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Cavernous sinus
The cavernous sinus is a very important intracranial, extradural anatomic region that contains many structures vital for visual function. Numerous disease processes along the skull base and in the cavernous sinus can have a major impact on vision or ocular motility.
Yet this anatomic structure remains quite unfamiliar to most ophthalmologists and orbital surgeons. It serves as a critical venous drainage route for both the orbit and the cranial base. 26 It also transmits arterial and neural structures from the intracranial compartment into the orbital apex.
The term cavernous sinus has been in use for more than 285 years, ever since Jacobus Winslow proposed it in 1734, reflecting his concept of a single trabeculated venous cavern similar to the corpus cavernosus of the penis. 58
His concept was incorrect, yet the term has persisted in the medical literature. It is clear from modern studies that the cavernous sinus is neither cavernous nor is it an intradural sinus, but rather it is a plexus or network of extremely thin-walled veins associated with adipose tissue. Parkinson 38 emphasized the inappropriateness of this term on anatomical grounds.
Hashimoto 18 recommended following Parkinson’s lead in using the term lateral sellar compartment 37 for this structure in its broader sense and restricting the term cavernous sinus to the more limited venous pathways within the lateral sellar compartment. In 2003, Tobenas-Dujardin et al. 50 proposed the term inter-periosto-dural space, which they believed would better reflect the real anatomic pattern.
However, this has not gained widespread usage. Although “lateral sellar compartment” might be anatomically more accurate, “cavernous sinus” remains in widespread use, especially outside the specialty of neurosurgery.
Furthermore, the International Federation of Associations of Anatomists did not adopt an alternative terminology for the cavernous sinus in its most recent edition of Terminologia Anatomica (2019). 49 Therefore, for the present chapter, we will use the classic terminology, using “cavernous sinus” for both the neural and venous components.
Embryology
The early development of the cavernous sinus is complex. Our current understanding is based on the seminal studies of Padget, 34 as well as more recent works. 14 , 25 By the 3-mm (28-day) embryonic stage, two longitudinal venous channels, the anterior cardinal veins, are laid down and extend along the ventrolateral surface of the developing brain, on the medial side of the cranial nerve roots.
Three pairs of venous channels develop from these to form the superior cerebral, middle cerebral, and inferior cerebral veins. Most of each cardinal vein atrophies, except for a segment of each vein in the region of the trigeminal ganglion that becomes the forerunner of the cavernous sinus, and another segment more posteriorly that becomes the internal jugular vein.
By the 8-mm (36-day) embryonic stage, the primitive supraorbital vein arises in the superficial tissues dorsal to the developing eye. It initially drains backward between the trigeminal and trochlear nerves into an anterior dural plexus, which will become the superior sagittal and transverse sinuses.
A new anastomosis appears from the supraorbital vein that diverts blood over the incipient annulus of Zinn into the venous plexus of the future cavernous sinus. By the 11-mm (40-day) stage, the initial formation of the chondrocranium is seen around the anterior notochord, surrounded by primitive mesenchyme.
At the 14.5-mm (44-day) stage, chondrification begins in the future greater and lesser wings of the sphenoid bone and the dorsum sellae. 50 At the same time the trigeminal (Gasserian) ganglion forms, along with its three major peripheral divisions. In the 23- to 25-mm (50-day) embryo, the hypophysis and diaphragma sellae become differentiated in the region of the developing cavernous sinus.
The lateral wall of the cavernous sinus is partially developed as a meningeal layer enclosing several cranial nerves, but the medial wall is not yet formed. In the 31-mm (56-day) embryo, a well-developed cavernous sinus with a definitive cavernous carotid artery and sympathetic plexus is present, containing two venous compartments, one on each side of the midline. Cranial nerves III, IV, VI, and the three branches of the trigeminal nerve (V1–V3) are all differentiated and located in their approximate adult relationships.
In the 70- to 90-mm (13- to 15-week) fetal stage, small ossification centers are seen in the body, greater wings, and lesser wings of the sphenoid bone. At the same time, ossification is beginning in the cartilaginous petrous portion of the temporal bone. 18 The primordium of the dura mater and subarachnoid membrane is already seen lining the area of the cavernous sinus on either side of the body of the sphenoid.
The pituitary gland is lined by an inner capsule and an outer meningeal layer, forming the definitive medial wall of the cavernous sinus. Many small, irregularly shaped lumens develop within the mesenchyme of the cavernous sinus region, and these venous channels gradually enlarge with further fetal development. These channels meander and intertwine and are lined only by an endothelial layer with no smooth muscle.
