Lateral Ventricle

Viewed from its lateral aspect, the lateral ventricle has a roughly C-shaped profile, with an occipital tail (see Fig. 5.1). The shape is a consequence of the developmental expansion of the frontal, parietal and occipital regions of the hemisphere (Ch. 3), which displaces the temporal lobe inferiorly and anteriorly. Both the caudate nucleus and the fornix, which lie in the wall of the ventricle, have adopted a similar morphology, so the tail of the caudate nucleus encircles the thalamus in a C shape, and the fornix traces the outline of the ventricle forward to the interventricular foramen.

The lateral ventricle is customarily divided into a body and anterior, posterior and inferior horns (Figs. 5.1, 5.3). The anterior (frontal) horn lies within the frontal lobe. It is bounded anteriorly by the posterior aspect of the genu and rostrum of the corpus callosum, and its roof is formed by the anterior part of the body of the corpus callosum. The anterior horns of the two ventricles are separated by the septum pellucidum. The coronal profile of the anterior horn is roughly that of a flattened triangle in which the rounded head of the caudate nucleus forms the lateral wall and floor (Fig. 5.4). The anterior horn extends back as far as the interventricular foramen.

Fig. 5.3 Horizontal section of the cerebrum dissected to remove the roofs of the lateral ventricles.

Fig. 5.4 Transverse MRI scan, at the level of the anterior horn of the lateral ventricle.

(Courtesy of Professor Alan Jackson, Department of Neuroradiology, University of Manchester, United Kingdom.)

The body lies within the frontal and parietal lobes and extends from the interventricular foramen to the splenium of the corpus callosum. The bodies of the lateral ventricles are separated by the septum pellucidum, which contains the columns of the fornices in its lower edge. The coronal profile of the body of the ventricle is a flattened triangle with an inward-bulging lateral wall, formed by the thalamus inferiorly and the tail of the caudate nucleus superiorly. The boundary between the thalamus and caudate nucleus is marked by a groove (see Fig. 5.3), which is occupied by a fascicle of nerve fibres, the stria terminalis, and the thalamostriate vein. The inferior limit of the body of the ventricle and its medial wall are formed by the body of the fornix. The fornix is separated from the thalamus by the choroid fissure. The choroid plexus occludes the choroid fissure and covers part of the thalamus and fornix. The body of the lateral ventricle widens posteriorly to become continuous with the posterior and inferior horns at the collateral trigone or atrium.

The posterior (occipital) horn curves posteromedially into the occipital lobe. It is usually diamond shaped or square in outline, and the two sides are often asymmetric. Fibres of the tapetum of the corpus callosum separate the ventricle from the optic radiation and form the roof and lateral wall of the posterior horn. Fibres of the splenium of the corpus callosum (forceps major) pass medially as they sweep back into the occipital lobe and produce a rounded elevation in the upper medial wall of the posterior horn. Lower down, a second elevation, the calcar avis, corresponds to the deeply infolded cortex of the anterior part of the calcarine sulcus.

The inferior (temporal) horn is the largest compartment of the lateral ventricle and extends forward into the temporal lobe. It curves around the posterior aspect of the thalamus (pulvinar); at first it passes downward and posterolaterally, and then it curves anteriorly to end within 2.5 cm of the temporal pole, near the uncus. Its position relative to the surface of the hemisphere usually corresponds to the superior temporal sulcus. The roof of the inferior horn is formed mainly by the tapetum of the corpus callosum, but also by the tail of the caudate nucleus and the stria terminalis, which extend forward in the roof to terminate in the amygdala at the anterior end of the ventricle.

The floor of the ventricle consists of the hippocampus medially and the collateral eminence, formed by the infolding of the collateral sulcus, laterally. The inferior part of the choroid fissure lies between the fimbria (a distinct bundle of efferent fibres that leaves the hippocampus) and the stria terminalis in the roof of the temporal horn (Fig. 5.5). The temporal extension of the choroid plexus fills this fissure and covers the outer surface of the hippocampus.

Fig. 5.5 Coronal section through the inferior horn of the lateral ventricle. Pia mater, red; ependyma, blue.

