Exophoria neutralization: Which prism is used to neutralize an exophoria on the cover test?

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

Answer: (A)

Neutralizing exophoria on the cover test relies on shifting the image so the outward drift is countered. To do this, you use a base-in prism, which bends light toward the nasal side and moves the image inward on the retina. This inward shift makes the eye converge slightly to maintain single vision, canceling the outward deviation seen during dissociation. When the base-in prism power is adjusted until there’s no movement on the cover/uncover tests, you’ve neutralized the exophoria. A base-out prism would push the image outward, worsening the outward drift and failing to neutralize. A Fresnel prism is just a type of prism and could be used with the correct orientation, but the direction needed for exophoria neutralization is base-in. Prism neutralization is indeed possible.