The Meninges

Figure 7-1. Development of the meninges. After the neural tube closes (A, B), cells from the neural crest and mesoderm (C, arrows) migrate to surround the neural tube and form the primordia of the dura and of the arachnoid and pia (D). A dermal sinus (E) is a malformation in which there is a channel from the skin into the meninges.

One developmental defect associated with closure of the neural tube and formation of the meninges in the lumbosacral area is the congenital dermal sinus (also called just dermal sinus) (Fig. 7-1E). This defect is caused by a failure of the ectoderm (future skin) to completely pinch off from the neuroectoderm and the primitive meninges that envelop it. As a result, the meninges are continuous with a narrow, epithelium-lined channel that extends to the skin surface (Fig. 7-1E). Dermal sinuses are sometimes discovered in young patients who have recurrent but unexplained bouts of meningitis. These lesions are surgically removed, and recovery is usually complete.

The ectomeninx around the brain is continuous with the skeletogenous layer that forms the skull. This relationship is maintained in the adult, in whom the dura is intimately adherent to the inner surface of the skull. In the spinal column, the ectomeninx is also initially continuous with the developing vertebrae. However, as development proceeds, the spinal ectomeninx dissociates from the vertebral bodies. A layer of cells remains on the vertebrae to form the periosteum, and the larger part of the ectomeninx condenses to form the spinal dura. The intervening space becomes the spinal epidural space (Fig. 7-2). This space is essential for the administration of epidural anesthetics.

Figure 7-2. The relation of the meninges to the brain and spinal cord and to their surrounding bony structures. The dura is represented in blue, the arachnoid in red. (From Haines DE: Neuroanatomy: An Atlas of Structures, Sections, and Systems, 8th ed. Baltimore, Lippincott Williams & Wilkins, 2012.)

OVERVIEW OF THE MENINGES

In general, the meninges consist of fibroblasts and varying amounts of extracellular connective tissue fibrils. The fibroblasts of each meningeal layer are modified to serve a particular function.

The human meninges are composed of the dura mater, arachnoid mater, and pia mater (Figs. 7-2 and 7-3). The outermost portion, the dura mater, also called the pachymeninx, is adherent to the inner surface of the skull but is separated from the vertebrae by the epidural space (Fig. 7-2). Around the brain the inner portions of the dura give rise to infoldings or septa, such as the falx cerebri and tentorium cerebelli (Fig. 7-2), which separate brain regions from each other. Major venous sinuses are found at the points where these septa originate. Spinal and cranial nerves, as they enter or exit the CNS, must pass through a cuff of the dura that is continuous with the connective tissue of the peripheral nerve. Blood vessels traverse the dura in similar fashion. Rostrally the dura sac is attached to the rim of the foramen magnum. Caudally the sac ends at about the level of the second sacral vertebrae and is attached to the coccyx by the filum terminale externum (or dural part of the filum terminale) (Fig. 7-2).

Figure 7-3. The structure of the meninges. Layers of the dura are shown in shades of gray, the arachnoid in shades of pink, and the pia in green. (From Haines DE: On a question of a subdural space. Anat Rec 230:3:21, 1991.)

The inner two layers of the meninges, the arachnoid mater and the pia mater (Figs. 7-2 and 7-3), are collectively known as the leptomeninges. This term is also commonly used in clinical medicine (as in leptomeningeal cysts and leptomeningitis). The arachnoid is a thin cellular layer that is attached to the overlying dura but, with the exception of the arachnoid trabeculae, is separated from the pia mater by the subarachnoid space. The arachnoid around the brain is directly continuous with the arachnoid lining the inner surface of the spinal dura (Fig. 7-2). Consequently, the spinal and cerebral subarachnoid spaces are also directly continuous with each other at the foramen magnum. The subarachnoid space contains cerebrospinal fluid (CSF) and vessels and is bridged by fibroblasts of various sizes and shapes that collectively form the arachnoid trabeculae. The arachnoid is avascular and does not contain nerve fibers.

The pia mater is located on the surface of the brain and spinal cord and closely follows all their various grooves and elevations (Figs. 7-2 and 7-3). Around the spinal cord, the pia mater contributes to the formation of the denticulate ligaments and the filum terminale internum (or pial part of the filum terminale) (Fig. 7-2).

