Meninges and ventricular system

The meninges consist of three concentric membranes that envelop the brain and spinal cord, supporting and protecting the delicate tissues they surround. The individual layers, from outside to inside, are the dura mater (pachymeninx), arachnoid mater and pia mater. The dura is an opaque, tough, fibrous coat that incompletely divides the cranial cavity into compartments and accommodates the dural venous sinuses. The arachnoid is much thinner than the dura and is mostly translucent. It loosely surrounds the brain, cranial nerves and vessels and spans fissures and sulci. The pia mater is a transparent, microscopically thin membrane that follows the contours of the brain and is closely adherent to its surface.

The dura is separated from the arachnoid by a narrow subdural space. The arachnoid is separated from the pia by the subarachnoid space, which varies greatly in depth; the larger expanses are termed subarachnoid cisterns. The subarachnoid space contains cerebrospinal fluid (CSF) which is secreted by the choroid plexuses of the cerebroventricular system, circulates within the subarachnoid space and is reabsorbed into the venous system through arachnoid villi associated with the dural venous sinuses.

Cranial and spinal meninges are continuous through the foramen magnum. The cranial meninges are described in this section and the spinal meninges are described in Chapter 47 .

Dura Mater

The dura mater is largely acellular and consists mainly of densely packed fascicles of collagen fibres arranged in laminae. The fascicles run in different directions in adjacent laminae, producing a lattice-like appearance that is particularly obvious in the tentorium cerebelli and around the defects or perforations that sometimes occur in the anterior portion of the falx cerebri.

The cranial dura differs from the spinal dura mainly in its relationship to the surrounding bones. It lines the cranial cavity and has an inner, meningeal, layer and an outer, endosteal layer. The two membranes are physically joined only at sites where veins pass from the brain into venous sinuses, e.g. the superior sagittal sinus. There is little histological difference between the layers: both contain fibroblasts, and the endosteal layer also contains osteoblasts. Focal calcification may occur in the falx cerebri and near the superior sagittal sinus. As a general rule, the meningeal layer covers neural tissues. It provides tubular sheaths for the cranial nerves as they pass out through the cranial foramina: the sheaths fuse with the epineurium as the nerves emerge from the skull. The dura fuses with the adventitia of major vessels, such as the internal carotid and vertebral arteries, at sites where they pierce it to enter the cranial cavity. The dural sheath of the optic nerve is continuous with the ocular sclera. The endosteal layer of dura adheres to the internal surfaces of the cranial bones, particularly at the sutures and the cranial base, and around the foramen magnum. It is continuous with the pericranium through the cranial sutures and foramina and with the orbital periosteum through the superior orbital fissure. Fibrous bands pass from the dura into the bones, so that it is difficult to remove the dura from the suture lines in young skulls: as the suture lines fuse, the dura becomes separated from them. The meningeal layer of dura is closely applied to the arachnoid mater over the surface of the brain: the layers are united except where they separate to enclose the venous sinuses that drain blood from the brain.

Extradural haematoma

The anatomical organization of the dura, and its relationships to the major venous sinuses, sutures and blood vessels, are clinically important. Separation of the dura from the cranial bones requires significant force, and consequently happens when high-pressure arterial bleeding takes place into the extradural or epidural space: this can result from damage to any meningeal vessel, commonly following skull fracture. The classic site for such injury is along the course of the middle meningeal artery, where a direct blow causing a fracture of the temporal and/or parietal bones can rupture the artery and cause rapid collection of blood into the extradural space. An extradural haematoma therefore acts as a rapidly expanding intracranial mass lesion that causes acute brain compression and displacement: it is a classic medical emergency that requires immediate diagnosis and surgery through a craniotomy for epidural blood clot evacuation and coagulation of the ruptured vessel ( Fig. 25.1 ). FLOAT NOT FOUND

$Fig. 25.1, A , A head CT scan showing a right-sided extradural (epidural) haematoma. The blood clot is biconvex. B , A bone flap from the same patient. Note the ramifying grooves in the inner table of the squamous parts of the temporal and parietal bones. A fracture line crossing these grooves has torn branches of the middle meningeal artery. C , An acute subdural haematoma. The crescent-shaped blood clot is causing a severe midline shift and brain herniation. D , A bilateral subacute subdural haematoma. E , A right-sided chronic subdural haematoma.$

Dural partitions

The meningeal layer of the dura is reflected inwards to form four septa, the falx cerebri, falx cerebelli, diaphragma sellae and tentorium cerebelli, that partially divide the cranial cavity into compartments.

