The optic chiasm is a critical intersection in the visual pathway. It facilitates receiving visual information so a person can see properly.
Damage to the optic chiasm can result in vision loss to specific areas of our visual field, depending on the location involved. This article further explains the structure and function of the optic chiasma and what happens when it is damaged.
What is the Optic Chiasm?
The optic chiasm, or optic chiasma, is an X-shaped structure at the base of the brain. The left and right eye’s optic nerves intersect here, combining visual information before moving further down the visual pathway to the brain.
After the optic nerve fibers combine visual information in the optic chiasm, they carry it to the brain. This process results in the visual images we see.
Where is the Optic Chiasm Located?
The optic chiasma is at the bottom of the brain, next to the pituitary gland and the hypothalamus. Because of these structures’ proximity, the optic chiasm can be damaged easily if underlying issues with the pituitary gland and hypothalamus exist.
The Circle of Willis encircles the optic chiasm. The Circle of Willis is a group of arteries that provides blood supply to the brain and is a common location for brain aneurysms.
What Does the Optic Chiasm Do?
The optic chiasm’s primary function is quickly combining electrical impulses from each optic nerve fiber before crossing. It aids in binocular vision and hand-eye coordination.
Once the electrical impulses cross at the optic chiasm, they move along opposite sides of the optical tract to the visual cortex in the brain. The merging of visual data from each eye allows each brain hemisphere to get the same visual field information.
Without the optic chiasm, we wouldn’t have:
- Binocular vision
- Stereopsis (depth perception)
- High visual acuity (distance vision)
- Ability to judge distance
- Visual control of our hands
- Ability to see 3D shapes
- Merging of visual and motor activity
Visual Pathway of the Optic Nerve
The visual pathway is the optic nerve's path from the eye to the brain. It includes essential structures, including the optic chiasm.
The optic nerve is the structure that carries light signals from your eye to your brain. It’s also called the second cranial nerve (CN II).
Each optic nerve starts in the retina, the tissue at the back of the eye that captures light. They then end at the brain's occipital lobe (visual cortex).
This is the process of the optic nerve’s travel through the visual pathway:
- Optic nerve fibers take in light impulses from retinal cells
- Nerve fibers enter the optic canal along the middle cranial fossa
- Nerve fibers from each eye merge at the optic chiasm and cross
- Nerve fibers enter the left and right optic tracts
- Each tract travels to its corresponding hemispheres in the thalamus
- The nerve fibers synapse and travel to the visual cortex
- The brain processes the data and responds accordingly
What Happens if the Optic Chiasm Is Damaged?
A damaged optic chiasm can result in various negative outcomes, including vision field loss in both eyes.
Other effects of a damaged optic chiasm or its surrounding structures include:
- Bitemporal hemianopsia. Loss of temporal peripheral vision in each eye
- Macular sparing. Loss of a wide area of the vision field but not the central vision
- Homonymous quadrantanopia. Quadrant visual field deficit affecting the same area in each eye
- Junctional scotoma. Blind spots caused by a lesion at the junction of the optic chiasm and nerve
Diseases that can affect the optic chiasm include:
- Multiple sclerosis
- Benign and cancerous tumors
- Vascular disorders (damage to surrounding blood vessels)
- Chiasmal optic neuritis (inflammation of the optic nerve at the optic chiasm)
A pituitary adenoma is a tumor of the pituitary gland. It’s the most common cause of chiasm damage.
While typically benign, they can grow large over time and pressure neighboring structures like the optic chiasm. The compression that a pituitary tumor causes on the optic chiasm can lead to corresponding temporal visual field loss and impairment.
Pressure on the optic nerve can lead to blindness, so it is important to seek medical attention if you are experiencing changes in your vision.
Common Questions about the Optic Chiasm
Does the pituitary gland affect the optic chiasm?
Since the optic chiasm is located next to the pituitary gland, it is susceptible to compression from pituitary tumors. This compression can cause vision loss in both eyes.
How big is the optic chiasm?
The optic chiasm is a small brain structure. It’s 10 to 20 mm in diameter, 4 to 13 mm wide, and 3 to 5 mm thick.
Is the optic chiasm part of the midbrain?
No. The optic chiasm is located at the bottom of the brain next to the pituitary gland and the hypothalamus.
What is the meaning of “chiasm”?
In Greek, chiasm means crossing. The definition of chiasm is the intersection or crossing of two tracts in the form of the letter X.
Is the optic chiasm the natural blind spot?
No. The optic chiasm is not the blind spot. The optic disc (where the optic nerve meets the retina) is a natural blind spot.
However, blind spots can develop if there are issues with the optic nerve in either one or both eyes.
The optic chiasm, located at the base of the brain, is where the optic nerves from your left and right eyes converge and cross. It’s an essential part of the visual pathway from the eyes to the brain.
The optic chiasm’s primary function is creating binocular vision, depth perception, and hand-eye coordination. When it sustains damage from disease or is compressed by a tumor, it can cause vision field loss or blindness.
Visiting your doctor if you experience any sudden changes to your vision is essential. A checkup can rule out structural changes to the optic chiasm or underlying medical conditions.
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