LATERAL VENTRICLE

Viewed from its lateral aspect, the lateral ventricle has a roughly C-shaped profile. The shape is a consequence of the developmental expansion of the frontal, parietal and occipital regions of the hemisphere, 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 that the tail of the caudate nucleus encircles the thalamus in a C-shape, and the fornix traces the outline of the ventricle forwards to the interventricular foramen.

The lateral ventricle is customarily divided into a body and anterior (frontal), posterior (occipital) and inferior (temporal) horns. The anterior 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 (Figs 16.2, 16.3). The anterior horn extends back as far as the interventricular foramen. 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 lateral wall of the body of the ventricle is formed by the caudate nucleus superiorly and the thalamus inferiorly. The boundary between the thalamus and caudate nucleus is marked by a groove (Fig. 16.2) which is occupied by a fascicle of nerve fibres, the stria terminalis, and by the thalamostriate vein (see Fig. 22.5). The inferior limit of the body of the ventricle and its medial wall are formed by the body of the fornix (Fig. 16.4). The fornix is separated from the thalamus by the choroidal fissure. The choroid plexus occludes the choroidal 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 horn curves posteromedially into the occipital lobe. It is usually diamond-shaped or square in outline, and the two sides are often asymmetrical. 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 horn is the largest compartment of the lateral ventricle and extends forwards into the temporal lobe. It curves round the posterior aspect of the thalamus (pulvinar), passes downwards and posterolaterally and then 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 forwards 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. 16.5). The temporal extension of the choroid plexus fills this fissure and covers the outer surface of the hippocampus.

Fig. 16.2 Horizontal section of the cerebrum dissected to remove the roofs of the lateral ventricles.

Fig. 16.3 Coronal MRI scan, at the level of the anterior horn of the lateral ventricle.

Fig. 16.4 The lateral and third ventricles, coronal section.

Fig. 16.5 The inferior horn of the lateral ventricle, coronal section.

THIRD VENTRICLE

The third ventricle is a midline, slit-like cavity which is derived from the primitive forebrain vesicle (Figs 16.1, 16.4, 16.6, 16.7; see Fig. 15.8). 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, and marks 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 which extends from one thalamus to the other.

Fig. 16.6 Sagittal MRI scan of the head.

Fig. 16.7 The third and fourth ventricles, sagittal hemisection of the brain.

Anteriorly, the third ventricle extends to the lamina terminalis (Fig. 16.7). 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 orientated anterior commissure, which crosses the midline. The anterior and posterior commissures are important neuroradiological landmarks. Prior to the introduction of modern imaging techniques, the fact that these commissures could be identified by ventriculography led to their use as reference markers for stereotaxic surgical procedures. This convention is now universal: the positions of the anterior and posterior commissures are used as the basic reference points for 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 choroidal fissure (Fig. 16.4). Above the roof is the body of the fornix. The posterior boundary of the ventricle is marked by a suprapineal recess above the pineal gland, a pineal (epiphysial) recess, which 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 1–2 mm in diameter. It extends throughout the dorsal quarter of the midbrain in the midline and is surrounded by the periaqueductal (central) grey matter (Fig. 16.7; see Fig. 19.19). 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.

FOURTH VENTRICLE

The fourth ventricle lies between the brain stem and the cerebellum (Figs 16.1B, 16.6, 16.7, 16.8; see Fig. 19.3). Rostrally it is continuous 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; CSF flows through it into the lateral extension of the pontine cistern. Occasionally, a lateral recess may not open.

Fig. 16.8 The floor of the fourth ventricle, dorsal aspect of the brain stem.

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 towards the cerebral aqueduct. Its posterior limit is called the obex. The inferior part of the ventricular floor is also triangular in shape and 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 motor nucleus of the vagus. 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 at 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 (Fig. 16.8). 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 mostly composed 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 through the foramen from the ventricle into the cisterna magna.