Third Ventricle

The third ventricle is a midline, slit-like cavity derived from the primitive forebrain vesicle (Figs. 5.1, 5.2, 5.6–5.8). The upper part of the lateral wall of the ventricle is formed by the medial surface of the anterior two-thirds of the thalamus, and the lower part is formed by the hypothalamus anteriorly and the subthalamus posteriorly. An indistinct hypothalamic sulcus extends horizontally on the ventricular wall between the interventricular foramen and the cerebral aqueduct, marking the boundary between the thalamus and hypothalamus. Dorsally, the lateral wall is limited by a ridge covering the stria medullaris thalami. The lateral walls of the third ventricle are joined by an interthalamic adhesion, or massa intermedia, a band of grey matter that extends from one thalamus to the other.

Fig. 5.6 Coronal section through the lateral and third ventricles. Pia mater of the tela choroidea, red; ependyma, blue.

Fig. 5.7 MRI scan of the head in the sagittal plane.

(Courtesy of Professor Alan Jackson, Department of Neuroradiology, University of Manchester, United Kingdom.)

Fig. 5.8 A, Sagittal hemisection of the brain showing the third and fourth ventricles. Pia mater, red; ependyma, blue. B, Sagittal hemisection through the brain.

(Hemisection by E. L. Rees; photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)

Anteriorly, the third ventricle extends to the lamina terminalis (see Fig. 5.8). This thin structure stretches from the optic chiasma to the rostrum of the corpus callosum and represents the rostral boundary of the embryonic neural tube. The lamina terminalis forms the roof of the small virtual cavity lying immediately below the ventricle, called the cistern of the lamina terminalis. This is important because it contains the anterior communicating artery, and aneurysm formation at this site may cause intraventricular haemorrhage through the thin membrane of the lamina terminalis. Above this, the anterior wall is formed by the diverging columns of the fornices and the transversely oriented anterior commissure, which crosses the midline. The anterior and posterior commissures are important neuroradiological landmarks. Before the introduction of modern imaging techniques, the anterior and posterior commissures could be identified by ventriculography. This led to their use as markers of the baseline for stereotactic surgical procedures. This convention is now universal, and the positions of the anterior and posterior commissures are the basic reference points for most surgical atlases of brain anatomy. The narrow interventricular foramen is located immediately posterior to the column of the fornix and separates the fornix from the anterior nucleus of the thalamus.

There is a small, angular, optic recess at the base of the lamina terminalis, just dorsal to and extending into the optic chiasma. Behind it, the anterior part of the floor of the third ventricle is formed mainly by hypothalamic structures. Immediately behind the optic chiasma lies the thin infundibular recess, which extends into the pituitary stalk. Behind this recess, the tuber cinereum and the mammillary bodies form the floor of the ventricle.

The roof of the third ventricle is a thin ependymal layer that extends from its lateral walls to the choroid plexus, which spans the choroid fissure (see Fig. 5.6). Above this is the body of the fornix. The posterior boundary of the ventricle is marked by a suprapineal recess above the pineal gland, by a pineal (epiphyseal) recess that extends into the pineal stalk and by the posterior commissure. Below the commissure, the ventricle is continuous with the cerebral aqueduct of the midbrain.

Cerebral Aqueduct

The cerebral aqueduct is a small tube, roughly circular in transverse section and approximately 2 mm in diameter. It extends throughout the dorsal quarter of the midbrain in the midline and is surrounded by the central, periaqueductal grey matter (see Fig. 5.8). Rostrally, it commences immediately behind and below the posterior commissure, where it is continuous with the caudal aspect of the third ventricle. Caudally, it is continuous with the lumen of the fourth ventricle at the junction of the midbrain and pons. The superior and inferior colliculi are dorsal to the aqueduct, and the midbrain tegmentum is ventral.

CASE 1 AQUEDUCTAL STENOSIS

A middle-aged woman complains of headache of several months’ duration; over time she exhibits mental slowing, papilledema with non-specific abducens palsies, spasticity with pyramidal tract signs and limb ataxia. She has a history of presumed viral meningoencephalitis years before but has otherwise been well.

Imaging demonstrates symmetric enlargement of the lateral and third ventricles (Fig. 5.9). The aqueduct is not visualized, and the fourth ventricle and cisterna magna are normal. A diagnosis of aqueductal stenosis is made, and ventricular shunting results in remarkable clinical improvement.

Fig. 5.9 MRI of aqueductal stenosis (dashed arrow) with marked ventricular enlargement. There is an incidental empty sella turcica as well (solid arrow).

