Cerebrospinal Fluid

23 Cerebrospinal Fluid

The meninges are composed of three connective tissue sheaths that envelope the central nervous system (CNS) and separate it from its bony encasement. From outside in, these are the dura mater, the arachnoid layer, and the pia mater. The cerebrospinal fluid (CSF), a waterlike substance in which the brain is bathed, is contained between the arachnoid layer and the pia mater in the subarach-noid space. There, the fluid serves to help the meninges to support and protect the semisolid brain and spinal cord and to remove metabolic waste products such as carbon dioxide, lactate, and hydrogen ions.

Meninges

Dura Mater

See Fig. 23.1.

The tough fibrous dura mater is composed of elongated fibroblasts and collagen fibrils. It contains blood vessels and nerves and is commonly described as comprising two parts, an outer endosteal layer (more properly called periosteal dura), which faces the skull, and an inner layer, the meningeal dura, which faces the brain. These two dural layers are closely attached to one another, except in certain parts where they separate to form the venous sinuses.

Pia Mater and Arachnoid Mater

The pia mater and arachnoid mater, which are collectively known as the leptomeninges, are separated from the dura mater by a potential space, known as the subdural space. Developmentally, the pia mater and arachnoid arise as a single layer of mesodermal tissue surrounding the brain and the spinal cord. This layer becomes separated as a fluid-filled space, the CSF-containing subarachnoid space, divides them. The trabeculae that pass between the pia and the arachnoid are remnants of these coverings’ common embryological origin.

Although the pia mater adheres to the surface of the brain, closely following the contours of its gyri and sulci, the arachnoid covers only its superficial surface. It follows from this that in certain areas around the brain the pia and arachnoid are separated widely; in such regions are formed cavities called the subarachnoid cisterns (not shown in figure).

Fig. 23.1 Structure of the meninges.

Dural Folds

See Fig. 23.2.

The dural folds are created by reflections of the dura mater along the contours of the brain. Three communicating spaces are thus formed in the cranial cavity, two supratentorial and one infratentorial. There are four major dural folds:

Falx cerebri The falx cerebri forms a vertical partition that runs within the longitudinal cerebral fissure separating the right and left cerebral hemispheres. Anteriorly, it is attached to the crista galli of the ethmoid bone; posteriorly, it is attached to the tentorium cerebelli.
Tentorium cerebelli The tentlike tentorium cerebelli divides the cranial cavity into middle and posterior fossae and separates the (supratentorial) occipital lobes from the (infratentorial) cerebellum. Its free edge forms the tentorial notch, through which the midbrain passes. Its peripheral fixed border is attached to the petrous portion of the temporal bones and the margins of the grooves for the transverse sinuses on the occipital bone.
Falx cerebelli The falx cerebelli (not shown in figure) extends vertically for a short distance in the posterior fossa between the cerebellar hemispheres.
Diaphragma sellae The small diaphragma sellae forms the roof of the sella turcica in which the pituitary is housed. A small opening in the sellae permits the pituitary stalk to pass to its attachment at the base of the brain.

Fig. 23.2 Dural folds.

Dural Venous Sinuses

See Fig. 23.3.

The venous sinuses are formed by the space between the periosteal and meningeal layers of the dura. They are described in Chapter 22. In short, they receive venous blood that is leaving the brain and empty it into the internal jugular veins, which exit the cranial cavity through the jugular foramen.

Fig. 23.3 (A) Venous sinuses, oblique view. (B) Venous sinuses, axial view.

Subarachnoid Cisterns

See Fig. 23.4.

Although they are often described as distinct compartments, the subarachnoid cisterns are in fact not truly anatomically distinct. Rather, these subarachnoid spaces are separated from each other by a trabeculated porous wall with various-sized openings. The contents of the major subarachnoid cisterns are as follows.