CIRCUMVENTRICULAR ORGANS

The walls of the ventricular system are lined with ependymal cells covering a subependymal layer of glia. At certain midline sites in the ventricular walls, collectively referred to as circumventricular organs, the blood–brain barrier is absent and specialized ependymal cells called tanycytes are present (Fig. 16.9). The functions of ependyma and tanycytes may include secretion into the CSF; transport of neurochemicals from subjacent neurones, glia or vessels to the CSF; transport of neurochemicals from the CSF to the same subjacent structures; chemoreception. In the adult, the ependymal and subependymal glial cell layers are the source of undifferentiated stem cells that are currently under intensive study for their potential neurorestorative properties.

Fig. 16.9 The locations of the circumventricular organs, median sagittal section of the brain.

The circumventricular organs include the vascular organ (organum vasculosum), subfornical organ, neurohypophysis, median eminence, subcommissural organ, pineal gland and area postrema.

The vascular organ lies in the lamina terminalis between the optic chiasma and the anterior commissure. Its external zone contains a richly fenestrated vascular plexus, which covers glia and a network of nerve fibres. The ependymal cells of the vascular organ, like those of other circumventricular organs, are flattened and have few cilia. The major inputs appear to come from the subfornical organ, locus coeruleus and a number of hypothalamic nuclei; the vascular organ projects to the median preoptic and supraoptic nuclei. The vascular organ is involved in the regulation of fluid balance and may also have neuroendocrine functions.

The subfornical organ lies at the level of the interventricular foramen. It contains many neurones, glial cells and a dense fenestrated capillary plexus, and is covered by flattened ependyma. It is believed to have widespread hypothalamic interconnections and to function in the regulation of fluid balance and drinking.

The neurohypophysis (posterior pituitary) is the site of termination of neurosecretory projections from the supraoptic and paraventricular nuclei of the hypothalamus. Neurones in these nuclei release vasopressin and oxytocin respectively into the capillary bed of the neurohypophysis, where the hormones gain access to the general circulation.

The median eminence contains the terminations of axons of hypothalamic neurosecretory cells. Peptides released from these axons control the hormonal secretions of the anterior pituitary via the pituitary portal system of vessels.

The subcommissural organ lies ventral to and below the posterior commissure, near the inferior wall of the pineal recess. The ependymal cells on the dorsal aspect of the cerebral aqueduct are tall, columnar and ciliated, with granular basophilic cytoplasm: they may be involved in the secretion of materials into the CSF from adjoining axonal terminals or capillaries.

The pineal gland (see Ch. 21) is a small structure approximately 8 mm in diameter situated rostro-dorsal to the superior colliculus and behind the stria medullaris. It is part of the epithalamus and consists of internal lobules of pinealocytes and sparse neurones covered by a pial capsule. It secretes melatonin which appears to be involved in the inhibition of puberty in children and in the in the regulation of the sleep–wake cycle in both children and adults.

The area postrema is a bilaterally paired structure, located at the caudal limit of the floor of the fourth ventricle. It is an important chemoreceptive area that triggers vomiting in response to the presence of emetic substances in the blood. The area postrema, along with the nucleus of the solitary tract and the dorsal motor nucleus of the vagus, makes up the so-called dorsal vagal complex, which is the major termination site of vagal afferent nerve fibres.

CHOROID PLEXUS

The vascular pia mater 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, is closely apposed 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 lateral, third and fourth ventricles (Figs 16.2, 16.4, 16.5). The body or stroma of the choroid plexus consists of many capillaries, separated from the subarachnoid space by the pia mater and choroid ependymal cells.

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 round 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 (Fig. 16.4).

From above, the tela choroidea is triangular with a rounded apex between the interventricular foramina, often indented by the anterior columns of the fornices. Its lateral edges are irregular, and contain choroid vascular fringes. At the posterior basal angles of the tela, these fringes continue and curve on into the inferior horn of the ventricle. When the tela is removed, a transverse slit (the transverse fissure) is left between the splenium and the junction of the ventricular roof with the tectum. The transverse fissure contains the roots of the choroid plexus of the third ventricle and of the lateral ventricles.

