The sigmoid sinus is a major, paired blood vessel within the head, with one on the left and one on the right side. Its primary role is to carry deoxygenated blood away from the brain. This vessel represents the final segment of the brain’s main venous drainage system before the blood exits the skull. It is an important part of the cranial circulation, ensuring efficient removal of blood from the central nervous system.
Defining the Dural Venous Sinus System
The sigmoid sinus belongs to the dural venous sinuses (DVS), a unique set of vessels unlike typical veins found elsewhere in the body. These sinuses are specialized channels formed between the two protective layers of the dura mater, the tough, outermost membrane covering the brain and spinal cord. The DVS network collects all the venous blood circulating within the cranial cavity.
A key distinction of the dural sinuses is their structure, as they lack the muscular walls and valves characteristic of conventional veins. This absence of valves means blood flow is more susceptible to changes in intracranial pressure. The sigmoid sinus is considered one of the terminal sinuses, collecting blood from upstream vessels before it leaves the cranium.
The S-Shaped Course and Precise Location
The location of the sigmoid sinus is defined by its continuation from the transverse sinus, a large vessel running along the back of the skull. Each sigmoid sinus begins where the transverse sinus curves downward and away from the tentorium cerebelli, the dural fold separating the cerebrum from the cerebellum. From this point, the vessel immediately adopts its namesake S-shaped course.
This distinctive S-curve allows the sinus to navigate the complex contours of the skull’s base, following a groove on the inner surface of the temporal bone. It runs adjacent to the mastoid part of the temporal bone, the bony prominence located just behind the ear. As it descends, it crosses the jugular process of the occipital bone. The sigmoid sinus terminates precisely at the jugular foramen, an opening in the skull base, where it transitions into the internal jugular vein, the major vessel that carries blood down the neck.
Function in Cerebral Blood Drainage
The sigmoid sinus acts as the final conduit for venous blood leaving a large portion of the brain. It collects blood delivered by the transverse sinus, which has already received inflow from several upstream vessels. This includes blood from the superior sagittal sinus and the straight sinus, which drain the cerebral hemispheres and deeper brain structures, respectively. The sigmoid sinus thus handles the bulk of the venous return from the posterior and lateral areas of the brain.
The sigmoid sinus also receives contributions from other vessels, including the superior petrosal sinus, which drains structures near the brainstem, and various veins from the cerebellum. It connects with veins outside the skull through tiny channels called emissary veins, which help regulate intracranial pressure. By channeling this extensive venous network, the sigmoid sinus directs deoxygenated blood from the posterior cranial cavity out of the skull.
Common Clinical Relevance
The anatomy of the sigmoid sinus makes it relevant to several clinical conditions involving blood flow and pressure. One serious issue is Dural Venous Sinus Thrombosis (DVST), which involves the formation of a blood clot within the sinus. A blockage in the sigmoid sinus can impair the drainage of blood from the brain, potentially leading to elevated intracranial pressure and, in severe cases, a hemorrhagic stroke.
Another common condition related to the sigmoid sinus is pulsatile tinnitus, the perception of a rhythmic sound synchronized with the heartbeat. This symptom often arises when the sigmoid sinus has anatomical variations, such as a thin or absent layer of bone (dehiscence) separating it from the inner ear. This lack of a bony barrier allows the sound of blood flowing turbulently through the sinus to be transmitted and heard. Tinnitus can also be caused by a narrowing or abnormality within the sinus itself, which alters the speed and character of the blood flow.