The pituitary gland is a small, pea-sized structure situated at the base of the brain, housed within a bony saddle-shaped cavity called the sella turcica. A pituitary tumor (adenoma) typically begins as a benign mass within this confined space. Although many adenomas remain small and cause no issues, those that grow larger can exert physical force on surrounding structures. One of the most common consequences of an enlarging pituitary tumor is the impairment of vision. This pressure on the visual pathway can lead to progressive and sometimes sudden loss of sight.
The Pituitary Gland’s Proximity to the Optic Chiasm
Vision loss from a pituitary tumor is a direct result of the gland’s anatomical relationship with the visual pathway. The pituitary gland is located immediately inferior to the optic chiasm, which is where the optic nerves from both eyes meet and partially cross.
Because the sella turcica is rigid, a growing tumor cannot expand downward or sideways. This forces the adenoma to grow upward, a process known as suprasellar extension. This upward growth causes the tumor mass to press directly against the underside of the optic chiasm, physically compressing the delicate nerve fibers within.
The optic chiasm is where fibers from the nasal (inner) half of each retina cross over to the opposite side of the brain. These crossing fibers carry visual information from the temporal, or peripheral, field of vision for each eye. When the tumor applies mechanical pressure to the center of the chiasm, it specifically damages these crossing fibers.
The position of the chiasm relative to the pituitary gland is not identical in every person, which can influence how a tumor affects vision. In about 80% of individuals, the chiasm is positioned directly above the gland, making it highly susceptible to compression. However, in a minority of people, the chiasm may be positioned slightly more anteriorly or posteriorly, which can change the initial presentation of visual symptoms.
Types of Vision Loss Caused by the Tumor
The most characteristic pattern of sight impairment caused by a pituitary tumor is known as bitemporal hemianopsia. This involves the loss of vision in the outer (temporal) half of the visual field in both eyes. Because the peripheral vision is lost on both sides, patients often describe the experience as having “tunnel vision.”
This pattern occurs because the tumor compresses the crossing nerve fibers in the optic chiasm, which transmit information from the outer visual fields. Individuals may not notice the gradual loss of their peripheral sight immediately, as the brain tends to compensate for the missing information. Symptoms often become noticeable only when a person has difficulty seeing cars approaching from the side or bumps into objects outside their central line of sight.
While bitemporal hemianopsia is the classic presentation, the pattern of vision loss can vary depending on the specific location of the compression. If the tumor presses unevenly on the chiasm, it may initially cause a defect in only one eye or affect only the superior (upper) part of the temporal visual field. If the tumor grows to press on one optic nerve before reaching the chiasm, it can cause unilateral vision loss, meaning the loss of sight is limited to a single eye.
Confirming the Diagnosis of Vision Impairment
Confirming a pituitary tumor’s impact on sight requires specialized eye examinations and advanced brain imaging. The initial step involves a detailed assessment of the visual fields, performed using a test called perimetry. This test maps the patient’s entire field of vision to determine the location and extent of any blind spots or defects.
Perimetry helps clinicians detect subtle areas of vision loss, particularly those in the peripheral field. Automated and kinetic perimetry are useful for mapping the full boundaries of the visual field, aiding in the detection of the peripheral deficits characteristic of chiasmal compression. These objective tests provide measurable documentation of the damage, which is used to monitor the effectiveness of treatment.
Brain imaging uses magnetic resonance imaging (MRI), which is the standard for visualizing the tumor. MRI provides detailed images that clearly show the size of the pituitary adenoma and its exact relationship to the optic chiasm. MRI confirms the physical compression and helps determine the best course of action.
Resolving Vision Issues Through Treatment
The goal of treating a pituitary tumor that affects vision is to relieve the pressure on the optic chiasm. For most large pituitary tumors, surgical removal is the most direct way to achieve this, often using transsphenoidal surgery. This minimally invasive procedure accesses the tumor through the nasal cavity and sphenoid sinus, allowing the surgeon to remove the mass and decompress the chiasm.
Surgical decompression leads to improvement in visual function for patients, with studies reporting improved visual acuity or visual fields in 75% to 80% of cases. The duration of vision loss before surgery can influence the outcome, as long-standing compression may lead to irreversible damage to the nerve fibers. However, even patients with severe preoperative defects often experience improvement after the pressure is removed.
For a prolactin-secreting tumor (prolactinoma), medical management is often the first-line treatment. Medications called dopamine agonists, such as cabergoline or bromocriptine, can effectively shrink the tumor size. This shrinkage reduces pressure on the optic chiasm, which can rapidly resolve visual field defects and restore visual acuity, often making surgery unnecessary. Radiation therapy is reserved for cases where surgery is incomplete or when the tumor is resistant to medical treatment.