DURA MATER

Periosteal and Meningeal Dura

The dura mater (pachymeninx) is composed of elongated fibroblasts and copious amounts of collagen fibrils (Fig. 7-3). This membrane contains blood vessels and nerves and is generally divided into outer (periosteal), inner (meningeal), and border cell portions. There is no distinct border between periosteal and meningeal portions of the dura (Fig. 7-3). Fibroblasts of the periosteal dura are larger and slightly less elongated than other dural cells. This portion of the dura is adherent to the inner surface of the skull, and its attachment is particularly tenacious along suture lines and in the cranial base. In contrast, the fibroblasts of the meningeal dura are more flattened and elongated, their nuclei are smaller, and their cytoplasm may be darker than that of periosteal cells. Although cell junctions are rarely seen between dural fibroblasts, the large amounts of interlacing collagen in periosteal and meningeal portions of the dura give these layers of the meninges great strength.

Dural Border Cell Layer

The innermost part of the dura is composed of flattened fibroblasts that have sinuous processes. Collectively these cells form the dural border cell layer (Fig. 7-3). The extracellular spaces between the flattened cell processes of dural border cells contain an amorphous substance but no collagen or elastic fibers. Cell junctions (desmosomes, gap junctions) are occasionally seen between dural border cells and cells of the underlying arachnoid.

Because of its loose arrangement, enlarged extracellular spaces, and lack of extracellular connective tissue fibrils, the dural border cell layer constitutes a plane of structural weakness at the dura-arachnoid junction. This layer is externally continuous with the meningeal dura and internally continuous with the arachnoid. Consequently, bleeding into this area of the meninges will disrupt and dissect open the dural border cell layer rather than invade the overlying dura or the underlying arachnoid. In the normal (and healthy) human, there is not a naturally occurring, or preexisting, space at the dura-arachnoid interface (Fig. 7-3).

Blood Supply

The arterial supply to the dura of the anterior cranial fossa originates from the cavernous portion of the internal carotid, the ethmoidal arteries (via the ethmoidal foramina), and branches of the ascending pharyngeal artery (via the foramen lacerum). The middle meningeal artery serves the dura of the middle cranial fossa and may be compromised when there is trauma to the skull. It is a branch of the maxillary artery and enters the skull through the foramen spinosum. The accessory meningeal artery (via the foramen ovale) and small branches from the lacrimal artery (via the superior orbital fissure) also serve the dura of the middle fossa. The dura of the posterior fossa is served by small meningeal branches of ascending pharyngeal and occipital arteries and by minute branches of the vertebral arteries.

The spinal dura is served by branches of major arteries (such as vertebral, intercostal, and lumbosacral) that are located close to the vertebral column. These small meningeal arteries enter the vertebral canal via the intervertebral foramina to serve the dura and adjacent structures.

Nerve Supply

The nerve supply to the dura of the anterior and middle fossae is from branches of the trigeminal nerve, Ethmoidal nerves and branches of the maxillary and mandibular nerves innervate the dura of the anterior fossa; the dura of the middle fossa is served mainly by branches from the maxillary and mandibular nerves. The dura of the posterior fossa receives sensory branches from dorsal roots C2 and C3 (and from C1 when this root is present) and may have some innervation from the vagus nerve. The tentorial nerve, a branch of the ophthalmic nerve, courses caudally to serve the tentorium cerebelli. Autonomic fibers to the vessels of the dura originate from the superior cervical ganglia and simply follow the progressive branching patterns of the vessels on which they lie.

Nerves to the spinal dura originate as recurrent branches of the spinal nerve located at that level. These delicate strands pass through the intervertebral foramina and are distributed to the spinal dura and to some adjacent structures.

Dural Infoldings and Sinuses

The periosteal dura lines the inner surface of the skull and functions as its periosteum. The meningeal dura is continuous with the periosteal dura but draws away from it at specific locations to form the dural infoldings (or reflections). The largest of these is the falx cerebri (Figs. 7-4 and 7-5A). It is attached to the crista galli rostrally, to the midline of the inner surface of the skull, and to the surface of the tentorium cerebelli caudally. The falx cerebri separates the right hemisphere from the left. The superior sagittal sinus is found where the falx cerebri attaches to the skull, the straight sinus where it fuses with the tentorium cerebelli, and the inferior sagittal sinus at its free edge (Fig. 7-4). Many large superficial veins located on the surface of the cerebral hemispheres empty into the superior sagittal sinus.

Figure 7-4. Midsagittal view of the skull showing the dural infoldings (reflections) and venous sinuses associated with each. (From Haines DE, Fredrickson RG: The meninges. In Al-Mefty O [ed]: Meningiomas. New York, Raven Press, 1991.)

Figure 7-5. Axial (A), coronal (B), and sagittal (C) T1-weighted magnetic resonance images showing the relationships of the falx cerebri (A, B) and the tentorium cerebelli (B, C). Note the positions of the right and left supratentorial compartments and the infratentorial compartment in relation to these large dural reflections in all three planes.

