Dural sinuses are channels inside the tough outer membrane of the brain that collect used blood and drain it out of the skull. Unlike regular veins, they’re formed by gaps between two layers of the membrane itself, creating rigid, open passages that funnel blood from the brain toward the neck. There are seven major dural sinuses, and together they serve as the brain’s primary drainage system.
Where Dural Sinuses Sit in the Skull
The brain is wrapped in a protective membrane called the dura mater, which has two layers: an outer layer that sticks to the inside of the skull bone, and an inner layer that hugs the brain’s surface. In certain spots, these two layers separate, and the space between them forms a channel lined with a smooth inner surface. These channels are the dural venous sinuses.
Because they’re built into the membrane rather than being freestanding blood vessels, dural sinuses don’t collapse the way regular veins can. Their walls are stiff, held open by the surrounding dura. They also lack valves, meaning blood can technically flow in either direction depending on pressure, though it normally moves toward the exit points at the base of the skull.
The Seven Major Sinuses
Some dural sinuses run along the midline of the brain, while others come in pairs on the left and right sides.
The superior sagittal sinus runs along the top of the brain from front to back, sitting above the large fold of dura that divides the brain’s two hemispheres. It’s the longest sinus and collects blood from the upper surfaces of both hemispheres. The inferior sagittal sinus runs along the bottom edge of that same fold and is much smaller. It merges with a large deep brain vein to form the straight sinus, which runs backward and downward through the dura.
All three of these midline sinuses, along with a small occipital sinus at the back of the skull, converge at a junction point called the confluence of sinuses. This junction sits near the back of the head where the skull bulges out slightly. From there, blood splits into the paired transverse sinuses, which run horizontally along each side of the skull. Each transverse sinus curves downward into an S-shaped sigmoid sinus, which exits the skull through an opening called the jugular foramen and becomes the internal jugular vein in the neck.
The cavernous sinuses are a different beast. They sit on either side of the pituitary gland, deep behind the eyes, and are connected to each other by smaller bridging channels. Rather than a simple open passage, each cavernous sinus is a complex web of venous spaces packed around several critical structures.
What Passes Through the Cavernous Sinus
The cavernous sinus is unusual because major nerves and an artery travel directly through or along its walls. The internal carotid artery, one of the brain’s main blood supplies, passes through the cavernous sinus surrounded by venous blood. Several cranial nerves also run through this area: the nerves controlling eye movement (cranial nerves III, IV, and VI), two branches of the nerve responsible for facial sensation (the ophthalmic and maxillary divisions of cranial nerve V), and a bundle of sympathetic nerve fibers that travel along the artery.
This tight packing of structures explains why problems in the cavernous sinus, such as blood clots or infections, can cause eye movement problems, facial pain, and vision changes all at once.
How Blood and Fluid Drain Through Them
Blood from the brain’s veins empties into the nearest dural sinus, and gravity plus pressure gradients keep it moving toward the jugular veins. The transition from the sigmoid sinus to the jugular vein happens gradually. The sinus widens into a bulb shape at the skull’s base, and over a short stretch the wall structure shifts from the rigid dura-lined channel to a true vein with its own layered wall, reinforced by spiral bands of connective tissue fibers.
Dural sinuses also play a role in recycling cerebrospinal fluid (CSF), the clear liquid that cushions the brain. Tiny finger-like projections called arachnoid granulations poke through the dura and into the sinuses, particularly the superior sagittal sinus. CSF from the space around the brain passes through these projections and enters the venous blood. This transfer is driven by a pressure difference: CSF pressure is slightly higher than the venous pressure inside the sinus, pushing fluid across. The cells of these projections form temporary internal channels, called vacuoles, that shuttle fluid from one side to the other without any permanent openings in the tissue.
Connections to the Outside of the Skull
Small veins called emissary veins pass through tiny holes in the skull and connect the dural sinuses to veins on the scalp and face. Like the sinuses themselves, emissary veins have no valves. Each one is named for the skull opening it passes through. The parietal emissary vein, for instance, links scalp veins to the superior sagittal sinus, while the mastoid emissary vein behind the ear connects to the sigmoid or transverse sinus.
These connections normally help regulate pressure, but they also create a potential route for infections to travel inward. The layer of loose tissue on the scalp where these veins reside is sometimes called the “danger area” because bacteria from a superficial infection, even something as minor as an infected hair follicle, can spread through an emissary vein into a dural sinus. In documented cases, infections reaching the cavernous sinus through the mastoid emissary vein have caused sinus thrombosis and meningitis.
What Happens When a Sinus Gets Blocked
A blood clot inside a dural sinus, called cerebral venous sinus thrombosis, blocks drainage and causes pressure to build up in the brain’s venous system. Symptoms range widely depending on which sinus is affected and how completely it’s blocked. The most common symptom is a headache, which can be the only sign. More severe cases cause seizures, weakness on one side of the body, confusion, or changes in consciousness.
The condition is more common in people with blood clotting disorders, those taking hormonal medications, and people with certain cancers. It can be tricky to identify because its symptoms overlap with arterial stroke and brain hemorrhage. Standard brain imaging may look normal or ambiguous, so diagnosis typically requires specialized imaging that specifically visualizes the venous system, such as magnetic resonance venography or CT venography, which can show where blood flow through the sinuses is interrupted.