The cavernous sinus is an anatomical structure deep within the skull, near the brain. It is part of the body’s vascular system, surrounded by neural tissues. Its proximity to nerves and blood vessels makes it a significant region.
Location and Structure
The cavernous sinus is a paired structure, one on each side of the sphenoid bone at the base of the brain. These sinuses extend from behind the eyes to the petrous part of the temporal bone, spanning approximately two centimeters in length and one centimeter in width. Unlike typical veins, it is a dural venous sinus, formed by layers of the dura mater, a protective brain membrane. Its interior has a mesh-like network of fibrous bands, creating interconnected spaces that give it its “cavernous” appearance. The two sinuses communicate through anterior and posterior intercavernous sinuses, situated around the pituitary gland.
Vital Structures Within
The cavernous sinus houses several structures. The internal carotid artery, a major blood vessel supplying the brain, traverses directly through the sinus; it is the only artery in the body completely surrounded by venous blood. Several cranial nerves also pass through or along the sinus walls: the oculomotor (CN III), trochlear (CN IV), abducens (CN VI), and the ophthalmic (V1) and maxillary (V2) divisions of the trigeminal nerve (CN V). The oculomotor, trochlear, and abducens nerves coordinate most eye movements, eyelid movement, and pupil size. The trigeminal nerve’s branches provide sensation to the upper and mid-regions of the face.
Physiological Role
The cavernous sinus serves as a drainage pathway for deoxygenated venous blood from various regions of the head. It collects blood from the brain, the orbits, and parts of the face. Blood flows into the sinus from tributaries like the superior and inferior ophthalmic veins, the superficial middle cerebral vein, and the sphenoparietal sinus. Veins connected to the cavernous sinus lack valves, allowing blood to flow in multiple directions. From the sinus, blood drains into the superior and inferior petrosal sinuses, then to the sigmoid sinus, and ultimately into the internal jugular vein, returning blood toward the heart.
Clinical Significance
Conditions affecting the cavernous sinus can have serious consequences. One is cavernous sinus thrombosis (CST), a blood clot within the sinus, often resulting from bacterial infections spreading from the face, nose, sinuses, ears, or teeth. The valveless nature of connecting veins facilitates infection spread. Symptoms include severe headache, eye pain, eye swelling or bulging, droopy eyelids, impaired eye movements, double vision, and facial numbness. Untreated CST can lead to vision loss, stroke, meningitis, or brain abscesses, requiring urgent medical attention.
Another condition is a carotid-cavernous fistula (CCF), an abnormal connection between the high-pressure carotid artery and the low-pressure cavernous sinus. This can occur due to trauma or an aneurysm rupture within the internal carotid artery. When a CCF forms, arterial blood shunts directly into the venous sinus, increasing pressure. Elevated pressure can lead to bulging and redness of the eyes, swelling around the eyes, blurred or double vision, and a persistent ringing sound in the ear correlating with heartbeat. These symptoms arise from impaired venous drainage and effects on nerves and blood vessels.