What causes loss of peripheral vision?

Many possibilities: Many things can cause loss of peripheral (side) vision: glaucoma is the most likely. Ocular pressure rise causes vascular and optic nerve damage. Another is a tumor or aneurysm near the pituitary gland, compressing the nerves from the eye as they first join and then re-divide on the way to the back of the brain. Oculo-vascular occlusions can do the same although most often this is only in one eye.