Which antitubercular drug is most commonly associated with optic neuritis and red-green color blindness as a dose-dependent adverse effect?

Explanation

## Ethambutol and Optic Neuritis **Key Point:** Ethambutol causes dose-dependent optic neuritis, characterized by decreased visual acuity and red-green color blindness (dyschromatopsia), making baseline and periodic ophthalmologic monitoring mandatory. ### Mechanism of Optic Toxicity - Ethambutol inhibits arabinosyl transferase, disrupting mycobacterial cell wall synthesis - At higher doses (>25 mg/kg/day), it accumulates in the optic nerve and causes direct retrobulbar optic neuritis - Red-green color discrimination is affected before loss of visual acuity ### Clinical Management | Feature | Details | | --- | --- | | **Onset** | Usually 1–3 months of therapy | | **Reversibility** | Partially reversible if drug stopped early; permanent damage if delayed | | **Monitoring** | Baseline visual acuity and color vision testing (Ishihara chart) | | **Recommended dose** | ≤15 mg/kg/day to minimize risk | | **Patient counseling** | Advise to report blurred vision, color vision changes, or visual field defects immediately | **High-Yield:** Ethambutol optic neuritis is a classic NEET PG question. The red-green color blindness is pathognomonic and a key distinguishing feature. **Clinical Pearl:** Always ask patients on ethambutol about visual symptoms at each visit. A simple bedside test: ask the patient to read colored text or identify colors on the Ishihara plate. **Warning:** Do NOT confuse ethambutol toxicity with isoniazid peripheral neuropathy or rifampicin hepatotoxicity — each drug has distinct adverse effect profiles.