Dural venous sinuses: the channels that drain blood from the brain

Dural venous sinuses are venous channels inside the skull, located between the two layers of the dura mater. They are valveless, so blood can flow in either direction between them and the veins that join them, and they form the main drainage route from the brain, mostly to the internal jugular veins. (Radiopaedia: Dural venous sinuses.)

The sinuses drain the cerebrum, cerebellum and brainstem, as well as the diploic veins of the skull bones and the emissary veins that connect them to veins outside the skull, and they empty into the internal jugular vein. (TeachMeAnatomy: The Dural Venous Sinuses.)

Radiopaedia lists five unpaired sinuses (superior sagittal, inferior sagittal, straight, occipital and intercavernous) and seven paired channels (transverse, sigmoid, superior petrosal, inferior petrosal, cavernous and sphenoparietal sinuses and the basilar venous plexus). (Radiopaedia: Dural venous sinuses.)

The cranial dura mater has two layers. The outer periosteal layer lines the inner surface of the skull bones; the inner meningeal layer is continuous with the dura around the spinal cord. The sinuses lie between these two layers. (TeachMeAnatomy: The Meninges.)

The meningeal layer folds inwards to form partitions. The falx cerebri hangs down between the right and left cerebral hemispheres. The tentorium cerebelli separates the occipital lobes from the cerebellum. The falx cerebelli separates the two cerebellar hemispheres, and the diaphragma sellae covers the pituitary fossa. Several sinuses run along the edges of these folds. (TeachMeAnatomy: The Meninges.)

The superior sagittal sinus is the largest dural venous sinus. It runs in the midline along the upper, attached border of the falx cerebri, from the frontal crest at the front of the skull to the internal occipital protuberance at the back, where it joins the confluence of sinuses. (TeachMeAnatomy: The Dural Venous Sinuses.)

It starts in front at the foramen caecum and receives the superficial cortical veins of the cerebral hemispheres. At its back end it usually turns to the right and drains into the right transverse sinus. Variations are common, especially in its front part, which can be duplicated or underdeveloped. (Radiopaedia: Superior sagittal sinus.)

The superior sagittal sinus also returns cerebrospinal fluid to the blood. The arachnoid mater pushes into the dural sinuses as the arachnoid granulations, where CSF is filtered back into the venous blood. (OpenStax Anatomy and Physiology 2e, 13.3: Circulation and the Central Nervous System.)

The inferior sagittal sinus is a smaller sinus that runs from front to back along the lower, free edge of the falx cerebri. It receives veins from the falx and small veins from the medial surfaces of the cerebral hemispheres and drains into the straight sinus. In one MR venography study it could not be seen in 14% of 204 patients. (Radiopaedia: Inferior sagittal sinus.)

The straight sinus lies at the junction of the falx cerebri and the tentorium cerebelli and is triangular in cross-section. Its front end receives the inferior sagittal sinus and the great cerebral vein (vein of Galen), which drains the deep parts of the brain, and it also receives some superior cerebellar veins. (Radiopaedia: Straight sinus.)

The straight sinus runs back and down towards the confluence of sinuses, but its exact ending varies: Radiopaedia gives the confluence in 56% of people, the left transverse sinus in 21% and the right transverse sinus in 13%. (Radiopaedia: Straight sinus.)

The confluence of sinuses (torcular Herophili) lies at the internal occipital protuberance. It is where the superior sagittal, straight and occipital sinuses meet, and from it the two transverse sinuses run out to each side. (TeachMeAnatomy: The Dural Venous Sinuses.)

The occipital sinus is one of the smallest dural sinuses. It lies on the inner surface of the occipital bone, in the attached margin of the falx cerebelli. It begins from small tributaries around the foramen magnum (the marginal sinus), some of which connect with the sigmoid sinus and the vertebral venous plexus, and it drains upwards into the confluence. It can be large or lie off the midline, which matters to surgeons planning an operation in the posterior cranial fossa. (Radiopaedia: Occipital sinus.)

Each transverse sinus runs out from the confluence in the lateral border of the tentorium cerebelli, grooving the occipital bone and the squamous temporal bone. In its front part it receives the inferior anastomotic vein (of Labbé). It ends by becoming the sigmoid sinus at the point where the superior petrosal sinus joins. (Radiopaedia: Transverse sinus.)

