Esophoria neutralization: Which prism neutralizes an esophoria on the cover test?

• A. BI prism
• B. BO prism
• C. Fresnel prism
• D. Prism neutralization not possible

Answer: (B)

Esophoria is an inward latent deviation that tends to pull the eye inward when fusion is disrupted. To neutralize it on the cover test, you want a prism that encourages the eyes to diverge slightly, reducing that inward drift. A base-out prism does exactly that: it shifts the image so the eyes must diverge a bit to fuse, counteracting the inward drift. When the amount of base-out prism matches the esophoria, single binocular vision is achieved on the test.

In practice, this is why base-out prisms neutralize esophoria, whereas base-in prisms would be used to counter exophoria. Fresnel prisms are just a convenient, temporary form of prism for trial, not a different neutralization principle.