The light a person perceives through his eyes is carried to the visual cortex by the means of the optic nerve. Most people believe that the optic nerve of one side carries signals related to vision to the same side of the visual cortex.
Still, others believe that it carries light perception to the contralateral half of the brain. But the truth is that both these assumptions are false.
It is true that the optic nerves cross and pass to the opposite side of the cortex. However, the arrangement of nerve fibers is somehow complex. In this article, we will talk in detail about this arrangement.
Optic chiasm is a structure located in the forebrain. It is the point at which the two optic nerves cross each other and travel to the opposite side of the brain. In this article, we will talk in detail about the location, structure, function and pathologic abnormalities of the optic chiasm.
Throughout history, the optic chiasm had been considered as an area of brain where two vessels cross each other. Many speculations were made about it. It was also believed that it is the crossing of two vessels that bring tears to the eyes.
At that time, it was thought that tears were made in the brain. These misconceptions continue to exist until it was proved that optic chiasm is a structure comprising of nervous tissue. It was put forward that the optic nerves from the two eyes undergo partial desiccation in optic chiasm.
Optic chiasm is located at the bottom or base of the brain. It is present just below the hypothalamus. It is located at the junction of anterior wall and floor of the hypothalamus.
The anterolateral angles are continuous with the optic nerves and the posterolateral angles are continuous with the optic tracts. It is surrounded by the arteries forming the circle of Willis and is present in between the circle.
Organization of Nerve Fibers
Let us first understand nerve fibers arising from the eyeball. The nerve fibers from the nasal half of eye correspond to the temporal of lateral half of eye field, while the temporal half of nerve fibers correspond to the nasal half or medial half of the eye field.
As discussed earlier, optic chiasm is a nervous structure at which fibers of the optic nerve cross. The fibers are arranged in such a way that the nasal fibers from both sides crossover and pass to the opposite side of the brain.
Thus, the fibers from the temporal half of retina or hemiretina remain on the same side whereas the fibers from the nasal half are crossed.
As a result, the left optic tract contains the nasal fibers from the right eye and the temporal fibers from the left eye. The opposite is true for the right optic tract.
As stated earlier, the optic chiasm is surrounded by the circle of Willis. The blood is supplied to optic chiasm by the small branches arising from the arteries forming the circle of Willis. These include the anterior, middle and posterior meningeal artery.
In order to understand why optic chiasm is important, let us first take a look at the visual pathway.
The nerve fibers from the temporal and nasal halves of eyeball leave the eyeball as an optic nerve. The fibers in this nerve are present separately. At optic chiasm, the fibers from the nasal half cross while the fibers from the temporal half remain uncrossed.
From the optic chiasm, fibers pass to the optic tract. The left optic tract contains uncrossed temporal fibers from the left eyeball and crossed nasal fibers from the right eyeball.
The optic tracts terminate at the lateral geniculate body. The optic radiations then pass from the lateral geniculate body to the visual cortex, carrying the visual sensation.
This crossing over of optic fibers is important in such a way that it allows the same hemispheric visual field to be perceived by visual cortex from both eyes. This can be understood from the example that the left visual cortex receives the temporal visual field from the right eye (by crossed right nasal fibers) and the nasal visual field from the left eye (via uncrossed left temporal fibers).
This allows the left visual cortex to form a binocular image of right hemispheric field. The net result of crossing at the optic chiasm is that the right visual cortex senses and processes left-hemispheric vision while the left visual cortex senses and processes right-hemispheric field of vision.
As the name indicates, this syndrome is caused by the lesions of optic chiasm. This syndrome is manifested by a set of signs and symptoms along with impaired vision. The degree of vision impairment depends on the location of lesion.
The causes of chiasmal syndrome may be intrinsic or extrinsic.
Extrinsic causes include compression of optic chiasm by some extrinsic factor that me a vascular aneurysm of a tumor such as pituitary adenoma, meningioma, etc. Pituitary adenomas are the most common cause of chiasmal syndrome.
The intrinsic causes include thickening of chiasma. The most common intrinsic etiologies of chiasmal syndrome are gliomas and multiple sclerosis.
All these etiologies are discussed briefly in the following subsections.
Pituitary adenoma is a benign tumor arising from the pituitary gland. They are the number one cause of optic chiasm abnormalities. They can sometimes cause impaired vision.
As the pituitary adenoma grows, it puts pressure on the optic nerve and the optic chiasm. This pressure compresses the nerve and causes vision impairment. It can even cause complete and permanent blindness.
The general sign and symptoms experienced by a person with pituitary adenoma include the following:
- Impaired smell perception
- Weight changes
- Sexual dysfunction
- Menstrual irregularities
The pituitary adenoma causes an increase in the secretion of pituitary hormones. It can result in corresponding changes in the body along with their own clinical presentations.
The pituitary adenomas can be detected easily by elevated levels of growth hormone in the patient’s blood. The diagnosis of pituitary adenoma is confirmed by MRI of the brain or CT scan.
Once the adenoma is detected as a macroadenoma, it can be treated via surgical excision of the tumor or the entire gland.
These are the benign tumors of meninges. They mostly arise from the arachnoid layer of meninges. The meningiomas of diaphragma sella and sphenoid can cause chiasmal syndrome. This is because they can compress the optic chiasm and result in a number of visual abnormalities.
The diagnosis of meningioma compressing optic chiasm is also made by radiological studies of the brain such as MRI or CT scan.
The treatment of meningiomas is surgical intervention or radiotherapy.
The gliomas are the benign tumors of glial cells occurring in the CNS. The gliomas of optic chiasm usually arise from the astrocytes.
The gliomas are slowly developing tumors and are more common in children. They are more frequently associated with type-1 neurofibroblastomas.
The diagnosis of optic chiasm glioma is made by MRI or CT scan findings.
The treatment involves resection of the optic nerve.
Optic chiasm gliomas have poor prognosis. The prognosis is worse if the tumor has become malignant and extended to the hypothalamus.
Other causes of Chiasmal Syndromes
We have discussed briefly some of the important causes of chiasmal syndromes. Below is a list of some other conditions that can sometimes lead to chiasmal syndromes.
- Multiple sclerosis
- Inflammatory processes
- Cysts in brain
- Aneurysm of blood vessels
Optic chiasm is a nervous structure located at the base of the brain just below the hypothalamus.
It receives optic nerves from both the eyeballs and continues as optic tracts.
Optic chiasm provides a site for the crossing over or desiccation of optic nerve fibers so that contralateral half of the visual field is perceived and processed by the visual cortex.
Optic chiasm is surrounded by the circle of Willis and derives its blood from the vessels forming this circle.
The nerve fibers in optic chiasm are organized in such a way that the fibers arising from the nasal half of the eyeball crossover and pass to the contralateral visual cortex. The overall effect is the production of binocular image in such a way that the contralateral hemispherical visual field is received and processed by the visual cortex.
Chiasmal syndromes are the abnormalities of optic chiasm that can severely impair vision or even can cause blindness.
Chiasmal syndromes may be due to pressure from an external structure or due to some intrinsic disease of the structure.
The important external factors responsible for chiasmal syndrome include pituitary adenoma and meningiomas.
The intrinsic causes include gliomas, multiple sclerosis, etc.
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