These venous channels communicate with other venous channels. Posteriorly they drain to the basilar venous sinus and then to the jugular bulb; posteroinferiorly with the inferior petrosal sinus and then into the pterygoid venous plexus through the foramen lacerum; and posterosuperiorly to the superior petrosal sinus and then into the sigmoid sinus. The cavernous sinuses on each side communicate with each other through one or more intercavernous sinuses situated between the dural layers, below the pituitary gland.
The Gasserian ganglion is situated posterior to the developing cavernous sinus on either side, over the tip of the petrous bone, and lateral to the dorsum sellae. The three branches of the trigeminal nerve run forward from the Gasserian ganglion.
The ophthalmic branch (V1) and the maxillary branch (V2) run anteriorly in the lateral wall of the cavernous sinus, within the loose inner connective tissue endosteal layer. The oculomotor (III) and trochlear (IV) nerves enter the cavernous sinus near the posterior clinoid process and also run anteriorly within the lateral wall to the superior orbital fissure (SOF).
The abducens nerve (VI) runs through the basilar venous plexus and then enters the cavernous sinus; it courses forward within the venous channels of the sinus just lateral to the internal carotid artery (ICA) and passes into the SOF. Third-order sympathetic nerve fibers enter the cranium through the foramen lacerum and become associated with these cranial nerves and vascular elements.
The ICA enters the skull base through the future carotid canal. It then penetrates the floor of the cavernous sinus inferolateral to the cartilaginous sphenoid bone. As the sella turcica develops, the ICA gradually assumes the S-shaped configuration seen in the adult.
During the 128- to 183-mm (18- to 23-week) stage of fetal development, further ossification occurs in the sphenoid bone as it expands in the anterolateral directions. By the 230-mm (28-week) fetal stage, a thick periosteum is seen over the surface of the sphenoid bone.
Dura is distinguishable along the lateral wall of the cavernous sinus as a definite meningeal layer separate from the overlying arachnoid membrane and the inner endosteal layer that is continuous with the periosteum of the sphenoid bone. Superiorly the meningeal layer folds to contribute to the diaphragma sellae over the pituitary gland. Within the mesenchyme of the cavernous sinus, large, well-defined venous lumens are now present.
The mesenchymal tissue between lumens gradually thins to become membranes separating the individual vascular channels. Small arteries and autonomic nerve fascicles are now apparent within these membranous walls.
In the 150- to 200-mm (21- to 25-week) fetal stage, blood flow through the cavernous sinus rapidly increases, probably due to alterations in neighboring venous pathways. Nerve fascicles become surrounded by collagen fibers forming sheaths.
Simultaneous with formation of the cavernous sinus is the development of the pituitary gland, which forms an important element adjacent to and above the bilateral cavernous sinuses. During the 2- to 3-mm (21-day) embryonic stage, the gland originates from two distinct ectodermal tissues.
A finger-like protrusion, called Rathke’s pouch, grows upward as a dorsal evagination from the stomodeum, or mouth, just anterior to the buccopharyngeal membrane. It differentiates into glandular epithelium characteristic of endocrine glands. The infundibulum is a ventral evagination from the floor of the third ventricle of the diencephalon just caudal to the developing optic chiasm from the same tissue. 1
It differentiates into the exocrine component of the pituitary gland. During the second month of embryonic development, Rathke’s pouch wraps around the infundibulum and differentiates into the anterior lobe, or adenohypophysis, of the pituitary gland. The infundibulum differentiates into the pituitary stalk and the posterior lobe, or neurohypophysis, of the gland.
Ultimately, the two portions grow together to form the definitive pituitary gland. As the cavernous sinus continues to develop, the enclosing dural and endosteal sheaths conform to the body of the pituitary gland to form the medial walls of the sinus, as well as the roof and the diaphragma sellae that separates the gland from the optic chiasm.
Anatomy of the adult cavernous sinus
The cavernous sinus is a paired structure located near the center of the head on either side of the sella turcica and pituitary gland and posterior to the sphenoid sinus. It is defined as the space between the SOF anteriorly, the posterior petroclinoid fold and clivus dura mater posteriorly, and the inner surface of the middle cranial fossa inferolaterally where the meningeal and periosteal layers of the dura meet and fuse. 12
The cavernous sinus measures approximately 10 to 20 mm in anteroposterior length, 8 to 12.5 mm in width, and 5 to 7 mm in height. 8 , 24 The lateral wall of the sinus is anatomically complex, composed of a superficial (outer) meningeal layer of dura and a deeper (inner) layer containing several cranial nerves.
The sinus is therefore surrounded by this dural envelope and contains a venous plexus, a short segment of the ICA, and the abducens nerve (VI). The venous plexus is fed by veins draining from the face, orbit, nasopharynx, cerebrum, cerebellum, and brainstem. It empties into the basilar venous system and the petrosal venous sinuses. Within the lateral wall of the cavernous sinus run the oculomotor (III) and trochlear (IV) nerves and the first two divisions (V1 and V2) of the trigeminal nerve. These latter structures, therefore, are not technically within the cavernous sinus but are only associated with its lateral wall.