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.

The choroid plexus of the fourth ventricle is similar in structure to that of the lateral and third ventricles. 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, the tela choroidea of the fourth ventricle, lies between the cerebellum and the inferior part of the roof of the ventricle. The choroid plexus of the fourth ventricle is T-shaped, with vertical and horizontal limbs but the precise form varies widely from a single vertical limb to an elongated ‘T’ which extends out through the foramina of Luschka into the cerebellopontine angle 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 to 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

CSF is a clear, colourless, liquid. In normal individuals CSF contains a very small amount of protein and differs from blood in its electrolyte content. It is not simply an ultrafiltrate of blood but is actively secreted by the choroid plexus epithelium. Choroid plexus epithelial cells have the characteristics of transport and secretory cells. Their apical surfaces, from which CSF is secreted, possess microvilli, and their basal surfaces exhibit interdigitations and folding. There are tight junctions at the apical ends of the epithelial cells, which are permeable to small molecular weight substances. Fenestrated capillaries in the stroma of the choroid plexus lie just beneath the epithelial cells. A blood–CSF barrier is sited at the choroid plexus epithelium.

Most of the CSF is secreted by the choroid plexuses in the lateral, third and fourth ventricles. However, there is also a small contribution from the ependymal lining of the ventricles and from the extracellular fluid from the brain parenchyma.

SUBARACHNOID SPACE

The subarachnoid space lies between the arachnoid and the pia mater. The cerebral subarachnoid space is connected with the fourth ventricle of the brain by three openings. The median aperture (foramen of Magendie) lies in the median plane in the inferior part of the roof of the fourth ventricle and provides communication with the cisterna magna. The paired lateral apertures (foramina of Luschka) are located at the ends of the lateral recesses of the fourth ventricle, and open into the subarachnoid space at the cerebellopontine angle, behind the upper roots of the glossopharyngeal nerves. The subarachnoid space therefore contains cerebrospinal fluid (CSF); it also contains the larger arteries and veins which traverse the surface of the brain (Fig. 16.10).

Fig. 16.10 The relationships between the dura, the leptomeninges and the blood vessels that enter and leave the cerebral cortex. The subarachnoid space is divided by trabeculae.

Trabeculae, in the form of sheets or fine filiform structures, traverse the subarachnoid space from the deep layers of the arachnoid mater to the pia mater and are also attached to large blood vessels within the subarachnoid space (Fig. 16.10). Each trabecula has a core of collagen and is coated by leptomeningeal cells. The trabeculae that cross the subarachnoid space may form compartments, particularly in the perivascular regions, which may facilitate directional flow of CSF through the subarachnoid space.

Arteries and veins in the subarachnoid space are coated by a thin layer of leptomeninges, often only one cell thick. Cranial and spinal nerves that traverse the subarachnoid space to pass out of cranial or intervertebral foramina are coated by a thin layer of leptomeninges which fuses with the arachnoid at the exit foramina.

Arachnoid and pia mater are in close apposition over the convexities of the brain, such as the cortical gyri, whereas concavities are followed by the pia but spanned by the arachnoid. This arrangement produces a subarachnoid space of greatly variable depth that is location dependent. The more expansive spaces are identified as subarachnoid cisterns (Fig. 16.11), which are continuous with each other through the general subarachnoid space. Subarachnoid trabeculae that cross the subarachnoid cisterns are long and filamentous.

Fig. 16.11 The principal subarachnoid cisterns, sagittal section.

