Optic chiasm: what are the problems associated with its lesion

As is often the case in anatomy, to understand what the optic chiasma is we refer to the Greek origin of its name: χιασμός, “chiasmos,” meaning “chi-shaped,” where chi refers to the Greek letter x. So we are talking about an anatomical formation of the point where the optic nerves cross, coming from the right eye and the left eye. It is because of this crossing that the typical elongated x-shape is created.

Starting from the retina, the nerve extends to the optic chiasm of the hemisphere contralateral to the starting eye. We are talking about an anatomical structure that with the following characteristics:

• 12 mm thick
• 8 mm long
• 4 mm in height
• is about 5 to 10 mm away from the pituitary gland.

This anatomical structure also represents the point at which the fibers of the two optic nerves partially intertwine (this is referred to as decussation), allowing half of the nerve fibers to pass from one optic nerve to the other (the other half of the fibers instead continues their course to the corresponding part of the brain). Specifically, the medial half of the fibers belonging to the two nerves, coming from the nasal half of the retina and carrying the images coming from the lateral half of the visual field, cross at x with the contralateral ones.

Looking for its precise location within the brain anatomy, we locate it at the midline relative to the location of the eyes, just below the hypothalamus.

Here is a short video published by Mind Time showing the anatomical structure of the visual apparatus and the location of the optic chiasm.

La funzione

We know that visual information from the outside world represents raw data that needs a process of processing that takes place within our nervous system. This process that restores the ability to see begins in the retina, for it is here that external impulses are received in the form of electrical impulses; the next step takes place in the optic tracts; it is through these that information travels to reach the brain.

However, the pathway of the optic tracts is not unique and linear; rather, it presents a kind of crossover that leads the two optic tracts to finish their “race” in the section of the brain opposite to the starting eye position.

Specifically, the information we receive from the outside world is received by the retina in the form of electrical impulses; here the images are projected in reverse thanks to the function of the lens: the lower half is projected onto the upper half of the retina and vice versa.

After leaving the retina, these electrical impulses travel through the optic nerves to the brain, where they are processed through a process that gives us sight. The two optic nerves, however, do not follow a single, straight path; rather, they curve and cross, heading toward the portion of the brain opposite the location of the eye.

The optic chiasm performs, therefore, the function of conveying information from one retina to the opposite portion of the brain.

Optic chiasm lesions

The fact that this anatomical structure is located inside the skull and in the lower area of the brain results in a high level of protection from trauma. It is very rare for an injury to occur in this area. When vision impairment is found, however, it is most likely that the injury is caused by compression of the chiasm, due to its proximity to the pituitary gland and third ventricle.

Causes

When this occurs, medical attention should be sought to identify the trigger that led to this compression. Common causes include:

• tumors of the pituitary gland;
• meningioma;
• increased pressure de cephalorachidian fluid of the third ventricle;
• arteriovenous malformations.

Symptoms

The peculiar anatomical arrangement, previously described, results in a visual deficit in the area of the visual field affected by the compression. Using a visual field test, one can then pinpoint the precise location of the optic tracts where the problem is located. One of the problems that can be encountered is hemianopsia.

If the compression involves the median part of the optic chiasm, bitemporal hemianopsia may occur, in which a loss of peripheral vision perception occurs, leaving only the central focus of the visual field intact.