Structure of the Optic Chiasm

• One optic nerve protrudes back from each eye. These nerves cross at the optic chiasm, directly below the pituitary gland, forming the shape of the letter X.
  
  

Neurological Arrangement of the Optic Chiasm

• The optic nerve fibers that provide a signal for the temporal, or outermost, visual field from each eye are located in the center of the optic chiasm. These fibers from each eye are located close together at that crossing.
  
  

Pituitary Gland Tumor

• A tumor of the pituitary gland is generally extremely small. Neurosurgery Today reports that they are typically measured in millimeters. As such, to have any affect on the vision in both eyes, they would have to affect nerve fibers that are close together. That makes the optic nerve fibers for the temporal visual field from each eye the most vulnerable.
  
  

Vision Loss From a Pituitary Gland Tumor

• The vision loss that nearly always results from a pituitary gland tumor is known as bitemporal hemianopsia. This occurs when the temporal visual field from each eye is damaged. This can only happen at the optic chiasm.
  
  

Testing For Bitemporal Hemianopsia

• Evidence of a pituitary gland tumor cannot generally be assessed when an optometrist or ophthalmologist views the inside of the eyeball. The only tests that can reveal a bitemporal hemianopsia are visual field tests.
  
  

Visual Field Testing

• The most basic screening test is known as confrontation visual field testing, which the eye care practitioner does with her hands. Automated, and more precise, tests of the visual field include Humphrey Visual Fields and Frequency Doubling Technology. Both of these involve the patient looking into an instrument and clicking a button when they see a specific target.