Meninges: structure and functions

The brain is surrounded by three membranes: the dura mater, the arachnoid mater, and the pia mater. The dura mater forms a strong outer framework, the arachnoid mater defines the boundaries of the cerebrospinal fluid spaces, and the pia mater tightly follows the contours of the convolutions and sulci and accompanies the vessels entering the brain. [1]

There are spaces between the membranes, and it is important to distinguish between "real" and "potential" spaces. The subarachnoid space normally exists and is filled with cerebrospinal fluid, vessels, and trabeculae. The epidural and subdural spaces within the skull are normally considered potential and "open" primarily during trauma or other pathology. [2]

This difference explains why the same symptom, such as a sudden headache, can mean different things. Blood in the epidural space is more often associated with damage to the dural artery, blood in the subdural space with rupture of the bridging veins, and blood in the subarachnoid space with a source within the cerebrospinal fluid space, including a ruptured aneurysm. [3]

From a practical standpoint, the meninges are more than just "brain packaging." They are a system of mechanical protection, venous and lymphatic drainage, immune surveillance, and control of cerebrospinal fluid composition—that is, part of the overall physiology of the brain. [4]

Table 1. 3 shells and their key roles

shell	Main properties	Main function
Solid	Dense connective tissue, duplications, sinuses	Mechanical protection, venous outflow, pain sensitivity
Cobweb	Thin, relatively avascular, forms granulations	Boundaries of cerebrospinal fluid spaces, participation in cerebrospinal fluid outflow
Soft	It is tightly attached to the brain and accompanies the vessels.	Brain surface trophism, perivascular spaces

Source for the table. [5]

The dura mater: folds, sinuses, vessels, and new data on lymphatics

The dura mater of the skull consists of two layers, which diverge in places to form the venous sinuses. Unlike normal veins, the sinuses lack valves and are supported by the tension of the dura mater layers, which facilitates the outflow of venous blood even during fluctuations in intracranial pressure.

Large dura mater duplications divide the cranial cavity into functional compartments. The falx cerebri separates the hemispheres, the tentorium cerebelli separates the occipital lobes from the cerebellum, and the diaphragm sella turcica forms a "roof" for the pituitary gland with an opening for the infundibulum. These folds are clinically important because they influence the spread of edema and the herniation scenarios.

The venous sinuses of the dura mater form a network that collects blood from the superficial and deep veins of the brain and directs it to the internal jugular vein through the sigmoid sinus. Key sinuses include the superior and inferior sagittal, straight, transverse, sigmoid, cavernous, and superior petrosal sinuses. [8]

Traditionally, the meninges were discussed almost exclusively in the context of veins, but in recent years, attention has increased to the meningeal lymphatic vessels. They are located primarily in the dura mater and are considered a pathway for the removal of molecules and immune signals from the cerebrospinal fluid spaces into the deep cervical lymph nodes, which is associated with inflammatory and degenerative diseases of the nervous system. [9]

Table 2. Duplications and sinuses of the dura mater: what is located where

Structure	Where is it located?	What does it provide?	Why is it important?
Falx cerebri	Interhemispheric fissure	Separation of the hemispheres, course of the sagittal sinuses	Subdural accumulation trajectories, risk of herniation
Tentorium cerebelli	Between the occipital lobes and the cerebellum	Division of the posterior cranial fossa, course of the transverse sinuses	Variants of wedging, surgical landmarks
Diaphragm of the sella turcica	Above the pituitary gland	Restriction of the suprasellar space	Compression of visual pathways in volumetric processes
Transverse and sigmoid sinuses	Posterior cranial fossa	Outflow to the internal jugular vein	Venous thrombosis, surgical risks
Cavernous sinus	On the sides of the sella turcica	Collection of venous blood from the base of the skull	Relationship with cranial nerves and clinical manifestations of lesions

Source for the table. [10]

