The transverse sinus is a paired venous channel located within the brain’s dura mater, specifically in the posterior cranial cavity. It is part of the dural venous sinuses, which are responsible for draining venous blood from the brain. The transverse sinus extends laterally from the confluence of sinuses, located near the internal occipital protuberance, and runs horizontally along the attached margin of the tentorium cerebelli. It curves downwards at the base of the skull, eventually transitioning into the sigmoid sinus. There is one transverse sinus on each side of the head, left and right, forming a crucial part of the brain’s venous drainage system.
Structure and Anatomy
Location and Structure
The transverse sinus is situated within the dura mater, which forms part of the cranial meninges. It lies along the posterior aspect of the cranial cavity, running laterally from the midline near the internal occipital protuberance, where the confluence of sinuses is located. This confluence is the meeting point of the superior sagittal sinus, straight sinus, and occipital sinus. The transverse sinus extends bilaterally from this confluence, following a curved, horizontal path along the attachment of the tentorium cerebelli to the occipital bone.
Course and Path
The sinus travels horizontally from the midline toward the lateral parts of the skull. It runs in the groove for the transverse sinus, a depression in the occipital bone, and continues to the junction of the occipital and temporal bones. At this point, the transverse sinus curves inferiorly and medially, transitioning into the sigmoid sinus, which eventually drains into the internal jugular vein.
Surrounding Structures
- Tentorium Cerebelli: The transverse sinus is located at the attached margin of the tentorium cerebelli, a fold of dura mater that separates the cerebrum from the cerebellum.
- Occipital Bone: The sinus runs along a groove in the occipital bone, where it is protected within the cranial cavity.
- Temporal Bone: As the sinus approaches the lateral portion of the skull, it passes near the mastoid part of the temporal bone before transitioning into the sigmoid sinus.
Sinus Walls
The transverse sinus is lined by endothelium and is situated between two layers of dura mater: the periosteal layer and the meningeal layer. The wall of the sinus does not have valves, allowing venous blood to flow freely in either direction, depending on pressure gradients.
Variations
There is considerable anatomical variation in the size and dominance of the transverse sinuses. Typically, one side (usually the right) is dominant, meaning it is larger and carries more blood. The other side may be smaller or even hypoplastic (underdeveloped), contributing less to venous drainage. The degree of asymmetry varies between individuals.
Tributaries
Several veins drain into the transverse sinus, including:
- Superior petrosal sinus: Drains from the cavernous sinus.
- Inferior cerebral veins: Drain the cerebrum’s lower parts.
- Inferior cerebellar veins: Drain the cerebellum.
- Diploic veins: Drain blood from the diploë of the skull bones.
- Emissary veins: Connect the venous system of the scalp with the intracranial sinuses.
Junction with Sigmoid Sinus
At its lateral extremity, the transverse sinus curves downward to become the sigmoid sinus. This transition occurs near the petrous part of the temporal bone and forms an S-shaped path before leading to the jugular foramen, where the sigmoid sinus continues as the internal jugular vein.
Function
Venous Drainage of the Brain
The primary function of the transverse sinus is to drain deoxygenated blood from the brain and return it to the systemic circulation. It collects venous blood from various sinuses within the cranial cavity and directs it toward the sigmoid sinus, which then empties into the internal jugular vein. This process is crucial for maintaining proper intracranial pressure and ensuring the removal of metabolic waste from the brain.
Collection of Blood from Multiple Sources
The transverse sinus acts as a central conduit for several major venous sinuses:
- Superior Sagittal Sinus: This sinus collects blood from the superficial parts of the brain, especially the cerebral hemispheres, and drains it into the confluence of sinuses, from which the transverse sinus emerges.
- Straight Sinus: Drains blood from the deep parts of the brain, including the deep cerebral veins and the inferior sagittal sinus.
- Occipital Sinus: Contributes venous blood from the posterior aspects of the brain and the cerebellum.
By collecting blood from these different sources, the transverse sinus serves as a key pathway in the brain’s venous system, efficiently channeling blood toward the internal jugular vein.
Maintaining Cerebral Circulation
The transverse sinus helps regulate cerebral circulation by ensuring that blood flow from the brain’s superficial and deep structures is properly drained. This function is essential for maintaining homeostasis in the brain, as any blockage or impairment in venous drainage could lead to increased intracranial pressure and complications such as brain swelling.
Prevention of Venous Blood Stagnation
Since the transverse sinus lacks valves, it allows for the free flow of blood based on pressure gradients. This characteristic prevents the stagnation of venous blood within the cranial cavity. The open, valveless system ensures that any increase in venous pressure can be alleviated by redirecting blood flow to less congested areas, thus contributing to the dynamic regulation of intracranial pressure.
Connection to the Emissary Veins
The transverse sinus also receives blood from the emissary veins, which connect the intracranial venous system with the veins of the scalp. This connection provides an alternative route for venous blood to exit the cranial cavity, helping to maintain proper circulation under various physiological conditions. The emissary veins also play a role in thermoregulation by allowing heat exchange between the venous blood in the scalp and the cranial cavity.
Role in Cerebrospinal Fluid Absorption
The transverse sinus indirectly aids in the absorption of cerebrospinal fluid (CSF). CSF is reabsorbed into the venous system via arachnoid granulations, small protrusions of the arachnoid mater that extend into the dural venous sinuses, including the transverse sinus. By facilitating the drainage of blood from the brain, the transverse sinus helps ensure that CSF is efficiently returned to the bloodstream, maintaining a balance between CSF production and absorption.
Pressure Regulation in the Brain
Through its connections with other venous sinuses and the internal jugular vein, the transverse sinus plays a vital role in regulating intracranial pressure. It provides a pathway for the drainage of excess blood and cerebrospinal fluid, which helps prevent conditions like hydrocephalus or increased intracranial pressure. When the pressure within the transverse sinus decreases or increases, it influences the overall venous outflow from the brain, affecting the equilibrium of pressure within the cranial cavity.
Clinical Significance
The transverse sinus is clinically significant due to its role in draining venous blood from the brain. Any disruption or blockage, such as in cerebral venous sinus thrombosis (CVST), can lead to increased intracranial pressure, causing symptoms like headaches, seizures, and even stroke. Thrombosis in the transverse sinus can result from conditions like dehydration, infection, trauma, or a hypercoagulable state.
The asymmetry of the transverse sinuses, where one side is often dominant, can influence the presentation of clinical conditions. For example, compression or damage to the dominant transverse sinus may have more severe consequences, leading to significant neurological deficits.
