During a 4 BO prism test with the prism over the right eye, the left eye does not move outward or re-fixate. This implies which of the following?

• A. Normal finding
• B. OD suppression
• C. OS suppression
• D. Diplopia

Answer: (B)

The main idea here is how suppression affects what you see when a small fusional vergence demand is placed with prisms. In a 4 prism diopter base-out test, you’re challenging the eyes to maintain single vision by diverging slightly to fuse the images.

If the brain suppresses one eye’s input to avoid diplopia, that eye won’t participate in the fusion response. So you’d see little or no outward movement or re-fixation from the eye that’s being suppressed in the behavioral response of the fellow eye.

In this scenario, with the prism over the right eye, the observed lack of outward movement or re-fixation from the left eye indicates that the brain is suppressing the input from the right eye (the eye with the prism) to avoid diplopia. That pattern is described as suppression of the eye with the prism, i.e., OD suppression. If both eyes were functioning normally, you’d expect fusional movements in both eyes to maintain single vision rather than a lack of movement in the right-eye image.