The sigmoid sinus is an S-shaped channel in the skull that carries deoxygenated blood away from the brain. It sits in a groove carved into the bone at the back and base of the skull, just behind the ear, and serves as one of the final stops in the brain’s venous drainage system before blood exits through the jugular vein in the neck. You have one on each side of your head.
Where It Sits in the Skull
The sigmoid sinus runs through the posterior cranial fossa, the deep bowl-shaped area at the back of the skull that houses the cerebellum and brainstem. It curves in an S-shape (hence the name, from the Greek letter sigma) along the inner surface of the mastoid bone, the bony bump you can feel just behind your earlobe. The sinus sits in a groove etched into this bone, separated from the mastoid air cells and middle ear structures by only a thin layer of bone, sometimes less than a millimeter thick.
This close relationship to the ear is one reason the sigmoid sinus matters clinically. It also explains why ear surgeons need to know exactly where it is before operating on the mastoid bone. In mastoid surgery, the sigmoid sinus forms the rear wall of the surgical cavity. Surgeons use the bony ridge where the digastric muscle attaches as a landmark to locate the junction where the transverse sinus curves downward into the sigmoid sinus.
How Blood Flows Through It
Your brain produces a large volume of venous blood that needs a route back to the heart. Unlike the rest of the body, which uses flexible veins, the brain drains through rigid channels called dural venous sinuses. These are spaces sandwiched between the two layers of the dura mater, the tough membrane lining the inside of the skull.
The flow path works like a river system. Blood from the brain’s surface and deep structures collects into the superior and inferior sagittal sinuses, which feed into the confluence of sinuses at the back of the skull. From there, blood flows laterally through the transverse sinuses, one on each side, running horizontally along the back of the skull. Each transverse sinus then curves downward and forward into the sigmoid sinus, which funnels blood into the internal jugular vein. The jugular vein exits the skull through an opening called the jugular foramen, carrying this blood down the neck and eventually back to the heart.
Size Variations Are Common
The two sigmoid sinuses are rarely identical. In a study of 200 patients using magnetic resonance venography (MRV), roughly half had some variation in their dural venous sinuses. The most common pattern was a smaller (hypoplastic) left transverse sinus paired with a smaller left sigmoid sinus, found in about 12.5% of patients. The right side showed the same paired pattern in about 6.5% of cases. When the transverse sinus on one side is underdeveloped, the sigmoid sinus on that same side tends to be smaller too, since it receives less blood flow.
These variations are normal and typically cause no symptoms. But they matter for imaging interpretation. A naturally small sigmoid sinus on one side can look like a partially blocked one on a scan. Radiologists need to distinguish between a sinus that has always been small and one that has been narrowed by a blood clot, because the treatment implications are completely different.
Sigmoid Sinus Thrombosis
A blood clot forming inside the sigmoid sinus is one of the more serious conditions that can affect it. This falls under the broader category of cerebral venous sinus thrombosis, a condition that blocks the brain’s venous drainage and can raise pressure inside the skull.
The most common symptom is headache, often one that worsens when lying down. Other symptoms depend on the size and extent of the clot but can include seizures, blurred vision lasting several seconds at a time, pulsatile tinnitus (a whooshing sound in the ear that matches your heartbeat), and changes in mental status. In severe cases, a clot can lead to bleeding in the brain, presenting as a sudden, explosive headache followed by rapid neurological decline.
Risk factors include inherited clotting disorders, pregnancy, oral contraceptive use, cancer, infections near the sinuses (such as meningitis or brain abscess), and COVID-19. The condition is diagnosed with CT venography or MR venography, both of which can visualize the clot directly. On a contrast-enhanced CT scan, a characteristic “empty delta sign” appears: the clot shows up as a dark filling defect surrounded by a bright rim of contrast dye.
Pulsatile Tinnitus and the Sigmoid Sinus
If you hear a rhythmic whooshing or thumping in one ear that pulses with your heartbeat, the sigmoid sinus may be involved. Sigmoid sinus dehiscence, where the thin bone separating the sinus from the middle ear wears away or is naturally absent, is one of the most commonly identified causes of pulsatile tinnitus, accounting for 25 to 35% of cases. A related condition, sigmoid sinus diverticulum, involves a small outpouching of the sinus wall that protrudes toward the ear.
Both conditions allow the turbulent flow of blood through the sinus to transmit sound directly to the structures of the middle and inner ear. Women are affected more often than men, and about half the time the problem occurs on the side with the dominant (larger) transverse-sigmoid sinus system, where higher blood volume creates more turbulent flow. Unlike many causes of tinnitus, these structural problems are treatable, which is why pulsatile tinnitus that matches the heartbeat warrants imaging to look specifically at the sigmoid sinus.
Why It Matters in Ear Surgery
The sigmoid sinus is a constant concern during mastoidectomy and other surgeries on the temporal bone. It defines the posterior boundary of the mastoid surgical cavity, with the roof of the cavity formed by the tegmen (the thin plate of bone separating the mastoid from the brain’s temporal lobe) and the front wall formed by the bony ear canal. Surgeons drilling into the mastoid bone must identify the sigmoid sinus early and work carefully around it.
The sinus also serves as a key landmark for neurosurgeons approaching tumors or other problems in the posterior cranial fossa. The junction where the transverse sinus becomes the sigmoid sinus, called the transverse-sigmoid sinus junction, is located using surface landmarks on the mastoid bone. Measurements from cadaver studies show the lower edge of this junction sits roughly 19 mm above and 7 mm behind a specific notch on the mastoid bone, giving surgeons a reliable way to plan their approach without accidentally cutting into the sinus.