The largest cistern, the cisterna magna or cerebellomedullary cistern, is formed where the arachnoid bridges the interval between the medulla oblongata and the inferior surface of the cerebellum. The cistern is continuous above with the lumen of the fourth ventricle through its median aperture, the foramen of Magendie, and below with the subarachnoid space of the spinal cord. The pontine cistern is an extensive space ventral to the pons, that is continuous below with the spinal subarachnoid space, behind with the cisterna magna and, rostral to the pons, with the interpeduncular cistern. The basilar artery runs through the pontine cistern into the interpeduncular cistern. As the arachnoid mater spans between the two temporal lobes it is separated from the cerebral peduncles and structures within the interpeduncular fossa by the interpeduncular cistern, which contains the circulus arteriosus (circle of Willis). Anteriorly, the interpeduncular cistern extends to the optic chiasma. The cistern of the lateral fossa is formed by the arachnoid as it bridges the lateral sulcus between the frontal, parietal and temporal opercula; it contains the middle cerebral artery. The cistern of the great cerebral vein (cisterna ambiens or superior cistern) lies posterior to the brain stem and third ventricle, and occupies the interval between the splenium of the corpus callosum and the superior cerebellar surface. The great cerebral vein traverses this cistern and the pineal gland protrudes into it.

Several smaller cisterns have been described, including the prechiasmatic and postchiasmatic cisterns, which are related to the optic chiasma, and the cistern of the lamina terminalis and the supracallosal cistern, all of which are extensions of the interpeduncular cistern and contain the anterior cerebral arteries. The subarachnoid space also extends through the optic foramen into the orbit where it is bounded by the optic nerve sheath. The latter is formed by fusion of the arachnoid and dura mater, and surrounds the orbital portion of each optic nerve as far as the back of the globe, where the dura fuses with the sclera of the eye.

CIRCULATION OF CEREBROSPINAL FLUID

The total CSF volume is approximately 150 ml, of which 125 ml is intracranial. The ventricles contain about 25 ml, most lying within the lateral ventricles, and the remaining 100 ml is located in the cranial subarachnoid space. CSF is secreted at a rate of 0.35–0.40 ml per minute, which means that normally about 50% of the total volume of CSF is replaced every five to six hours. An effective means of removal from the cranial cavity is thus essential. CSF flows from the lateral ventricles to the third ventricle and then through the cerebral aqueduct to the fourth ventricle. Mixing of CSF from different choroidal sources occurs and is probably assisted by cilia on the ependymal cells lining the ventricles and by arterial pulsations. CSF leaves the fourth ventricle through the medial and lateral apertures to enter the subarachnoid space of the cisterna magna and the subarachnoid cisterns over the front of the pons respectively. The movement of CSF in the subarachnoid space is complex and is characterized by a fast flow component and a much slower bulk flow component. During systole, the major arteries lying in the basal cisterns and other subarachnoid spaces dilate significantly and exert pressure effects on the CSF, causing rapid CSF flow around the brain and out of the cranial cavity into the upper cervical vertebral canal. The pressure wave which causes this outflow of CSF is dispersed through the spinal CSF space, which acts as a capacitance vessel, and is transmitted to the dural venous sinuses through the arachnoid granulations. As the blood within the major arteries passes into the brain in late systole and diastole, CSF re-enters the skull from the spine. This CSF flow occurs at rapid rates and is repeated during every heart cycle. In addition, there is a slow bulk flow of CSF, with a time course measured in hours, which results in circulation of CSF over the cerebral surface in a superolateral direction. CSF is absorbed into the venous system by active diffusion into cerebral capillaries which occurs as a result of the CSF/interstitial pressure differential.

Arachnoid villi and granulations

Arachnoid granulations are focal outpushings of the arachnoid mater and subarachnoid space through the wall of dural venous sinuses, very often close to points where veins enter the sinus: granulations have been reported in the superior sagittal, transverse, superior petrosal and straight sinuses in decreasing frequency. They are most prominent along the margins of the great longitudinal fissure, commonly in the lateral lacunae of the superior sagittal sinus, and also protruding directly into the lumen of the sinus (see Ch. 27). They increase in number and size with increasing age and may appear as focal filling defects in the venous sinuses on angiograms and contrast enhanced CT scans and focal signal alterations in the venous sinuses on MR examination.

