A patient receiving antitubercular therapy complains of difficulty distinguishing traffic light colours. Visual acuity testing confirms bilateral colour vision loss. Which drug and which toxic effect are responsible?

• A Isoniazid, optic atrophy
• B Rifampicin, cortical visual impairment
• C Streptomycin, vestibular toxicity
• D Ethambutol, retrobulbar neuropathy ✓

Explanation

Ethambutol causes dose-dependent retrobulbar neuropathy affecting the optic nerve fibres. The earliest and most sensitive sign is impaired red-green colour discrimination, followed by reduced visual acuity and central scotomas. Risk rises with doses above 15 mg/kg/day and with renal impairment, so baseline and monthly visual assessment is advised. Isoniazid can rarely cause optic neuritis, but its classic toxicity is peripheral neuropathy due to pyridoxine depletion.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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