22 Microsurgical Anatomy of the Cavernous Sinus
Eduardo Carvalhal Ribas and Alexandre Yasuda
Abstract
The cavernous sinuses are venous structures, located one at each side of the sella turcica, and are mostly related to the intracranial surface of the sphenoid and temporal bones. This small region has a complex anatomy, and its deep understanding made surgical approach of lesions inside it possible.
Each cavernous sinus harbors several neurovascular structures. The internal carotid artery (ICA) enters the cavernous sinus as it passes between the foramen lacerum and the petrolingual ligament and exits this region passing through the upper ring of the carotid collar. The intracavernous segment of the ICA has a tortuous trajectory and the sinus is subdivided into four venous spaces, according to the position of these compartments in relation to the ICA. The oculomotor, trochlear, ophthalmic, and abducens nerves and the sympathetic plexus around the intracavernous carotid artery have their trajectory through the cavernous sinus.
In this chapter, the anatomy of the cavernous sinus is reviewed, with attention to its walls and their subdivision into triangles according the location of cranial nerves and ligaments of this region. Also, the anatomy of the middle fossa and paraclinoid triangles, found lateral and posterior to the cavernous sinus respectively, is explained.
Keywords: Keywords: cavernous sinus, skull base, internal carotid artery, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve
22.1 Introduction
The cavernous sinuses are venous structures, located one at each side of the sella turcica, that contain several neurovascular structures. Anteriorly and laterally, it receives the venous drainage of the orbit, sylvian fissure, and anterior and middle fossae by several veins (e.g., superior and inferior ophthalmic veins, deep middle cerebral vein, superficial sylvian vein and the veins of the foramen rotundum, foramen ovale, and foramen spinosum), and posteriorly it opens into the basilar, superior petrosal, and inferior petrosal sinuses. Both cavernous sinuses can also be interconnected by the anterior, posterior, and inferior intercavernous sinuses, which are found anterosuperior, posterosuperior, and inferior to the pituitary gland, respectively. The intercavernous sinuses can occur with varying degrees and, when they are present and form a circular communication around the diaphragma sellae, it is named circular sinus.
Among several anatomical descriptions in the literature, controversy exists regarding whether the structure of this venous cavity is a trabeculated venous channel or a venous plexus.1 , 2 , 3 Both concepts are partially correct and co-exist because some veins that drain to this region converge to form large venous spaces, while others maintain their integrity inside the cavernous sinus.
The first anatomical descriptions of the cavernous sinus began in the 17th century, reporting the existence of an intracranial segment of the internal carotid artery (ICA) before entering the dura mater. Johann Jakob Wepfer described this arterial segment covered by a sinus at the sphenoid bone in 1658,4 and later anatomists began to note its relations to neural structures as well.5 , 6 The term “cavernous sinus” was used for the first time by Jacques Winslow, in 1732, relating the fine fibrous trabeculae of this region to the similar architecture found at the corpus cavernosum of the penis.
Deep understanding of its challenging anatomy was made only in the 20th century, and unlocked the possibility to surgically approach lesions inside the cavernous sinus. Although Krogius is generally credited with the first surgical approach, in 1895,7 the later work of Dwight Parkinson in direct repairing carotid-cavernous fistulas with preservation of the carotid artery established surgical excursions inside the cavernous sinus as a reasonable neurosurgical procedure.2 , 8 , 9 , 10 Following his steps, modern neurosurgeons also described surgical approaches to treat aneurysms and tumors inside the cavernous sinus.11 , 12 , 13 , 14
22.2 The Sinus
The shape of the cavernous sinus resembles a boat, being narrowest anteriorly and widest posteriorly, and has five walls: medial, lateral, superior (roof), posterior, and anterior. The medial wall faces the sella turcica, pituitary gland, and body of the sphenoid bone, while the lateral wall faces the temporal lobe. Both medial and lateral walls converge inferiorly, at the superior margin of the second division of the trigeminal nerve (maxillary nerve), and as a result this division is not included inside the cavernous sinus. The superior wall (or roof) faces the basal cisterns and the anterior clinoid process (ACP). The posterior wall extends from the dorsum sellae medially to the ostium of Meckel’s cave laterally and faces the posterior fossa.
