The brain and spinal cord are wrapped in three concentric layers of connective tissue — the meninges — and suspended in a fluid that surrounds them, cushions them, and bathes them with nutrients and signaling molecules. The arrangement is so essential that injury or disease at any layer produces recognizable, often life-threatening clinical pictures: epidural hematoma, subdural hematoma, subarachnoid hemorrhage, meningitis, hydrocephalus, idiopathic intracranial hypertension. Each is a disorder of the coverings of the CNS, and each carries diagnostic and management implications that turn on a precise understanding of the relevant anatomy.

This page covers the meninges, the ventricular system, the cerebrospinal fluid, and the subarachnoid cisterns. The arrangement of these structures determines how blood, pus, tumor, and CSF distribute within the cranial cavity, and it determines what the clinician sees on imaging and at the bedside.

The Three Meningeal Layers

From outermost to innermost:

Dura Mater

A thick, tough connective tissue layer composed of two fused sheets — an outer periosteal layer (which serves as the periosteum of the inner skull) and an inner meningeal layer. The two layers are fused throughout most of their extent, separating only to form the dural venous sinuses (described below) and to fold inward as the dural reflections that compartmentalize the cranial cavity. The dural folds include:

Falx cerebri: a vertical fold in the longitudinal fissure, separating the two cerebral hemispheres. Attached anteriorly to the crista galli, posteriorly to the tentorium cerebelli at the straight sinus.
Tentorium cerebelli: a horizontal fold separating the occipital lobes from the cerebellum. Defines the supratentorial and infratentorial compartments. Has a free anterior edge (the tentorial incisura) through which the midbrain passes.
Falx cerebelli: a small vertical fold in the midline of the posterior fossa, partially separating the cerebellar hemispheres.
Diaphragma sellae: a horizontal sheet roofing the sella turcica, with an opening for the pituitary stalk.

The dura is innervated by branches of the trigeminal nerve and the upper cervical roots — the source of the headache associated with meningeal irritation and the pattern of referred pain in conditions affecting the dura.

Arachnoid Mater

A delicate, avascular membrane that lies beneath the dura. It is separated from the dura by a thin “subdural space” (under normal conditions, a potential rather than actual space) and from the underlying pia by the subarachnoid space, which contains CSF and the cerebral vessels.

Two distinctive features of the arachnoid:

Arachnoid trabeculae: spider-web-like strands of connective tissue that traverse the subarachnoid space, anchoring the arachnoid to the pia. The name “arachnoid” comes from this web-like appearance.
Arachnoid granulations (villi): small protrusions of arachnoid through the dura into the dural venous sinuses (especially the superior sagittal sinus), allowing CSF absorption back into the venous system.

Pia Mater

A thin, vascular membrane that adheres closely to the surface of the brain and spinal cord, following every gyrus and sulcus into the depths. The pia is the layer in direct contact with the CNS surface and carries the small blood vessels that supply the cortex.

Where the arachnoid bridges over a sulcus and the pia descends into it, the space between is filled with CSF and constitutes the subarachnoid space.

The Meningeal Spaces

Three potential or actual spaces are defined by the meningeal layers:

Epidural (extradural) space: the potential space between the dura and the inner table of the skull. Becomes an actual space only when pathology fills it — most importantly an epidural hematoma from rupture of the middle meningeal artery in temporal bone fracture. The bleed accumulates rapidly because of arterial pressure and pushes the dura inward, producing the characteristic lentiform (lens-shaped) appearance on CT.
Subdural space: the potential space between the dura and the arachnoid. Becomes an actual space when bridging veins between the cortex and the dural sinuses tear, producing a subdural hematoma. The bleed accumulates more slowly because of venous pressure and spreads diffusely along the dural surface, producing the crescentic shape on CT.
Subarachnoid space: an actual space, between the arachnoid and the pia, containing CSF, the cerebral arteries and veins, and the cranial nerves. Hemorrhage into this space — most commonly from rupture of an aneurysm at the Circle of Willis — produces subarachnoid hemorrhage, with CSF distributed throughout the basal cisterns and over the cerebral surface.

The Dural Venous Sinuses

The venous sinuses run between the two layers of the dura. They drain the brain, the meninges, and parts of the skull and face. The major sinuses:

Superior sagittal sinus: runs in the upper margin of the falx cerebri from front to back. Drains most of the cortical veins of the upper convexity. The arachnoid granulations enter it.
Inferior sagittal sinus: runs in the lower margin of the falx cerebri, joining the great cerebral vein to form the straight sinus.
Straight sinus: runs in the line of attachment between the falx cerebri and the tentorium cerebelli, draining into the confluence of sinuses (torcular Herophili).
Transverse sinuses: run in the lateral attachment of the tentorium, from the confluence of sinuses outward.
Sigmoid sinuses: continuations of the transverse sinuses, curving downward and forward to exit the skull as the internal jugular veins.
Cavernous sinuses: lie on either side of the sella turcica. Contain CN III, IV, V₁, V₂, VI, and the internal carotid artery, plus the sympathetic plexus around the carotid. Drained anteriorly by the ophthalmic veins and posteriorly into the petrosal sinuses.

