All vision relies on the optic nerve and its ability to relay the images our eyes detect to our brains for processing. There are twelve paired nerves that relay information from the body to the brain and the optic nerve is the second pair of this twelve. Thus, it is frequently referred to as cranial nerve II.
The optic nerve is a part of the body’s central nervous system. As such, damage to this nerve cannot be repaired. Any impaired vision that results from injury or illness that affects the optic nerve is permanent.
At the center point of the back of the eye, fibers from the retina form the optic nerve. Behind the eyes, the optic nerve from each eye joins together, forming the optic chiasm. Visual signals cross over to the other side of the brain in the chiasm and, from here, travel to nine primary visual nuclei along both sides of the brain.
The retina is lined with a collection of tiny organs that resemble rods and cones. The rods detect light and the cones color. These visual images are transferred to the brain via retinal fibers and then the optic nerve.
Although it isn’t obvious, thanks to the placement of both our eyes, every eye has a blind spot, where there are no rods or cones. It is this spot that the optic nerve connects with the eye.
Some experts believe a large part of visual processing occurs in the eye itself because there are about 100 million photoreceptors (rods and cones) in the retina but only 1.2 million retinal nerve fibers traveling to the brain. Others dispute this, saying the signal capacity of the fibers of the optic nerve is great enough to carry only signals to the brain, where all the processing is done.
Regardless of where the processing is done, damage anywhere along the optic nerve results in permanent loss of vision, with vision loss equal to severity of the damage. When the optic nerve becomes damaged, the pupil is also affected. Optic nerve damage influences the pupil’s reflex ability, which hinders diagnosis of many eye disorders. Pupil reflex is vital also in diagnosing other disorders that affect the body system wide.
Visual impairment caused by damage to the optic nerve depends upon where along the nerve the damage occurs. When damage occurs in front of the optic chiasm (between eye and chiasm), loss of vision will be discerned in the eye on the same side (left or right) as the damage itself. When the chiasm itself is damaged, visual loss affects both eyes.
Damage to the optic nerve beyond the chiasm causes visual loss in the eye opposite the damage. In this case, damage to the optic nerve on the right side of the brain causes visual loss in the left eye.
The chiasm is situated near the pituitary gland, leaving it vulnerable to damage when certain pituitary tumors are present. Traumatic injury, inflammation, toxicity, stroke, and compression from an aneurysm or tumor can all cause damage to the optic nerve to the extent vision is impaired. Injury can also occur when Leber’s Hereditary Optic Neuropathy, a congenital condition, is present.
The three most common medical conditions that affect the optic nerve include glaucoma, which is a neuropathy causing loss of peripheral vision but which can progress to loss of central vision as well. Multiple sclerosis patients risk optic neuritis, or inflammation of the optic nerve, which commonly happens before age 50, and patients at heightened risk for cardiovascular disease risk anterior ischemic optic neuropathy after age 50.