A 52-year-old man on intensive phase antitubercular therapy (2HRZE) complains of difficulty distinguishing traffic light colours after six weeks. Which drug is responsible, and what should be done?

• A Isoniazid; add pyridoxine and continue unchanged
• B Pyrazinamide; check uric acid and stop the drug
• C Ethambutol; stop the drug immediately and document baseline red-green colour vision ✓
• D Rifampicin; reassure as this is a harmless discolouration effect

Explanation

Ethambutol causes dose-dependent retrobulbar neuritis, presenting earliest as impaired red-green colour discrimination before visual acuity falls. Baseline colour vision testing and monthly symptom review are standard, and the drug must be stopped promptly because the deficit can become irreversible. Isoniazid causes peripheral neuropathy and hepatitis, pyrazinamide causes hyperuricaemia and hepatotoxicity, and rifampicin causes orange body fluid discolouration, not visual change.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.