Last Updated: August 23, 2020
The pituitary gland and sella are located below the center of the brain in the center of the cranial base (Fig. 8.1). Access to the sella is limited from above by the optic nerves and chiasm and the circle of Willis, from laterally by the cavernous sinuses and internal carotid arteries, and from behind by the brainstem and basilar artery. The vital structures protecting its superior, lateral, and posterior borders have led to the preferred surgical routes to tumors of the gland being from below through the nasal cavity and sphenoid sinus or from anteriorly between the frontal lobe and the floor of the anterior cranial fossa. This chapter focuses on the microsurgical anatomy important in performing the various subcranial and transcranial approaches to the sellar region. The chapter is divided into two sections: the first section deals with the relationships in the cranial base around and below the sella, and the second section deals with the relationships in the suprasellar and third ventricular regions (17). Special emphasis is placed on the transnasal route to the sella, because this route is the one most commonly selected for dealing with pituitary tumors.
The sella can be reached by several routes through the nasal cavity (11, 17). The nasal cavity, wider below than above, is bounded above by the anterior cranial fossa, laterally by the orbit and the maxillary sinus, and below by the hard palate (Figs. 8.2 and 8.3). This cavity is divided sagittally by the nasal septum, which is formed anteriorly and superiorly by the perpendicular plate of the ethmoid and inferiorly and posteriorly by the vomer, with an anterior bony deficiency occupied by septal cartilage. The nasal cavity opens anteriorly onto the face through the anterior nasal aperture and posteriorly into the nasopharynx by way of the posterior nasal apertures. Each posterior nasal aperture, measuring approximately 25 mm vertically and 13 mm transversely, is bordered above by the anterior aspect of the sphenoid body, below by the posterior margin of the hard palate formed by the horizontal plate of the palatine bones, medially by the nasal septum formed by the vomer, and laterally by the medial pterygoid plate.
The lateral nasal wall usually has three medially directed projections: the superior, middle, and inferior nasal conchae, below each of which is a corresponding superior, middle, or inferior nasal meatus (Figs. 8.2 and 8.3). The paired sphenoethmoidal recesses, located above and behind the superior nasal conchae and in front of the upper anterior aspect of the sphenoid body, are the site of the paired sphenoid ostia, which communicate between the nasal cavity and the sphenoid sinus. The upper half of the lateral nasal wall, corresponding to the medial orbital wall, is composed, from anterior to posterior, of the frontal process of the maxilla, the lacrimal bone, and the orbital plate of the ethmoid bone. The extremely thin lacrimal and ethmoid bones, occupied by the ethmoid air cells, separate the nasal cavity from the orbit. The nasolacrimal groove and canal, the site of the lacrimal sac and nasolacrimal duct, respectively, pass downward in front of the anterior end of the middle nasal concha and open into the inferior nasal meatus. The frontoethmoidal suture, located at the junction of the roof and medial orbital wall, is situated at the level of the roof of the nasal cavity and the cribriform plate. The anterior and posterior ethmoidal foramina, which transmit the anterior and posterior ethmoidal arteries and nerves, are located in or just above the frontoethmoidal suture. These arteries and nerves exit the ethmoidal foramina and enter the anterior cranial fossa at the lateral edge of the cribriform plate. The anterior ethmoidal artery, a terminal branch of the ophthalmic artery, supplies the mucosa of the anterior and middle ethmoidal sinuses and the dura covering the cribriform plate and the planum sphenoidale. It gives rise to the anterior falcine artery intracranially. The posterior ethmoidal artery, usually smaller than the anterior ethmoidal artery and absent in up to 30% of the ophthalmic arteries, feeds the mucosa of the posterior ethmoidal sinus and the dura of the planum sphenoidale. The average distance between the anterior lacrimal crest of the maxilla’s frontal process and the anterior ethmoidal foramen is 22 to 24 mm; between the anterior and posterior ethmoidal foramina, 12 to 15 mm; and between the posterior ethmoidal foramen and the optic canal, 3 to 7 mm (11). In midline transfacial procedures, these arteries may be divided between the periorbita and the medial orbital wall. Care should be taken to prevent damaging the optic nerve, which is sometimes located immediately behind the posterior ethmoidal foramen.
The lower part of the lateral nasal wall is formed, from anterior to posterior, by the maxilla, the perpendicular plate of the palatine bone, and the medial pterygoid plate. The eustachian tube opens into the nasopharynx along the posterior edge of the medial pterygoid plate. The root of the middle nasal concha attaches to the lateral nasal wall near the junction of the orbit and the maxillary sinus. Thus, the medial wall of the maxillary sinus is bounded medially by the middle and inferior nasal meatus and the inferior nasal concha (Figs. 8.2 and 8.3). The maxillary sinus communicates with the middle nasal meatus through an opening located in the medial wall just below the roof of the sinus.
The pterygopalatine fossa is situated just outside the lateral wall of the nasal cavity between the posterior wall of the maxillary sinus anteriorly and the pterygoid process posteriorly (Figs. 8.2–8.4). The pterygopalatine fossa contains the pterygopalatine ganglion, which receives the vidian nerve (nerve of the pterygoid canal), the segment of the maxillary nerve and its branches located just anterior to the foramen rotundum, and the internal maxillary artery and its terminal branches. This fossa communicates laterally with the infratemporal fossa through the pterygomaxillary fissure and medially with the nasal cavity via the sphenopalatine foramen, which transmits the corresponding nerve and vessels. The internal maxillary artery exits the infratemporal fossa to enter the pterygopalatine fossa by passing through the pterygomaxillary fissure. The greater and lesser palatine arteries and nerves arise from the maxillary artery and nerve and descend in the greater and lesser palatine canals, which are separated medially from the nasal cavity by the thin perpendicular plate of the palatine bone.
The sphenoid bone is located in the center of the cranial base (Figs. 8.3 and 8.5) (21, 22). The intimate contact of the body of the sphenoid bone with the nasal cavity below and the pituitary gland above has led to the transsphenoidal route being the operative approach of choice for most sellar tumors. The neural relationships of the sphenoid bone are among the most complex of any bone. The olfactory tracts, gyrus rectus, and posterior part of the frontal lobe rest against the smooth upper surface of the lesser wing; the temporal lobe rests against the inner surface of the greater wing; the pons and mesencephalon lie posterior to the clival portion; the optic chiasm lies posterior to the chiasmatic sulcus; and the IInd through VIth cranial nerves are intimately related to the sphenoid bone and all exit the cranium through the optic canal, superior orbital fissure, foramen rotundum, or foramen ovale, all foramina located in the sphenoid bone (Fig. 8.6).
The sphenoid bone has many important arterial and venous relationships: the carotid arteries groove each side of the sphenoid bone and often form a serpiginous prominence in the lateral wall of the sphenoid sinus, the basilar artery rests against its posterior surface, the circle of Willis is located above its central portion, and the middle cerebral artery courses parallel to the sphenoid ridge of the lesser wing. The cavernous sinuses rest against the sphenoid bone, and intercavernous venous connections line the walls of the pituitary fossa and dorsum sellae.
