A patient receiving antitubercular therapy complains of difficulty distinguishing red from green colours after three months of treatment. Visual acuity remains normal. Which drug is responsible, and what is the appropriate action?

• A Isoniazid, reduce the dose
• B Rifampicin, stop the drug permanently
• C Pyrazinamide, continue with monitoring
• D Ethambutol, stop the drug and review ✓

Explanation

Ethambutol causes a dose-dependent retrobulbar optic neuropathy whose earliest manifestation is impaired red-green colour discrimination, often preceding loss of visual acuity. Baseline visual assessment and monthly symptom enquiry are advised, particularly when doses exceed 15 mg/kg or renal function is impaired. The drug should be stopped immediately once toxicity is suspected, since continuation can produce irreversible blindness. Pyridoxine prevents isoniazid neuropathy, not ocular toxicity, and neither rifampicin nor pyrazinamide affects colour vision.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.