Answer
The pituitary gland lies in the sella turcica, a depression of the sphenoid bone--this location is just behind the eyes. A pituitary tumor that is large enough to press on the optic chiasm may impair vision in several ways..
First, central vision may be lost. As a consequence, the patient will have difficulty reading. The patient may also see an opaque white light in the center of the visual field. Another symptoms of this chiasmal syndrome may be bilateral loss of peripheral vision ( bitemporal hemianopsia). Central scotoma in one or both eyes may also develop. In this sign an area of lost or diminished vision surrounds an area of norma vision. A scotoma develops first in one eye, usually the left eye (OS).
Work Step by Step
The pituitary gland is located in the sella turcica of the sphenoid bone; this location is just behind the eyes.. A swelling or a protrusion of a pituitary tumor anteriorly may press on the optic chiasm. If the tumor is large enough, it may impinge on the oculomotor nerve(CN III) and other cranial nerves close by (CN IV, CNV, CN VI).
The optic nerve (CN II) carries visual impulses from the retina back into the brain to be interpreted as light and visual images. The two cranial nerves partially cross at the optic chiasm. The nasal half (medial) of the right optic nerve crosses over to the left side(contralaterally) to join the temporal half (left half) of the left optic nerve to form the left optic tract. On the other side the nasal half of the left optic nerve crosses over to join the temporal half of the right optic nerve to form the right optic tract. These tracts go to the lateral geniculate nuclei in the thalamus on each side; thence the optic radiation travels to the optic cortex in the occipital lobe.
If the pituitary tumor is centrally located it will first press on the nerve fibers that have decussated (crossed) that is, the nasal half of each optic nerve. The early result will be impairment of peripheral vision in both eyes a condition known as bitemporal hemianopsia. Other signs and symptoms that may develop include central vision loss, loss of ability to focus and difficulty in reading . Patients also report seeing an opaque white lite in the central visual field. Other symptoms that may develop as the tumor expands include scotoma, diplopia and nystagmus.
Non-visual symptoms that may also be caused by pituitary tumor pressure on the optic chiasm include headache, nausea and vomiting.
