American Board of Ophthalmology (ABO) Practice Exam

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What prism direction is used in reverse slab off in the lens with the most plus?

Base up
Base down
Base inward
Base outward

The correct answer is: Base down

In reverse slab-off, the prism direction is applied to the lens with the most plus power in a specific way. The purpose of this technique is to effectively manage and balance the vision for patients who have significant differences in refractive error between the two eyes, particularly in cases of aniseikonia. When the choice specifies "reverse slab-off," it refers to the method where the added prism is designed to neutralize the image size difference that occurs due to a higher positive lens (-plus) in one eye, which leads to a larger image size compared to the eye with less plus power. To counter this effect, the base direction of the prism is oriented downward in the more plus lens, thereby reducing the effect of the plus power on the retinal image size. This downward orientation, or base down, effectively helps to bring the images into better alignment, enhancing visual comfort and mitigating any disparities that might exist due to differing lens powers. By using base down in the lens with the most plus, it allows for optimal adjustment of the visual output, aiding the patient in achieving better binocular vision.