Discussion: Aqueductal stenosis may be congenital or acquired later in life, presumably as a result of viral or bacterial infection with ependymitis and subsequent occlusion of the aqueduct. It is often asymptomatic until adulthood, ultimately presenting with a non-specific syndrome of hydrocephalus involving primarily the anterior ventricular system, as visualized in this case with appropriate neuroimaging.

Differential diagnoses include hydrocephalus secondary to choroid plexus papilloma with overproduction of CSF, pinealoma, invasive tumour of the brain stem or cerebellum, intraventricular tumour such as ependymoma of the fourth ventricle, chronic basilar meningitis obliterating the foramina of Luschka and Magendie, Arnold–Chiari malformation and Dandy–Walker syndrome with enlargement of the fourth ventricle. The syndrome of so-called normal-pressure hydrocephalus is evidenced classically by progressive memory deficits and dementia; ataxia; pyramidal tract signs, especially in the legs; and urinary tract dysfunction. CSF pressure is normal, and there is no papilledema. This disorder is most likely due to obliteration of the cerebral subarachnoid space secondary to prior trauma, meningitis, or subarachnoid haemorrhage, and neuroimaging is diagnostic in the majority of cases, many of which respond dramatically to ventricular shunting.

Fourth Ventricle

The fourth ventricle lies between the brain stem and the cerebellum (Figs. 5.10, 5.11). It is continuous rostrally with the cerebral aqueduct and caudally with the central canal of the spinal cord. In sagittal section, the fourth ventricle has a characteristic triangular profile, and the apex of its tented roof protrudes into the inferior aspect of the cerebellum. The ventricle is at its widest at the level of the pontomedullary junction, where a lateral recess on both sides extends to the lateral border of the brain stem. At this point, the lateral aperture of the fourth ventricle (foramen of Luschka) provides access to the subarachnoid space at the cerebellopontine angle, and CSF flows through it into the lateral extension of the pontine cistern. Occasionally, a lateral recess may not open.

Fig. 5.10 Dorsal aspect of the brain stem. The floor of the fourth ventricle has been exposed by cutting the cerebellar peduncles and removing the cerebellum.

(By permission from Mettler, F.A. 1948. Neuroanatomy, 2nd ed. CV Mosby, St. Louis.)

Fig. 5.11 Dorsal aspect of the brain stem, including the floor of the fourth ventricle.

The floor of the fourth ventricle is a shallow diamond-shaped or rhomboidal depression (rhomboid fossa) on the dorsal surfaces of the pons and the rostral half of the medulla. It consists largely of grey matter and contains important cranial nerve nuclei. The precise location of some nuclei is discernible from surface features. The superior part of the ventricular floor is triangular in shape and is limited laterally by the superior cerebellar peduncles as they converge toward the cerebral aqueduct. Its posterior limit is called the obex. The inferior part of the ventricular floor is also triangular in shape. It is bounded caudally by the gracile and cuneate tubercles, which contain the dorsal column nuclei, and more rostrally by the diverging inferior cerebellar peduncles. A longitudinal median sulcus divides the floor of the fourth ventricle. Each half is itself divided, by an often indistinct sulcus limitans, into a medial region known as the medial eminence and a lateral region known as the vestibular area. The vestibular nuclei lie beneath the vestibular area. In the superior part of the ventricular floor, the medial eminence is represented by the facial colliculus, a small elevation produced by an underlying loop of efferent fibres from the facial nucleus, which covers the abducens nucleus. Between the facial colliculus and the vestibular area, the sulcus limitans widens into a small depression, the superior fovea. In its upper part, the sulcus limitans constitutes the lateral limit of the floor of the fourth ventricle. Here a small region of bluish grey pigmentation denotes the presence of the subjacent locus coeruleus. Inferior to the facial colliculus, at the level of the lateral recess of the ventricle, a variable group of nerve fibre fascicles, known as the striae medullaris, runs transversely across the ventricular floor and passes into the median sulcus. In the inferior area of the floor of the fourth ventricle, the medial eminence is represented by the hypoglossal triangle (trigone), which lies over the hypoglossal nucleus. Laterally, the sulcus limitans widens to produce an indistinct inferior fovea. Caudal to the inferior fovea, between the hypoglossal triangle and the vestibular area, is the vagal triangle (trigone), which covers the dorsal vagal nucleus. The triangle is crossed below by a narrow translucent ridge, the funiculus separans, which is separated from the gracile tubercle by the small area postrema. The funiculus and area postrema are both covered by thickened ependyma containing tanycytes; the area postrema also contains neurones. The blood–brain barrier is modified in both sites.

