Optical Chiasm: Definition and Related Pathologies

It is a space in the form of X, located in the forebrain, directly opposite the hypothalamus.

Crucial to vision, the left and right optic nerves intersect in the chiasm, thus creating an X shape.

Half of the axons of each nerve (its long filiform portions) enter the opposite tract in this place, which makes it a partial decussation (crossing).

The optic chiasm lies between the anterior perforated spaces and behind the lower anterior part of the longitudinal fissure. It is formed by the union of the optical sections. The optic nerves arise from the optic chiasm.

The optic chiasm is an X-shaped space just in front of the pituitary gland, where the optic nerve fibers pass to the brain. The fibers of the nasal half of each retina cross, but those of the temporal sides do not.

Specifically, the fibers of the nasal half of the left eye and the temporal half of the right eye form the right optic tract; And the fibers of the nasal half of the right eye and the temporal half of the left form the left optic tract.

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

The optic nerve fibers on the nasal sides of each retina cross to the opposite side of the brain through the optic chiasm (decussation of the middle fibers).

The temporal hemiretin projections remain on the same side. The inferonasal retina is related to the anterior portion of the optic chiasm while the superonasal fibers of the retina are related to the posterior portion of the optic chiasm.

Benign (not cancerous) and typically occurring before age 20, symptoms of optic glioma may include:

  • Smell or loss of vision
  • Squint eyes or involuntary eye movements.
  • High intracranial pressure (pressure inside the skull).
  • Loss of appetite and reduction of fat as part of the diencephalic syndrome, a rare condition (usually caused by a tumor in the brain diencephalon) in which a person does not gain weight, regardless of normal diet and development.

After a preliminary diagnosis, a brain angiogram or a biopsy can be performed. An angiogram involves the use of x-rays to see vessels that have been injected with dye. A biopsy is a sampling of tissue that will be used for laboratory tests.

A head scan can be done to confirm the location and size of the glioma. Visual field tests (tests of eye skills) 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.

The macroscopic anatomy of the optic nerves and the chiasm has been studied, and the differences in the tension of the crossed and non-crossed fibers after the chiasmatic displacement have been investigated.

The anterior and posterior attachments of the medial and lateral fibers of the nerves have been studied. The chiasm has been dissected under low power microscopy and a three-dimensional image of it was developed.

The bitemporal hemianopia, as well as the associated or independent hemianopic scotomas, result from the stretching of the crossed fibers in the chiasm.

Binasal hemianopia is the result of compression of the non-crossed fibers in the optic nerve or chiasm by the internal or anterior cerebral carotid arteries.

The compression is effective because it is very localized and, probably as a result of the pulsation, it deeply cuts the nerve with a resulting acute distortion of the fibers; It is likely that loose lateral fibers are less affected by more widespread compression.

When this defect develops in the upper part of an existing bitemporal hemianopia, it is believed that its usual cause remains the same.

The crossed and uncrossed fibers of the optic chiasm differ not only anatomically in the retina areas in which they arise, but also physically.

Tension is the force that causes the bitemporal hemianopia and the pressure that produces defects of the nasal field.

Relations

The Circle of Willis surrounds the pituitary stem and the optic chiasm. It is in relation, above, with the terminal sheet; Behind, with the cinereum tuber; On each side, with the previous perforated substance.

Within the chiasm, the optic nerves undergo partial decussation. The fibers that form the medial part of each tract and the posterior part of the chiasm have no connection with the optic nerves.

They simply cross in the chiasm and connect the medial geniculate bodies of the two sides; They form the corner of Guidden.

The remaining and main part of the chiasm consists of two sets of fibers, crossed and not crossed.

The crossed fibers, which are the most numerous, occupy the central part of the chiasm and pass from the optic nerve on one side to the optic tract on the other, in the chiasm with similar fibers of the opposite optic nerve.

The uncrossed fibers occupy the lateral part of the chiasm and pass from the nerve on one side to the tract on the same side.

The crossed fibers of the optic nerve tend to occupy the medial side of the nerve and the fibers do not cross the lateral side. In the optical tract, however, the fibers are much more intermixed.

Level:

  • Lateral: anterior perforated substance.
  • Previous: optical nerves.
  • Posterior: optic tracts, tuber cinereum, pituitary stem, mammillary bodies, perforated posterior substance.
  • Superior: supraoptic recess of the third ventricle, anterior commissure.
  • Superposterior: hypothalamus.
  • Lower: infundibular recess of the third ventricle.

Blood supply

The small branches and superior pituitary artery supply the chiasm and the intracranial portion of the optic nerves, while the optic tracts are supplied by small branches of the anterior choroidal arteries and PCom.

Related pathology

The lesions that compress the chiasm classically produce the visual field defect of the bitemporal hemianopia, causing the loss of temporal fields.