A 58-year-old man on HRZE therapy for pulmonary TB complains at his 4-month review of difficulty distinguishing the red and green capsules in his blister pack. Visual acuity testing shows bilateral reduction and fundoscopy is normal. The most appropriate next step is:

• A Stop ethambutol immediately and refer to an ophthalmologist ✓
• B Continue all drugs and reassure, as this resolves spontaneously
• C Stop isoniazid and start pyridoxine 100 mg daily
• D Add topical corticosteroid drops and continue therapy

Explanation

Loss of red-green colour discrimination is the earliest sign of ethambutol-induced retrobulbar (optic) neuritis, a dose- and duration-dependent toxicity. The drug must be stopped promptly because continued exposure can progress to irreversible visual loss. Isoniazid causes peripheral neuropathy, not colour blindness, and fundoscopy being normal early does not exclude toxic optic neuropathy.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.