Falx cerebri

The falx cerebri is a strong, crescent-shaped sheet that lies in the sagittal plane and occupies the longitudinal fissure between the two cerebral hemispheres ( Fig. 25.2 ). The crescent is narrow anteriorly, where the falx is fixed to the crista galli, and broad posteriorly, where it blends with the tentorium cerebelli: the straight sinus runs along this line of attachment to the tentorium (see Fig. 25.2 ). The anterior part of the falx is thin and may have a number of irregular perforations. Its convex upper margin is attached to the internal cranial surface on each side of the midline, as far posteriorly as the internal occipital protuberance. The superior sagittal sinus runs in a cranial groove within the dura along this margin, and the falx cerebri is attached to the lips of this groove. The lower edge of the falx is free and concave, and contains the inferior sagittal sinus.

Fig. 25.2, A , The cerebral dura mater, its reflections and associated major venous sinuses. B , An anatomical dissection of a silicon-injected specimen.

Falx cerebelli

The falx cerebelli is a small midline fold of dura mater that lies below the tentorium cerebelli and projects forwards into the posterior cerebellar notch between the cerebellar hemispheres. Its base is directed upwards and is attached to the posterior part of the inferior surface of the tentorium cerebelli in the midline. Its posterior margin contains the occipital sinus and is attached to the internal occipital crest. The lower apex of the falx cerebelli frequently divides into two small folds that disappear at the sides of the foramen magnum.

Diaphragma sellae

The diaphragma sellae is a small, circular, horizontal sheet of dura mater that forms a roof over the sella turcica and covers the pituitary gland (hypophysis). The infundibulum (pituitary stalk) passes into the pituitary fossa through a central opening in the diaphragm: there is wide individual variation in the size of this opening. The diaphragma sellae is an important landmark in pituitary surgery because large pituitary tumours extend above it and may adopt a characteristic dumbbell shape. A trans-sphenoidal approach is currently the preferred option for accessing pituitary tumours, irrespective of whether there is suprasellar extension beyond the diaphragma sellae.

Tentorium cerebelli

The shape of the tentorium cerebelli (see Fig. 25.2B ) is reminiscent of a single-poled tent, as its name suggest. It lies between the cerebellum and the occipital lobes of the cerebral hemispheres, and divides the cranial cavity into supratentorial and infratentorial compartments that contain the forebrain and hindbrain, respectively. Its concave anterior edge is free and separated from the dorsum sellae of the sphenoid bone by a large curved hiatus, the tentorial incisure or notch, which is occupied by the midbrain and the anterior part of the superior aspect of the cerebellar vermis. The convex outer limit of the tentorium is attached posteriorly to the lips of the transverse sulci of the occipital bone and to the posterior inferior angles of the parietal bones, where it encloses the transverse sinuses. Laterally, it is attached to the superior borders of the petrous parts of the temporal bones, where it contains the superior petrosal sinuses. On each side, near the apex of the petrous temporal bone, the lower layer of the tentorium is evaginated anterolaterally under the superior petrosal sinus to form a recess, Meckel’s cave, between the endosteal and meningeal dural layers in the middle cranial fossa. The cave is entered by the posterior root of the trigeminal nerve and contains cerebrospinal fluid and the trigeminal ganglion; the evaginated meningeal layer fuses anteriorly with the anterior part of the ganglion.

The arrangement of the dura mater in the central part of the middle cranial fossa is complex. The tentorium forms the medial part of the floor of the middle cranial fossa. On both sides, the rim of the tentorial incisure is attached to the apex of the petrous temporal bone. It continues forwards as a ridge of dura, the anterior petroclinoidal ligament, that is attached to the anterior clinoid process: this ligament marks the junction of the roof and the lateral wall of the cavernous sinus. The periphery of the tentorium cerebelli (attached to the superior border of the petrous temporal bone), crosses under the free border of the tentorial incisure at the apex of the petrous temporal bone, and continues forwards to the posterior clinoid process as a rounded ridge of dura mater, the posterior petroclinoidal ligament. The dural extension between the anterior and posterior petroclinoidal ligaments forms the roof of the cavernous sinus. On either side, it is pierced superiorly by the oculomotor nerve and posteriorly by the trochlear nerve: both nerves proceed anteroinferiorly into the lateral wall of the cavernous sinus.

