The sigmoid sinus is a dural venous sinus that forms an essential part of the brain’s venous drainage system. It is responsible for channeling deoxygenated blood from the transverse sinus to the internal jugular vein.
Location
The sigmoid sinus is located at the posterior part of the cranial cavity, following an S-shaped course within the dura mater. It originates at the transverse sinus, running along the temporal bone and the mastoid portion of the temporal bone. From there, it curves downward, passing through the jugular foramen, where it becomes continuous with the internal jugular vein. The sinus is situated near the petrous part of the temporal bone, the occipital bone, and the cerebellum, making it a key structure in the posterior cranial fossa.
Structure and Anatomy
The sigmoid sinus is a prominent venous structure in the brain’s dural venous sinus system, responsible for carrying blood from the transverse sinus to the internal jugular vein. Its distinct S-shaped course and key anatomical relationships make it an essential component of cerebral venous drainage. Below is a detailed description of the anatomy of the sigmoid sinus.
General Structure
The sigmoid sinus is a dural venous sinus, meaning it lies between the two layers of the dura mater, the outermost membrane covering the brain. It is a continuation of the transverse sinus and takes an S-shaped curve as it descends toward the base of the skull. Like other venous sinuses, the sigmoid sinus is valveless, allowing blood to flow freely in response to pressure gradients.
Course
The sigmoid sinus begins as a continuation of the transverse sinus, located at the posterior aspect of the cranial cavity. The transverse sinus runs horizontally along the tentorium cerebelli, a dural fold separating the cerebrum from the cerebellum. The transition from the transverse sinus to the sigmoid sinus occurs at the junction where the transverse sinus curves downward near the mastoid part of the temporal bone.
The sigmoid sinus follows a distinctive S-shaped curve as it descends, passing behind the petrous part of the temporal bone and close to the mastoid air cells. It continues downward, eventually exiting the cranial cavity through the jugular foramen, where it becomes the internal jugular vein. This S-shaped curve makes the sigmoid sinus one of the most recognizable sinuses in neuroanatomy.
Tributaries
The sigmoid sinus receives blood from several tributaries as it courses through the posterior cranial fossa. These include:
- Transverse sinus: The most significant tributary, the transverse sinus drains the superior sagittal sinus and the straight sinus, carrying venous blood from the superficial and deep parts of the brain.
- Superior petrosal sinus: This smaller venous sinus drains the cavernous sinus and empties into the sigmoid sinus near its origin at the transverse sinus.
- Mastoid emissary vein: This vein connects the sigmoid sinus to the veins of the scalp and the posterior auricular vein. The mastoid emissary vein plays a role in equalizing venous pressure between the inside and outside of the skull.
- Occipital emissary veins: These veins drain blood from the occipital region of the skull and can also empty into the sigmoid sinus.
Jugular Foramen and Transition to the Internal Jugular Vein
The sigmoid sinus terminates as it passes through the jugular foramen, an opening in the skull located between the petrous portion of the temporal bone and the occipital bone. The jugular foramen is a key anatomical structure that allows venous blood to exit the cranial cavity. As the sigmoid sinus passes through the foramen, it becomes continuous with the internal jugular vein, the major vein responsible for returning deoxygenated blood from the brain to the heart.
The jugular foramen also serves as a passageway for several important cranial nerves, including the glossopharyngeal nerve (CN IX), vagus nerve (CN X), and accessory nerve (CN XI), which travel alongside the sigmoid sinus as it transitions into the internal jugular vein.
Relations to Surrounding Structures
The sigmoid sinus is closely related to several important anatomical structures within the posterior cranial fossa. These include:
- Petrous part of the temporal bone: The sigmoid sinus runs along the posterior aspect of the petrous ridge, a part of the temporal bone that houses the structures of the inner ear.
- Mastoid air cells: These are small, air-filled cavities within the mastoid part of the temporal bone. The sigmoid sinus lies near these air cells, which can be relevant in conditions such as mastoiditis, where infection can spread to the sinus.
- Cerebellum: The sigmoid sinus lies close to the cerebellum, which is located in the posterior cranial fossa. This proximity is important in the context of cerebellar and posterior fossa surgery.
- Cranial nerves: As it exits the skull through the jugular foramen, the sigmoid sinus is in close proximity to the glossopharyngeal (CN IX), vagus (CN X), and accessory (CN XI) nerves. These nerves exit the skull alongside the internal jugular vein.
