A patient has 15 prism diopters of exophoria at near and vergence ranges of 17/25/18 BI and 6/9/4 BO. Based on Sheard's criterion, what prism amount should be prescribed?

A patient has 15 prism diopters of exophoria at near and vergence ranges of 17/25/18 BI and 6/9/4 BO. Based on Sheard's criterion, what prism amount should be prescribed?

• A. 4 pd BO
• B. 4 pd BI
• C. 8 pd BO
• D. 8 pd BI

Answer: (D)

Sheard's criterion guides prism prescription by ensuring the available fusional vergence (the patient’s own ability to converge or diverge) can comfortably compensate the remaining heterophoria after the prism is applied. For exophoria at near, you want the prism to provide enough convergence so that the residual deviation stays within the patient’s convergence reserves, with a safe cushion between the residual phoria and what the eyes can flexibly fuse. Here the near exophoria is 15 prism diopters. The patient has a strong base-in vergence capacity (convergence) with a break of 25 Δ and recovery of 18 Δ, indicating generous convergence reserves. If you apply a base-in prism of 8 Δ, the gross demand drops from 15 Δ to about 7 Δ, which can be comfortably managed within the available convergence reserves. This leaves a healthy margin for fusional adaptation and reduces symptoms. A smaller correction, like 4 Δ, would leave a larger residual exophoria (about 11 Δ) that may still challenge the reserves and symptoms could persist. Base-out prisms would aggravate a left-to-right misalignment and are not appropriate for exophoria. Therefore, 8 prism diopters of base-in prism is the best fit.

Sheard's criterion guides prism prescription by ensuring the available fusional vergence (the patient’s own ability to converge or diverge) can comfortably compensate the remaining heterophoria after the prism is applied. For exophoria at near, you want the prism to provide enough convergence so that the residual deviation stays within the patient’s convergence reserves, with a safe cushion between the residual phoria and what the eyes can flexibly fuse.

Here the near exophoria is 15 prism diopters. The patient has a strong base-in vergence capacity (convergence) with a break of 25 Δ and recovery of 18 Δ, indicating generous convergence reserves. If you apply a base-in prism of 8 Δ, the gross demand drops from 15 Δ to about 7 Δ, which can be comfortably managed within the available convergence reserves. This leaves a healthy margin for fusional adaptation and reduces symptoms. A smaller correction, like 4 Δ, would leave a larger residual exophoria (about 11 Δ) that may still challenge the reserves and symptoms could persist. Base-out prisms would aggravate a left-to-right misalignment and are not appropriate for exophoria.

Therefore, 8 prism diopters of base-in prism is the best fit.