endoscope with placement of the ventricular catheter into the fourth ventricle parallel to the brainstem.

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