A patient has 10 pd of esophoria with BI vergence 6/10/4 and BO vergence 21/28/22. What is the prism amount and direction prescribed using Percival's criterion?

• A. 3 pd BO
• B. 3 pd BI
• C. 12 pd BO
• D. 12 pd BI

Answer: (A)

Percival's criterion guides prism prescription by ensuring the residual vergence demand after adding prism stays within the patient’s available fusional vergence reserves, while using the smallest prism that achieves this and distributes the load between divergence and convergence systems.

Here the patient has an esophoria of 10 prism diopters. Their divergence reserves (base-out) are relatively large: near break about 28 and recovery about 22. The convergence reserves (base-in) are smaller, with near break about 10 and recovery around 4. To relieve the inward drift without pushing either vergence system to its limit, you want a prism in the direction that reduces convergence demand but not so large that you overtax the convergence system or misbalance fusion. A small base-out prism of 3 diopters accomplishes this: it reduces the immediate need for convergence from 10 pd to about 7 pd, a level easily handled by the available base-in and base-out reserves (still well within the convergence break of 10 and far below the divergence break of 28). Larger prisms would risk overloading the divergence or convergence reserves and destabilizing fusion.

Therefore, the prism amount and direction prescribed is 3 diopters base-out.