In the anterior view, the sphenoid bone resembles a bat with wings outstretched (Fig. 8.5). It has a central portion called the body; two lesser wings, which spread outward from the superolateral part of the body; two greater wings, which spread upward from the lower part of the body; and two pterygoid processes with their medial and lateral pterygoid plates directed downward from the body. The body of the sphenoid bone is more or less cubical and contains the sphenoid sinus. The superior orbital fissure, through which the oculomotor, trochlear, abducens, and ophthalmic nerves pass, is formed on its inferior and lateral margins by the greater wing and on its superior margin by the lesser wing. The inferior surface of the lesser wing forms the posterior part of the roof of each orbit, and the exposed surface of the greater wing forms a large part of the lateral wall of the orbit, the floor of the middle fossa, and the roof of the infratemporal fossa. The optic canals are situated above and are separated from the superomedial margin of the superior orbital fissure by the optic strut, a bridge of bone that extends from the lower margin of the base of the anterior clinoid process to the body of the sphenoid. The narrowest part of the optic canal is closer to the orbital than the intracranial end. The optic canals average 5 mm in length, and are of a conical configuration, tapering to a narrow waist near the orbit end. The sphenoid ostia open from the nasal cavity into the sinus. The infratemporal crest divides the inferior from the lateral parts of the greater wing and separates the temporal fossae. The lateral pterygoid muscles arise between the infratemporal crest and the lateral pterygoid plate. The area lateral to the infratemporal line gives origin to the temporalis muscle. The pterygoid (vidian) canal courses from anterior to posterior through the junction of the pterygoid process and the sphenoid body.
In the superior view, the pituitary fossa occupies the central part of the body and is bounded anteriorly by the tuberculum sellae and posteriorly by the dorsum sellae (Figs. 8.1 and 8.6). The chiasmatic groove (sulcus), a shallow depression between the optic foramina, is bounded posteriorly by the tuberculum sellae and anteriorly by the planum sphenoidale. The frontal lobes and the olfactory tracts rest against the smooth upper surface of the lesser wing and the planum sphenoidale. The posterior margin of the lesser wing forms a free edge, the sphenoid ridge, which projects into the sylvian fissure to separate the frontal and temporal lobes. The anterior clinoid processes are located at the medial end of the lesser wings, the middle clinoid processes are lateral to the tuberculum sellae, and the posterior clinoid processes are situated at the superolateral margin of the dorsum sellae. The dorsum sellae is continuous with the clivus. The upper part of the clivus is formed by the sphenoid bone, and the lower part is formed by the occipital bone. The carotid sulcus extends along the lateral surface of the body of the sphenoid.
The depth of the sella turcica is the greatest distance between the floor and a perpendicular line connecting the tuberculum and dorsum. Sellar length, defined as the greatest anterior-posterior diameter of the pituitary fossa, may occur at the level of the tuberculum sellae or below. Sellar width is defined as the width of the horizontal plateau of the sellar floor between the carotid sulcus. The volume is calculated by applying the simplified mathematical formula for the volume of an ellipsoid, namely, volume (cm3) = 0.5 (length × width × depth in mm)/1000. The upper limit of normal depth is 13 mm; length, 17 mm; width, 15 mm; and volume, 1100 mm (15).
The superior aspect of each greater wing is concave upward and is filled by the pole of each temporal lobe. The foramen rotundum, ovale, and spinous are located, from anterior to posterior, near the junction of the body and greater wing. When viewed from inferiorly, the vomer, a separate bone, frequently remains attached to the anterior half of the body of the sphenoid, and its most anterior portion separates the sphenoid ostia.
The pterion and the keyhole are two important anatomic landmarks in the region of the greater wing in the lateral view (Fig. 8.5). The pterion is located over the upper part of the greater wing and approximates the site of the lateral end of the sphenoid ridge. The keyhole is located just behind the junction of the temporal line and the zygomatic process of the frontal bone, several centimeters anterior to the pterion. A burr hole placed over the pterion will be located at the lateral end of the sphenoid ridge. A burr hole placed at the keyhole will expose the orbit in its lower part and dura over the frontal lobe in its upper part.
The sphenoid sinus separates the cavernous sinuses, the cavernous segments of the carotid arteries, and the optic, extraocular, and trigeminal nerves. In addition, it separates the pituitary gland from the nasal cavity. The sphenoid sinus is subject to considerable variation in size and shape and to variation in the degree of pneumatization (Figs. 8.5, 8.7, and 8.8) (4, 8). It is present as minute cavities at birth, but its main development takes place after puberty. In early life, it extends backward into the presellar area and subsequently expands into the area below and behind the sella turcica, reaching its full size during adolescence. As the sinus enlarges, it may partially encircle the optic canals. When the sinus is exceptionally large, it extends into the roots of the pterygoid processes or greater wing of the sphenoid bone and may even extend into the basilar part of the occipital bone. As age advances, the sinus frequently undergoes further enlargement associated with absorption of its bony walls. Occasionally there are gaps in its bone, with the mucous membrane lying directly against the dura mater.
There are three types of sphenoid sinus in the adult: conchal, presellar, and sellar types, depending on the extent to which the sphenoid bone is pneumatized (Fig. 8.5). In the conchal type, the area below the sella is a solid block of bone without an air cavity. In the presellar type of sphenoid sinus, the air cavity does not penetrate beyond a vertical plane parallel to the anterior sellar wall. The sellar type of sphenoid sinus is the most common, and here the air cavity extends into the body of sphenoid below the sella and as far posteriorly as the clivus. In our previous study in adult cadavers, this sinus was of a presellar type in 24% and of the sellar type in 76% (15). The conchal type is most common in children before the age of 12 years, at which time pneumatization begins within the sphenoid sinus. In the conchal type, which is infrequent in the adult, the thickness of bone separating the sella from the sphenoid sinus is at least 10 mm.
The depth of the sphenoid sinus is defined as the distance from the ostium of the sphenoid sinus to the closest part of the sella (Fig. 8.8). In the adult, the average anterior-posterior diameter of the cavity has been found to be 17 mm (range, 12–23 mm) (4). This measurement defines the length of the path within the sinus through which instruments must pass to reach the sellar wall and is important when selecting instruments for transsphenoidal surgery. The speculum most commonly used for transsphenoidal surgery is 9 cm in length and its tip should be placed anterior to the sphenoid sinus. In reaching the floor of the sella turcica, the depth of the sphenoid sinus (2 cm or more) is added to the 9 cm length of the speculum. Thus, after traversing a distance of 11 to 12 cm, the dissecting instruments must then enter the sella turcica and be able to reach above the sella if a suprasellar tumor is present. The distance may be greater in the presence of acromegaly; therefore, it is important that transsphenoidal instruments have shafts at least 12 cm in length. Some transsphenoidal instruments have shafts 9.5 cm in length, barely long enough to reach through the speculum into the sphenoid sinus. The fact that important neural and vascular structures are exposed either in the lateral sinus wall, directly lateral to the sella, or above the diaphragma sellae, especially if the latter is defective, has led the author to prefers blunt rather than sharp ring curettes for dissection in these areas.
Another measurement important in transsphenoidal surgery is the thickness of the anterior sellar wall and sellar floor. In our previous study, in the sellar type of sinus, the thickness of the anterior sellar wall ranged from 0.1 to 0.7 mm (mean, 0.4 mm) as compared with 0.3 to 1.5 mm (mean, 0.7 mm) for the presellar type. The thickness of bone covering the sinus was defined at the planum sphenoidale, tuberculum sellae, anterior sellar wall, sellar floor, and the clivus. The thickest bone was found at the clivus and tuberculum sellae and the thinnest along the anterior sellar wall (15, 21).
The septae within the sphenoid sinus vary greatly in size, shape, thickness, location, completeness, and relation to the sellar floor (Fig. 8.9). The cavities within the sinus are seldom symmetrical from side to side and are often subdivided by irregular minor septae. The septae are often located off the midline as they cross the floor of the sella. In our previous study, a single major septum separated the sinus into two large cavities in only 68% of specimens, and even in these cases, the septae were often located off the midline or were deflected to one side (15). The most common type of sphenoid sinus has multiple small cavities in the large paired sinuses. The smaller cavities are separated by septae oriented in all directions. Computed tomography or magnetic resonance imaging of the sella provide the definition of the relationship of the septae to the floor of the sella needed for transsphenoidal surgery. Major septae may be found as far as 8 mm off the midline (15).
