Which criterion is more effective for prescribing prism in patients with an exophoria?

• A. Sheard's criterion
• B. Percival's criterion
• C. Both criteria
• D. Neither criterion

Answer: (A)

The main idea here is choosing prism in a way that keeps fusion comfortable by matching the eye’s vergence demands with its available vergence reserves. For an exophoria, the critical reserve is the convergence (positive fusional vergence) that counters the outward drift.

Sheard's criterion says the compensating fusional vergence in the direction of the phoria should be at least twice the magnitude of the phoria. In practice, this means selecting a prism amount that leaves the patient with a robust convergence reserve to deal with the exophoric demand, reducing symptoms without overloading the vergence system. This approach directly ties prism prescription to the patient’s actual ability to converge and fuse, which tends to predict better relief for exophoria.

Percival's criterion uses a balance between the two fusional vergences but doesn’t specifically target the convergence reserve most needed for an exophoric deviation. Because of that, it’s less reliable for guiding prism in exophoria, which is why Sheard's criterion is the preferred choice.