If one eye has BU prism and the other eye has BD prism, should you add or subtract for vertical imbalances?

If one eye has BU prism and the other eye has BD prism, should you add or subtract for vertical imbalances?

• A. Add
• B. Subtract
• C. Ignore
• D. Cannot determine

Answer: (A)

Vertical prism effects add when the prisms in the two eyes pull in opposite directions. A BU prism in one eye shifts that eye’s image upward, while a BD prism in the other eye shifts its image downward. The relative vertical offset between the two eyes’ images is the sum of the magnitudes, so you add the powers to find the vertical imbalance. For example, BU 4 PD with BD 2 PD yields a total vertical imbalance of 6 PD. If the prisms were in the same direction (both BU or both BD), you’d subtract to get the net disparity, and if there’s nothing to compare, you’d ignore or can’t determine in that scenario.

Vertical prism effects add when the prisms in the two eyes pull in opposite directions. A BU prism in one eye shifts that eye’s image upward, while a BD prism in the other eye shifts its image downward. The relative vertical offset between the two eyes’ images is the sum of the magnitudes, so you add the powers to find the vertical imbalance. For example, BU 4 PD with BD 2 PD yields a total vertical imbalance of 6 PD. If the prisms were in the same direction (both BU or both BD), you’d subtract to get the net disparity, and if there’s nothing to compare, you’d ignore or can’t determine in that scenario.