The internal carotid artery rests directly against the lateral surface of the body of the sphenoid bone, and its course is marked by a groove in the bone, the carotid sulcus, that defines the course of the intracavernous portion of the carotid artery. As the sphenoid sinus expands and its walls resorb, the carotid sulcus produces a prominence within the sinus wall below the floor and along the anterior margin of the sella (Figs. 8.6, 8.8, and 8.10) (4, 15). This prominence is most pronounced with maximal pneumatization of the sphenoid and varies from a small focal bulge to a serpiginous elevation marking the full course of the carotid artery along the lateral wall of the sphenoid sinus. The carotid prominence can be divided into three parts: the retrosellar, infrasellar, and presellar segments. The first part, the retrosellar segment, is located in the posterolateral part of the sinus. This segment of the prominence is present only in well-pneumatized sellar-type sinuses in which the air cavity extends laterally in the area below the dorsum. The second part, the infrasellar segment, is located below the sellar floor. The third part, the presellar segment, is located anterolateral to the anterior sellar wall. Of the 50 specimens we examined, 98% had presellar, 80% had infrasellar, and 78% had retrosellar prominences (15, 17). Any part of the prominence may be present and the others absent. If all three parts are present and connected, they form a serpiginous bulge marking the full course of the carotid artery. In the normal sinus, the presellar part extends to the anterior sellar wall. The anterior sellar wall is located anterior to the carotid prominence when the sella is greatly expanded by tumor.
Only the presellar part of the carotid prominence is present in a presellar type of sphenoid sinus, and it is this part that is also most frequently present in the sellar type of sinus. The corresponding arterial segments are slightly longer than the segments of the prominence because of tortuosity of the artery. This tortuosity, although present, is limited by the dural walls of the cavernous sinus, particularly if the artery is encircled by a ring of bone formed by the union of the anterior and middle clinoid processes. Serial coronal sections through the cavernous sinus show that the artery does not always nestle into the bony carotid sulcus on the intracranial surface of the sphenoid bone, but is separated from it by an extension of the cavernous sinus.
The bone separating the artery and the sphenoid sinus is thinner over the anterior than the posterior parts of the carotid prominence and is thinnest over the part of the artery just below the tuberculum sellae. In our study, a layer of bone less than 0.5 mm thick separated the artery and sinus in nearly 90% of sinuses, and areas of absence of bone between the artery and the sinus were present in nearly 10%. Such defects in the bone separating the sphenoid sinus and carotid arteries may occur bilaterally. In our study of 50 sinuses, the bone separating the artery and the sinus was as thick as 1.0 mm (4). The bone over the carotid arteries was frequently as thin or thinner than that separating the anterior surface of the pituitary gland and the sphenoid sinus. The intracranial surface of the sphenoid bone was covered by periosteum, and this and the sinus mucosa separated the air cavity and carotid arteries if no bone was present.
The proximity of the carotid prominences to the midline is important in pituitary surgery. The transverse separation between the carotid prominences of each side was measured at the level of the tuberculum sellae, anterior sellar wall, sellar floor, dorsum sellae, and clivus. The shortest distance between both carotid bulges into the sphenoid sinus is usually located at the level of the tuberculum sellae (Figs. 8.4, 8.7, and 8.10). In our specimens, the shortest distance between both carotid prominences of each side was located just below the tuberculum in 72%, at the level of the floor of the sella in 20%, and at the clivus in 8% (4).
The optic canals protrude into the superolateral portion of the sinus. The superior orbital fissure produces a smooth wide prominence in the midlateral wall below the optic canal, and the maxillary nerve frequently protrudes into the inferolateral part. There are areas where no bone separates the optic sheath and sinus mucosa (Figs. 8.2, 8.10, and 8.11). In nearly 80% of the optic nerves, less than 0.5 mm of bone separated the optic nerve and sheath from the sinus. Care must be taken to avoid damage to the nerves in the transsphenoidal approach, if a dehiscence of the bone covering exposes them in the sinus. Injury to nerves exposed in the sinus wall may explain some cases of unexpected visual loss after transsphenoidal surgery (13, 20).
A pneumatized diverticulum of the sinus, called the opticocarotid recess, often extends laterally into the optic strut between the optic canal and the prominence overlying the carotid artery and superior orbital fissure (Figs. 8.2, 8.10, and 8.11). This pneumatization may extend through the optic strut into the anterior clinoid process, thus creating a channel through which cerebrospinal fluid can leak into the sinus after an anterior clinoidectomy with resulting cerebrospinal fluid rhinorrhea.
The diaphragma sellae forms the roof of the sella turcica (Fig. 8.1). It covers the pituitary gland, except for a small central opening in its center, which transmits the pituitary stalk. The diaphragma is more rectangular than circular, tends to be convex or concave rather than flat, and is thinner around the infundibulum and somewhat thicker at the periphery. It frequently is a thin, tenuous structure that would not be an adequate barrier for protecting the suprasellar structures during transsphenoidal operation. In an earlier anatomic study, Renn and Rhoton (15) found that the diaphragma was at least as thick as one layer of dura in 38% and in most cases it furnishes an adequate barrier during transsphenoidal hypophysectomy. In the remaining 62%, the diaphragma was extremely thin over some portion of the pituitary gland. It was concave when viewed from above in 54% of the specimens, convex in 4%, and flat in 42%. Even when flat, it lies below the plane of the upper surface of the anterior clinoid process so that a medially projecting supradiaphragmatic lesion, such as an aneurysm, may seem on neuroradiological studies to be located below the anterior clinoid and within the cavernous sinus when they are above the diaphragm in the subarachnoid space.
The opening in the diaphragm’s center is large when compared with the size of the pituitary stalk. The diaphragmal opening was 5 mm or more in 56% of our cases, and in these, it would not form a barrier during transsphenoidal pituitary surgery. The opening was round in 54% of the cases, and elliptical with the short diameter of the ellipse oriented in an anterior-posterior direction in 46%. A deficiency of the diaphragma sellae is assumed to be a precondition to formation of an empty sella. An outpouching of the arachnoid protruded through the central opening in the diaphragma into the sella turcica in approximately half of the patients. This outpouching, if opened, represents a potential source of postoperative cerebrospinal fluid leakage (13).
When exposed from above by opening the diaphragma, the superior surface of the posterior lobe of the pituitary gland is lighter in color than the anterior lobe (Fig. 8.1). The anterior lobe wraps around the lower part of the pituitary stalk to form the pars tuberalis (16, 21). The posterior lobe is softer, almost gelatinous, and is more densely adherent to the sellar wall. The anterior lobe is firmer and is more easily separated from the sellar walls. The gland’s width is the same or more than either its depth or its length in most patients. Its inferior surface usually conforms to the shape of the sellar floor, but its lateral and superior margins vary in shape because these walls are composed of soft tissue rather than bone. If there is a large opening in the diaphragma, the gland tends to be concave superiorly in the area around the stalk. The superior surface may become triangular as a result of being compressed laterally and posteriorly by the carotid arteries. As the anterior lobe is separated from the posterior lobe, there is a tendency for the pars tuberalis to be retained with the posterior lobe. Small intermediate lobe cysts are frequently encountered during separation of the anterior and posterior lobes.