In the anteromedial part of the middle cranial fossa, the dura ascends as the lateral wall of the cavernous sinus. It reaches the ridge produced by the anterior petroclinoidal ligament and runs medially as the roof of the cavernous sinus, where it is pierced by the internal carotid artery. The interclinoidal ligament, between the anterior and posterior clinoid processes, forms the medial limit of the roof of the cavernous sinus and continues medially with the diaphragma sellae. At, or just below, the opening in the diaphragma for the pituitary stalk, the dura of the diaphragma and the suprasellar arachnoid blend with each other and with the capsule of the pituitary gland: the subarachnoid space does not extend into the sella turcica.

Transtentorial coning

Normally, the arrangement of dural partitions such as the falx cerebri and tentorium cerebelli may help to stabilize the brain within the cranial cavity. However, when there is focal brain swelling or a focal space-occupying lesion within the brain or cranial cavity, mass effect and raised intracranial pressure may cause the brain to herniate under the falx cerebri or, more significantly, through the tentorial incisure. In this case, the medial temporal lobe, and particularly the uncus, will compress the oculomotor nerve, midbrain and the posterior cerebral arteries. This life-threatening event and neurosurgical emergency, occurring in patients with supratentorial space-occupying lesions, is known as transtentorial uncal herniation. Similarly, space-occupying lesions in the smaller infratentorial compartment may cause upward herniation of the cerebellar vermis through the tentorial hiatus (upward transtentorial herniation) or downward herniation of the cerebellar tonsils through the foramen magnum (tonsillar herniation) (see Fig. 28.23 ); these neurosurgical emergencies require a suboccipital craniectomy.

Innervation of the cranial dura mater

The cranial dura mater is innervated mainly by branches of the three divisions of the trigeminal nerve, the second and third cervical spinal nerves and the cervical sympathetic trunk ( Fig. 25.3 ) .

Fig. 25.3, The innervation of the cranial meninges.

Fig. 25.3, B , The innervation of the cranial meninges.

Less well established meningeal branches have been described arising from the vagus and hypoglossal nerves, and possibly from the facial and glossopharyngeal nerves.

In the anterior cranial fossa, the dura is innervated by meningeal branches of the anterior and posterior ethmoidal nerves and anterior filaments of the meningeal rami of the maxillary (nervus meningeus medius) and mandibular (nervus spinosus) divisions of the trigeminal nerve. Nervi meningeus medius and spinosus are largely distributed to the dura of the middle cranial fossa, which also receives filaments from the trigeminal ganglion. The nervus spinosus re-enters the cranium through the foramen spinosum with the middle meningeal artery, and divides into anterior and posterior branches that accompany the main divisions of the artery and supply the dura mater in the middle cranial fossa and, to a lesser extent, the anterior fossa and calvarium. The anterior branch communicates with the meningeal branch of the maxillary nerve. The posterior branch also supplies the mucous lining of the mastoid air cells. The nervus spinosus contains sympathetic postganglionic fibres from the middle meningeal plexus. The nervus tentorii, a recurrent branch of the intracranial portion of the ophthalmic division of the trigeminal nerve, supplies the supratentorial falx cerebri and the tentorium cerebelli. Intraoperative mechanical stimulation of the falx may trigger the trigeminocardiac reflex ( ). The space between the anterior half of the straight sinus and the medial tentorial notch can be considered a safe surgical area where innervation is scarce, whereas the entire region of the transverse sinus and the posterior half of the straight sinus are densely innervated by the nervus tentorii ( ). The dura in the posterior cranial fossa is innervated by ascending meningeal branches of the upper cervical nerves that enter through the anterior part of the foramen magnum (second and third cervical nerves) and through the hypoglossal canal and jugular foramen (first and second cervical nerves).