Arachnoid villi and arachnoid granulations differ in terms of size and complexity. Villi are microscopic structures that are present in the superior sagittal sinus of the fetus and newborn infant. Granulations are visible to the naked eye by the age of 18 months in the parieto-occipital region of the superior sagittal sinus, and by the age of 3 years in the laterally located sinuses of the posterior fossa; they become more lobulated and complex with increasing age, and may become calcified, when they are known as Pacchionian bodies.

At the base of each arachnoid granulation, a thin neck of arachnoid mater projects through an aperture in the dural lining of the venous sinus and expands to form a core of collagenous trabeculae and interwoven channels (Fig. 16.12). The core is surmounted by an apical cap of arachnoid cells, some 150 μm thick. Channels extend through the cap to reach the subendothelial regions of the granulation. The cap region of each granulation is attached to the endothelium of the sinus over an area some 300 μm in diameter, whereas the rest of the core of the granulation is separated from the endothelium by a fibrous dural cupola.

Fig. 16.12 An arachnoid granulation.

(Modified by permission from Springer-Verlag, Principles of Pediatric Neurosurgery, Vol 4: Morphology of CSF drainage pathways in man. Raimondi (ed), Kida and Weller 1994.)

The function of arachnoid granulations remains controversial. It has long been thought that arachnoid villi and arachnoid granulations constitute the major pathway for the resorption of CSF from the subarachnoid space into the blood, although the relative contribution each might make to the process is not known and the exact mechanism of resorption has not been determined. Recent studies using radionuclide cisternography and cardiac gated MRI have shown that CSF may also be resorbed throughout the ventricles and subarachnoid space, through the walls of pial and subarachnoid vessels. Moreover, there is a growing body of experimental evidence that significant volumes of CSF may be absorbed into extracranial lymphatics in animals: radiolabelled proteins injected into the CSF (rabbits, cats), brain parenchyma (rabbits) or ventricles (sheep) have been recovered from cervical lymph, possibly by traveling along extensions of the subarachnoid space that surround the olfactory and optic nerves. It has been suggested that extracranial lymphatics play a major role in CSF reabsorption at relatively low pressures, but that other routes, including arachnoid granulations, become increasingly important as CSF pressure rises. A connection between parenchymal interstitial fluid and extracranial lymphatics in humans has yet to be convincingly demonstrated.

An alternative, and not necessarily mutually exclusive, view of arachnoid granulations is that they are involved in dissipating the pressure wave which occurs in the subarachnoid space during arterial systole (caused by the pulsation of the arterial vessels within the space). Since the dural venous sinuses lie within thick non-compliant dural coverings, pressure changes within them functionally represent the extracranial venous compartment: systolic dilatation of the arachnoid granulations would therefore dissipate intracranial pressure changes into the extracranial venous system.

Hydrocephalus

Obstruction of the circulation of CSF leads to accumulation of fluid within the ventricular system (hydrocephalus), which causes compression of the brain (Fig. 16.13). 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 causes enlargement of the lateral ventricles; obstruction of the cerebral aqueduct leads to enlargement of both the lateral ventricles and the third ventricle; and obstruction or congenital absence of the apertures of the fourth ventricle leads to enlargement of the entire ventricular system. Hydrocephalus may also occur in the presence of intact communications between the ventricular and subarachnoid spaces (communicating hydrocephalus). The conventional view is that obstructions within the subarachnoid space impair CSF circulation and lead to CSF malabsorption, causing either obstructive or communicating hydrocephalus. Analyses of the intracranial hydrodynamics related to the pulse pressure have suggested that this is an oversimplification, and that increased pulse pressure in the brain capillaries maintains the ventricular enlargement in communicating hydrocephalus and chronic obstructive hydrocephalus (see Greitz 2004 for further details of the haemodynamic theory).

Fig. 16.13 T2 weighted sagittal image of the brain in a patient with obstructive hydrocephalus due to a congenital stenosis of the cerebral aqueduct. The image shows marked dilatation of the third ventricle and proximal aqueduct.