Pituitary Gland and Carotid Artery
The distance separating the medial margin of the carotid artery and the lateral surface of the pituitary gland is an important consideration in transsphenoidal surgery (Figs. 8.6, 8.7, and 8.10). There is often a separation between the lateral surface of the gland and the carotid artery. In our cases where the artery did not indent the gland, the distance between the gland and artery varied from 1 to 7 mm (average, 2.3 mm); however, in approximately one in four cases, the artery protruded through the medial wall of the cavernous sinus to indent the gland (Fig. 8.1J) (9, 15). In these cases, the gland loses its spherical shape and conforms to the wall of the artery, often developing protrusions above or below the artery. In these cases, it would be difficult to remove the entire gland during transsphenoidal hypophysectomy. Such residual fragments may explain the pituitary function that remains after attempted hypophysectomy. Intrasellar tumors are subjected to the same forces, which prevent them from being spherical, and the increased pressure within the tumor increases the degree to which the tumor insinuates into surrounding crevices and tissue planes. Separation of these extensions from the main mass of gland or tumor may explain cases in which the tumor and elevated pituitary hormone levels persist or recur after adenoma removed.
The proximity of the carotid arteries to the midline is extremely important in pituitary surgery. In a previous study, the shortest distance between the two carotid arteries was found in the supraclinoid area in 82% of the cases, in the cavernous sinus along the side of the sella in 14%, and in the sphenoid sinus in 4% (15). Arterial bleeding during transsphenoidal surgery has been reported as due to carotid artery injury, but may also have arisen from a tear in an arterial branch of the carotid, such as the inferior hypophyseal artery, or by avulsion of a small capsular artery from the carotid artery (13).
Intercavernous Venous Connections
Venous sinuses that interconnect the paired cavernous sinuses may be found in the margins of the diaphragma and around the gland (Figs. 8.1, 8.6, 8.11, and 8.12) (15). The intercavernous connections within the sella are named on the basis of their relationship to the pituitary gland; the anterior intercavernous sinuses pass anterior to the hypophysis, and the posterior intercavernous sinuses pass behind the gland. Actually, these intercavernous connections can occur at any site along the anterior, inferior, or posterior surface of the gland, or all connections between the two sides may be absent. The anterior intercavernous sinus may cover the whole anterior wall of the sella. The anterior sinus is usually larger than the posterior sinus, but either or both may be absent. If the anterior and posterior connections coexist, the whole structure constitutes the “circular sinus.” Entering an anterior intercavernous connection that extends downward in front of the gland during transsphenoidal operation may produce brisk bleeding. However, this usually stops with temporary compression of the channel with hemostatic foam or with light coagulation, which serves to glue the walls of the channel together.
A large intercavernous venous connection called the basilar sinus passes posterior to the dorsum sellae and upper clivus connecting the posterior aspect of both cavernous sinuses (Figs. 8.6, 8.7, and 8.11). The basilar sinus is the largest and most constant intercavernous connection across the midline. The superior and inferior petrosal sinuses join the basilar sinus. The abducent nerve often enters the posterior part of the cavernous sinus by passing through the basilar sinus.
The cavernous sinuses are located on each side of the sphenoid sinus, sella, and pituitary gland (Figs. 8.6, 8.7, and 8.11). They extend from the superior orbital fissure in front to the petrous apex behind and surround the horizontal portion of the carotid artery. The medial wall of the paired cavernous sinuses form the lateral boundary of the sella. Sellar tumors frequently extend into the cavernous sinus (9). The cavernous sinuses are described in greater detail in Chapter 9.
The intracavernous portion of the carotid artery begins lateral to the posterior clinoid process where it leaves the foramen lacerum and turns abruptly forward to enter into the cavernous sinus (Figs. 8.6–8.8, 8.10, and 8.11). It then passes forward in a horizontal direction for approximately 2 cm and terminates by passing upward along the medial side to the anterior clinoid process, where it penetrates the roof of the cavernous sinus. The cavernous carotid is relatively fixed by the bony ring formed by the anterior and middle clinoid processes and the carotid sulcus, but despite this, large extensions of pituitary tumor may produce lateral displacement of the artery.
The branches of the intracavernous portion of the carotid artery that supply the sellar contents are the meningohypophyseal trunk, the largest intracavernous branch, which gives rise to the inferior hypophyseal artery, and McConnell’s capsular arteries, which arise directly from the internal carotid artery (Figs. 8.6, 8.7, and 8.11). The meningohypophyseal trunk arises at the level of the dorsum sellae at or just before the apex of the first curve of the carotid where it turns forward after leaving the carotid canal. The inferior hypophyseal artery arises from the meningohypophyseal trunk and passes medially to the posterior pituitary capsule and lobe, and anastomoses with its mate of the opposite side after supplying the dura of the sellar floor. McConnell’s capsular arteries are frequently absent and, if present, arise from the medial side of the carotid artery and pass to the capsule of the gland or the dura lining the anterior wall and floor of the sella.
The location of the nerves in the wall of the cavernous sinus are, from superior to inferior, the IIIrd cranial nerve superiorly and then the trochlear, ophthalmic, and abducens nerves (Figs. 8.6 and 8.10). The oculomotor, trochlear, and ophthalmic nerves lie between the two dural leaves of the lateral sinus wall. The abducens courses within the sinus on the medial side of the ophthalmic nerve and is adherent to the carotid artery medially and the ophthalmic nerve laterally. The IIIrd and IVth cranial nerves enter the dural roof of the cavernous sinus with the IIIrd nerve in front and medial to the IVth nerve. The IIIrd nerve enters the cavernous sinus slightly lateral and anterior to the dorsum sellae, almost directly above the meningohypophyseal trunk. The ophthalmic nerve enters the cavernous sinus wall inferiorly and slopes slightly upward to depart through the superior orbital fissure (Fig. 8.6). The VIth cranial nerve enters the lower part of the posterior wall of the sinus, bends laterally around the proximal portion of the intracavernous carotid, and runs parallel to the ophthalmic nerve between the ophthalmic nerve and intercavernous carotid. It usually enters the sinus as a single bundle, but may also be split into two bundles in the subarachnoid space before reaching the sinus. After entering the sinus, it may split into multiple, as many as five, rootlets as it courses between the internal carotid artery and ophthalmic nerve, but these collect together to form a single bundle that passes through the superior orbital fissure.
SUPRASELLAR AND THIRD VENTRICULAR REGION
This section of the chapter deals with the neural, arterial, and venous relationships in the suprasellar and third ventricular regions that are important in planning surgery for pituitary adenomas and tumors arising in the sella. The anatomy important to dealing with tumors within the third ventricle is dealt with in Chapter 5.
Ventricular and Cisternal Relationships
Tumors arising in the sella often extend upward into the suprasellar cisterns to compress the floor of the third ventricle and involve the circle of Willis and deep cerebral venous system (Fig. 8.13) (25). The area involved by those tumors arising in the sellae corresponds to the anterior incisural space located between the free edges of the tentorium and the front of the midbrain. The anterior incisural space corresponds roughly to the suprasellar area. From the front of the midbrain it extends obliquely forward and upward around the optic chiasm to the subcallosal area. It opens laterally into the sylvian fissure and posteriorly between the uncus and the brainstem.
The part of the anterior incisural space located below the optic chiasm has posterior and posterolateral walls (14, 19). The posterior wall is formed by the cerebral peduncles. The posterolateral wall is formed by the anterior third of the uncus, which hangs over the free edge above the oculomotor nerve. The infundibulum of the pituitary gland crosses the anterior incisural space to reach the opening in the diaphragma sellae. The part of the anterior incisural space situated above the optic chiasm is limited superiorly by the rostrum of the corpus callosum, posteriorly by the lamina terminalis, and laterally by the part of the medial surfaces of the frontal lobes located below the rostrum.
