👁 What is Ethambutol-Induced Optic Neuropathy (EON)?
Ethambutol is a first-line treatment for tuberculosis (TB).
Its most serious adverse effect is toxic optic neuropathy, referred to as EON.
🔬 Pathophysiology
- Acts as a chelating agent → depletes essential metal ions
- Inhibits mitochondrial function in retinal ganglion cells
→ Leads to axonal degeneration and optic nerve damage
⚠ Risk Factors
FactorRisk ↑
| Dose | >15 mg/kg |
| Duration | >2 weeks |
| Renal impairment | Drug is renally excreted |
| Elderly, low body weight, smokers | ↑ susceptibility |
🇰🇷 Korean data: incidence ~1.5%
🧠 EON can occur even with standard dosing and short use
🧠 Clinical Presentation
CategoryFinding
| Vision | Bilateral central vision loss |
| Color vision | Red-green dyschromatopsia |
| Visual fields | Central scotoma, centrocecal scotoma, bitemporal hemianopsia |
| OCT | Thinning of temporal RNFL |
| VEP | Delayed P100 latency |
| Fundoscopy | Optic disc pallor (late stage) |
🛑 Management & Prognosis
- Immediate discontinuation of the drug is crucial
- Most recover in 3–4 months, but some take over a year
- Up to 50% may experience permanent vision loss
✅ Summary
- EON is a sight-threatening complication of ethambutol
- Occurs unpredictably, even at low doses
- Early recognition + drug discontinuation = best chance of recovery
- Ophthalmic screening during therapy is highly recommended
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