Neuro-ophthalmology Illustrated Chapter 8 – Optic Neuropathies 10

Questions:
116. What are the characteristic features of toxic and nutritional optic neuropathies?
117. Is visual loss in toxic or nutritional optic neuropathy commonly the result of a single cause?
118. Is treatment by stopping the toxic agent or replacing a missing vitamin that is causing an optic neuropathy usually effective in improving visual function?
119. What are the common toxins linked to optic neuropathies?
120. What are the nutritional causes linked to optic neuropathies?
121. What is the most common agent in toxic optic neuropathy?
122. What are the findings of methanol-related optic neuropathy and what is its course?
123. What are the characteristics of ethylene glycol-related optic neuropathy?
124. What is the most common medication that causes toxic optic neuropathy?
125. What are the characteristics of ethambutol-related optic neuropathy?
126. What are the characteristics of amiodarone-related optic neuropathy?
127. What should be always be excluded when considering the diagnosis of toxic optic neuropathy?
128. What is often the first sign of vitamin B12 deficiency?
129. What are the characteristics of Vitamin B12 deficiency?

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Questions with answers:
116. What are the characteristic features of toxic and nutritional optic neuropathies?
Characteristic clinical features include progressive, symmetrical central visual loss, reduced perception of color, the optic nerve may be normal or may appear slightly swollen. Temporal optic disk pallor is seen by 6 weeks after the onset of visual loss.

117. Is visual loss in toxic or nutritional optic neuropathy commonly the result of a single cause?
Definitively proven cases of optic nerve damage caused by a single recognized toxic agent or a deficiency in a single identified nutrient are rare.

118. Is treatment by stopping the toxic agent or replacing a missing vitamin that is causing an optic neuropathy usually effective in improving visual function?
In some cases, stopping the toxic agent or vitamin supplementation may result in improvement of visual function. However, visual loss is often irreversible.

119. What are the common toxins linked to optic neuropathies?
Among the toxins linked to optic neuropathies are methanol, ethylene glycol, lead, organic solvents, tobacco (usually cigars), ethambutol, amiodarone, linezolid, and disulfiram.

120. What are the nutritional causes linked to optic neuropathies?
Vitamin B12 and folate deficiencies.

121. What is the most common agent in toxic optic neuropathy?
Methanol ingestion is the most widely recognized cause of toxic optic neuropathy. It is also most often accidental (mistaken for or has been added to ethyl alcohol).

122. What are the findings of methanol-related optic neuropathy and what is its course?
The findings of methanol-related optic neuropathy are acute optic neuropathy with swollen disks, headaches nausea, and metabolic acidosis. The course is one of progressive visual loss, abdominal pain, respiratory distress, confusion, coma, and death 18 to 48 hours after ingestion.

123. What are the characteristics of ethylene glycol-related optic neuropathy?
Ethylene glycol is the active ingredient in automobile antifreeze. It may be consumed accidentally or in a suicide attempt. It is very toxic to the optic nerves. Symptoms and signs resemble those of methanol intoxication ( (acute optic neuropathy with swollen disks, headaches nausea and metabolic acidosis). In addition, renal insufficiency is common with ethylene glycol.

124. What is the most common medication that causes toxic optic neuropathy?
Ethambutol

125. What are the characteristics of ethambutol-related optic neuropathy?
Toxicity is usually dose-related and develops in patients after at least 2 months, with a mean of 7 months. Visual loss is bilateral and slowly progressive. Bitemporal visual field defects and color vision loss may be present early. Visual function may improve after stopping the drug. Unrecognized reduced renal function is often a contributing factor and the toxicity may be masked by cataracts.

126. What are the characteristics of amiodarone-related optic neuropathy?
Although it is still debated, there is good evidence that sequential bilateral optic neuropathies may develop in patients treated with amiodarone, an antiarrhythmic agent. Toxicity is dose-related and usually occurs in patients who have been treated with it for a few months. It is classically associated with disk edema resembling nonarteritic anterior ischemic optic neuropathy. Visual loss may be subacute or slowly progressive. Visual function may improve after stopping the medication (which should be done only after consulting with the patient’s cardiologist).

127. What should be always be excluded when considering the diagnosis of toxic optic neuropathy?
The diagnosis of toxic optic neuropathy is usually a diagnosis of exclusion. Rule-out other causes of bilateral optic neuropathies, such as compression, vitamin B12 deficiency, and hereditary optic neuropathies.

