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Optic chiasma

The optic chiasm  or optic chiasma is an X-shaped space, located in the forebrain, directly in front of the hypothalamus. Crucial to vision, the left and right optic nerves intersect at the chiasm, thus creating the hallmark X-shape. One-half of each nerve's axons (their long, threadlike portions) enters the opposite tract at this location, making it a partial decussation (crossing).

While rare, optic gliomas — tumors — can grow within the chiasm or in conjunction with hypothalamic tumors. Gliomas can directly affect the optic nerves.

Benign (noncancerous) and typically occurring before age 20, symptoms of optic glioma can include:

  • Bulging eyes or vision loss
  • Squinting or involuntary eye movement
  • Elevated intracranial pressure (pressure inside the skull)
  • Loss of appetite and fat reduction as part of diencephalic syndrome, a rare condition (usually caused by a tumor in the diencephalon of the brain) where a person doesn’t gain weight, regardless of normal eating and development

Following a preliminary diagnosis, a cerebral angiography or biopsy may be performed. An angiography involves the use of X-rays to view vessels that have been injected with dye. A biopsy is a sampling of tissue to be used for lab testing. A head CT scan may be conducted to confirm the location and size of the glioma. Visual field tests (testing of the eye’s abilities) can help determine if the tumor has spread deeper into the brain. Treatment options include: chemotherapy, corticosteroids, or radiation. In some cases surgery may be required to remove the glioma from the optic chiasm.

Written and medically reviewed by the Healthline Editorial Team
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In Depth: Optic chiasma

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