Chiasmatic Configuration and Tuberculum Sellae
The relation of the chiasm to the sella is an important determinant of the ease with which the pituitary fossa may be exposed by the transfrontal surgical route. The normal chiasm overlies the diaphragma sellae and the pituitary, the prefixed chiasm overlies the tuberculum sellae, and the postfixed chiasm overlies the dorsum sellae. In approximately 70 percent of cases, the chiasm is in the normal position. Of the remaining 30 percent, about half are prefixed and half postfixed.
A prominent tuberculum sellae may restrict access to the sella even in the presence of a normal chiasm. The tuberculum may vary from being almost flat to protruding upward as much as 3 mm toward the anterior margin of a normal chiasm.
Carotid Artery, Optic Nerve, and Anterior Clinoid Process
An understanding of the relations between the carotid artery, the optic nerve, and the anterior clinoid process is fundamental to all surgical approaches to the sellar and parasellar areas. 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 a posterolateral course toward its bifurcation into the anterior and middle cerebral arteries.
The optic nerve proximal to its entrance into the optic canal is covered by a reflected leaf of dura, the falciform process, which extends medially from the anterior clinoid process across the top of the optic nerve. The length of nerve covered by dura only, at the intracranial end of the optic canal, may vary from less than 1 mm to as much as 1 cm. Thus, coagulation of the dura above the optic nerve just proximal to the optic canal, on the assumption that bone separates the dura from the nerve, could lead to nerve injury. Compression of the optic nerves 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. The full length of the optic canal must be unroofed before its narrowest point is passed, because the narrowest part is closer to the orbital than to the intracranial end. The optic canals average 5 mm in length and are conical, tapering to a narrow waist near the orbit. The ophthalmic artery is found inferomedial to the optic nerve when the periosteum lining the optic canal is opened.
All the arterial components of the circle of Willis and the adjacent carotid artery give origin to multiple perforating branches, which may become stretched over suprasellar tumors. The supraclinoid portion of the carotid artery, in addition to giving off the posterior communicating and anterior choroidal arteries, also gives off perforating branches, which include the superior hypophyseal artery and other branches passing to the optic nerve. chiasm, anterior hypothalamus, and anterior perforated substance. The posterior part of the circle of Willis and the upper centimetre of the basilar artery also send a series of perforating arteries through the suprasellar area into the diencephalon and midbrain. and these arteries may become stretched around suprasellar tumors. The largest perforating branches arising from the posterior part of the circle of Willis are the thalamoperforate and medial posterior choroidal arteries.
The origin and proximal segment of the ophthalmic artery 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 preforaminal segment. The artery arises above the cavernous sinus in most cases, but it may also arise within the cavernous sinus, and rarely it is absent.
The posterior communicating artery arises from the posteromedial wall of the carotid artery. It courses posteromedially above and medial to the oculomotor nerve toward the interpeduncular fossa and gives rise to multiple perforating branches, which may be stretched over the tip of a suprasellar tumor.
The origin and initial segment of the anterior choroidal artery may be visible between the posterior communicating artery and the bifurcation of the internal carotid artery. The initial segment of the anterior choroidal artery, which is directed posterolaterally below the optic tract, may be displaced upward and laterally by sellar tumors.
Each anterior cerebral artery courses over the superior surface of the optic chiasm or nerve to join the anterior communicating artery. 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 longer ones pass anteriorly over the nerves. In some cases, visual loss may be caused by displacement of the chiasm against these arteries before it is caused by direct compression of the visual pathway 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 multiple branches which terminate in the superior surface of the optic chiasm, the anterior hypothalamus, the anterior perforated substance, and the region of the optic tract.
The recurrent artery of Heubner also arises from the anterior cerebral artery in the region of the anterior communicating artery and runs above the chiasm adjacent to the anterior cerebral artery. The recurrent artery courses anterior to the anterior cerebral artery in about two-thirds of cases (it is seen when the frontal lobe is elevated prior to visualizing the anterior cerebral artery).
The diaphragma sellae forms the roof of the sella turcica. It covers the pituitary gland, except for a small central opening that 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. The opening in its center is large compared to the size of the pituitary stalk. The diaphragma is frequently a thin, tenuous structure, which would not be an adequate barrier for the protection of the suprasellar structures during trans-sphenoidal surgery. An outpouching of the arachnoid protrudes through the central opening in the diaphragma into the sella turcica in about half the specimens. Although this diverticulum can usually be retracted unruptured during trans-sphenoidal surgery, it represents a potential source of postoperative cerebrospinal fluid leakage.
