Within the head are small cavities or spaces referred to as sinuses, and they primarily serve as drainage for venous blood. Some are found within the cranial bones and others, in soft tissues of the head. Several of such sinuses are located within the dura mater (a bilaminar membrane, also called pachymeninx, that is one of the three coverings of the brain constituting the cranial meninges). They include the:
- superior sagittal sinus
- inferior sagittal sinus
- straight sinus
- occipital sinus
- transverse sinuses,
- cavernous sinus
- basilar sinus
- superior petrosal sinuses
- inferior petrosal sinuses
- sigmoid sinuses
This article will focus on the sigmoid sinuses, together with some relevant clinical information.
Course and Drainage
The sigmoid sinuses are a pair of endothelium-lined spaces located between the periosteal and the meningeal layers of the dura mater. They follow an S-shaped course in the posterior cranial fossa, forming deep grooves in the temporal and occipital bones.
Each sigmoid sinus turns anteriorly and then continues inferiorly as the internal jugular vein (IJV) after traversing the jugular foramen, thus expanding into the jugular bulb which occupies the posterior and largest compartment in the jugular foramen. The sigmoid sinus commences as the termination of the transverse sinus, deeply grooving the inner surface of the mastoid part of the petrous bone, as well as the temporal and occipital bones.
It is connected with the exterior in its upper part by the mastoid emissary vein, which joins the posterior auricular vein and in its lower part by a vein, which passes through the posterior condylar foramen to join the sub-occipital plexus of veins. It receives the superior petrosal sinus at its upper end and the occipital sinus at its lower end. Cerebellar veins drain into it, and it receives veins also from the mastoid air cells. Thrombophlebitis in these veins may lead to cerebellar abscess (pus) from mastoid infection.
Occlusion of dural venous sinuses may result from thrombi (clots), thrombophlebitis (venous inflammation), or tumors (for example, meningiomas). However, the transverse, cavernous, and superior sagittal sinuses are the most susceptible to thromboses.
Also, metastasis of tumor cells and pus in abscesses to the dural sinuses is possible. Because the sigmoid sinuses and all of the other dural venous sinuses are valveless, compression of the thorax, abdomen, or pelvis, as occurs during heavy coughing and straining, may force venous blood from these regions into the internal vertebral venous system, and from it into the dural venous sinuses. As a result, pus and tumor cells in these regions may spread to the vertebrae and brain through the basilar and occipital sinuses, which directly communicate with the internal vertebral venous plexuses.
Fractures of the cranial base may affect or even tear the internal carotid artery, producing an arteriovenous fistula within the sinus. Of the dural venous sinuses, the cavernous sinus is the most prone to this injury. When such cranial base fractures occur, arterial blood rushes into the cavernous sinus, enlarging it and forcing retrograde blood flow into its venous tributaries, especially the ophthalmic veins. As a result, the eyeball protrudes (a condition known as exopthalmos) and the conjunctiva becomes engorged (a condition referred to as chemosis).