The anterior incisural space opens laterally into the part of the sylvian fissure situated below the anterior perforated substance. The anterior limb of the internal capsule, the head of the caudate nucleus, and the anterior part of the lentiform nucleus are located above the anterior perforated substance. The interpeduncular cistern, which sits in the posterior part of the anterior incisural space between the cerebral peduncles and the dorsum sellae, communicates anteriorly with the chiasmatic cistern, which is located below the optic chiasm. The interpeduncular and chiasmatic cisterns are separated by Liliquist’s membrane, an arachnoidal sheet extending from the dorsum sellae to the anterior edge of the mamillary bodies. The chiasmatic cistern communicates around the optic chiasm with the cisterna laminae terminalis, which lies anterior to the lamina terminalis.
The optic and oculomotor nerves and the posterior part of the olfactory tracts pass through the suprasellar region and anterior incisural space (Figs. 8.6, 8.11, and 8.13). Each olfactory tract runs posteriorly and splits just above the anterior clinoid process to form the medial and lateral olfactory striae, which course along the anterior margin of the anterior perforated substance.
The optic nerves and chiasm and the anterior part of the optic tracts cross the anterior incisural space. The optic nerves emerge from the optic canals medial to the attachment of the free edges to the anterior clinoid processes and are directed posteriorly, superiorly, and medially toward the optic chiasm. From the chiasm, the optic tracts continue in a posterolateral direction around the cerebral peduncles to enter the middle incisural spaces. The optic nerve proximal to its entrance into the optic canal is covered by a reflected leaf of dura mater, the falciform process, which extends medially from the anterior clinoid process across the top of the optic nerve. The length of nerve covered only by the dura of the falciform process at the intracranial end of the optic canal may vary from less than 1 mm to as much as 1 cm (15). Coagulation of the dura above the optic nerve just proximal to the optic canal on the assumption that bone separates the dura mater from the nerve could lead to nerve injury. Compression of the optic nerve against the sharp edge of the falciform process may result in a visual field deficit, even if the compressing lesion does not damage the nerve enough to cause visual loss. Normally, the optic nerve is separated medially from the sphenoid sinus by a thin layer of bone, but if the sinus is well pneumatized, this bone is absent, and the optic nerves may protrude directly into the sphenoid sinus, separated from the sinus by only mucosa and the dural sheath of the nerve (4, 15).
The optic chiasm is situated at the junction of the anterior wall and floor of the third ventricle (Fig. 8.13). The anterior cerebral and anterior communicating arteries, the lamina terminalis, and the third ventricle are located above the chiasm. The tuber cinereum and the infundibulum are posterior to, the internal carotid arteries are lateral to, and the diaphragma sellae and pituitary gland are below the optic chiasm. The suprachiasmatic recess of the third ventricle is located between the chiasm and lamina terminalis. The infundibular recess extends into the base of the pituitary stalk behind the optic chiasm. The relationship of the chiasm to the sella is an important determinant of the ease with which the pituitary fossa can be exposed by the transfrontal surgical route (Figs. 8.13 and 8.14). The normal chiasm overlies the diaphragma sellae and the pituitary gland, the prefixed chiasm overlies the tuberculum, and the postfixed chiasm overlies the dorsum. In approximately 70% of our cases, the chiasm is in the normal position; of the remaining 30%, approximately half are “prefixed” and half “postfixed” (15).
A prominent tuberculum sellae may restrict access to the sellae, even in the presence of a normal chiasm. The tuberculum may vary from being almost flat to protruding upward as much as 3 mm, and it may project posteriorly to the margin of a normal chiasm (15). A prefixed chiasm, a normal chiasm with a small area between the tuberculum and the chiasm, and a superior protruding tuberculum sellae do not limit exposure by the transsphenoidal approach, but they limit the access to the suprasellar area provided by the transcranial approach. There are several methods of gaining access to the suprasellar area when these variants are present. One is to expose the sphenoid sinus from above by opening through the tuberculum and planum sphenoidale, thus converting the approach to a transfrontal-transsphenoidal exposure. If the chiasm is prefixed and the tumor is seen through a thin, stretched arterial wall of the third ventricle, the lamina terminalis may be opened to expose the tumor, but this exposure is infrequently used for pituitary adenomas, and they more commonly form craniopharyngiomas and gliomas involving the third ventricle. If the space between the carotid artery and the optic nerve has been enlarged, by a lateral or parasellar extension of tumor, the tumor may be removed through this space (16, 23).
An understanding of the relationship of the carotid artery, optic nerve, and anterior clinoid process is fundamental to all surgical approaches to the sellar and parasellar areas (Figs. 8.6 and 8.13). The carotid artery and the optic nerve are medial to the anterior clinoid process. The artery exits the cavernous sinus beneath and slightly lateral to the optic nerve. The optic nerve pursues a posteromedial course toward the chiasm, and the carotid artery pursues a posterolateral course toward its bifurcation below the anterior perforated substance into the anterior and middle cerebral arteries in the area.
The oculomotor nerve arises in the interpeduncular cistern from the midbrain on the medial side of the cerebral peduncle and courses between the posterior cerebral and superior cerebellar arteries (Figs. 8.6 and 8.13). The oculomotor nerve courses in the lateral wall of the interpeduncular cistern and forms the pillars to which Liliquist’s membrane attaches. Liliquist’s membrane arises from the arachnoid membrane covering the dorsum sellae and separates the chiasmatic and interpeduncular cisterns. The uncus of the temporal lobe is situated lateral to the oculomotor nerve. The oculomotor nerve pierces the roof of the cavernous sinus and slopes downward in the superolateral corner of the cavernous sinus.
The trochlear nerve is the longest and thinnest cranial nerve (Fig. 8.6). It arises from the midbrain below the inferior colliculi and passes around the brainstem near the junction of the midbrain and pons to reach the lower margin of the tentorial edge. The trochlear nerve pierces the medial edge of the tentorium and enters the roof of the cavernous sinus just behind the anterior tentorial attachment.
The abducens nerve arises at the lower margin of the pons and passes above, below, or is split into two bundles by the anteroinferior cerebellar artery (Fig. 8.6). It passes upward in the prepontine cistern and turns forward at the upper border of the petrous apex, where it pierces the dura to enter the posterior part of the cavernous sinus.
The trigeminal nerve arises on the posterior fossa from the midpons. The posterior root passes above the petrous apex to enter Meckel’s cave, which is located lateral to the cavernous sinus. Meckel’s cave extends forward to the level of the trigeminal ganglion. The nerve divides into the three divisions at the anterior edge of the ganglion. The ophthalmic division courses in the lower anterior part of the cavernous sinus. The maxillary nerve courses just below the cavernous sinus, where its medial side produces a prominence in the lateral wall of the sphenoid sinus, just before exiting the foramen rotundum, to enter the pterygopalatine fossa.
The arterial relationships in the suprasellar area are among the most complex in the head, because this area contains all the components of the circle of Willis (6) (Figs. 8.15 and 8.16). Numerous arteries, including the internal carotid and basilar arteries and the circle of Willis and its branches, may be stretched around tumors in this area. The posterior part of the circle of Willis and the apex of the basilar artery are located in the anterior incisural space below the floor of the third ventricle; the anterior part of the circle of Willis and the anterior cerebral and anterior communicating arteries are intimately related to the anterior wall of the third ventricle; both the anterior and posterior cerebral arteries send branches into the roof of the third ventricle; the internal carotid, anterior choroidal, anterior and posterior cerebral, and anterior and posterior communicating arteries give rise to perforating branches that reach the walls of the third ventricle and anterior incisural space; and all the arterial components of the circle of Willis and the adjacent segments of the carotid and basilar arteries and their perforating branches may be stretched around suprasellar extensions of pituitary tumors (17).