Meningeal branches from the vagus nerve apparently originate from the superior vagal ganglion and are distributed to the dura mater in the posterior cranial fossa. Meningeal branches from the hypoglossal nerve leave the nerve in its canal and supply the diploë of the occipital bone, the dural walls of the occipital and inferior petrosal sinuses, and much of the floor and anterior wall of the posterior cranial fossa. These meningeal rami may not contain vagal or hypoglossal fibres but ascending, mixed sensory and sympathetic fibres from the upper cervical nerves and superior cervical sympathetic ganglion. All meningeal nerves contain a postganglionic sympathetic component, either from the superior cervical sympathetic ganglion or by communication with its perivascular intracranial extensions. The role of the autonomic nerve supply of the cranial dura mater is uncertain.

Sensory nerve endings are restricted to the dura mater and cerebral blood vessels, and are not found in either the brain itself, or in the arachnoid or pia mater. Stimulation of these nerve endings causes pain, as evidenced during awake craniotomy procedures, and is the basis of certain forms of headache. The density of dural innervation, particularly around the dural venous sinuses, increases during the later part of gestation, peaking at term and subsequently decreasing during the first year of postnatal life ( ).

Dural venous sinuses

Dural venous sinuses form a complex network of venous channels that drain blood from the brain and cranial bones ( Fig. 25.4 ; see Figs 25.2 , 26.10, 26.11, 26.13 ). They lie between the endosteal and meningeal layers of dura mater, are lined by endothelium, lack valves and their walls are devoid of muscular tissue ( ). They develop initially as venous plexuses and, to a variable degree, most adult sinuses retain that form. Plexiform arrays of small veins adjoin the superior and inferior sagittal and straight sinuses, and, less frequently, the transverse sinuses. Ridges of ‘spongy’ venous tissue, venous lacunae, often project into the lumina of the superior sagittal and transverse sinuses ( , ).

Fig. 25.4, The major venous sinuses at the base of the skull. A , The sinuses coloured dark blue have been opened up. B , C . Digital subtraction angiography, venous phase. B , Sagittal view.

The plexiform nature of the cranial venous sinuses and their wide connections with cerebral and cerebellar veins vary considerably, particularly in earlier years, e.g. in infancy, the falx cerebelli may contain large plexiform channels and venous lacunae that augment the occipital sinus. These variations must be established for each patient by catheter-based angiography, magnetic resonance (MR) venography or computed tomographic (CT) venography, when clinical necessity arises. The existence of arteriovenous shunts has been inferred from the demonstration of a connection between the middle meningeal arteries and the superior sagittal sinus ( ). Dural arteriovenous fistulae are thought to be acquired lesions that form in an area of thrombosis within a sinus. If the sinus remains completely thrombosed, venous drainage from these lesions takes place through cortical veins: when the sinus remains closed, the elevated pressure within the cortical veins receiving the arterial flow carries the risk of haemorrhage. If the sinus is partially open, venous drainage is usually into the involved sinus.

Fig. 25.4, C , Posterior view; note the asymmetry of the transverse sinuses.

The named sinuses are the superior and inferior sagittal, straight, transverse, sigmoid, occipital, cavernous, intercavernous, superior and inferior petrosal, sphenoparietal, basilar and marginal.

Superior sagittal sinus

The superior sagittal sinus runs in the attached convex margin of the falx cerebri. It grooves the internal surface of the frontal bone, the adjacent margins of the two parietal bones and the squamous part of the occipital bone ( Fig. 25.5 ; see Figs 25.2 , 26.10 , 26.11B,C , 26.13 ). It begins near the crista galli, a few millimetres posterior to the foramen caecum, and receives primary tributaries from the cortical veins draining the superior part of the frontal, parietal and occipital lobes, and the anterior part of the orbital surface of the frontal lobe. The sinus is triangular in cross-section, with the apex directed downwards, and is continuous with the falx cerebri. Its lumen is invaded in its intermediate third by projections from its dural walls that may divide the lumen into superior and inferior channels. It is narrow anteriorly, and widens gradually to approximately 1 cm in diameter as it runs posteriorly to enter the confluence of the sinuses, situated to one side (usually the right) of the internal occipital protuberance (see Fig. 25.4 . (The confluence of the sinuses is also known as the torcular Herophili or the torcula, although the term actually describes the bony gutter in which the confluence lies). At the confluence of the sinuses the superior sagittal sinus usually deviates to become continuous with the right transverse sinus, but also usually connects with the occipital and contralateral transverse sinuses. The size and degree of communication of the channels meeting at the confluence are highly variable; any sinus involved may be duplicated, narrowed or widened near the confluence (see Fig. 26.13B ).