The clinical importance of the sinuses: thrombosis of any sinus produces a recognizable syndrome. Superior sagittal sinus thrombosis classically causes headache with bilateral parasagittal infarcts. Cavernous sinus thrombosis causes painful ophthalmoplegia with proptosis. Transverse sinus thrombosis is increasingly recognized as a cause of idiopathic intracranial hypertension-like presentations.

The Ventricular System

The ventricles are the fluid-filled cavities within the brain. They are derived from the cavity of the embryonic neural tube and form a continuous communicating system:

Two lateral ventricles: paired C-shaped cavities, one in each cerebral hemisphere. Each has a frontal horn, body, atrium (trigone), occipital horn, and temporal horn. Communicate with the third ventricle through the foramen of Monro (interventricular foramen).
Third ventricle: a midline slit between the two thalami. Communicates with the fourth ventricle through the cerebral aqueduct (of Sylvius).
Fourth ventricle: a tent-shaped cavity behind the pons and upper medulla, with the cerebellum as its roof. Communicates with the subarachnoid space through the midline foramen of Magendie and the paired lateral foramina of Luschka.

Cerebrospinal Fluid

Production

CSF is produced primarily by the choroid plexus, a vascular fringe of specialized epithelium lining portions of the ventricular walls. Choroid plexus is found in the body and atrium of each lateral ventricle, the roof of the third ventricle, and the roof of the fourth ventricle. A smaller amount of CSF is produced by transependymal flow from the brain interstitium.

Total CSF production is approximately 500 mL per day. Total CSF volume at any moment is about 150 mL (about 25 mL in the ventricles, 125 mL in the subarachnoid space). The volume therefore turns over about three times per day.

Circulation

CSF flows from the lateral ventricles through the foramina of Monro into the third ventricle, then through the cerebral aqueduct into the fourth ventricle, then through the foramina of Magendie and Luschka into the basal cisterns. From there it circulates upward over the cerebral convexities and downward around the spinal cord. The flow is driven by the pulsatile production of CSF (choroidal arterial pulsations).

Absorption

CSF is absorbed primarily by the arachnoid granulations into the dural venous sinuses, especially the superior sagittal sinus. A smaller amount is absorbed along the cranial and spinal nerve roots, into the cervical lymphatics.

The recently described glymphatic system — perivascular spaces lined by aquaporin-4-expressing astrocyte endfeet — is now recognized as an additional pathway for CSF-interstitial fluid exchange and clearance of metabolic waste, particularly during sleep. Disruption of glymphatic flow has been implicated in neurodegenerative disease, particularly Alzheimer disease.

Composition

CSF differs from plasma in several ways:

Lower protein: about 15-45 mg/dL in lumbar CSF, much lower than plasma. Reflects the blood-CSF barrier formed by the choroid plexus and the blood-brain barrier.
Higher chloride and lower potassium than plasma.
Glucose: about two-thirds of plasma glucose; CSF/serum glucose ratio is normally about 0.6.
Very few cells: 0-5 lymphocytes per microliter in normal CSF.
Slightly acidic: pH about 7.32, reflecting the bicarbonate-buffered relationship with brain metabolism.

Subarachnoid Cisterns

Where the arachnoid bridges across the brain’s natural depressions, the subarachnoid space enlarges into named cisterns. The major cisterns:

Cisterna magna: between the cerebellum and the dorsal surface of the medulla. Receives CSF from the foramina of Magendie and Luschka. The site of cisternal puncture in some clinical scenarios.
Quadrigeminal (superior) cistern: above the colliculi of the dorsal midbrain. Contains the great cerebral vein and the pineal gland.
Ambient cistern: wraps around the lateral aspect of the midbrain. Contains the posterior cerebral artery and the basal vein.
Interpeduncular cistern: between the cerebral peduncles. Contains the basilar artery bifurcation and the origins of the posterior cerebral arteries.
Suprasellar (chiasmatic) cistern: above the sella, in front of the brainstem. Contains the optic chiasm, the optic tracts, the proximal anterior cerebral arteries, and the pituitary stalk.
Prepontine cistern: anterior to the pons. Contains the basilar artery, CN V exiting the pons, and CN VI emerging from the pontomedullary junction.
Cerebellopontine angle cistern: at the lateral pontomedullary angle. Contains CN VII and VIII entering the internal auditory meatus, and CN V superiorly. The site of vestibular schwannoma.
Sylvian (lateral) cisterns: extend into the Sylvian fissures. Contain the middle cerebral arteries.

The cisterns are clinically important because:

Subarachnoid hemorrhage from an aneurysm fills the basal cisterns first, producing the typical “starburst” pattern on CT.
Basal meningitis (tuberculous, fungal) preferentially involves the basal cisterns.
The cisterns are the locations of many cranial nerves and major vessels — pathology here often presents with multifocal cranial neuropathies and vascular complications.
Cisternostomy and ventriculocisternostomy are surgical approaches that exploit cisternal anatomy.

