Label The Cranial Dura Septa And Associated Sinuses

The intricate architecture of the cranial dura mater houses essential structures that support and protect the brain. Understanding the dural septa and associated sinuses is crucial for medical professionals in fields like neurology, neurosurgery, and radiology. This article provides a detailed overview of these structures, their functions, and their clinical significance.

Introduction to the Cranial Dura Mater

The dura mater is the outermost of the three layers of meninges surrounding the brain and spinal cord. It is a thick, strong, and fibrous membrane that provides significant protection to the central nervous system. Unlike the other meningeal layers (arachnoid mater and pia mater), the dura mater is composed of two layers in the cranial cavity:

• Periosteal Layer: This outer layer adheres to the inner surface of the skull and serves as the periosteum for the cranial bones.
• Meningeal Layer: This inner layer is a true membrane that covers the brain and spinal cord.

In most areas, these two layers are fused together. However, in certain regions, they separate to form the dural venous sinuses, which are critical for draining blood from the brain. Additionally, the meningeal layer folds inward to create the dural septa, partitioning the cranial cavity and providing further support to the brain.

Dural Septa: Divisions and Support

The dural septa are infoldings of the meningeal layer of the dura mater that divide the cranial cavity into compartments. These septa not only provide structural support but also limit the displacement of the brain during movement or trauma. The major dural septa include:

1. Falx Cerebri: This is the largest of the dural septa and is a sickle-shaped fold that lies in the longitudinal fissure, separating the two cerebral hemispheres.
2. Tentorium Cerebelli: This tent-like structure separates the cerebrum from the cerebellum.
3. Falx Cerebelli: A small, sickle-shaped fold that separates the two cerebellar hemispheres.
4. Diaphragma Sellae: The smallest of the dural septa, it covers the pituitary gland and the sella turcica.

1. Falx Cerebri

The falx cerebri (Latin for "sickle of the brain") is a large, crescent-shaped fold of dura mater that descends vertically in the longitudinal fissure between the left and right cerebral hemispheres.

• Attachment: Anteriorly, it is attached to the crista galli of the ethmoid bone. Posteriorly, it merges with the tentorium cerebelli.
• Sinuses: The falx cerebri contains two major venous sinuses:
  • Superior Sagittal Sinus: Runs along the superior margin of the falx cerebri.
  • Inferior Sagittal Sinus: Runs along the inferior margin of the falx cerebri.

Function: The primary function of the falx cerebri is to limit the movement of the cerebral hemispheres in relation to each other, thus preventing injury during head movements.

2. Tentorium Cerebelli

The tentorium cerebelli (Latin for "tent of the cerebellum") is a broad, tent-shaped dural fold that separates the occipital lobes of the cerebrum from the cerebellum and brainstem.

• Attachment: It attaches to the petrous part of the temporal bone, the superior border of the petrous ridge, and the internal surface of the occipital bone. Anteriorly, it attaches to the clinoid processes of the sphenoid bone.
• Notch: The anterior free edge of the tentorium cerebelli forms the tentorial notch (also known as the incisura), an opening through which the brainstem passes.
• Sinuses: The tentorium cerebelli contains several venous sinuses:
  • Transverse Sinuses: Run along the posterolateral attachment of the tentorium.
  • Superior Petrosal Sinuses: Run along the superior border of the petrous part of the temporal bone.
  • Straight Sinus: Located along the line of attachment of the falx cerebri to the tentorium cerebelli.

Function: The tentorium cerebelli supports the occipital lobes, prevents the weight of the cerebrum from compressing the cerebellum and brainstem, and creates two main compartments within the cranial cavity: the supratentorial and infratentorial compartments.

3. Falx Cerebelli

The falx cerebelli is a small, sickle-shaped dural fold that projects downward from the inferior surface of the tentorium cerebelli, separating the two cerebellar hemispheres.

• Attachment: It attaches to the internal occipital crest and extends into the posterior cerebellar notch.
• Sinus: The falx cerebelli contains the occipital sinus in its posterior margin.

