What is the formula for prescribing prism using Sheard's criterion?

• A. 2/3 phoria - 1/3 (compensating fusional vergence)
• B. 1/3 phoria - 2/3 (compensating fusional vergence)
• C. 1/2 phoria - 1/2 (compensating fusional vergence)
• D. 3/4 phoria - 1/4 (compensating fusional vergence)

Answer: (A)

Sheard's criterion estimates how much prism to prescribe by balancing the latent misalignment (phoria) with the person’s ability to fuse using compensating fusional vergence. The idea is to reduce the vergence demand enough so that the remaining residual phoria can be comfortably compensated by the patient’s fusional reserves. The prism amount is calculated as two-thirds of the phoria minus one-third of the compensating fusional vergence. If this value is positive, prescribe that amount of prism; if zero or negative, prism is not indicated by this criterion. For example, with a phoria of 6 prism diopters and compensating fusional vergence of 3 diopters, the prism would be 2/3×6 − 1/3×3 = 4 − 1 = 3 prism diopters. The direction of the prism would be chosen to counteract the deviation (e.g., base direction that reduces the misalignment).