The surface of the posterior lobe of the pituitary gland is lighter in color than the anterior lobe. The upper part of the anterior lobe wraps around the lower part of the pituitary stalk to form the pars tuberalis. When the gland is removed from the sella, the posterior lobe will be found to be more densely adherent to the sellar wall than the anterior lobe. In most specimens the gland is as wide or wider than it is deep or long. 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. Intermediate lobe cysts are frequently encountered during separation of the anterior and posterior lobes.
The distance separating the medial margin of the carotid artery and the lateral surface of the pituitary gland usually varies from 1 to 3 mm; however, in some specimens the artery protrudes medially and indents the gland. Heavy arterial bleeding during trans-sphenoidal hypophysectomy has been reported to be caused by injury to a branch of the carotid artery (e.g., the inferior hypophyseal artery) or by avulsion of a small capsular branch from the carotid artery.
If the carotid arteries indent the lateral surface of the gland, the gland loses its rounded shape and conforms to the wall of the artery, often developing protrusions above or below the artery. Separation of these protrusions from the main mass of the gland or tumor may explain cases in which functioning gland or tumor remains after hypophysectomy and tumor removal.
Intercavernous Venous Connections
Venous sinuses may be found in the margins of the diaphragm and around the gland. The intercavernous connections within the sella are named on the basis of their relation 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 may occur at any site along the anterior, inferior, or posterior surface of the gland. 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 trans-sphenoidal surgery may produce brisk bleeding. However, this usually stops with temporary compression of the channel or with light monopolar diathermy, which serves to glue the walls of the channel together.
A large intercavernous venous connection called the basilar sinus consistently passes posterior to the dorsum sellae and upper clivus. The basilar sinus connects the posterior aspect of both cavernous sinuses and is the largest and most constant intercavernous connection across the midline. The superior and inferior petrosal sinuses join the basilar sinus. The abducens nerve often enters the posterior part of the cavernous sinus by passing through the basilar sinus.
The sphenoid sinus varies considerably in size, shape, and degree of pneumatization. It is present as minute cavities at birth; 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. Later in life it often 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. In a study in adult cadavers, this sinus was found to be of the presellar type in 24 percent and of the sellar type in 75 percent. In the infrequent conchal type, the thickness of bone separating the sella turcica from the sphenoid sinus is at least 10 mm.
The carotid artery frequently produces a serpentine prominence into the sinus wall below the floor and along the anterior margin of the sella. The optic canals usually protrude into the superolateral portion of the sinus, and the second and third divisions of the trigeminal nerve into the inferolateral part. A diverticulum of the sinus, called the opticocarotid recess, often projects laterally between the optic canal and the carotid prominence.
Removing the mucosa and bone from the lateral wall of the sinus exposes the dura covering the medial surface of the cavernous sinus and optic canals. Opening this dura exposes the carotid arteries and optic and trigeminal nerves within the sinus. The sixth cranial nerve is located between the lateral side of the carotid artery and the medial side of the first trigeminal division. The second and third trigeminal divisions are seen in the lower margin of the opening through the lateral wall of the sphenoid sinus. In about one-half of sinuses, there are spots where bone only 0.5 mm thick or less separates the optic and trigeminal nerves and the carotid arteries from the mucosa of the sphenoid sinus, and in a few cases no bone separates these structures. The absence of such bony protection in the walls of the sinus may explain some of the cases of cranial nerve deficits and carotid artery injury reported following trans-sphenoidal surgery. The bone is often thinner over the carotid arteries than over the anterior margin of the pituitary gland.
The septa in the sphenoid sinus vary greatly in size, shape, thickness, location, completeness, and relation to the sellar floor. The cavities in the sinus are seldom symmetrical from side to side and are often subdivided by irregular minor septa. The septa are often located off the midline as they cross the floor of the sella. In a previous study, a single major septum separated the sinus into two large cavities in only 68 percent of specimens, and even in these cases the septum was often located off the midline or was deflected to one side. The most common type of sphenoid sinus has multiple small cavities in the large paired sinuses. The smaller cavities are separated by septa oriented in all directions.
Computed tomography of the sella is routinely used to define the relation of the septa to the floor of the sella for trans-sphenoidal surgery. Major septa may be found as much as 8 mm off the midline.