A patient requires a +6.00 sph. Rx for reading glasses. How much BI prism per eye would you give to help convergence?

• A. 8 BI per eye
• B. 8 BO per eye
• C. 6 BI per eye
• D. 6 BO per eye

Answer: (A)

The idea being tested is how plus power for near work affects vergence and how prism can compensate. When you prescribe reading glasses with a +6.00 diopter addition, the eyes’ accommodation demand drops, which in turn reduces accommodative convergence. If the goal is to keep comfortable, single vision at near, you can add base-in prism to increase the inward turning (convergence) effort without changing the lens power. Base-in prisms effectively boost the convergence demand, helping fuse the near image despite the reduced accommodative convergence from the plus lens. For a +6.00 near addition, about 8 prism diopters of base-in prism per eye is a common starting value to support convergence while reading. Base-out prisms would not help in this situation, as they tend to reduce convergence and make near fusion harder. Six prism diopters of base-in is usually not enough to offset the reduction in accommodative convergence from a +6.00 addition, which is why 8 BI per eye is preferred here.