Which of the following anti-tuberculous drugs is most likely to cause optic neuritis and color blindness (red-green) as an adverse effect?

Explanation

## Ethambutol and Optic Neuritis **Key Point:** Ethambutol is a first-line anti-tuberculous drug with a characteristic adverse effect profile centered on the optic nerve. ### Mechanism of Toxicity Ethambutol causes dose-dependent optic neuritis by inhibiting mycobacterial arabinosyl transferases, but the exact mechanism of human optic toxicity remains incompletely understood. The effect is reversible if caught early but can progress to permanent vision loss if the drug is not discontinued. ### Clinical Features of Ethambutol-Induced Optic Neuritis - **Red-green color blindness** (dyschromatopsia) — earliest and most sensitive sign - Decreased visual acuity - Central scotoma - Peripheral vision loss (in severe cases) - Symptoms typically appear after 2–7 months of therapy **High-Yield:** Red-green color blindness is the hallmark and often the first manifestation; patients should be counseled to report any change in color perception immediately. ### Monitoring and Prevention - **Baseline ophthalmologic assessment** before starting ethambutol - **Monthly color vision testing** (Ishihara chart) during therapy - **Immediate discontinuation** if color vision changes or visual acuity declines - Risk increases with doses >25 mg/kg/day and renal impairment ### Reversibility If detected early and the drug is stopped promptly, optic neuritis is usually reversible. Delayed recognition can lead to permanent blindness. **Clinical Pearl:** In patients with renal failure or those on high-dose ethambutol, ophthalmologic monitoring is mandatory and should be more frequent. [cite:Harrison 21e Ch 205]