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

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

Answer: (B)

When you prescribe prism for a phoria, the goal is to keep the vergence demand within the patient’s fusion reserves so single, comfortable vision can be maintained without causing diplopia.

Esophoria involves a tendency to over-converge. The convergence system is often the limiting factor, and how the demand is distributed between the two eyes matters a lot. Percival's criterion is focused on balance: it guides you to choose a prism magnitude that does not force either eye to use more than its share of the vergence reserves (typically aiming to keep the required vergence demand within half of each eye’s available reserve). This balancing approach is particularly robust for esophoria, where misbalancing the convergence demand can readily trigger symptoms or diplopia.

Sheard's criterion, on the other hand, is centered on the relationship between phoria magnitude and the fusional reserve in the direction of the phoria, and it works well for exophoria (where the divergence reserve is the key limiter). For esophoria, this ratio can be less reliable because the convergence reserves are the limiting factor and the distribution of load between the eyes is more critical.

So, for esophoria, prioritizing a prism prescription that distributes the convergent demand within the eyes’ vergence reserves (Percival's criterion) tends to yield a safer and more comfortable magnit ude.