Dural Attachments
The sigmoid sinus is securely attached to the dura mater along its entire course. The dura mater provides structural support to the sinus, ensuring that it remains patent (open) for blood flow. The attachment of the sinus to the dura mater also helps to stabilize the sinus during changes in intracranial pressure or physical movement.
Venous Drainage Network
The sigmoid sinus is part of a larger venous drainage network that includes other major dural sinuses. It acts as a conduit for blood from the transverse sinus, carrying deoxygenated blood from the brain’s superficial and deep structures. After passing through the jugular foramen and becoming the internal jugular vein, the blood is transported out of the cranial cavity and toward the heart.
Confluence of Sinuses: Venous blood from the superior sagittal sinus, straight sinus, and occipital sinus converges at the confluence of sinuses. From there, blood is directed into the transverse sinuses, which then flow into the sigmoid sinuses.
This network ensures that venous blood is efficiently drained from the brain, reducing the risk of venous congestion and maintaining normal intracranial pressure.
Anatomical Variations
The anatomy of the sigmoid sinus can vary between individuals. For example, the size and shape of the sinus may differ, and variations in the connection between the transverse and sigmoid sinuses are common. Additionally, the relationship between the sigmoid sinus and the jugular bulb (the dilated part of the internal jugular vein just below the jugular foramen) can vary, which may be relevant in surgical planning or in cases of vascular pathology.
Some individuals may have differences in the symmetry of the sinuses, with one side being more dominant in terms of venous drainage. These variations are typically benign but can be important to recognize during imaging studies or neurosurgical procedures.
Function
The sigmoid sinus plays an essential role in the brain’s venous drainage system, responsible for directing deoxygenated blood from the brain back to the systemic circulation. Its strategic location and course allow it to perform several vital functions in maintaining proper cerebral circulation and pressure regulation. Below is a detailed explanation of its functions.
Draining Venous Blood from the Transverse Sinus
The primary function of the sigmoid sinus is to drain venous blood from the transverse sinus. The transverse sinus collects blood from several key regions of the brain, including the superior sagittal sinus, straight sinus, and occipital sinus. These sinuses drain blood from the cerebral hemispheres, cerebellum, and other structures in the posterior cranial fossa.
After blood is collected by the transverse sinus, it flows into the sigmoid sinus, which directs the blood downward toward the jugular foramen, where it transitions into the internal jugular vein. This continuous drainage is crucial for maintaining proper blood flow and preventing the accumulation of venous blood within the brain.
Transport of Blood to the Internal Jugular Vein
The sigmoid sinus is the final segment in the venous drainage system before the blood leaves the cranial cavity through the internal jugular vein. As the sigmoid sinus passes through the jugular foramen, it becomes the internal jugular vein, the major vein responsible for returning deoxygenated blood from the brain to the heart.
This function is critical because it ensures that venous blood, which contains metabolic waste products and carbon dioxide, is efficiently transported out of the brain and into the systemic venous system for reoxygenation. The smooth transition from the sigmoid sinus into the internal jugular vein is essential for maintaining the proper flow of blood from the brain to the body.
Drainage from the Posterior Cranial Fossa
The sigmoid sinus is located in the posterior cranial fossa, an area of the skull that houses important structures like the cerebellum, brainstem, and occipital lobes. The sinus helps drain venous blood from these regions, which are critical for motor coordination, balance, and vital functions like breathing and heart rate.
Venous blood from the cerebellum and other parts of the posterior cranial fossa is collected by smaller veins and directed into the transverse and sigmoid sinuses. The sigmoid sinus plays a crucial role in ensuring that blood from these deep brain structures is efficiently drained and transported out of the cranial cavity.
Communication with Other Venous Sinuses
The sigmoid sinus is part of a larger network of dural venous sinuses that work together to ensure efficient venous drainage from the brain. In addition to receiving blood from the transverse sinus, the sigmoid sinus also receives blood from the superior petrosal sinus, which drains the cavernous sinus and other structures near the base of the brain.
The sigmoid sinus functions as a major conduit, collecting venous blood from multiple sources and directing it into the internal jugular vein. This communication between sinuses ensures that venous blood from various regions of the brain, including both superficial and deep structures, is efficiently transported out of the cranial cavity.