The two transverse sinuses are often unequal. In the study quoted by Radiopaedia, the left was hypoplastic in 39% of people and absent in 20%, while the right was hypoplastic in only 6% and absent in 4%; both were symmetrical in 31%. This variation can make it hard to diagnose thrombosis on scans. (Radiopaedia: Transverse sinus.)

The sigmoid sinus continues from the transverse sinus where the tentorium ends. It runs down in an S-shaped groove behind and medial to the mastoid air cells and ends in the jugular bulb in the posterior part of the jugular foramen. It is connected to veins outside the skull by the mastoid and condylar emissary veins. (Radiopaedia: Sigmoid sinus.)

In OpenStax's summary, the transverse sinuses lie near the lambdoid suture and flow into the sigmoid sinuses, which then flow into the internal jugular veins. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

The cavernous sinuses lie on either side of the pituitary fossa and the body of the sphenoid bone, extending from the apex of the orbit in front to the apex of the petrous temporal bone behind. The two sides are joined by the intercavernous sinuses. (Radiopaedia: Cavernous sinus.)

The oculomotor nerve (CN III), trochlear nerve (CN IV) and the ophthalmic (V1) and maxillary (V2) divisions of the trigeminal nerve run in the lateral wall, in that order from top to bottom. The internal carotid artery passes through the sinus itself, surrounded by a plexus of sympathetic nerves, and the abducens nerve (CN VI) runs through the sinus on the inferolateral side of the artery. (Radiopaedia: Cavernous sinus.)

The cavernous sinus receives the superior and inferior ophthalmic veins from the orbit, the superficial middle cerebral vein, the sphenoparietal sinus and the intercavernous sinuses. It drains backwards through the superior petrosal sinus to the transverse sinus and through the inferior petrosal sinus to the jugular bulb. Emissary veins through the foramen ovale and nearby openings connect it with the pterygoid venous plexus outside the skull. (Radiopaedia: Cavernous sinus.)

The overall route is: cerebral and cerebellar veins, then the dural venous sinuses, then the internal jugular vein and the general circulation. The superior and inferior sagittal and straight sinuses lead to the confluence; the transverse sinuses carry blood from the confluence to the sigmoid sinuses, which leave the skull through the jugular foramen as the internal jugular veins. The inferior petrosal sinus joins the internal jugular vein at the jugular foramen. (TeachMeAnatomy: The Dural Venous Sinuses.)

OpenStax describes the same pathway: blood from the brain drains through the superior sagittal sinus to the confluence of sinuses, then through the transverse and sigmoid sinuses into the jugular veins, which return it towards the heart. (OpenStax Anatomy and Physiology 2e, 13.3: Circulation and the Central Nervous System.)

Radiopaedia describes the internal jugular vein as formed by the union of the sigmoid sinus and the inferior petrosal sinus in or just below the jugular foramen, at the jugular bulb. (Radiopaedia: Internal jugular vein.)

Cerebral venous thrombosis is a blockage of the veins or sinuses inside the skull. It is rare, accounting for about 0.5% of all cerebrovascular disease. Headache is the most common symptom (about 90%), and focal neurological deficits and seizures are frequent. A predisposing factor, such as hormonal causes including the oral contraceptive pill, a blood clotting disorder, or a local or systemic cause, is found in most cases (about 87.5%). On contrast CT, a clot in the superior sagittal sinus can show as the "empty delta sign". (Radiopaedia: Cerebral venous thrombosis.)

Cavernous sinus thrombosis is a clot within the cavernous sinus, most often caused by infection spreading from the orbit, the paranasal sinuses or the "danger zone" of the face through the connection between the facial vein and the superior ophthalmic vein. Features include headache, swelling around one eye, bulging of the eye (proptosis), sensitivity to light and cranial nerve palsies, with the abducens nerve (CN VI) most often affected. It can spread quickly to cause meningitis. (TeachMeAnatomy: The Dural Venous Sinuses.)

A subdural haematoma is a collection of venous blood between the dura and the arachnoid mater. It usually results from tearing of the cerebral veins where they cross to enter the dural venous sinuses. (TeachMeAnatomy: The Meninges.)