Under Percival's criterion, which prism direction is typically prescribed for esophoria?

• A. BO prism
• B. BI prism
• C. No prism
• D. Both prisms

Answer: (A)

Percival's criterion guides prism choice by picking the direction that minimizes the fusional effort needed to maintain single binocular vision across viewing conditions. When a patient shows an inward drift of the eyes (esophoria), the goal is to reduce that inward tendency. A base-out prism shifts the image outward, effectively helping the eyes align by reducing the need for convergence. This makes it easier for the patient to fuse the images with less vergence demand, so it’s the direction that best suits esophoria under Percival's criterion.

Using base-in would push the image inward, increasing convergence demand and making fusion harder, which is not desirable for esophoria. Not prescribing any prism leaves the phoria unaddressed, and choosing both directions isn’t consistent with the criterion, which selects a single, most effective direction.