The optic nerve is an extension of the brain that connects the retina of the eye to the brain. The retina contains photoreceptors, cells that are activated by light and that connect to other retinal cells called ganglion cells. These, in turn, send long projections called axons into the brain. By this route, the optic nerve sends visual impulses to the brain.
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Optic neuritis is the most common optic nerve disease to affect young people. The average age at the first attack is 31 years, but teenagers and persons over 40 may develop this disease for the first time. It is an inflammatory disease of uncertain cause, which affects more women than men. The presenting symptoms are blurred central vision (the vision used to read and see fine detail), reduced sensation of light brightness, and reduced color vision. Commonly there is aching pain in the eye made worse by eye movement.
The pain often goes away within a week or so but the blurred vision lasts for weeks to several months. Three fourths of the patients have nearly complete return of central vision, as measured by reading the eye chart. However, there is usually some permanent residual haziness, which patients describe as “looking through smoke” even though outlines of objects are sharp. Fifteen percent of patients remain blind in the affected eye. Recurrences in the same or the other eye are uncommon, and residual foggy vision following these is more prominent than after a first attack. Thus, optic neuritis can be a serious problem that leads to significant visual disability.
There is a close relationship between optic neuritis and multiple sclerosis (MS).
MS is one of several illnesses classed as demyelinating disease. These conditions are characterized by degeneration of the fatty covering called myelin that surrounds nerves in the central nervous system (CNS) and ensures normal conduction of nerve impulses. We believe that in demyelinating diseases the body’s immune system has attacked and damaged the myelin sheaths (nerve coverings). Normally, of course, the immune system protects the body against invasion from the outside by disease-producing organisms such as bacteria, fungi, and viruses. On occasion the immune system perceives a normal body tissue to be a foreign invader and mounts a destructive attack on that tissue using proteins called antibodies and specialized white blood cells called killer cells. This constitutes the mechanism of a class of diseases called autoimmune. MS is most likely a disease of this type in which the abnormal attack on tissue is limited to the nervous system. The optic nerve is frequently the first area of the CNS to be attacked by this disease.
The diagnosis of MS is based on both of the following criteria: (l) remissions and exacerbations—attacks of the disease come and go—and (2) multiple areas of the nervous system are involved in separate attacks. In other words, the disease is multiple both in time and site. There are many demyelinating illnesses that are “one shot.” They come once and never recur. At the time of the first attack, even if multiple areas of the nervous system are affected, it is not possible to diagnose the condition as MS until there is a recurrence.
The percentage of optic neuritis patients who later develop MS or necrologic symptoms other than visual loss varies with length of follow-up in large studies. When patients are followed up for more than 15 years after an attack of optic neuritis, more than 60% develop other neurologic symptoms. One possibility is that all patients with optic neuritis have a limited form of MS, in which many are not destined to have any other attacks. Another possibility is that optic neuritis is a syndrome— a collection of signs and symptoms with various underlying causes and mechanisms—in which only a fraction of patients have MS.
For decades, physicians have treated optic neuritis with steroids, administered either orally or intravenously. In recent years, some physicians have given ultra-high-dose steroids intravenously. It has never been proved that treatment with any form of steroid regimen improves the long-range outcome of optic neuritis. In addition, steroids may cause adverse side effects. To help determine the best treatment of optic neuritis, the Neuro-Ophthalmology Service at the University of Illinois Hospital Eye and Ear Infirmary is participating in a multicenter study, funded by the National Eye Institute.