Optic Chiasm

What is the optic chiasm?

The optic chiasm is where the optic nerves from each eye meet and cross. It’s responsible for transmitting visual information. This intersection helps your brain combine what both of your eyes see. Because of this meeting point, when you open your eyes, you get a single image.

The optic nerve is the cable that connects your eye to your brain. Both optic nerves meet and cross at the chiasm. It sits at the base of your brain. It’s surrounded by blood vessels and glands.

Because of its location, it’s at risk of damage to the surrounding glands or blood vessels. This might lead to vision changes or even vision loss.

What does the optic chiasm do?

The optic chiasm functions to:

• Create a unified field of vision: Each eye has a slightly different perspective. Your brain can combine them, so you see one image instead of two different ones.
• Enable depth perception (binocular vision): Because the field of view from both eyes overlaps, your brain can pick up on small differences between the two images, like distance and depth. This helps you see things in three dimensions and understand how far away things are.

Where is the optic chiasm located?

Your optic chiasm is at the bottom of your brain. It sits below your hypothalamus and just above your pituitary gland. It’s in an area called the suprasellar cistern. This is close to your third ventricle (fluid-filled cavities). It’s also surrounded by a loop of arteries known as the circle of Willis.

What are the regions of the optic chiasm?

Several important parts come together at the optic chiasm to help you see:

• Optic nerves: These are the nerves that carry visual messages from each eye. Each optic nerve contains fibers from two parts of the retina. You have nasal fibers (from the inner half of your eye closest to your nose) and temporal fibers (from the outer half of your eye, farthest from your nose).
• Optic chiasm: This is the spot where some of the nerve fibers cross over to the other side of your brain. Only the nasal fibers cross. The temporal fibers stay on the same side. This crossing helps your brain understand where things are in your left and right visual fields.
• Optic tracts: After the fibers cross (or don’t cross), they continue as optic tracts. Each optic tract now carries visual information from both eyes, but only for one side of your vision (left or right). For example, the right optic tract carries messages about what you see in your left visual field.

What does the optic chiasm look like?

It has an “X” shape. “Chiasm” comes from the Greek letter “chi” (X). The shape happens when optic nerve fibers from both eyes cross. On average, it’s about a half inch or 13.5 millimeters wide.

What are the common conditions that affect the optic chiasm?

Different conditions can press on the optic chiasm and affect your vision. These include:

• Tumors like craniopharyngioma, meningioma, optic nerve glioma, pituitary adenoma and pilocytic astrocytoma
• A bulge or weak spot in an artery or blood vessel (aneurysm)
• Sudden bleeding or a loss of blood flow to your pituitary gland (pituitary apoplexy)

Other conditions can affect how your optic chiasm works, including:

• Autoimmune conditions (like neurosarcoidosis): Your immune system causes inflammation around the chiasm.
• Demyelinating diseases (like optic neuritis or multiple sclerosis): Damage to the protective cover around nerves affects signals needed for vision.
• Non-decussating retinal fugal fiber syndrome (achiasmatic syndrome): A very rare syndrome where nerve fibers don’t cross at the optic chiasm.
• Ocular albinism: A genetic condition that reduces the natural coloring (pigmentation) of eye structures and causes visual pathways to develop abnormally.

What happens if the optic chiasm is damaged?

Damage may lead to vision changes, like:

• Blind spots
• Blurred vision
• Difficulty seeing colors
• Loss of the outer sides of your vision in both eyes