Equalization of Intracranial Pressure
The sigmoid sinus plays a role in regulating intracranial pressure (ICP) by ensuring the proper outflow of venous blood from the brain. Intracranial pressure is influenced by the balance between the production and drainage of cerebrospinal fluid (CSF), as well as the volume of venous blood within the brain. If venous drainage is impaired, it can lead to increased ICP, which can cause headaches, visual disturbances, and, in severe cases, brain herniation.
By draining blood from the brain and facilitating its exit through the internal jugular vein, the sigmoid sinus helps maintain normal ICP. Proper venous drainage through the sigmoid sinus is essential to prevent the buildup of pressure within the cranial cavity, which could otherwise lead to neurological complications.
Draining Blood from the Mastoid Region
The sigmoid sinus is closely related to the mastoid part of the temporal bone, and it receives venous blood from the mastoid emissary veins. These veins connect the venous system of the scalp and skull with the intracranial venous sinuses, providing an alternate route for venous blood to drain from the outside of the skull.
By receiving blood from the mastoid region, the sigmoid sinus plays a role in the drainage of venous blood from the mastoid air cells, which are small air-filled spaces within the temporal bone. This function is particularly important because infection or inflammation in the mastoid air cells (such as in mastoiditis) can lead to complications that affect the sinus. The sigmoid sinus provides a pathway for draining blood from this region, helping to maintain proper circulation within the temporal bone and surrounding areas.
Collaboration with Emissary Veins
The sigmoid sinus connects with emissary veins, which provide a link between the extracranial venous system and the intracranial venous sinuses. The mastoid emissary vein is one of the most significant of these connections, allowing venous blood to flow between the posterior auricular vein (which drains the scalp) and the sigmoid sinus.
These emissary veins act as safety valves, helping to balance venous pressure between the inside and outside of the skull. This collateral circulation function ensures that venous blood can be rerouted in cases of increased intracranial pressure or blockage in the venous system, preventing venous congestion and maintaining normal circulation.
Venous Outflow from the Cerebellum
As the sigmoid sinus passes through the posterior cranial fossa, it helps facilitate the venous outflow from the cerebellum. The cerebellum is responsible for coordinating movement and balance, and it requires proper venous drainage to function efficiently. The sigmoid sinus collects venous blood from the cerebellar veins and other tributaries, ensuring that blood from this region is removed from the cranial cavity and directed toward the heart.
Proper venous drainage from the cerebellum is essential to prevent venous congestion, which could lead to increased intracranial pressure or cerebellar dysfunction. The sigmoid sinus plays a key role in maintaining the circulation of venous blood from the cerebellum and other posterior cranial structures.
Pressure Regulation During Postural Changes
The sigmoid sinus, along with other venous sinuses, helps regulate intracranial venous pressure during postural changes, such as standing up, lying down, or bending over. Changes in body position can affect venous return from the brain to the heart, and the sigmoid sinus plays a role in ensuring that venous blood flow remains constant despite these changes in pressure.
The sigmoid sinus allows for the equalization of pressure within the cranial venous system, helping to prevent dizziness or other symptoms that can occur when blood flow from the brain is momentarily disrupted by changes in posture.
Draining Blood from the Occipital Region
The sigmoid sinus also helps drain venous blood from the occipital region of the brain, which is responsible for visual processing. Venous blood from the occipital lobes is directed into the transverse sinus, which flows into the sigmoid sinus. The sigmoid sinus ensures that this blood is efficiently transported out of the brain and into the internal jugular vein, maintaining proper circulation within the occipital region.
Clinical Significance
The sigmoid sinus is clinically significant due to its critical role in draining venous blood from the brain and its close proximity to several important anatomical structures.
- Cerebral Venous Sinus Thrombosis (CVST): A thrombus (blood clot) in the sigmoid sinus can lead to cerebral venous sinus thrombosis, causing venous congestion, increased intracranial pressure, headaches, seizures, and, in severe cases, neurological deficits or stroke. Early diagnosis and treatment are essential to prevent serious complications.
- Mastoiditis and Sigmoid Sinus Thrombophlebitis: Because the sigmoid sinus is located near the mastoid air cells, infections like mastoiditis can spread to the sinus, leading to sigmoid sinus thrombophlebitis. This condition can obstruct venous drainage, resulting in neurological symptoms and requiring urgent medical intervention.
- Surgical Considerations: The sigmoid sinus is located near the temporal bone, and its proximity to critical structures like the cerebellum and cranial nerves makes it important in surgeries involving the posterior cranial fossa. Injury to the sinus during surgery can result in significant blood loss and complications related to venous drainage.