There are usually two or three lateral venous lacunae on each side of the midline, named frontal (small), parietal (large) and occipital (intermediate) lacunae; they tend to become confluent in the elderly, producing a single elongated lacuna on each side. Fine fibrous bands cross the lacunae, and numerous arachnoid granulations project into them ( Fig. 25.6 ; see Fig. 25.5 ). The lacunae mainly drain diploic and meningeal veins. The cortical veins typically pass beneath the lacunae on their way to the sinus. The superior anastomotic vein (of Trolard), the largest cortical vein that connects the superficial middle cerebral (Sylvian) vein and the superior sagittal sinus, often runs in the precentral, central or postcentral sulci. Near its posterior end, the superior sagittal sinus receives veins from the pericranium that pass through parietal foramina. Acute and complete thrombosis of the superior sagittal sinus is an extremely severe condition causing acute elevation of the intracranial pressure and herniation. Slow and progressive occlusion of the sinus, as is typical for sagittal meningiomas, may be compensated by the development of collateral venous drainage with no clinical consequences.

Fig. 25.6, A coronal section through the vertex of the skull to show the relationships between the superior sagittal sinus, meninges and arachnoid granulations.

Inferior sagittal sinus

The inferior sagittal sinus is located in the posterior half or two-thirds of the free margin of the falx cerebri (see Figs 25.2 , 26.11C ). It increases in size posteriorly, ends in the straight sinus, and receives veins from the falx and sometimes from the medial surfaces of the cerebral hemispheres (anterior pericallosal veins).

Straight sinus

The straight sinus lies in the junction of the falx cerebri and the tentorium cerebelli (see Figs 25.2 , 26.10 , 26.11A,C ). It runs posteroinferiorly as a continuation of the inferior sagittal sinus and drains into the transverse sinus, most commonly into the left sinus. It is not (or is only tenuously) continuous with the superior sagittal sinus. Its tributaries include the great cerebral vein (of Galen), formed by the convergence of the internal cerebral and basal veins, and some superior cerebellar veins which may drain initially into the medial tentorial sinus (a short sinus within the tentorium).

Transverse sinus

The transverse sinuses are attached to the internal occipital protuberance (see Figs 25.2 , 25.4 , 26.11 , 26.13 ). They begin at the confluence of the sinuses: one, usually the right, is directly continuous with the superior sagittal sinus, and the other is directly continuous with the straight sinus. The right transverse sinus is typically larger and drains blood from the superficial parts of the brain. The left transverse sinus mainly drains blood from the deep parts of the brain. On each side, the sinuses run in the attached margin of the tentorium cerebelli, first on the squama of the occipital bone, then on the mastoid angle of the parietal bone. Each follows a gentle anterolateral curve, increasing in size as it does so, to the posterolateral part of the petrous temporal bone, where it turns down as a sigmoid sinus that ultimately becomes continuous with the internal jugular vein. The transverse sinuses receive tributaries from the lateral temporal surface and basal surface of the temporal and occipital lobes. The inferior anastomotic vein (of Labbé) is the largest vein that connects the veins of the Sylvian fissure with the transverse sinus. All tributaries of each transverse sinus may drain first into a lateral tentorial sinus. The superior petrosal sinuses drain into the transition between the transverse and sigmoid sinuses on each side (see Figs 25.2 , 25.4 ).

Sigmoid sinus

The sigmoid sinuses are continuations of the transverse sinuses, beginning where these leave the tentorium cerebelli ( Fig. 25.7 ; see Figs 25.4 , 26.11 , 26.13C ). Each sigmoid sinus curves inferomedially in a groove on the mastoid process of the temporal bone, crosses the jugular process of the occipital bone and turns forwards to the superior jugular bulb, lying posteriorly in the jugular foramen. Anteriorly, a thin plate of bone separates its upper part from the mastoid antrum and mastoid air cells. The sigmoid sinus is connected with pericranial veins via mastoid and condylar emissary veins. Neurosurgical approaches to the lateral aspect of the posterior fossa (cerebellopontine angle) are classified as retrosigmoid, when the craniectomy is located just behind the sigmoid sinus, and presigmoid, when the mastoid bone in front of the sinus is drilled away to provide a more anterior corridor into the posterior fossa.

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