The venous sinuses are specialized blood vessels located primarily within the skull. They are channels that collect deoxygenated blood and return it toward the heart. Unlike the familiar veins found elsewhere in the body, these sinuses possess unique structural characteristics and play a dual role in maintaining the delicate fluid balance required for brain health.
Defining the Venous Sinuses and Their Structure
The venous sinuses, specifically known as dural venous sinuses, are channels situated between the two layers of the dura mater, the tough, protective covering surrounding the brain. The outer periosteal layer and the inner meningeal layer of the dura mater separate in certain areas to create these distinct blood-collecting spaces. This location provides structural support and protection.
A key structural feature differentiating these sinuses from standard veins is the composition of their walls, which lack the muscular tissue layer, or tunica media. Furthermore, dural venous sinuses are valveless, meaning they do not have the one-way flaps that prevent the backflow of blood. The absence of muscular walls and valves means blood flow within the sinuses is largely influenced by gravity and changes in pressure within the cranial cavity. This unique structure allows them to collect blood from the cerebral veins, which drain the brain tissue itself.
The Role in Cerebrospinal Fluid Drainage
Beyond collecting venous blood, the dural sinuses perform a unique physiological function related to cerebrospinal fluid (CSF). The brain and spinal cord are constantly bathed in this clear fluid, which is produced to cushion the central nervous system and remove waste. The CSF must be continuously reabsorbed back into the bloodstream at the same rate it is produced.
This reabsorption process occurs largely at the dural venous sinuses through specialized structures called arachnoid granulations. These granulations are small outpouchings of the arachnoid mater—the middle layer of the meninges—that project into the sinus channels. The granulations act as a one-way transfer system, allowing CSF to move from the space surrounding the brain into the lower-pressure venous blood. This mechanism links the venous system directly to the regulation of fluid pressure and homeostasis within the skull.
Major Venous Sinuses and Their Connections
The dural venous sinuses form an interconnected network, with several major named channels contributing to the drainage pathway. The Superior Sagittal Sinus is an unpaired channel running along the midline at the very top of the brain, collecting blood from the brain’s upper surface. Blood then flows posteriorly to the confluence of sinuses, where it meets the Straight Sinus and other channels.
From the confluence, the blood is directed into the paired Transverse Sinuses, which run horizontally along the back of the skull. These sinuses continue forward and curve downward to become the S-shaped Sigmoid Sinuses. The left and right Sigmoid Sinuses exit the skull base and drain into the paired Internal Jugular Veins.
Causes and Symptoms of Sinus Thrombosis
A clinical concern involving these vessels is Cerebral Venous Sinus Thrombosis (CVST), which occurs when a blood clot forms within a dural venous sinus. This blockage prevents blood from draining out of the brain, causing pressure to build up in the blood vessels and potentially leading to swelling or bleeding in the brain tissue. Because the sinuses lack collateral drainage pathways, a blockage quickly increases intracranial pressure.
Common risk factors for CVST involve conditions that increase the blood’s tendency to clot, known as hypercoagulability. These include inherited clotting disorders, the use of oral contraceptives containing estrogen, and the elevated estrogen levels associated with pregnancy and the postpartum period. Infections in the head or face, such as ear infections or sinusitis, can also lead to clot formation by spreading to the nearby sinuses.
The symptoms of CVST are highly variable but often begin with a severe headache that tends to worsen over time. The increased pressure can also cause visual changes and trigger seizures. Other symptoms mimic a stroke:
- Weakness or numbness on one side of the body.
- Difficulty speaking.
- Decreased level of consciousness.
- Blurred vision or double vision.