The carotid artery is the most medial structure within the cavernous sinus. The internal carotid artery exits the cavernous sinus along the medial surface of the anterior clinoid process to reach the anterior incisural space (Figs. 8.4, 8.6, 8.15, and 8.16). After entering this space, it courses posteriorly, superiorly, and laterally to reach the site of its bifurcation below the anterior perforated substance. It is first below and then lateral to the optic nerve and chiasm. It sends perforating branches to the optic nerve, chiasm, and tract and to the floor of the third ventricle. These branches pass across the interval between the internal carotid artery and the optic nerve and may serve as an obstacle to the operative approaches directed through the triangular space between the internal carotid artery, the optic nerve, and the anterior cerebral artery. The internal carotid artery also gives off the superior hypophyseal artery, which runs medially below the floor of the third ventricle to reach the tuber cinereum and joins its mate of the opposite side to form a ring around the infundibulum.
The ophthalmic artery, the first branch of the internal carotid artery above the cavernous sinus, usually arises and enters the optic canal below the optic nerve (Figs. 8.7, 8.8, 8.10, and 8.11). Its origin and proximal segment may be visible below the optic nerve without retracting the nerve, although elevation of the optic nerve away from the carotid artery is usually required to see the segment proximal to the optic foramen. The artery arises from the supraclinoid segment of the carotid artery in most cases, but some arise within the cavernous sinus or rarely as a branch of the middle meningeal artery (9, 12, 15).
The posterior communicating artery arises from the posterior wall of the internal carotid artery and courses posteromedially below the optic tracts and the floor of the third ventricle to join the posterior cerebral artery (Figs. 8.15 and 8.16). Its branches penetrate the floor between the optic chiasm and the cerebral peduncle and reach the thalamus, hypothalamus, subthalamus, and internal capsule. Its posterior course varies, depending on whether the artery provides the major supply to the distal posterior cerebral artery. If it is normal, with the posterior cerebral artery arising predominately from the basilar artery, it is directed posteromedially above the oculomotor nerve toward the interpeduncular fossa. If the posterior cerebral artery has a fetal-type configuration in which it arises predominantly from the carotid artery, the posterior communicating artery is directed posterolaterally above or below and lateral to the oculomotor nerve.
The anterior choroidal artery arises from the posterior surface of the internal carotid artery above the origin of the posterior communicating artery (Figs. 8.15 and 8.16). It is directed posterolaterally below the optic tract between the uncus and cerebral peduncle. It passes through the choroidal fissure behind the uncus to supply the choroid plexus in the temporal horn, sending branches into the optic tract and posterior part of the third ventricular floor that reach the optic radiations, globus pallidus, internal capsule, midbrain, and thalamus.
The anterior cerebral artery arises from the internal carotid artery below the anterior perforated substance and courses anteromedially above the optic nerve and chiasm to reach the interhemispheric fissure, where it is joined to the opposite anterior cerebral artery by the anterior communicating artery (Figs. 8.15 and 8.16). The junction of the anterior communicating artery with the right and left A1 segments is usually above the chiasm rather than above the optic nerves. The shorter A1 segments are stretched tightly over the chiasm, and the larger ones pass anteriorly over the nerves. Displacement of the chiasm against these arteries may result in visual loss before that caused by direct compression of the visual pathways by the tumor. The arteries with a more forward course are often tortuous and elongated, and some may course forward and rest on the tuberculum sellae or planum sphenoidale. The anterior cerebral and anterior communicating arteries give rise to perforating branches that terminate in the whole anterior wall of the third ventricle and reach the adjacent parts of the hypothalamus, fornix, septum pellucidum, and striatum. The recurrent branch of the anterior cerebral artery, frequently encountered in the area, arises from the anterior cerebral artery in the region of the anterior communicating artery, courses laterally above the bifurcation of the internal carotid artery, and enters the anterior perforated substance.
The bifurcation of the basilar artery into the posterior cerebral arteries is located in the posterior part of the suprasellar area below the posterior half of the floor of the third ventricle (Figs. 8.11, 8.15, and 8.16). A high basilar bifurcation may indent the floor. The posterior cerebral artery courses laterally around the cerebral peduncle, above the oculomotor nerve, and passes between the uncus and the cerebral peduncle to reach the quadrigeminal cistern. Its branches reach the floor, roof, and posterior and lateral walls of the third ventricle. The thalamoperforating arteries are a pair of larger perforating branches that arise from the proximal part of the posterior cerebral artery in the sellar region and enter the brain through the posterior part of the third ventricular floor and the lateral walls. The author is aware of several cases in which damage to the thalamoperforating branches occurred during transsphenoidal surgery after opening of the posterosuperior part of the tumor capsule, with resulting coma and death.
Veins do not pose a formidable obstacle to operative approaches to the suprasellar area and lower part of the third ventricle as they do in the region of the roof and posterior third ventricle, because the veins in the suprasellar region are small. The suprasellar area is drained, almost totally, by tributaries of the basal vein. The basal veins are formed by the union of veins draining the suprasellar area, and proceed posteriorly between the midbrain and the temporal lobes to empty into the internal cerebral or great vein. The internal cerebral veins course in the roof of the third ventricle and are only infrequently involved in pituitary adenomas. They originate just behind the foramen of Monro and course posteriorly within the velum interpositum. They join above or posterior to the pineal body to form the great vein.
DISCUSSION AND OPERATIVE APPROACHES
The fact that the pituitary fossa is usually separated from the sphenoid sinus by only a thin layer of bone led to the transsphenoidal route being used for operations on sellar tumors as early as 1907 (Figs. 8.2, 8.8, 8.10, and 8.11) (2, 10, 24). The approach subsequently fell into disfavor because of the high incidence of complications and the difficulty in operating through such a deep, narrow exposure. The modern redevelopment of this approach began with Dott and Bailey of Edin burgh (3) who learned the technique from Cushing and later taught it to Guiot of France (7). Guiot reintroduced this technique, using radiofluoroscopy to visualize the depth and position of the surgical instruments, and this, in combination with the illumination and magnification provided by the operating microscope, afforded the possibility of a safer approach and more accurate visualization of normal and pathological tissues (7, 8). Guiot taught the method to Hardy of Montreal (8) and both Guiot and Hardy’s work reflects the improvement in the procedure brought about by the use of the operating microscope and radiofluoroscopy. Since its reintroduction, it has become the method of choice for removal of nearly all micro- and macroadenomas of the pituitary gland and selected other sellar tumors, including tumors with suprasellar extensions if they extend from and are centered above an enlarged sella turcica. The use of the transsphenoidal approach for clival and upper midline posterior fossa lesions is reviewed in Chapter 6 of the Millennium issue of Neurosurgery (18).
Several routes through the nasal cavity have been used to reach the sphenoid sinus (Fig. 8.17). The sublabial approach is directed under the lip and submucosally along the nasal septum to the sphenoid sinus. The transseptal approach avoids the oral cavity and is directed through a small incision along one side of the columella and submucosally along the septum. The endonasal approach, used in recent years by the author, is directed through one nostril, between the concha laterally and the nasal septum medially, and does not require an incision in the nose before reaching the anterior face of the sphenoid. This discussion, in addition to examining the sellar anatomy, also focuses on the microsurgical anatomy of the nasal cavity and other surgical routes to the sella.
