A 42-year-old woman on intensive phase antitubercular therapy for 6 weeks complains of progressive blurring of vision and difficulty telling red from green during sewing. Visual acuity testing confirms reduced acuity with a centrocecal scotoma. Which drug is responsible, and what is the immediate management?

• A Isoniazid, add pyridoxine and continue therapy
• B Rifampicin, continue as the effect is self-limiting
• C Ethambutol, stop the drug immediately and review the regimen ✓
• D Streptomycin, substitute kanamycin

Explanation

Ethambutol causes dose-dependent retrobulbar optic neuropathy, and the earliest detectable abnormality is loss of red-green color discrimination, followed by reduced acuity and a centrocecal scotoma. The drug must be stopped at once because continuation can lead to irreversible blindness; recovery usually follows withdrawal if detected early. Pyridoxine corrects isoniazid-induced peripheral neuropathy, not optic neuropathy, and rifampicin causes harmless orange discoloration of body fluids rather than visual loss.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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Written and medically reviewed by the StethoPrep medical team.