Optic Chiasm

Optic nerve tract. BSIP/UIG

The optic chiasm is an X-shaped structure formed by the crossing of the optic nerves in the brain. The optic nerve connects the brain to the eye.

At the optic chiasm, nerve fibers from half of each retina cross over to the opposite side of the brain. The fibers from the other half of the retina travel to the same side of the brain. Because of this junction, each half of the brain receives visual signals from the visual fields of both eyes.

To biologists, the optic chiasm is thought to be a turning point in evolution. It is thought that the crossing and uncrossing optic nerve fibers that travel through the optic chiasm developed in such a way to aid in binocular vision and eye-hand coordination.

How disease affects the optic nerve tract:

  • When a disease or lesion affects the optic nerve before it reaches the optic chiasm in the brain, the defect in the the vision will show up on only one side or one eye and can affect the entire field of that eye.
  • Diseases that cause damage to the optic chiasm cause a bitemporal visual field defect. A bitemporal visual field defect is a decrease or absence of vision on the temporal half or side of the field of vision in each eye. People that suffer from a bitemporal visual field defect sometimes do not notice it until one eye is covered because when both eyes are open, the over-lapping visual fields of each eye will mask the defect. 
  • If the disease affects the optic tract after the chiasm, the person will have a defect in their vision in both eyes, but the defect will after the same half of the visual field.

What diseases can affect the optic chiasm?

The most common disorder affecting the optic chiasm is a pituitary adenoma. Pituitary adenomas are tumors that can affect vision, sometimes causing vision loss.

As they grow in size, pituitary adenomas can put pressure on important structures in the body, such as the optic nerve. Putting pressure on the optic nerve may cause blindness, so it is crucial for eye doctors to detect pituitary tumors before they cause damage to vision.

The pituitary gland is about the size of a bean and is attached to the base of the brain behind the nasal area. it sits right under the optic chiasm. Although small, the pituitary controls the secretion of many different types of hormones. It helps maintain growth and development and regulates many different glands, organs and hormones. Changes in hormones can cause significant changes in our bodies.

Besides vision changes such as double visiondrooping eyelids and visual field loss, pituitary adenomas also may cause the following symptoms:

  • Forehead headaches
  • Nausea or vomiting
  • Change in sense of smell
  • Sexual dysfunction
  • Depression
  • Unexplained weight changes
  • Change in menses or early menopause

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