Hydrocephalus

An imbalance of CSF production and absorption produces hydrocephalus — accumulation of CSF with enlargement of the ventricular system. Several types:

Communicating (non-obstructive) hydrocephalus: CSF flows freely between the ventricles and subarachnoid space, but absorption is impaired. Causes include subarachnoid hemorrhage (the most common acquired cause), meningitis, and congenital deficiency of arachnoid granulations.
Non-communicating (obstructive) hydrocephalus: an obstruction blocks CSF flow somewhere along the ventricular pathway. Common sites: foramen of Monro, cerebral aqueduct (aqueductal stenosis), foramina of Magendie/Luschka, fourth ventricle.
Normal pressure hydrocephalus (NPH): ventricular enlargement with normal CSF pressure, in older adults. The classical triad is gait apraxia, urinary incontinence, and cognitive decline. Treatable with ventricular shunting in selected patients.
Hydrocephalus ex vacuo: ventricular enlargement from atrophy of surrounding brain tissue, not from CSF obstruction. Not true hydrocephalus, since there is no abnormality of CSF dynamics.

Management of hydrocephalus typically involves CSF diversion — ventriculoperitoneal shunting, ventriculoatrial shunting, or endoscopic third ventriculostomy for obstructive cases.

Idiopathic Intracranial Hypertension (IIH)

Elevated intracranial pressure without ventricular enlargement, mass lesion, or evident cause. Predominantly affects young women with elevated body mass index. Presents with headache, papilledema, and sometimes visual loss from chronic optic nerve compression. The pathophysiology is debated; current theories center on impaired CSF absorption, often associated with transverse sinus stenosis. Treatment includes weight loss, acetazolamide, and in refractory cases CSF shunting or transverse sinus stenting.

Lumbar Puncture and CSF Analysis

The lumbar puncture is the principal clinical access to CSF. The needle is inserted between L3-L4 or L4-L5 (below the conus medullaris) into the lumbar cistern of the subarachnoid space. Several pieces of information come from a standard LP:

Opening pressure: normally 5-20 cm H₂O in the lateral recumbent position. Elevated in meningitis, subarachnoid hemorrhage, IIH, and many other conditions.
Appearance: normally clear. Cloudy CSF suggests high cell counts (meningitis); xanthochromic (yellow) CSF suggests prior subarachnoid bleeding.
Cell counts: normally 0-5 lymphocytes per microliter. Elevated counts indicate inflammation (lymphocytic predominance in viral meningitis, demyelinating disease; neutrophilic predominance in bacterial meningitis).
Glucose: low in bacterial, tuberculous, fungal, and carcinomatous meningitis; normal in most viral meningitis.
Protein: elevated in many inflammatory and infectious conditions; markedly elevated in Guillain-Barré syndrome (albuminocytological dissociation), CIDP, and obstructive lesions.
Special studies: Gram stain and culture, PCR for specific pathogens, oligoclonal bands (for MS), cytology (for malignant cells), biomarkers (tau, amyloid beta for Alzheimer disease, 14-3-3 for prion disease), antibody panels (paraneoplastic, autoimmune encephalitis).

🔍 Did You Know?

The classical lentiform (lens-shaped) appearance of an epidural hematoma on CT, and the crescentic appearance of a subdural hematoma, reflect the different layers in which the bleeds collect. The epidural hematoma is bound by the firm attachment of the dura to the inner table of the skull at the cranial sutures — it cannot cross sutures, which gives it the bulging, lens-shaped appearance. The subdural hematoma is bound by the dural reflections (falx cerebri, tentorium) — it cannot cross the midline but can spread along the inner skull surface, giving it the long crescentic appearance. This single anatomical fact distinguishes the two on imaging within seconds of looking at the scan.

Pitfalls and Pearls

Epidural hematoma is typically lentiform and does not cross sutures. Subdural hematoma is crescentic and does not cross the midline. The shapes reflect the bound layers.
The “lucid interval” classically precedes epidural hematoma decompensation — brief loss of consciousness, recovery, then rapid deterioration as the arterial bleed expands. Look for it in head injury history.
Subarachnoid hemorrhage fills the basal cisterns, producing the characteristic starburst pattern on CT. The pattern can suggest the site of the aneurysm.
Cavernous sinus thrombosis presents with painful ophthalmoplegia, V₁ sensory loss, proptosis, and chemosis. Often from spreading face or sinus infection.
Lumbar puncture is safely performed below L2 in adults because the conus medullaris ends above this level.
Always measure opening pressure on LP. Elevated pressure may be the first clue to IIH, cryptococcal meningitis, or cerebral venous thrombosis.
Albuminocytological dissociation (high protein with few cells) is classic for Guillain-Barré syndrome.
Xanthochromia takes 6-12 hours to develop after SAH. A normal CSF less than 6 hours after a thunderclap headache does not exclude SAH.
The classical triad of NPH is gait apraxia, urinary incontinence, and cognitive decline. Imaging shows ventriculomegaly out of proportion to atrophy. Selected patients respond dramatically to shunting.
The arachnoid granulations are the principal site of CSF absorption into the dural venous sinuses. Their dysfunction underlies most communicating hydrocephalus.

References

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