If vertical prism is unacceptable, what other condition must be true?

• A. There is >1 mm difference in the height of the PRPs in addition to vertical prism >0.33 PD
• B. There is >2.5 mm difference
• C. PRP height difference must be less than 1 mm
• D. There is no other condition

Answer: (B)

When a vertical prism is not acceptable, another factor to consider is how far the prism reference points (PRPs) are vertically offset between the two eyes. A substantial difference in PRP heights helps offset the unwanted vertical prismatic effect and maintains comfortable binocular alignment. The practical threshold used in this context is that the PRP height difference should exceed 2.5 mm. If the PRP heights are not offset by this amount, the vertical prism can lead to problematic image separation or diplopia for the patient.

So, the best choice states there is more than a 2.5 mm difference in PRP heights. The other options don’t fit because smaller height differences (like 1 mm or less, or a claim of no other condition) wouldn’t adequately compensate for the vertical prism, and asserting no other condition ignores the compensatory role of PRP height disparity.