Why is a baseline vision test required before initiating ethambutol therapy?

• A. To monitor liver function
• B. To assess kidney function
• C. To detect pre-existing visual impairment and to monitor for optic neuritis during therapy
• D. To measure blood pressure

Answer: (C)

Ethambutol can damage the optic nerve, causing changes in vision and color perception that may become permanent if not caught early. Establishing a baseline vision assessment before starting therapy provides a reference point for all aspects of visual function—acuity, color vision, and field of view. With this starting point, any future decline or new color-vision deficits can be detected promptly, allowing timely dose adjustment or discontinuation to prevent lasting harm. It also helps identify any pre-existing visual impairment so the risk of optic neuritis during treatment can be monitored more effectively. This test is specifically about vision, not liver or kidney function or blood pressure, which are monitored for other safety reasons.