Introduction

See the figure Tracing the Visual Pathways.

The small photoreceptors of the retina (the inner surface at the back of the eye) sense light and transmit impulses to the optic nerve. The optic nerve carries impulses to the brain. A problem anywhere along the optic nerve or damage to the areas at the back of the brain that sense visual information can result in loss of vision. A common cause of damage to the optic nerve is a tumor of the pituitary gland that presses on the nerve.

The two optic nerves carry signals from the eyes to the back of the brain. At a structure in the brain called the optic chiasm, each nerve splits, and half of its fibers cross over to the other side. Because of this anatomic arrangement, damage along the optic nerve pathway causes specific patterns of vision loss. By understanding the pattern of vision loss, a doctor can often determine where in the pathway the problem is.

See the sidebar Some Patterns of Vision Loss.