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.
Optic Canal
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.
Suprasellar Arteries
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).
Diaphragma Sellae
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.
Pituitary Gland
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.
Sphenoid 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.
| 