The tentorium cerebelli is the second largest of the dural infoldings (Figs. 7-4 and 7-5B, C). It attaches rostrally to the clinoid processes, rostrolaterally to the petrous portion of the temporal bone (location of the superior petrosal sinus), and caudolaterally to the inner surface of the occipital bone and a small part of the parietal bone (location of the transverse sinus) (Figs. 7-4 and 7-5B, C). The tent shape of the tentorium divides the cranial cavity into supratentorial (above the tentorium) and infratentorial (below the tentorium) compartments (Fig. 7-5B; see also Fig. 7-10). The supratentorial compartment is divided into right and left halves by the falx cerebri (Fig. 7-5A, B). The sweeping edges of the right and left tentoria, as they arch from the clinoid processes to join at the straight sinus, form the tentorial notch (Fig. 7-6). The occipital lobe is above the tentorium, the cerebellum is below it, and the midbrain passes through the tentorial notch.

Figure 7-6. View of the cranial base from the dorsal aspect showing the tentorium cerebelli (and its associated sinuses) and the diaphragma sellae. Also indicated are the positions of grooves formed by some of the major sinuses. The red-shaded area indicates the position of the tentorial incisura (tentorial notch), which is the space continuation between the supratentorial compartments and the infratentorial compartment.

Located below the tentorium cerebelli on the midline of the occipital bone is the falx cerebelli (Fig. 7-4). This small dural infolding extends into the space found between the cerebellar hemispheres and usually contains a small occipital sinus.

The smallest of the dural infoldings, the diaphragma sellae (Figs. 7-4 and 7-6), forms the roof of the hypophyseal fossa and encircles the stalk of the pituitary. The cavernous sinuses are found on either side of the sella turcica, and the anterior and posterior intercavernous sinuses are found in their respective edges of the diaphragma sellae.

It is emphasized that venous sinuses are endothelium-lined spaces that communicate with each other. In addition, large veins from the surface of the brain empty into the venous sinuses. As they enter the sinus, these veins are attached to a cuff of dura. Consequently, a blow to the head (or a minor bump to the head in an aged person) may cause the brain to shift just enough in the subarachnoid space to tear a vein at the point where it enters the sinus. This tear may allow venous blood to enter the subarachnoid space or may create a hematoma within the dural border cell layer at the dura-arachnoid interface, a subdural hematoma.

Compartments and Herniation Syndromes

The interior of the cranial cavity is divided into a supratentorial compartment located superior to the tentorium cerebelli and consisting of right and left halves (separated by the falx cerebri) and a single infratentorial compartment located inferior to the tentorium cerebelli (Fig. 7-5). The concept of supratentorial and infratentorial compartments, with an understanding of their contents and relationships, is an essential element in the diagnosis of what are commonly called herniation syndromes. In general, a herniation syndrome occurs when there is an intracranial event (hemorrhage, rapid tumor growth, traumatic brain injury) that causes an increase in intracranial pressure, forcing the comparatively gelatinous brain over the edge of a dural reflection. These syndromes are considered in more detail in later chapters.

The following are examples of herniation syndromes related to the supratentorial compartments. A lesion in one cerebral hemisphere may expand toward the midline, deform the falx cerebri, and force the cingulate gyrus under the edge of the falx into the opposite hemisphere; this is a subfalcine or cingulate herniation. In this example, the deficits may reflect occlusion of the adjacent anterior cerebral artery. Central (or transtentorial) herniation is the situation in which the diencephalon is forced downward through the tentorial incisure or notch. This is a neurologic emergency, and in about 90% of patients there is serious disability or death. Uncal herniation is the case when a rapidly expanding lesion, usually a hematoma, forces the uncus, a medial structure of the temporal lobe, over the edge of the tentorium cerebelli with resultant damage to the midbrain. The most common deficits are (1) a decreased level of consciousness, (2) dilation of the pupil and a loss of most eye movement reflecting damage to the ipsilateral oculomotor nerve, and (3) a contralateral hemiplegia reflecting damage to the descending corticospinal fibers. However, this early stage is likely to be followed by serious complications or death.

Examples of herniation syndromes related to the infratentorial compartment include upward cerebellar herniation and tonsillar herniation. In upward cerebellar herniation, a mass or pressure increase in the posterior fossa may force the cerebellum upward through the tentorial incisura, inflicting damage to the midbrain. In tonsillar herniation, the tonsils of the cerebellum are forced downward into and possibly through the foramen magnum. The resulting pressure on the medulla may damage respiratory centers and result in sudden death. All of the herniation syndromes are potentially serious, and all measures should be taken to avoid their occurrence.