Final answer:
A large pituitary tumor causes a visual field defect known as bilateral hemianopia, which is the loss of peripheral vision on both sides, due to the tumor pressing against the optic chiasm.
Step-by-step explanation:
A large pituitary tumor typically causes a specific type of visual field defect known as bilateral hemianopia, which is the loss of the lateral or side vision. This happens because the pituitary gland, positioned in the sella turcica of the sphenoid bone, sits directly inferior to the optic chiasm. When a tumor grows in the pituitary gland, it can press against and disturb the fibers crossing within the optic chiasm. The affected fibers are responsible for conveying peripheral visual information to the opposite side of the brain, which leads to an interruption in signal transmission, resulting in the perceptual loss of vision on the sides.
It is important to note that this condition is different from "tunnel vision" as the superior and inferior peripheral fields are usually not affected. The disruption caused by the tumor affects only the axons that carry information from the lateral visual field, hence the term bilateral hemianopia. In this condition, the patient's central vision remains intact while they are unable to detect objects that are to their extreme left or right. Understanding the relationship between the optic chiasm's location and the pituitary gland is critical for recognizing how pituitary tumors can lead to such distinctive visual deficits.