The anatomy of the skull base is complex with multiple neurovascular structures in a small space. Understanding all of the intricate relationships begins with understanding the anatomy of the sphenoid bone. The cavernous sinus contains the carotid artery and some of its branches; cranial nerves III, IV, VI, and V1; and transmits venous blood from multiple sources. The anterior skull base extends to the frontal sinus and is important to understand for sinus surgery and sinonasal malignancies. The clivus protects the brainstem and posterior cranial fossa. A thorough appreciation of the anatomy of these various areas allows for endoscopic endonasal approaches to the skull base.
Key points
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The sphenoid bone is at the center of the skull base and understanding its anatomy from multiple perspectives is important to understanding endonasal approaches.
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Within the sphenoid sinus, the lateral opticocarotid recess is a key landmark for identifying the locations of the parasellar carotid artery and optic nerve.
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The tuberculum sella is the anterior and superior limit of the sella. Limited removal of the tuberculum during pituitary surgery helps avoid CSF leak, while complete removal allows for access to the suprasellar space.
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The major neurovascular structures of the cavernous sinus are located in the lateral compartment and lateral wall.
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The clivus can be divided into thirds. The upper third corresponds to the dorsum sella, the middle third is in the sphenoid sinus below the sella, and the lower third behind the nasopharynx.
Introduction
The nasal cavity has been used as a corridor to access the midline skull base since the turn of the twentieth century. The transsphenoidal route to the pituitary was initially explored in the 1890s because of the high mortality associated with early transcranial approaches. Although the transsphenoidal approach showed some promise, it fell largely out of favor as neurosurgeons became increasingly proficient with transcranial approaches. It was not until the 1960s when Gerard Guiot was able to publish excellent results with the transsphenoidal approach that it began to regain popularity. He was also the first to report trying to use an endoscope in transsphenoidal surgery, but he abandoned it because of poor visualization compared with the microscope.
It was also in the 1960s that Harold Hopkins introduced his rod-lens system, greatly improving on the prior 100 years of endoscopy. Combined with cameras and evolving video technology, endoscopy took a big step forward in the medical field. Otolaryngologists began using the rod-lens in the nasal cavity and endoscopic sinus surgery was born. Simultaneously, neurosurgeons began using these new endoscopes as adjuncts to their microscopic resections. These two parallel developments finally came together in the 1990s when the first multidisciplinary endoscopic skull base teams were formed.
Endoscopic endonasal surgery has now become an invaluable alternate means of accessing and treating pathology of the skull base. It offers a direct route for accessing the anterior, middle, and posterior cranial fossa. It has been shown to be safe and effective, but requires a detailed understanding of the intricate anatomy to be successful. This article reviews the anatomy of the midline skull base from the frontal sinus to the clivus, with special attention to the endonasal perspective.
Introduction
The nasal cavity has been used as a corridor to access the midline skull base since the turn of the twentieth century. The transsphenoidal route to the pituitary was initially explored in the 1890s because of the high mortality associated with early transcranial approaches. Although the transsphenoidal approach showed some promise, it fell largely out of favor as neurosurgeons became increasingly proficient with transcranial approaches. It was not until the 1960s when Gerard Guiot was able to publish excellent results with the transsphenoidal approach that it began to regain popularity. He was also the first to report trying to use an endoscope in transsphenoidal surgery, but he abandoned it because of poor visualization compared with the microscope.
It was also in the 1960s that Harold Hopkins introduced his rod-lens system, greatly improving on the prior 100 years of endoscopy. Combined with cameras and evolving video technology, endoscopy took a big step forward in the medical field. Otolaryngologists began using the rod-lens in the nasal cavity and endoscopic sinus surgery was born. Simultaneously, neurosurgeons began using these new endoscopes as adjuncts to their microscopic resections. These two parallel developments finally came together in the 1990s when the first multidisciplinary endoscopic skull base teams were formed.
Endoscopic endonasal surgery has now become an invaluable alternate means of accessing and treating pathology of the skull base. It offers a direct route for accessing the anterior, middle, and posterior cranial fossa. It has been shown to be safe and effective, but requires a detailed understanding of the intricate anatomy to be successful. This article reviews the anatomy of the midline skull base from the frontal sinus to the clivus, with special attention to the endonasal perspective.
Sphenoid bone
The sphenoid bone sits at the center of the skull base, and knowing its anatomy is central to understanding endonasal approaches. The sphenoid bone has been described as resembling a bat with its wings outstretched ( Fig. 1 ). It consists of a central body, which is cuboidal in shape and houses the sphenoid sinus at its center. The sella turcica is located superiorly and the upper clivus posteriorly. The lesser wings extend from the superolateral aspect of the body and the greater wings from the inferior aspect of the body. The superior orbital fissure is the space between the greater and lesser wings. The paired pterygoid processes and pterygoid plates project downward from the body on either side.
The lesser wings extend laterally to form part of the floor of the anterior cranial fossa. The inferior surfaces of the lesser wings form the posterior roof of the orbits. Medially the lesser wings join with the planum sphenoidale, which forms the roof of the sphenoid sinus. The planum articulates anteriorly with the cribriform plate. At the posteromedial ends of the lesser wings are the anterior clinoid processes and the optic canals. The optic canals are separated from the superomedial aspect of the superior orbital fissure by a piece of bone known as the optic strut. The optic strut extends from the body of the sphenoid to the base of the anterior clinoid. Between the two optic foramina is a shallow depression known as the chiasmatic groove or prechiasmatic sulcus. This groove is bounded by the planum sphenoidale anteriorly and tuberculum sella posteriorly. At the junction of the prechiasmatic sulcus and planum sphenoidale is a bony ridge known as the limbus of the sphenoid.