The arachnoid membrane and subarachnoid space: where cerebrospinal fluid lives

The arachnoid mater lies beneath the dura mater and usually does not extend deeply into the sulci, "covering" them like a thin membrane. Because of this, the subarachnoid space forms between the arachnoid and pia mater, filled with cerebrospinal fluid, blood vessels, and the trabecular meshwork, which holds the brain in a "suspended" state. [11]

The depth of the subarachnoid space varies: where the arachnoid tissue departs from the surface of the brain, cisterns—expanded pockets of cerebrospinal fluid—form. These cisterns are important not only anatomically: they contain large arteries and nerves, and during hemorrhage or inflammation, they are often the first to "fill." [12]

Clinically, it is crucial that the subarachnoid space is continuous between the skull and the spinal canal. This explains why blood from a subarachnoid hemorrhage and inflammatory changes from meningitis can spread along the cerebrospinal fluid pathways, and why a diagnostic lumbar puncture reflects processes within the skull. [13]

The arachnoid mater forms granulations that protrude into the area of the venous sinuses and lateral lacunae. Classically, these serve as the main pathway for cerebrospinal fluid absorption into the venous system, but modern data show that the outflow of cerebrospinal fluid is multichannel and also includes lymphatic pathways.

Table 3. Frequent cisterns and what usually passes through them

Tank	Location	What is most important to remember
Cerebellar cerebellar	Between the cerebellum and the medulla oblongata	Large reservoir of cerebrospinal fluid, connection with the spinal canal
Chiasmatic	In the area of the optic chiasm	Proximity to the visual pathways and vessels of the base of the brain
Interpeduncular	Between the legs of the brain	Passage of the base vessels, sensitivity to hemorrhage
Sylvian fissure and lateral cistern	Lateral groove	Proximity to the middle cerebral vessels

Source for the table.

Cerebrospinal fluid: formation, barriers, circulation and outflow

Cerebrospinal fluid is produced primarily by the choroid plexuses of the ventricles and circulates through the ventricular system to the subarachnoid space. Its total volume in adults is approximately 150 ml, and its composition is maintained relatively stable by specialized barriers and transport mechanisms. [16]

It is important to distinguish between two barriers. The blood-brain barrier is located at the level of the brain capillaries and is defined by tight junctions of endothelial cells, while the blood-cerebrospinal fluid barrier is located at the level of the epithelium of the choroid plexus and arachnoid membrane, which control the composition of the cerebrospinal fluid. [17]

Cerebrospinal fluid (CSF) normally contains very few cells and protein. Clinical reference ranges for adults often include 0-5 cells per microliter and protein levels of approximately 15-45 mg per deciliter, but reference ranges can vary between laboratories and depend on age and collection site. [18]

For a long time, it was believed that the main route of CSF outflow was through the arachnoid granulations into the venous sinuses. The modern picture is broader: pathways through the dura mater and meningeal lymphatic vessels, as well as perineural routes, including the nasal lymphatic system, have been described. [19]

A separate modern twist is the idea that some of the arachnoid granulations may act as lymphatic "conduits," connecting the cerebrospinal fluid space with the dura mater and bone marrow tissues of the cranial vault. This helps explain why the anatomy of the granulations is so variable and why cerebrospinal fluid drainage is not reducible to a single mechanism. [20]

Table 4. Simplified diagram of cerebrospinal fluid movement and main “exits”

Stage	Where does cerebrospinal fluid go?	What provides movement
Education	Ventricles	Choroid plexuses and transport through the epithelium
Transition to outdoor spaces	Subarachnoid space	Aperture of the 4th ventricle and circulation through the cisterns
Reservoirs	Foundation cisterns and convexital spaces	Vascular pulsations, respiratory and postural factors
Venous outflow tract	Venous sinuses and lacunae	Arachnoid granulations and pressure gradients
Lymphatic and perineural pathways	Meningeal lymphatic vessels, nasal lymphatic pathways	Drainage to the cervical lymph nodes and through the perineural canals