The roof of the fourth ventricle is formed by the superior and inferior medullary veli. Superiorly, a thin sheet of tissue, the superior medullary velum, stretches across the ventricle between the converging superior cerebellar peduncles (see Fig. 5.10). The superior medullary velum is continuous with the cerebellar white matter and is covered dorsally by the lingula of the superior vermis. The inferior medullary velum is more complex and is composed mostly of a thin sheet, devoid of neural tissue, formed by ventricular ependyma and the pia mater of the tela choroidea, which covers it dorsally. A large median aperture (foramen of Magendie) is present in the roof of the ventricle as a perforation in the posterior medullary velum, just inferior to the nodule of the cerebellum. CSF flows from the ventricle through the foramen into the cerebellomedullary cistern.

Circumventricular Organs

The walls of the ventricular system are lined with ependymal cells, beneath which lies a subependymal layer of glia. At certain sites, collectively referred to as circumventricular organs (Fig. 5.12), specialized ependymal cells called tanycytes are also present. Ependyma and tanycytes may be involved in secretion into the CSF; transport of neurochemicals from subjacent neurones, glia or vessels to the CSF; transport of neurochemicals from CSF to the same subjacent structures; and chemoreception. In addition, in the adult, the ependymal and subependymal glial cell layers are the source of undifferentiated stem cells (Mercier, Kitasako, and Hatton 2002), currently under intensive study for their potential neurorestorative properties.

Fig. 5.12 Median sagittal section of the brain indicating the locations of the circumventricular organs.

The circumventricular organs are midline sites in the ventricular walls (McKinley et al 2003), where the blood–brain barrier is absent. They include the vascular organ (organum vasculosum), subfornical organ, neurohypophysis, median eminence, subcommissural organ, pineal gland and area postrema.

Choroid Plexus

In the roofs of the third and fourth ventricles and in the medial wall of the lateral ventricle along the line of the choroid fissure, the vascular pia mater lies in close apposition to the ependymal lining of the ventricles, without any intervening brain tissue. It forms the tela choroidea, which gives rise to the highly vascularized choroid plexuses from which CSF is secreted into the ventricles.

Choroid plexuses are located in the lateral ventricles, the third ventricle and the fourth ventricle (see Figs. 5.3, 5.5, 5.6).

In the lateral ventricle, the choroid plexus extends anteriorly as far as the interventricular foramen, through which it is continuous across the third ventricle with the plexus of the opposite lateral ventricle. From the interventricular foramen, the plexus passes posteriorly, in contact with the thalamus, curving around its posterior aspect to enter the inferior horn of the ventricle and reach the hippocampus. Throughout the body of the ventricle, the choroid fissure lies between the fornix superiorly and the thalamus inferiorly (see Fig. 5.6).

From above, the tela choroidea is triangular, with a rounded apex between the interventricular foramina, often indented by the anterior columns of the fornices (see Fig. 5.3). Its lateral edges are irregular and contain choroid vascular fringes. At the posterior basal angles of the tela, these fringes continue and curve into the inferior horn of the ventricle; centrally, the pial layers depart from each other as described earlier. When the tela is removed, a transverse slit (the transverse fissure) is left between the splenium and the junction of the ventricular roof and the tectum. It marks the posterior limit of the extracerebral space enclosed by the posterior extensions of the corpus callosum above the third ventricle. The latter contains the roots of the choroid plexus of the third ventricle and of the lateral ventricles, enclosed between the two layers of pia mater (see Fig. 5.6). The choroid plexus of the third ventricle is attached to the tela choroidea, which is, in effect, the thin roof of the third ventricle as it develops during fetal life. In coronal sections of the cerebral hemispheres, the choroid plexus of the third ventricle can be seen in continuity with the choroid plexus of the lateral ventricles (see Fig. 5.6).

