To neutralize an esophoria on the cover test, which prism base direction is used?

• A. Base In
• B. Base Out
• C. Base Up
• D. Base Down

Answer: (B)

When a phoria shows a tendency for the eye to drift inward (an esophoria), you counter that inward tendency with a prism that pushes the image outward. A prism bends light toward its base, so placing a base-out prism in front of the eye shifts the perceived image in a way that the eye must turn slightly outward to fuse. This outward shift neutralizes the inward drift seen on the cover test. Using a base-in prism would push the image inward and worsen the inward drift, and vertical prisms don’t address the horizontal misalignment involved in esophoria. So the prism base direction used is base-out.