Function: The falx cerebelli helps to stabilize the cerebellar hemispheres and limit their movement.

4. Diaphragma Sellae

The diaphragma sellae is the smallest of the dural septa, forming a horizontal roof over the sella turcica, a saddle-shaped depression in the sphenoid bone that houses the pituitary gland.

• Attachment: It attaches to the tuberculum sellae and the anterior and posterior clinoid processes.
• Opening: It has a central opening that allows the pituitary stalk (infundibulum) to pass through, connecting the hypothalamus to the pituitary gland.

Function: The diaphragma sellae protects the pituitary gland and helps to keep it in place within the sella turcica.

Dural Venous Sinuses: Drainage Pathways

The dural venous sinuses are endothelial-lined spaces located between the periosteal and meningeal layers of the dura mater. These sinuses serve as the primary drainage pathway for blood from the brain, eventually leading to the internal jugular veins. The major dural venous sinuses include:

1. Superior Sagittal Sinus (SSS)
2. Inferior Sagittal Sinus (ISS)
3. Straight Sinus (SS)
4. Transverse Sinuses (TS)
5. Sigmoid Sinuses (SiS)
6. Occipital Sinus (OS)
7. Cavernous Sinuses (CS)
8. Superior Petrosal Sinuses (SPS)
9. Inferior Petrosal Sinuses (IPS)

1. Superior Sagittal Sinus (SSS)

The superior sagittal sinus is a large, unpaired sinus that runs along the superior margin of the falx cerebri.

• Location: It begins at the crista galli and extends posteriorly to the confluence of sinuses.
• Drainage: It receives blood from the superior cerebral veins and cerebrospinal fluid (CSF) from the arachnoid granulations.

2. Inferior Sagittal Sinus (ISS)

The inferior sagittal sinus is a smaller, unpaired sinus that runs along the inferior margin of the falx cerebri.

• Location: It runs posteriorly and joins the great cerebral vein of Galen to form the straight sinus.
• Drainage: It primarily drains blood from the falx cerebri and the medial aspect of the cerebral hemispheres.

3. Straight Sinus (SS)

The straight sinus is an unpaired sinus that runs along the attachment of the falx cerebri to the tentorium cerebelli.

• Formation: It is formed by the union of the inferior sagittal sinus and the great cerebral vein of Galen.
• Drainage: It drains into the confluence of sinuses.

4. Transverse Sinuses (TS)

The transverse sinuses are paired sinuses that run horizontally along the posterolateral attachment of the tentorium cerebelli.

• Location: They originate at the confluence of sinuses and curve forward and medially to become the sigmoid sinuses.
• Drainage: They drain blood from the superior sagittal sinus, straight sinus, and other smaller sinuses.

5. Sigmoid Sinuses (SiS)

The sigmoid sinuses are paired, S-shaped sinuses that continue from the transverse sinuses.

• Course: They pass through the jugular foramen to become the internal jugular veins.
• Drainage: They drain blood from the transverse sinuses and receive blood from the inferior cerebral and cerebellar veins.

6. Occipital Sinus (OS)

The occipital sinus is a small, unpaired sinus that runs along the falx cerebelli.

• Location: It begins near the foramen magnum and drains into the confluence of sinuses.
• Drainage: It drains blood from the posterior cranial fossa.

7. Cavernous Sinuses (CS)

The cavernous sinuses are paired, complex venous structures located on either side of the sella turcica.

• Location: They receive blood from the superior and inferior ophthalmic veins, superficial middle cerebral vein, and sphenoparietal sinus.
• Contents: They contain the internal carotid artery and several cranial nerves (CN III, CN IV, CN V1, CN V2, and CN VI).
• Drainage: They drain into the superior and inferior petrosal sinuses.

8. Superior Petrosal Sinuses (SPS)

The superior petrosal sinuses are paired sinuses that run along the superior border of the petrous part of the temporal bone.

