Toxic Optic Neuropathy
- Subacute or chronic binocularly symmetrical optic neuropathy caused by systemic medications or toxins
- Common causes: ethambutol treatment, alcohol ingestion (See Nutritional Deficiency Optic Neuropathy )
- Uncommon causes: methanol ingestion, linezolid treatment
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Core clinical features
- Slowly progressive painless binocular visual loss (faster in methanol toxicity)
- Bilateral symmetrical visual acuity loss
- Bilateral central or centrocecal scotomas
- Acquired color vision loss
- Pupil size and constriction are normal (no afferent pupil defect)
- Optic discs appear normal, slightly swollen, or show temporal pallor
- Peripapillary nerve fiber layer edema in methanol toxicity
- Causative agent is often evident
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Possible accompanying clinical features
- Peripheral neuropathy
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Imaging features
- Brain imaging is normal or shows reduced optic nerve caliber bilaterally
- Nutritional deficiency optic neuropathy
- Maculopathy
- Hereditary optic neuropathy (dominantly-inherited or Leber hereditary optic neuropathy)
- Optic neuritis
- Compressive optic neuropathy
- Chiasmal or bilateral retrogeniculate vision loss
- Psychogenic visual loss
- Exclude maculopathy with ophthalmoscopy and retinal imaging
- Exclude nutritional and hereditary optic neuropathies
- Look for exposure to the following toxins: ethambutol, alcohol, methanol, linezolid, thallium, toluene, vincristine, desferrioxamine, ethylene glycol, halogenated hydroxyquinolones, disulfiram
- Promptly discontinue exposure to potential toxins
- In suspected methanol toxicity, obtain blood chemistry for acidosis and order brain MRI, which often discloses high T2/FLAIR signal abnormalities in basal ganglia and parieto-occipital white matter
- Treat methanol toxicity with base solutions to correct acidosis and facilitate excretion of formic acid, which causes the damage; in severe intoxications with reduced vision, may also need to treat with alcohol dehydrogenase inhibitors like fomepizole or, if not available, with ethanol, which competes with methanol for alcohol dehydrogenase; hemodialysis may also be necessary
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Tip: if a toxic agent is not definitely identified, brain imaging and other evaluation for alternative causes of optic neuropathy is necessary
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Tip: the causative toxin may not initially be recognized
- Visual recovery depends on the type of toxin and the extent of damage at discovery
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Trap: a standard ethambutol dose (< 25mg/kg/day) may cause optic neuropathy, especially in underweight patients, those with renal disease, and those being dialyzed (because ethambutol is not eliminated in the dialysate)
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Tip: early diagnosis of ethambutol toxicity is critical because it is minimally reversible; therefore, patients should undergo pre-treatment visual screening and monitoring of visual acuity and color vision at regular intervals while on treatment
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Tip: novel anti-neoplastic agents may have toxic effects with still-undefined frequency