Optic Neuropathy
Optic neuropathy is damage of the optic nerve due to a blockage of its blood supply, to nutritional deficiencies, or to toxins.
Blockage of the blood supply to the part of the optic nerve within the eye can lead to death or dysfunction of optic nerve cells and is called ischemic optic neuropathy. Two types can occur: nonarteritic and arteritic.
Nonarteritic ischemic optic neuropathy usually occurs in people older than 50. Risk factors include high blood pressure, diabetes, and atherosclerosis. Rarely, it occurs in younger people with severe migraines. Arteritic ischemic optic neuropathy usually occurs in people older than 70. The blood supply to the optic nerve is blocked due to inflammation of the arteries (arteritis), most notably the temporal artery, which causes temporal arteritis (giant cell arteritis (see Section 5, Chapter 69)).
Damage to the optic nerve can also be caused by exposure to a substance that is harmful to the optic nerve, such as lead, methanol, ethylene glycol (antifreeze), tobacco, or arsenic. This type is sometimes called toxic amblyopia. Optic neuropathy may be caused by a nutritional deficiency (sometimes called nutritional amblyopia), especially of vitamin B12. Alcoholics are particularly susceptible, although the cause may be malnutrition, not alcohol. Rarely, optic neuropathy may be caused by drugs such as chloramphenicol, isoniazid, ethambutol, and digoxin.
Loss of vision may be rapid (over minutes or hours), but it can develop gradually over 2 to 7 days. Depending on the cause, vision may be impaired in one or both eyes. Vision in the involved eye or eyes can range from almost normal to complete blindness. In people with optic neuropathy caused by exposure to a toxin or to a nutritional deficiency, both eyes are usually affected. A small area of vision loss at the center of the visual field slowly enlarges and can progress to complete blindness. People with temporal arteritis tend to be older, and their loss of vision tends to be more severe.
About 40% of people with nonarteritic ischemic optic neuropathy experience spontaneous improvement over time. In this condition, repeat episodes in the same eye are extremely rare. Involvement of the other eye is estimated to occur in 10 to 34% of affected people over a 5-year period.
Diagnosis involves examination of the back of the eyes with an ophthalmoscope. Determining the cause involves obtaining a careful history of possible exposures to toxic substances and determining whether the person is suffering from any of the disorders known to be a risk factor. If temporal arteritis is suspected as a cause, blood tests and a biopsy of the temporal artery may be done to confirm the diagnosis.
In people with nonarteritic ischemic optic neuropathy, treatment involves controlling blood pressure, diabetes, cholesterol levels, and other factors that affect the blood supply to the optic nerve. In people with arteritic ischemic optic neuropathy due to temporal arteritis, high doses of corticosteroids are given to prevent loss of vision in the second eye, which occurs in 25 to 50% of people within days to weeks if treatment is not started. The role of aspirin in preventing involvement of the second eye is being investigated, although at this time there is no evidence to support its use.
People with optic neuropathy caused by exposure to chemicals or drugs should avoid tobacco, alcohol, or other responsible toxic chemical or drug if it is known. If alcohol use is a contributing cause, the person should eat a well-balanced diet and take vitamin supplements. If lead is the cause, chelating drugs (such as succimer or dimercaprol) help remove it from the body.
In people with optic neuropathy caused by a nutritional deficiency, treatment usually involves correction of the deficiency with dietary supplements. However, if the cause is vitamin B12 deficiency, treatment with dietary supplements alone is not enough. Vitamin B12 deficiency is typically treated with injections of supplemental vitamin B12. Unless the optic nerve shows evidence of wasting (atrophy), recovery of some of the lost vision is expected.
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