Sublabial Transsphenoidal Approach
The head is positioned in a pinion head holder, with lateral fluoroscopy, with the head tilted or rotated so that the surgeon’s direction of view is through the patient’s nasal cavity sphenoid face in front of the sella. The whole procedure is performed using the operating microscope. The upper lip is elevated to expose the buccogingival margin, which is incised transversely from one canine fossa to the other. In the subperiosteal dissection, the upper lip is elevated to expose the osseous nasal floor, the nasal spine, and the lower edge of the lateral rami of the maxilla. The nasal spine and the lateral rami are preserved. With experience, the surgeon can work around rather than removing the nasal spine. We also avoid breaking off the thin edge of the lateral rami during the opening of the speculum, because healing may produce an annoying callus beside the nose. Using subperiosteal and subperichondral dissection, the mucosa is elevated from the nasal septum along the path directed to the sphenoid face.
The nasal speculum is inserted under fluoroscopic control between the septal mucosa on one side and the adjacent septum, with the handle facing superiorly (Figs. 8.17 and 8.18). Dissection misdirected into the area of the cribriform plate may result in a leak of cerebrospinal fluid or a loss of the sense of smell. When the speculum is correctly positioned, the sphenoid ostia situated on each side of the perpendicular plate of the ethmoid will usually lie at the superior end of the exposure. The ostia should mark the upper margins of the opening into the sphenoid sinus. Opening the speculum pushes the remaining cartilaginous and osseous septum to one side and the mucosa on the septum to the other side. Opening the blades of the speculum will provide sufficient compression of the conchae for adequate exposure. Removal of the nasal conchae is not required. If the speculum has been inserted correctly, the junction of the vomer and perpendicular ethmoid plate with the sphenoid face will be oriented vertically in the center of the area between the blades of the speculum, as is essential to maintaining the approach in the midline. There is no need to reposition the speculum during the remainder of the procedure. In the past, it was common to remove a piece of the nasal septum using a knife, scissors, or a Ballinger swivel knife to be used as a splint for closing the sella at the end of the operation, in which case, an anterior strut of septal cartilage should be preserved to maintain a normal postoperative nasal contour. However, the author avoids taking any of the septum and uses the bone harvested in opening the face of the sphenoid sinus to close the sella. A small biodegradable burr hole cover may also be cut to the appropriate size for use as a splint for closing the sella.
The transseptal approach used a short incision adjacent to the columella, usually on the right side, at the mucocutaneous junction (Fig. 8.17). The incision is carried down to the anterior part of the cartilaginous septum and, by using subperichondral dissection, the exposure is advanced submucosally along the anterior edge of the columella to the left side of the nasal septum. The submucosal dissection is directed, by using fluoroscopy, to the sphenoid face. The posterior septum is separated from the sphenoid face in an area of sufficient size, usually 1.0 to 1.5 cm, to allow the tips of the speculum blades to be advanced submucosally along the sphenoid face. With an osteotome, a piece of bone large enough to serve as a splint for sellar closure can usually be harvested from the sphenoid face at the time of opening the sphenoid sinus.
The author has used this type of transsphenoidal approach for the last four years. The view of the microscope is directed through one nostril between the nasal septum and nasal conchae to the sphenoid face below the ostia (Figs. 8.4 and 8.17– 8.19). No incision is needed in the anterior part of the nasal cavity. In the endonasal approach, a hand-held nasal speculum, inserted under fluoroscopy into one nostril between the conchae and nasal septum, is opened to compress the conchae and septum sufficiently that the endonasal transsphenoidal speculum can be advanced through one nostril to the sphenoid face. Removal of the conchae is not required. The junction of the nasal septum with the sphenoid face is the most reliable landmark for maintaining the exposure in the midline. The sphenoid ostia are situated on each side of the perpendicular ethmoid plate and mark the upper limit of the opening into the sphenoid sinus. Dissection misdirected into the area of the cribriform plate is to be avoided. The endonasal speculum is placed in the nasal cavity so that the crest on the sphenoid face at the septum’s junction with the perpendicular ethmoid plate is positioned vertically between the tips of the speculum blades.
Management of Sphenoid Sinus and Sella
The anterior wall of the sinus is opened with a small bayonetted osteotome to yield an exposure in the anterior wall approximately 1.0 to 1.5 cm in diameter. The opening may be tailored to extend predominantly to either side, depending on the size, shape, and location of the tumor. The opening is enlarged to allow passage of instruments of sufficient size and angle to remove the tumor.
The configuration of the interior of the sinus and the location of the septa are noted. The septae may vary in number and position and frequently deviate from the midline (Figs. 8.4, 8.9, and 8.19). The septae are not to be used as a guide to the midline, but may be used as landmarks based on where the preoperative computed tomography or magnetic resonance imaging show them to be located in relation to the sella and the tumor. The mucosa of the sphenoid sinus is pushed laterally from the septae as needed to expose the anterior sellar wall. The mucosa is preserved because it facilitates normal sinus drainage. Usually, the mucosa on each side of the septae near the midline of the sinus can be pushed laterally to obtain a clear view of the sinus cavity and the anterior sellar wall. The floor of the sella turcica is usually seen as a smooth bulge in the superior part of the sinus if the opening into the sinus has been positioned correctly based on fluoroscopy. The sella should lie directly ahead of the tips of the speculum on fluoroscopy. The chiasmatic sulcus may produce a subsidiary bulge into the sinus and this, lying superior to the sellar floor, should not be mistaken for the sella turcica. The prominences overlying the optic canals, superior orbital fissure, and maxillary nerve, located in the lateral part of the presellar portion of the sphenoid sinus, are not normally visible with the operating microscope and should not be searched for if the exposure is in the midline; however, they can often be identified using straight and angled endoscopes (Figs. 8.2, 8.8, 8.10, and 8.20). The prominences in the sinus walls overlying the carotid arteries are frequently exposed at the lateral edge of the anterior sellar wall and are not to be confused with the prominence overlying a tumor. The carotid prominences are located just lateral and anterior to the anterior wall of the sella (Figs. 8.2, 8.8, and 8.10). In a few cases, the nerves and arteries will be covered by only sinus mucosa and dura, and in others, the bone covering the optic nerves will be very thin. Therefore, the nerves passing through the optic canal, superior orbital fissure, and foramen rotundum could be injured in the transsphenoidal approach if vigorous attempts are made to strip or curette the mucosa from the walls of the sinus, or if the nasal speculum is advanced to lie within the sinus and is forcefully opened against its lateral walls. The nasal speculum should not be advanced into the sinus, because this does not increase the exposure and may cause damage to the poorly protected arteries and nerves within the sinus walls. Septae within the sinus are removed as needed to expose the anterior sellar wall. The bulge of the sellar floor is usually identifiable, unless the sinus is of a presellar or conchal type, in which case the sellar bulge may not be apparent. However, the floor of the sella turcica should be directly ahead of the long axis of the blades if the speculum has been positioned correctly on lateral fluoroscope and the vertical septal crest is positioned between the tips of the speculum blades.
A thin sellar floor facilitates the transsphenoidal approach. In nearly all adults, the floor will be less than 1 mm thick, and in two-thirds of adults, it will be less than 0.5 mm thick (1, 15). In the latter case, breaching the floor is accomplished by pressure on the bone with a small dissector, curette, or gentle tap on an osteotome. If the bone is thicker than 1 mm, it may be necessary to use the firm, but controlled, tap of a hammer against the osteotome or to remove the anterior wall with a drill.