When viewed from above, the center of the sphenoid body contains the sella turcica, a saddle shaped depression that houses the pituitary gland. It is bound anteriorly by the tuberculum sella and posteriorly by the dorsum sella. The dorsum sella also makes up the upper clivus. The posterior clinoid processes project from the superolateral aspects of the dorsum sella. On either side of the sella are the cavernous sinuses and the carotid arteries. The medial wall of the cavernous sinus marks the lateral boundary of the sella.
In addition to the paired anterior and posterior clinoids, there is also a pair of middle clinoids. The middle clinoid is a variable length piece of bone that extends from the superolateral aspect of the sella toward the apex of the anterior clinoid. Its length can vary from being nonexistent to attaching to the anterior clinoid to form a complete osseous ring known as the caroticoclinoidal ring. The most common finding is a short segment of bone that does not extend all the way to the anterior clinoid. It is located at the inner bend of the anterior genu of the parasellar carotid as it loops around this middle clinoid. It is an important endonasal landmark because it marks the level of the roof of the cavernous sinus and the transition point between the cavernous and paraclinoidal segments of the internal carotid artery (ICA).
The roof of the sella is formed by the diaphragma sella, a dural structure that extends from the tuberculum sella to the dorsum sella. It is continuous laterally with the roof of the cavernous sinus. In the center of the diaphragma is an opening known as the pituitary aperture, which transmits the pituitary stalk. This opening is variable in size and thickness. The arachnoid layer is located just above the diaphragma and so typically there is no cerebrospinal fluid (CSF) within the sella.
Sphenoid sinus
The sphenoid sinus varies considerably in size, shape, pneumatization, and septation. The degree of pneumatization varies with age, with most pneumatization occurring in adolescence. There are three types of pneumatization patterns in adults: conchal, presellar, and sellar. The conchal type has little to no pneumatization. The presellar type has some pneumatization but does not extend beyond the plane of the tuberculum sella. The sellar type is the most common and the cavity extends beyond the floor of the sella and often into the clivus creating a depression called the clival recess ( Fig. 2 A).
With greater pneumatization, the major sphenoid landmarks become more apparent (see Fig. 2 ). In the center of the sphenoid sinus is the sella turcica, which is a bulge into the sinus when viewed from below. Beneath the sella is the middle clivus and in most cases the clival recess. On either side of the clival recess is the bone that covers the vertical paraclival segment of ICA. As the carotid artery courses superiorly and enters the cavernous sinus, it turns and runs in a horizontal plane anteriorly before turning again and looping back on itself to go posteriorly. This loop occurs around the previously mentioned middle clinoid. This anterior genu of the carotid, which corresponds with the paraclinoidal segment, is the most superficial when viewed from the sphenoid sinus, and it often forms a prominence in the lateral sinus wall on either side of the sella.
Above the prominence of the carotid runs the optic nerve prominence as it courses from the cisternal segment to the optic canal (see Fig. 2 ). Just lateral and between the carotid and optic nerve there is often a depression known as the opticocarotid recess (OCR). The OCR is the result of pneumatization of the previously described optic strut. With extensive pneumatization, the OCR can be quite prominent. In addition to helping identify the location of the carotid artery and optic nerve, it also demarcates the level at which the optic nerve enters the true optic canal and becomes most susceptible to injury. Also, there can be dehiscence of the bone overlying the carotid artery or the optic nerve and these should be noted before any surgery involving the sphenoid sinus.
Pituitary gland
The human pituitary gland is composed of two embryologically, anatomically, and functionally distinct parts. There is an anterior lobe or adenohypophysis, and a smaller posterior lobe or neurohypophysis. The anterior lobe develops from an invagination of oral ectoderm known as Rathke’s pouch. It is a glandular structure that is responsible for the production and release of growth hormone, prolactin, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and follicle-stimulating hormone.
In contrast, the posterior lobe develops as a direct extension of neural ectoderm from the floor of the third ventricle. It is a projection of unmyelinated axons and specialized glial cells called pituicytes from the hypothalamus. It is not a glandular structure like the anterior gland, but rather stores and releases oxytocin and vasopressin, which is produced by hypothalamic cell bodies. Grossly it is often distinguishable from the anterior gland by its lighter color.
The anterior gland receives its blood supply from the superior hypophyseal arteries. They arise from the medial aspect of the supraclinoidal segment of the carotid artery approximately 5 mm distal to the ophthalmic artery ( Fig. 3 A). Each superior hypophyseal artery typically gives off three branches: one to the optic nerve (recurrent branch), one to the undersurface of the optic chiasm and upper infundibulum (anastomotic branch), and one to the lower infundibulum and diaphragm (descending branch). The branch to the upper infundibulum anastomoses with the branch from the contralateral superior hypophyseal artery to form a capillary network. A portal venous system drains the capillary plexus of the superior hypophyseal arteries, which delivers blood to the anterior gland. This allows the delivery of hypothalamic prohormones to the adenohypophysis. The posterior gland has been thought to receive most of its blood supply from the inferior hypophyseal arteries. The inferior hypophyseal artery is a branch of the meningohypophyseal trunk, which is the first branch of the internal carotid within the cavernous sinus ( Fig. 3 B). Recent evidence, however, has shown that sacrifice of both inferior hypophyseal arteries does not cause posterior pituitary dysfunction (ie, diabetes insipidus), suggesting alternative vascular supply by the superior hypophyseal arterial system. Venous drainage from the anterior and posterior gland comes together and drains into the cavernous sinus.