Cerebellomedullary Cistern (Cisterna Magna)

This cistern lies between the cerebellum and the medulla. It receives CSF from the fourth ventricle via the median foramen of Magendie and the two lateral foramina of Luschka. The cerebellomedullary cistern contains:

The vertebral artery and the origin of the posterior inferior cerebellar artery
The ninth, tenth, eleventh, and twelfth cranial nerves
The choroid plexus

The Prepontine Cistern

The prepontine cistern surrounds the ventral aspect of the pons. It contains

The basilar artery and the origin of the anteroinferior cerebellar artery
The origin of the superior cerebellar arteries
The sixth cranial nerve

The Cerebellopontine Cistern

The cerebellopontine cistern is situated in the lateral angle between the cerebellum and the pons. It contains

The seventh and eighth cranial nerves
The anteroinferior cerebellar artery
The fifth cranial nerve and the petrosal vein

The Interpeduncular Cistern

The interpeduncular cistern is situated between the two cerebral peduncles. It contains

The bifurcation of the basilar artery
Peduncular segments of the posterior cerebral arteries
Peduncular segments of the superior cerebellar arteries
Perforating branches of the posterior cerebral arteries
The posterior communicating arteries
The basal vein of Rosenthal
The third cranial nerve, which passes between the posterior cerebral and superior cerebellar arteries

The Crural Cistern

The crural cistern is situated around the ventrolateral aspect of the midbrain. It contains

The anterior choroidal artery
The medial posterior choroidal artery
The basal vein of Rosenthal

The Chiasmatic Cistern

The chiasmatic cistern is situated just ventral to the optic chiasm. It contains

The anterior aspect of the optic chiasm and optic nerves
The hypophyseal stalk
The origin of the anterior cerebral arteries

The Carotid Cistern

The carotid cistern is situated between the carotid artery and the ipsilateral optic nerve. It contains

The internal carotid artery
The origin of the anterior choroidal artery
The origin of the posterior communicating artery

The Sylvian Cistern

The sylvian cistern is situated in the fissure between the frontal and temporal lobes. It contains

The middle cerebral artery
The middle cerebral (sylvian) veins
The fronto-orbital veins
Collaterals to the basal vein of Rosenthal

The Lamina Terminalis Cistern

The lamina terminalis cistern is situated just rostral to the third ventricle. It contains

The anterior cerebral arteries (the A1 segment and the proximal A2 segment)
The anterior communicating artery
Heubner’s artery
The hypothalamic arteries
The origin of the fronto-orbital arteries

The Quadrigeminal Cistern

The quadrigeminal cistern is situated dorsal to the mid-brain. It contains

The great vein of Galen
The posterior pericallosal arteries
The third portion of the superior cerebellar arteries
Perforating branches of the posterior cerebral and the superior cerebellar arteries
The third portion of the posterior cerebral arteries. (These paired arteries approach one another, and then turn posteriorly under the splenium of the corpus callosum. After traveling beneath the corpus cal-losum, they then bifurcate into two cortical branches, the calcarine and parieto-occipital branches.)

The Ambient Cistern

The ambient cistern is situated along the lateral aspects of the midbrain. It is composed of a supratentorial and an infratentorial compartment. Its supratentorial portion contains

The basal vein of Rosenthal
The posterior cerebral artery

Its infratentorial portion contains:

The superior cerebellar artery
The fourth nerve

The Lumbar Cistern

The lumbar cistern extends from the conus medullaris (L1–L2) to about the level of the second sacral vertebra. It contains the filum terminale and the nerve roots of the cauda equina. It is from this cistern that CSF is withdrawn during lumbar puncture.

It is of clinical significance that cerebral arteries, veins, and cranial nerves must pass through the subarachnoid space, and that these structures maintain their menin-geal investment until around their point of exit from the skull. The optic nerve, for example, is surrounded by sub-arachnoid space up until its attachment to the eyeball. CSF pressure is thus transmitted to the head of the optic nerve, which may be visualized by the direct ophthalmoscope, as a marker of intracranial hypertension.