The dura mater that covers the middle fossa, lateral to the cavernous sinus, has two layers: an endosteal layer, which faces the bone; and a meningeal layer, which faces the brain. When these layers reach the inferior edge of the cavernous sinus, the endosteal layer separates into two parts (inner and outer).15
The outer part of endosteal layer, together with the meningeal layer, ascends and forms the lateral wall of the cavernous sinus. The cranial nerves located at the lateral wall of the sinus are embedded in the outer part of the endosteal layer. Surgically, the meningeal layer can be peeled away from the outer part of the endosteal layer, leading to the exposure of these nerves at the lateral wall of the cavernous sinus inside the outer part of endosteal layer.
The medial wall of the cavernous sinus is divided into a sellar part and a sphenoidal part. The inner part of the endosteal layer passes below the inferior aspect of the cavernous sinus and covers the body of the sphenoid bone, constituting the sphenoidal part of the medial wall. This membrane also continues medially across the sellar floor, below the pituitary gland, and reaches the other side. The sellar part of the medial wall is a continuation of the diaphragma sellae that folds downward around the lateral surface of the anterior lobe of the pituitary gland and is constituted by the meningeal layer15 (see Fig. 22.1a, b).
Fig. 22.1 Superior view of anterior and middle fossae. (a) The cavernous sinuses are venous structures, located one at each side of the sella turcica and covered by dura mater. (b) The lateral wall of the cavernous sinus is formed by two sheets of dura mater: the meningeal layer and the outer part of endosteal layer. The meningeal layer can be peeled away from the outer part of the endosteal layer, leading to the exposure of the cranial nerves that are embedded in the lateral wall of the cavernous sinus. Ant. Clin., anterior clinoid process; Gr. Wing, greater sphenoid wing.; Less. Wing, lesser sphenoid wing. Anatomical dissections performed by Eduardo Carvalhal Ribas, MD, at Dr. Rhoton’s laboratory. (Reproduced with permission from the Rhoton Collection [http://rhoton.ineurodb.org], CC BY-NC-SA 4.0 [http://creativecommons.org/licenses/by-nc-sa/4.0].)
22.3 Osseous Relationships
The anterior edge of the sinus is directly underneath the ACP, and continues inferiorly along the medial limit of the superior orbital fissure and posterior to the optic strut. The ACP is a bony process of the sphenoid bone that projects posteriorly, and is attached to this bone at three sites: an anterior root continues laterally with the lesser sphenoid wing; a medial anterior root continues in direction of the planum sphenoidale, passing over the optic nerve and forming the roof of the optic canal; and a medial posterior root, also called optic strut, continues passing below the optic nerve and forms the floor of the optic canal. The optic strut separates the optic canal (medial) from the superior orbital fissure (lateral).
The posterior edge of the sinus extends from the lateral margin of the dorsum sellae. This posterior edge stretches from the posterior clinoid superiorly, to the junction of the body of the sphenoid bone with petrous apex inferiorly, and to the medial margin of the trigeminal impression and Meckel’s cave laterally. The posterior clinoids are bony processes found, one at each side, at the most superior and lateral edges of the dorsum sellae.
The superior and inferior aspects of the anterior and posterior edges of the cavernous sinus can be connected by lines to define the superior and inferior limits of the sinus. The superior limit is defined by a line extending from the inferior surface of the anterior clinoid to the posterior clinoid. The inferior limit is drawn from most inferior and medial aspect the superior orbital fissure to the most superior aspect of the petroclival fissure, passing this line lateral to the carotid sulcus and medial to the foramen rotundum. As a result, the foramina ovale, rotundum, and spinosum are not inside the cavernous sinus, but venous channels passing through and around these foramina form a pericavernous venus plexus that empties into the cavernous sinus.
As the intracavernous carotid artery courses along the lateral aspect of the sphenoid bone, it creates a groove at the intracranial surface of the sphenoid bone called carotid sulcus. The middle clinoid is a bony process that projects upward from the medial and anterior aspect of the carotid sulcus ( Fig. 22.2a, b).
