The optic chiasma (from the Greek khiasma, meaning crossing) corresponds to the area of ​​crossing of the optic nerves within the brain.

Structure of the optic nerves. Like all cranial nerves in humans, the optic nerve is an even nerve. The optic nerves are sensory nerves allowing the assimilation and integration of visual perception. Each optic nerve originates from the retinal ganglion cells, which originate from the two semi-retinas of each retina (1).

Structure of the optic chiasm. Fibers from the ganglion cells in the nasal retina and the temporal retina come together and leave the eyeball to form the two optic nerves. Each optic nerve passes through the optic hole of the sphenoid bone and extends medially and posteriorly to the skull. The two nerves meet in front of and above the pituitary gland to form the optic chiasm. At this point, fibers from the nasal retina cross while those from the temporal retina continue in the same path to form the two optic tracts (2).

Termination. The optic tracts continue posteriorly before terminating as synapses within the lateral geniculate body of the thalamus (2).

Role in visual perception. The optic chiasm makes it possible to transmit the visual information obtained by the left hemisphere of the two retinas to the right hemisphere of the brain; and through the right hemisphere of both retinas to the left hemisphere of the brain. The information is first processed by the thalamus and then transmitted to the cerebral cortex (3).

Optic neuropathies. They correspond to all the pathologies affecting the optic nerves and in particular the optic chiasma. When the optic chiasm is affected, the subject may experience partial loss of vision. This occurs for example in case of bitemporal hemianopia, affecting the lateral visual field. The origin of neuropathies can be infectious, tumor or genetic (4).

Optic neuritis. This optic neuropathy is linked to inflammation of the optic nerve, especially in the optic chiasm. In the most severe cases, it can cause partial or complete loss of sight (4). The causes of optic neuritis are varied.

• Multiple sclerosis. This pathology is an autoimmune disease of the central nervous system, causing inflammatory reactions. The immune system attacks myelin, the sheath surrounding nerve fibers, including those of the optic nerve (5).

Optic chiasm compression. Certain pathologies can cause compression of the optic chiasm and impair vision. This phenomenon is notably observed in certain cases of brain tumors, aneurysms of the ophthalmic artery or of the supraclinoid carotid artery (4).

Drug treatments. Depending on the pathology diagnosed, certain drugs may be prescribed such as corticosteroids or beta-blockers.

Surgical treatment. In the most serious cases, a surgical intervention can be put in place.

Physical examination. The examination of pupillary function is systematically carried out during an ophthalmological evaluation.

Medical imaging exam. To complete the diagnosis, an MRI or CT scan can be done.

Much research is currently being done to cure blindness patients. Certain systems such as the bionic eye or the artificial retina have already shown their effectiveness in partially restoring sight to certain patients. These systems pick up light messages and transmit them to the brain via the optic nerve (6). At this stage of research, the patient’s optic nerve must be intact to allow information to be transmitted to the brain.