Source for the table. [21]

The pia mater and perivascular spaces: the relationship of the membranes to brain "cleansing"

The pia mater adheres tightly to the surface of the brain and extends into the sulci, following the microrelief. It is rich in vessels and forms sheaths around the arteries and veins that enter the brain from the subarachnoid space. [22]

As a vessel passes from the surface to the interior, a perivascular space, often referred to as the Virchow-Robin space, forms around it. These spaces are considered important "exchange channels" between the cerebrospinal fluid, the interstitial fluid of the brain, and the pathways for metabolite excretion. [23]

The glymphatic system describes the coordinated movement of fluids through perivascular spaces, which facilitates the removal of soluble waste products and the distribution of intrathecal drugs. Imaging data in humans indicate that the efficiency of this transport can be reduced by impaired intracranial compliance and altered pulsatility. [24]

This topic is clinically important not as a “fashion” but as an explanation for magnetic resonance imaging observations: dilated perivascular spaces are more common in small vessel diseases and some neurodegenerative processes and are considered as a potential marker of impaired fluid drainage. [25]

Table 5. What changes when perivascular transport is disrupted

Sign	What could this reflect?	Where is it most common?
Dilated perivascular spaces	Slowing of fluid exchange, vascular changes	Small vessel disease, age-related changes
Violation of "directional" perivascular movement	Decreased delivery of dissolved substances to tissue	Conditions with impaired pulsation and compliance
Associations with cognitive decline	Multifactorial effect, including vascular contribution	Neurodegenerative and vascular syndromes

Source for the table. [26]

Diseases of the membranes and spaces: how anatomy helps recognize dangerous scenarios

Meningitis is an inflammation of the meninges, most commonly affecting the leptomeninges (the arachnoid and pia mater) and the cerebrospinal fluid space. Clinical guidelines emphasize the principle of early antibacterial therapy when acute bacterial meningitis is suspected, often with ceftriaxone or cefotaxime as an empirical basis, and also discuss the role of dexamethasone, which is preferably administered with the first dose of antibiotics in appropriate scenarios. [27]

Subarachnoid hemorrhage is dangerous because blood enters the cerebrospinal fluid space, quickly irritating the membranes and vessels and can lead to vasospasm and secondary ischemic damage. Current guidelines on aneurysmal subarachnoid hemorrhage emphasize the need for rapid diagnosis and early identification of the bleeding source. [28]

An epidural hematoma is located between the bone and the dura mater and is most often associated with injury to the middle meningeal artery. This condition can develop rapidly and requires urgent evaluation because the dura mater is fixed in the suture area and the hematoma grows "inward," compressing the brain. [29]

A subdural hematoma is located between the dura mater and arachnoid mater and is typically associated with rupture of the bridging veins. The concept of "potential space" is particularly important here: normally, there is virtually none, but during bleeding, the layers separate, forming a significant volume. [30]

Spontaneous intracranial hypotension is associated with a cerebrospinal fluid leak, most often at the level of the spinal meninges, and presents with orthostatic headache. Characteristic features on contrast-enhanced magnetic resonance imaging, including diffuse enhancement of the dura mater and possible subdural collections, are important for diagnosis. [31]

Table 6. Space, typical problem and main verification method

Where does the process take place?	A typical example	What usually helps to confirm
Epidural space	Epidural hematoma	Computed tomography of the head
Subdural space	Subdural hematoma	Computed tomography or magnetic resonance imaging
Subarachnoid space	Subarachnoid hemorrhage	CT scan followed by vascular imaging
Leptomeninges and cerebrospinal fluid	Bacterial meningitis	Lumbar puncture and cerebrospinal fluid analysis, culture and polymerase chain reaction as indicated
Dura mater and cerebrospinal fluid pressure	Spontaneous intracranial hypotension	Magnetic resonance imaging of the brain and spine with contrast

Source for the table. [32]