Fig. 22.2 The cavernous sinuses are located lateral to the sella turcica, and are mostly related to the intracranial surface of the sphenoid and temporal bones. The several neurovascular structures contained inside the cavernous sinuses go through corresponding foramina of the skull base. (a) Superior view of the skull base. (b) Lateral view of the sphenoid bone. Ant. Clin., anterior clinoid process; Body, sphenoid body; Car. Sulcus, carotid sulcus; Dorsum, dorsum sellae; For., foramen; Gr. Wing, greater sphenoid wing; Less. Wing, lesser sphenoid wing; Pet. Apex, petrous apex; Planum, planum sphenoidale; Post. Clin., posterior clinoid process; Sella, sella turcica; Sup. Orb. Fiss., superior orbital fissure; Tuberc., tuberculum sellae. (Reproduced with permission from the Rhoton Collection [http://rhoton.ineurodb.org], CC BY-NC-SA 4.0 [http://creativecommons.org/licenses/by-nc-sa/4.0].)
22.4 Nerves
The oculomotor, trochlear, ophthalmic, and abducens nerves and the sympathetic plexus around the intracavernous carotid artery are present inside the cavernous sinus. The abducens nerve and the sympathetic plexus are the only nerves that have a purely intracavernous course, while the others are related to the lateral wall of the sinus.
The oculomotor nerve enters the roof the cavernous sinus and courses at its lateral wall. It has a short cistern around and becomes incorporated at the lateral wall of the sinus when it reaches the lower margin of the ACP. This nerve subdivides into inferior and superior divisions just before passing through the superior orbital fissure, and reaches the orbit to innervate the medial, superior, and inferior rectus muscles, inferior oblique muscle, levator palpebrae superioris muscle, and pupilloconstrictor muscle.
The trochlear nerve penetrates the cavernous sinus at the posterolateral edge of its roof, near the junction of the anterior and posterior petroclinoid dural folds. It also has a thin cistern around and is found inferior to the oculomotor nerve at the lateral wall of the sinus. The trochlear nerve’s trajectory changes more anteriorly, at the level of the ACP, from laterally to medially as it crosses to pass between the superior surface of the oculomotor nerve and the inferior aspect of the ACP. This nerve exits the cavernous sinus passing through the superior orbital fissure and enters the orbit, where its trajectory continues more medially and innervates the superior oblique muscle.
Sympathetic fiber bundles form a plexus around the intracavernous carotid artery, and are large enough to be seen without high magnification. These fibers, which first cover the carotid, are distributed along the abducens nerve and later over the ophthalmic nerve on its way to reach the orbit. Finally, these fibers innervate the pupillodilator muscle through the long ciliary nerves and by passing through the ciliary ganglion. These fibers can also have a more direct course, passing directly from the carotid plexus to the ciliary ganglion or continuing along the ophthalmic artery to the orbit.
The inferior limit of the cavernous sinus, formed by the junction of its lateral and medial walls, is at a line that connects the inferior and medial aspect of the superior orbital fissure to the most superior aspect of the petroclival fissure, medial to the foramina rotundum and ovale. This line passes under the superior one-third of the gasserian ganglion and the upper portion of the medial wall of Meckel’s cave, including these structures partially inside the cavernous sinus and excluding their inferior parts from the sinus. The ophthalmic nerve (first division of the trigeminal nerve) is embedded within the inner layer of the lateral wall of the cavernous sinus, and travels below the trochlear nerve to reach the superior orbital fissure and exits the sinus, where it divides into three branches: lacrimal, frontal, and nasociliary. The maxillary nerve (second division of the trigeminal nerve) and the mandibular nerve (third division of the trigeminal nerve) are found inferior to the cavernous sinus and exit the intracranial space by passing through the foramina rotundum and ovale, respectively.
The greater superficial petrosal nerve (GSPN) emerges from the geniculate ganglion, located at the petrous part of the temporal bone, and carries parasympathetic preganglionic fibers from the facial nerve. On the other hand, the deep petrosal nerve arises from the internal carotid plexus and runs through the carotid canal, carrying postsynaptic sympathetic nerve fibers. Both nerves join in the cartilagenous substance which fills the foramen lacerum, without passing through this foramen, and form the vidian nerve (nerve of the pterygoid canal). The vidian nerve travels inside the pterygoid canal, a bony tunnel of the sphenoid bone located at floor of sphenoid sinus, and exits in the pterygopalatine fossa, where it joins the pterygopalatine ganglion (also known as the sphenopalatine ganglion). Finally, fibers from this ganglion will distribute sympathetic and parasympathetic innervation to blood vessels, lacrimal gland, and mucous glands. These nerves are not inside the cavernous sinus, but are important to understand the anatomy of this region ( Fig. 22.3a, b).