The bony boundaries of the cavernous sinus
The cavernous sinus lies within the middle cranial base. The latter is bounded anteriorly and laterally by the greater wing of the sphenoid bone and posteriorly by the clivus and the anterior aspect of the petrous portion of the temporal bone.
The body of the sphenoid bone makes up the floor of the middle cranial fossa and contains the sella turcica, situated between the anterior and posterior clinoid processes. The sella turcica consists of the tuberculum sellae anteriorly between the cranial openings of the optic canal. Behind it is the pituitary fossa, and the posterior extent of the sella is bounded by the dorsum sellae.
The cavernous sinus lies lateral to the body of the sphenoid bone and over the top of the petrous apex of the temporal bone.
The posterior portion of the sinus rests against the lateral edge of the dorsum sellae, and its anterior portion extends to the SOF beneath the anterior clinoid process and the lesser wing of the sphenoid. Laterally the sinus extends to the junction of the sphenoid body and the greater wing but does not include the foramen rotundum, foramen ovale, and the foramen spinosum.
The latter three foramina are located just lateral to the lateral wall of the cavernous sinus. Inferiorly, the sinus extends to the lower border of the carotid sulcus, a groove along the lateral aspect of the sphenoid body in which lies the intracavernous portion of the ICA.
Lateral to the anterior clinoid process and extending superolaterally beneath the lesser sphenoid wing is the SOF, which marks the anteriormost extent of the cavernous sinus.
It opens into the orbital apex and transmits cranial nerves III, IV, VI, and branches of the ophthalmic division of the trigeminal nerve (V1). Just posterior to and slightly inferior to the SOF, on the floor of the middle cranial fossa, is the foramen rotundum, lateral to the sphenoid sinus. It lies lateral to the cavernous sinus and transmits the maxillary division (V2) of the trigeminal nerve into the pterygopalatine fossa. The foramen ovale lies about 1 cm posterior and lateral to the foramen rotundum and carries the mandibular branch (V3) of the trigeminal nerve into the infratemporal fossa.
The foramen lacerum is an irregular opening posteromedial to the foramen ovale and transmits the internal jugular vein as it exits the cranium. In the petrous apex, near its junction with the sphenoid and occipital bones, lies the carotid canal, which continues anteromedially to open into the foramen lacerum.
Anteriorly, the anterior clinoid process is a rounded projection extending from the lesser wing of the sphenoid bone. It extends above the anterior roof of the cavernous sinus and forms the lateral wall of the optic canal. Inferomedially, the lesser sphenoid wing and clinoid process are joined by the optic strut to the body of the sphenoid bone.
The strut separates the optic canal from the SOF. It also forms the floor of the optic canal and the anterior roof of the cavernous sinus. The posterior face of the optic strut has a depression to accommodate the anterior bend of the intracavernous carotid artery beneath the anterior clinoid process.
The dural folds
The cavernous sinus has four walls that mark its boundaries and delimit its anatomic extent. Dural folds help define the boundaries of the cavernous sinus and provide important landmarks for surgery in this anatomic location. The anterior border of the cavernous sinus consists of the anterior clinoid processes and the SOF. 7
Dural structures extend from the upper and lower portions of the anterior clinoid process and surround the ICA, forming upper and lower rings in the region where the artery forms a sharp anterior bend. The segment of the carotid artery that lies between the upper (distal) and lower (proximal) dural rings is the clinoid portion and lies within the anteriormost portion of the cavernous sinus. The floor of the sinus is composed of endosteum (periosteum), which also covers the body of the sphenoid bone, and is continuous with periosteum of the middle cranial fossa.
The medial wall of the sinus is divided into a lower sphenoidal portion and an upper sellar portion. The lower sphenoidal part overlies the body of the sphenoid bone and a horizontal groove for the carotid artery, the carotid sulcus.
It is covered by endosteum continuous with periosteum of the floor of the middle cranial fossa. The bone separating the sphenoid sinus from the cavernous sinus is very thin in this region, <0.5 mm in most individuals, 24 and may even have spontaneous dehiscences so that the sphenoid sinus may be separated from the cavernous sinus only by layers of endosteum and sinus mucosa.
The upper sellar portion of the medial wall is lined by a meningeal layer continuous with the diaphragma sellae above. Controversy exists as to the existence of the endosteal layer in this region. Songtao et al. reported a distinct inner layer (lamina propria) between the dural layer and the pituitary gland, which also contributed to the medial wall in two-thirds of the specimens studied.
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