128. What is often the first sign of vitamin B12 deficiency?
Optic neuropathy

129. What are the characteristics of Vitamin B12 deficiency?

Severe vitamin B12 deficiency (usually from impaired absorption) may produce bilateral slowly progressive optic neuropathies and which may precede the anemia and other neurologic symptoms and signs.

The information below is from Neuro-ophthalmology Illustrated-2nd Edition. Biousse V and Newman NJ. 2012. Thieme

8.8 Toxic and Nutritional Optic Neuropathies
Toxic and nutritional optic neuropathies generally have similar clinical features and may even coexist in the same patient.

8.8.1 Features
Characteristic clinical features include progressive, symmetrical central visual loss; reduced perception of color; and cecocentral scotomas. Acutely, the optic nerve may be normal or may appear slightly swollen. Temporal optic disc pallor is seen at least 6 weeks after the onset of visual loss.

8.8.2 Causes
Causation in many cases is likely multifactorial. In humans, definitively proven cases of optic nerve damage caused by a single recognized toxic agent or a deficiency in a single identified nutrient are rare. In some cases, discontinuation of the agent or vitamin supplementation may result in improvement of visual function. However, visual loss is often irreversible.
Among the suspected toxins linked to optic neuropathies are methanol, ethylene glycol, cobalt, lead, zinc, organic solvents, toluene, tobacco (usually, cigars), ethambutol, amiodarone, linezolid, and disulfiram. Nutritional causes include vitamin B12, folate, and copper deficiencies. Several of the leading types of toxic and nutritional optic neuropathies are described in the following section.

8.8.3 Types
Methanol

Methanol ingestion is the most widely recognized cause of toxic optic neuropathy. It is also most often accidental (mistaken for or added to ethyl alcohol). Methanol-related optic neuropathy is acute, and the optic nerves are usually swollen. Visual loss from bilateral optic neuropathies is associated with headaches and nausea. Worse visual loss, abdominal pain, respiratory distress, confusion, and, ultimately, coma, and death happen 18 to 48 hours after ingestion. Metabolic acidosis is one of the hallmarks of methanol ingestion.

Ethylene Glycol
Ethylene glycol is the active ingredient in automobile antifreeze. It may be consumed accidentally or in a suicide attempt. It is very toxic to the optic nerves. Symptoms and signs resemble those of methanol intoxication, except for renal insufficiency, which is common with ethylene glycol.

Ethambutol

Ethambutol, an antimycobacterial drug used to treat tuberculosis, is the medication most often implicated in toxic optic neuropathy (▶Fig. 8.38).

Toxicity is usually dose related (develops in patients taking the drugs for a few months—usually at least 2 months, with a mean of 7 months). Bitemporal visual field defects may reflect early involvement of the chiasm, and color vision is affected early. Visual loss is bilateral and slowly progressive. Visual function may improve after discontinuation of the drug.

Amiodarone
Although it is still debated, there is good evidence that sequential bilateral optic neuropathies may develop in patients treated with amiodarone, an antiarrhythmic agent. Toxicity is dose related and usually occurs in patients who have been treated with it for a few months. It is classically associated with disc edema resembling nonarteritic anterior ischemic optic neuropathy (▶Fig. 8.39). Visual loss may be subacute or slowly progressive. Visual function may improve after discontinuation of the medication (which can be done only after consulting with the patient’s cardiologist.

Pearls
The diagnosis of toxic optic neuropathy is usually a diagnosis of exclusion. Workup ruling out other causes of bilateral optic neuropathies, such as compression, vitamin B12 Deficiency, and hereditary optic neuropathies, should always be obtained.

Vitamin B12 Deficiency
Severe vitamin B12 deficiency (usually from impaired absorption) may produce bilateral, slowly progressive optic neuropathies (▶Fig. 8.40). The optic neuropathies are often the first sign of vitamin B12 deficiency and may precede the anemia and other neurologic symptoms and signs.

Copper Deficiency
Chronic copper deficiency is very rare and can cause many hematological manifestations, such as myelodysplasia, anemia, leukopenia, and neutropenia, as well as ataxia, peripheral neuropathy, and bilateral optic neuropathies.

Pearls
Always check vitamin B12 and copper levels in patients with progressive bilateral visual loss and bilateral optic atrophy.

Reference: 1. Neuro-ophthalmology Illustrated-2nd Edition. Biousse V and Newman NJ. 2012. Thieme

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