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The ophthalmic nerve is one of the three branches of the fifth cranial nerve, also known as the trigeminal nerve. It functions to relay images and impulses to the brain from the retina, resulting in sight. Within the brain is a structure called the optic chiasm, where the nerve splits and nerve fibers from each side of the brain cross to the other. Any damage to the ophthalmic nerve will almost always result in sight disruption, with severe damage causing permanent blindness. Several medical conditions, such as diabetes, multiple sclerosis (MS) and glaucoma, can cause damage to the nerve and cause severe vision impairment.
The retina, through its nerve- and photoreceptor-rich macula, senses light and relays images via electrical impulses to the brain through the ophthalmic nerve. The nerves associated with the photoreceptors group together to form this nerve, the beginning of which is the optic disc, at the back of each eye. Approximately 1.2 million nerve fibers can be found within each ophthalmic nerve bundle. The nerve exits the back of each eye through the optic canal and then passes on to the optic chiasm, where half of each bundle crosses over to the other side of the brain. The two halves meet and combine again at the back of the brain, where the impulses are translated into sight.
The ophthalmic nerve is encased in a myelin sheath and then swathed in three layers of meninges, membranes that surround and protect the central nervous system. Its unusual pathway from eye to brain means damage at specific locations will result in particular vision disturbances. For instance, if the nerve is damaged before the optic chiasm, the sight in only one eye will be affected. If the optic chiasm itself is damaged, peripheral vision in both eyes will likely be lost. Problems farther along one side of the nerve, nearer to the brain, will result in the opposite side of the visual field in each eye being lost.
Damage to the ophthalmic nerve may occur as the result of a blocked blood supply caused by high blood pressure or diabetes, or inflammation of the arteries. Inflammation of the ophthalmic nerve itself, called optic neuritis, if often caused by MS; and papilledema, or swelling of the nerve, is often brought on by head trauma, meningitis or a brain abscess or tumor. Toxic or nutritional amblyopia occurs when nutritional deficiency or a poison such as lead damages the ophthalmic nerve, causing a slow loss of vision over weeks or months. In some cases of ophthalmic nerve damage, vision can be partially or totally restored once the underlying problem is treated.