• Location: They connect the cavernous sinus to the sigmoid sinus.
• Drainage: They drain blood from the cavernous sinus and the middle ear.

9. Inferior Petrosal Sinuses (IPS)

The inferior petrosal sinuses are paired sinuses that run along the petro-occipital fissure.

• Location: They connect the cavernous sinus to the internal jugular vein.
• Drainage: They drain blood from the cavernous sinus, the internal ear, and the brainstem.

Clinical Significance

Understanding the anatomy of the dural septa and associated sinuses is crucial for diagnosing and managing various neurological conditions. Some clinical implications include:

1. Subdural Hematoma: Bleeding between the dura mater and the arachnoid mater, often caused by trauma. The falx cerebri and tentorium cerebelli can limit the spread of the hematoma.
2. Epidural Hematoma: Bleeding between the skull and the dura mater, often associated with skull fractures. The periosteal layer of the dura is tightly adhered to the skull, limiting the spread.
3. Subarachnoid Hemorrhage: Bleeding into the subarachnoid space, often caused by ruptured aneurysms. Blood can spread throughout the CSF pathways, including the dural sinuses.
4. Sinus Thrombosis: Formation of a blood clot within the dural venous sinuses, which can lead to increased intracranial pressure, cerebral edema, and neurological deficits.
5. Mass Effect and Herniation: Lesions such as tumors or abscesses can cause mass effect, leading to displacement of brain tissue. The dural septa can influence the pattern of herniation. For example, subfalcine herniation occurs under the falx cerebri, while transtentorial herniation occurs through the tentorial notch.
6. Cavernous Sinus Syndrome: Damage to the cavernous sinus, often due to tumors, infections, or thrombosis, can affect the cranial nerves that pass through it, leading to ophthalmoplegia, facial pain, and vision changes.
7. Empty Sella Syndrome: A condition in which the sella turcica is filled with CSF, causing the pituitary gland to be compressed. This can result from a defect in the diaphragma sellae.
8. Hydrocephalus: Blockage of CSF flow can lead to hydrocephalus, and the dural sinuses play a role in CSF absorption through the arachnoid granulations.
9. Intracranial Hypertension: Elevated pressure within the skull, which can be caused by a variety of factors, including dural sinus thrombosis, tumors, and hydrocephalus.

Imaging Techniques

Several imaging techniques are used to visualize the dural septa and venous sinuses:

• Computed Tomography (CT): CT scans can identify fractures, hematomas, and other abnormalities that affect the dura mater and adjacent structures. CT angiography can be used to visualize the dural venous sinuses and detect thrombosis.
• Magnetic Resonance Imaging (MRI): MRI provides detailed images of the brain and surrounding structures, including the dural septa and venous sinuses. MR venography is particularly useful for visualizing the venous sinuses and detecting thrombosis.
• Digital Subtraction Angiography (DSA): DSA is an invasive imaging technique that involves injecting contrast dye into the blood vessels to visualize the dural venous sinuses. It is often used to diagnose and treat dural arteriovenous fistulas (dAVFs).

Development of Dural Structures

The development of the dural septa and venous sinuses is a complex process that occurs during embryonic and fetal development. The dura mater originates from the mesoderm and undergoes significant remodeling to form the various septa and sinuses.

• The falx cerebri begins to develop early in gestation and gradually extends downward into the longitudinal fissure.
• The tentorium cerebelli develops as an extension of the dura mater between the developing cerebrum and cerebellum.
• The dural venous sinuses form through a process of angiogenesis and remodeling of the dural vasculature.

Conclusion

The cranial dura mater, with its septa and associated sinuses, plays a vital role in protecting and supporting the brain. The dural septa divide the cranial cavity into compartments, limiting brain displacement and providing structural support. The dural venous sinuses serve as the primary drainage pathway for blood from the brain. A thorough understanding of these structures is essential for medical professionals in diagnosing and managing various neurological conditions. Advanced imaging techniques such as CT and MRI are invaluable tools for visualizing these structures and detecting abnormalities.