The choroid plexus of the fourth ventricle is similar in structure to that of the lateral and third ventricles. Thus, the roof of the inferior part of the fourth ventricle develops as a thin sheet in which the pia mater is in direct contact with the ependymal lining of the ventricle. This thin sheet forms the tela choroidea of the fourth ventricle, lying between the cerebellum and the inferior part of the roof of the ventricle. The choroid plexus of the fourth ventricle is T-shaped, having vertical and horizontal limbs, but this form varies widely. The vertical (longitudinal) limb is double, flanks the midline and is adherent to the roof of the ventricle. The limbs fuse at the superior margin of the median aperture (foramen of Magendie) and are often prolonged on the ventral aspect of the cerebellar vermis. The horizontal limbs of the plexus project into the lateral recesses of the ventricle. Small tufts of plexus pass through the lateral apertures (foramina of Luschka) and emerge, still covered by ependyma, in the subarachnoid space of the cerebellopontine angle.

The blood supply of the choroid plexus in the tela choroidea of the lateral and third ventricles is usually via a single vessel from the anterior choroidal branch of the internal carotid artery and several choroidal branches of the posterior cerebral artery. The two sets of vessels anastomose to some extent. Capillaries drain into a rich venous plexus served by a single choroidal vein. The blood supply of the fourth ventricular choroid plexus is from the inferior cerebellar arteries.

Cerebrospinal Fluid

Hydrocephalus

Obstruction of the circulation of CSF leads to the accumulation of fluid (hydrocephalus), which causes compression of the brain (Fig. 5.13). Within the brain, critical points at which obstruction may occur correspond to the narrow foramina and passages of the ventricular system. Thus, obstruction of the interventricular foramen, as with an intraventricular tumour, causes enlargement of one or both lateral ventricles. Obstruction of the cerebral aqueduct, which may be congenital, due to atresia of the aqueduct, or acquired, as in ependymitis accompanying chronic infection (e.g. tuberculous meningitis), leads to enlargement of both lateral ventricles and the third ventricle. Obstruction or congenital absence of the apertures of the fourth ventricle leads to enlargement of the entire ventricular system. Obstruction or restriction of CSF circulation can also occur within the subarachnoid space as a result of meningeal adhesions caused by meningitis. When this occurs at the level of the tentorial notch, passage of CSF from the posterior fossa to its sites of reabsorption is restricted.

Fig. 5.13 MRI scan shows an enhancing mass, which is a meningioma growing from the meninges at the edge of the foramen magnum. The tumour is benign but is compressing the brain stem and causing secondary hydrocephalus.

(Courtesy of Professor Alan Jackson, Department of Neuroradiology, University of Manchester, United Kingdom.)

CASE 3 TUBERCULOUS MENINGITIS WITH OBSTRUCTIVE HYDROCEPHALUS

A 16-year-old boy experiences increasing bifrontal headaches for 6 weeks and then develops recurrent vomiting, diplopia and an unsteady gait. Shortly before hospitalization he becomes confused, then stuporous. At the time of admission to the hospital, he exhibits a stiff neck with signs of meningeal irritation, mild facial diplegia, bilateral sixth nerve palsies and papilledema. Cerebellar function cannot be assessed. Reflexes are exaggerated throughout; the plantar responses are silent. He has a low-grade fever, and chest X-ray demonstrates several lesions in both lung fields, consistent with a diagnosis of tuberculosis. Magnetic resonance imaging (MRI) of the head shows enlargement of the lateral third ventricles, along with contrast enhancement of the basal meninges. Spinal fluid is under increased pressure and has a cloudy, opalescent appearance. The CSF clots on standing and contains 125 leukocytes per cubic millimetre, predominantly lymphocytes; it has a protein content of 400 mg% and a sugar content of less than 20 mg%. Within 48 hours, his CSF polymerase chain reaction (PCR) is reported as consistent with tuberculosis, and the CSF culture subsequently yields acid-fast organisms.

Discussion: This is the typical appearance and course of tuberculous meningitis; the CSF findings themselves are characteristic of a subacute to chronic bacterial meningitis. Tuberculous meningitis must be differentiated from other chronic meningitides, such as syphilitic or cryptococcal. Some viral infections, in particular herpes and mumps, may produce a similar set of changes within the spinal fluid. Metastatic leptomeningeal invasion and sarcoid must also be considered in the differential diagnosis. Bacteriological studies are diagnostic.