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Toxic amblyopia
is reduction in visual acuity believed to be the result of a toxic
reaction in the orbital portion (papillomacular bundle) of the optic
nerve. It can be caused by various toxic and nutritional factors
and probably unknown factors. The main symptom is painless vision
loss. Diagnosis is by history and visual field examination. Treatment
is avoiding suspected toxic agents and improving nutrition.
Etiology
Toxic amblyopia is usually bilateral and symmetric. In alcoholics, undernutrition may be the underlying cause. True tobacco-induced amblyopia is rare. Lead, methanol, chloramphenicol , digoxin , ethambutol , and many other chemicals can damage the optic nerve. Deficiencies of protein and antioxidants are likely risk factors. Toxic amblyopia may occur with other nutritional disorders, such as Strachan's syndrome (polyneuropathy and orogenital dermatitis).
Symptoms and Signs
Vision blurring and dimness typically develop over days to weeks. An initially small central or pericentral scotoma slowly enlarges, typically involving both the fixation and the blind spot (centrocecal scotoma), and progressively interferes with vision. Total blindness may occur in methanol ingestion, but other nutritional causes typically do not cause profound vision loss. Retinal abnormalities do not usually occur, but temporal disk pallor may develop late.
Diagnosis
A history of undernutrition or toxic or chemical exposure combined with typical bilateral scotomata on visual field testing justifies treatment. Laboratory testing for lead, methanol, and other suspected toxins is done.
Prognosis
Vision may improve if the cause is treated or removed quickly. Once the optic nerve has atrophied, vision usually does not recover.
Treatment
The underlying cause is treated. Exposure to toxic substances should stop immediately. Chelation therapy is indicated in lead poisoning. Dialysis, fomepizole , ethanol, or a combination is used for methanol poisoning. Treatment with oral or parenteral B vitamins before vision loss becomes severe may reverse the condition in cases in which undernutrition is the presumed cause. Low-vision aids (eg, magnifiers, large-print devices, talking watches) may be helpful. The role of antioxidants has not been fully characterized. Their use could be justified on a theoretic basis; however, there is no proof of efficacy, and the at-risk population that should receive such supplements has not been defined.
Last full review/revision July 2008 by James Garrity, MD
Content last modified July 2008
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