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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.

Tracing the Visual Pathways Nerve signals travel along the optic nerve from each eye. The two optic nerves meet at the optic chiasm. There, the optic nerve from each eye divides, and half of the nerve fibers from each side cross to the other side. Because of this arrangement, the brain receives information via both optic nerves for the left visual field and for the right visual field.

Some Patterns of Vision Loss Depending on where the damage to the visual pathway occurs, the type of vision loss varies. For instance, if the optic nerve is damaged somewhere between an eyeball and the optic chiasm, the person may become blind in only that eye. If the optic chiasm is damaged, both eyes lose some vision—the right eye loses vision in the right portion of its visual field, and the left eye loses vision in the left portion of its visual field. In hemianopia, damage farther back in the optic nerve pathway (which is often the result of a stroke or a tumor) produces yet another pattern of vision loss. Half of the visual field in both eyes is affected. For example, with damage to the left portion of the brain, both eyes lose the right half of their visual field.

Last full review/revision February 2003