How can a pituitary tumor cause loss of the outer visual fields in both eyes?

A pituitary tumor can cause loss of the outer visual fields in both eyes—a condition known as bitemporal hemianopia—due to its anatomical location near the optic chiasm.12

The optic chiasm is the point where the optic nerves from both eyes meet and partially cross. Specifically, the nerve fibers originating from the inner (nasal) half of each [retina] cross over to the opposite side of the brain. Because these nasal retinal fibers are responsible for receiving images from the outer (temporal) parts of the visual field, they are particularly vulnerable when a tumor grows upward from the pituitary gland and compresses the chiasm.132

As the tumor exerts pressure on these crossing fibers, it disrupts the transmission of sensory impulses from the outer visual fields, resulting in the characteristic bitemporal visual field defect.12