EFFECTS OF
PITUITARY TUMORS ON THE VISUAL APPARATUS
The optic chiasm lies above the diaphragma sellae. The most common sign that a pituitary tumor has extended beyond the confines of the sella turcica is a visual defect caused by the growth pressing on the optic chiasm. The most frequent disturbance is a bitemporal hemianopsia, which is produced by the tumor pressing on the crossing central fibers of the chiasm and sparing the uncrossed lateral fibers. The earliest changes are usually enlargement of the blind spot; loss of color vision, especially for red; and a wedge-shaped area of defective vision in the upper-temporal quadrants, which gradually enlarges to occupy the whole quadrant and subsequently extends to include the lower temporal quadrant as well.
The type of visual defect produced
depends on the position of the chiasm in relation to the pituitary gland and
the direction of tumor growth. In about 10% of the cases, the chiasm may be
found almost entirely anterior or posterior to the diaphragma sellae instead of
in its usual position, which is directly above the diaphragma. There are also
lateral displacements of the chiasm, which may cause either its right or its
left branch to lie above the diaphragma. If the chiasm is abnormally fixed, the
adenoma may grow upward for a long time before it seriously disturbs vision.
Bilateral central scotomas are caused by damage to the posterior part of the
chiasm, and their occurrence suggests that the chiasm is prefixed and that the
tumor is large. In other cases of prefixed chiasm, the tumor may extend in such
a direction as to compress the optic tract rather than the chiasm, thus
producing a homonymous hemianopsia. However, homonymous defects do not always
indicate a prefixed chiasm; they may also be produced by lateral extension into
the temporal lobe below a normally placed chiasm. Other visual defects that may
occur include unilateral central scotoma; dimness of vision (amblyopia) in one
eye caused by compression of one optic nerve; and an inferior quadrantal
hemianopsia, presumably resulting from a large tumor causing the anterior
cerebral arteries to cut into the dorsal surface of a normally placed chiasm.
Primary optic atrophy is present in
most cases, but it may be absent when the lesion is behind the chiasm. Although
papilledema is rare, it may occur with large tumors that cause increased
intracranial pressure. If pressure on the visual pathway is relieved (e.g.,
with surgery or pharmacotherapy), the visual fields may return to normal.
However, vision recovery is caused partly by the degree and duration of the
optic tract deformation. Field defects can be detected on gross examination by
observing the angle at which an object, such as the examiner’s finger, becomes
visible when the patient looks straight ahead. Quantitative perimetry is necessary for exact plotting of the size and
shape of the field defect.
In some cases of pituitary tumor showing
expansive growth sufficient to enlarge the sella, the visual pathway escapes
damage because the sellar diaphragm is tough and prevents expansion toward the
chiasm. In these cases, the pituitary tumor may extend laterally into the
cavernous sinus or inferiorly into the sphenoid sinus. This structure shows
considerable variation, from a dense,
closely knit membrane to a small rim with a wide infundibular opening. In most
cases, the diaphragm does yield to pressure from below. Usually, the chiasm
lies directly on the diaphragm and is separated from it by only a potential
cleft. Frequently, particularly where there is a well-developed chiasmatic
cistern, the optic chiasm may be as high as 1 cm above the diaphragm, which
allows an invading tumor considerable room for expansion
before it presses on the visual pathway.