If a drill is used to breach the sellar floor, the drilling is stopped while there is still a thin plate of bone that can be removed with a small bayonetted bone curette. The sellar opening should not extend along the anterior wall to the chiasmatic sulcus or tuberculum sellae, because an opening in this area is difficult to close, and may be associated with a high incidence of cerebrospinal fluid leakage. The opening in the sellar wall may be enlarged with a Kerrison rongeur if needed. The bone from the face of the sphenoid is saved for use in closing the sella.
By using a No. 11 scalpel blade on a bayonet knife handle, a short vertical incision is made in the dura in the midline. A small blunt right-angled ring curette is inserted through the small vertical dural opening, and the dura is separated from the anterior surface of the gland or tumor. After freeing the dura, a 45-degree-angle alligator scissor, rather than a knife, is selected to open the dura in an x-shaped cut from corner to corner, because a pointed knife may damage the carotid arteries in the far lateral corners of the exposure. The sellar dura is lifted away from the gland or tumor with the distal blade of the 45-degree-angled scissors, so that it can be seen that the cut does not extend into any structure deep to the dura. In some cases, the carotid artery may protrude through the medial wall of the cavernous sinus and indent the tumor and the gland and should not be mistaken for the tumor. Incising the dura on the diagonal from corner to corner provides a wider opening than a cruciate incision directed vertically and horizontally. The upper leaf of dura may be further incised in the midline if exposure over the top of the gland is needed.
After removing the tumor, the thickness of the diaphragma, the size of its opening, and the arrangement of the arachnoid around the pituitary stalk may be apparent. Excessive exploration and dissection, after the tumor is removed, is not a necessary part of most operations for pituitary adenomas and is to be avoided if possible because it may open into the cerebrospinal fluid-filled cisterns. In closing, the sella is filled with a pledget of crushed fat taken through a small skin incision from the subcutaneous tissues of the abdomen. A stent of thin bone, taken from the sphenoid face, is fashioned to fit through the sellar opening to close the defect in the sellar floor. The stent is fitted inside the sellar opening so that it fits snugly inside and overlaps the margin of the sellar opening from inside so that intrasellar and intracranial pressure will press the bone against the opening. The author avoids taking any nasal septum and uses a biodegradable burr hole corner, cut to the appropriate size and shape, if the bone from the sphenoid face is not available, as is frequent with reoperations. The sphenoid mucosa is repositioned and the ostia are inspected to make sure they are open. The sphenoid ostia may be enlarged if they seem small. No packing is placed in the sphenoid sinus unless the tumor has eroded the sellar walls and has filled the sphenoid sinus.
The transsphenoidal speculum is removed after the surgeon has ascertained that hemostasis in the sella and sphenoid sinus is satisfactory. The long hand-held speculum is reinserted, and the walls of the nasal passage on the side of the approach is inspected to make sure that hemostasis in the nasal mucosa is satisfactory and that there is an adequate opening in the anterior wall of the sphenoid sinus, preferably at the site of the ostia, for drainage of sinus secretions. The handheld speculum is then inserted into the opposite nasal passage to make certain there is no bleeding on that side. If hemostasis is excellent, as is almost always the case, no packing is left in the nose. The sphenoid ostia are also inspected to make sure the sphenoid ostia are of adequate size to promote sinus drainage, because an inadequate route of sinus drainage may lead to infection or development of a mucocele. The sinus mucosa is preserved, if possible, because it is essential to normal sinus drainage. Secretions collect in a sinus from which all the mucosa has been stripped unless the sinus has been obliterated with fat and closed.
The posterior wall of the sphenoid sinus forming the clivus may also be opened in the area below the sella with an osteotome or drill (Figs. 8.2 and 8.20). After the initial opening is completed, it can be enlarged with a Kerrison rongeur. If necessary, the clival opening can be extended upward by removing the floor of the sella turcica and downward by removing the floor of the sphenoid sinus.
All of these procedures are performed by using the magnified view and intense light provided by the operating microscope, which may at times be supplemented with the use of endoscopes advanced into the sphenoid, where they are helpful in identifying the structures in the wall of the sphenoid sinus. Advancing the endoscope into the sella is of less value because the tumor, bleeding, and the downwardly herniating diaphragm often obscure the endoscopic view (Fig. 8.20).
Subfrontal Exposure of the Suprasellar Region
A tumor situated between, above, or below the optic nerves and chiasm in the chiasmatic and lamina terminalis cisterns may be approached through a small frontal bone flap with frontal lobe elevation to expose the involved optic nerves and chiasm (Figs. 8.21 and 8.22). The bicoronal (Souttar) scalp incision from the subfrontal approach is located behind the hairline. The scalp flap is reflected forward as a single layer and a small frontal bone flap is positioned just above the supraorbital margin and extending up to the lateral edge of the superior sagittal sinus. The lateral burr hole, above the orbital rim, is positioned at the keyhole site and the medial burr hole usually extends through the front and back wall of the frontal sinus.
This approach is selected for a pituitary adenoma only if the sphenoid sinus is not pneumatized, the sella is small or not of sufficient size to reach the suprasellar extension of the tumor, or if there is an unusual suprasellar extension of the tumor that cannot be reached by the transsphenoidal approach. The subfrontal approach is most commonly used for lesions located between the upper surface of the pituitary gland and lower surface of the optic nerves and chiasm. A pterional craniotomy would be performed if there was a need to expose the area lateral to the anterior clinoid process and supraclinoid segment of the internal carotid artery, and an orbitozygomatic craniotomy might be considered if there was major involvement of the cavernous sinus.
After the area below the frontal lobe is reached, one of four routes may be followed to the tumor. The most commonly used route is the subchiasmatic approach directed between the optic nerves and below the chiasm (Figs. 8.21 and 8.22). Other routes that may be used are the opticocarotid approach directed between the carotid artery and the optic nerve; the transfrontal transsphenoidal approach achieved by drilling off the planum sphenoidale to approach the sella by a combined route; and the lamina terminalis approach directed above the chiasm, medial to the optic tracts, and usually above the anterior communicating artery (20). The subchiasmatic approach is most commonly used because most tumors elevate the chiasm and open the space between the optic nerves and chiasm. The optic carotid approach would be used if the tumor opened or protruded through the space between the optic nerve and carotid artery and the tumor was difficult to reach by the suprachiasmatic approach. The transfrontal-transsphenoidal approach would be used if the chiasm was prefixed and there was a large amount of tumor in the sphenoid sinus, and the lamina terminalis approach would be selected if the chiasm was prefixed and the tumor presented through a thin stretched lamina terminalis.
The fact that the carotid arteries, trigeminal nerves, and optic nerves may be exposed by removing the thin wall of the sphenoid sinus offers the possibility of another surgical approach to these structures; tumors located below and medial to the optic nerve within the optic canal could be approached from this direction; fractures through the optic canal compressing the optic nerve might be decompressed through the sphenoid sinus; and the second trigeminal division might be approached from this direction (Figs. 8.4, 8.8, 8.10, and 8.19). The length of carotid artery exposed in the wall of the sphenoid sinus offers the possibility that the intracavernous segment might be exposed by the transsphenoidal approach for trapping procedures, inserting catheters for obliteration of fistulas, or for specialized contrast studies. The close proximity of the cavernous sinus to the lateral wall of the sphenoid sinus offers the possibility that the cavernous sinus might be entered through the thin sphenoidal wall for insertion of wire or other materials used to thrombose arteriovenous fistulas within the cavernous sinus.
Contributor: Albert L. Rhoton, Jr., MD
Content from: Rhoton AL, Jr. The sellar region. Neurosurgery 2002;51(4 Suppl):335-374, 10.1097/00006123-200210001-00009. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons.
The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton, Jr., MD.
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