To neutralize an exophoria on the cover test, which prism base direction is used?

To neutralize an exophoria on the cover test, which prism base direction is used?

• A. Base In
• B. Base Out
• C. Base Up
• D. Base Down

Answer: (A)

Exophoria is a tendency for the eyes to drift outward when fusion is disrupted. To neutralize this on the cover test, you want a prismatic effect that makes the eyes converge a bit. A base-in prism does exactly that: its light is bent toward the nose, producing a converging effect that counteracts the outward drift and helps the patient maintain single binocular vision. Using a base-out prism would push the eyes further apart, which won’t neutralize the exophoria, and base-up or base-down prisms address vertical misalignments, not horizontal exophoria.

Exophoria is a tendency for the eyes to drift outward when fusion is disrupted. To neutralize this on the cover test, you want a prismatic effect that makes the eyes converge a bit. A base-in prism does exactly that: its light is bent toward the nose, producing a converging effect that counteracts the outward drift and helps the patient maintain single binocular vision. Using a base-out prism would push the eyes further apart, which won’t neutralize the exophoria, and base-up or base-down prisms address vertical misalignments, not horizontal exophoria.