A microstrabismus is defined as a deviation smaller than which value that is not visible on cover test?

• A. < 5 pd
• B. < 10 pd
• C. < 15 pd
• D. < 20 pd

Answer: (B)

Microstrabismus describes a very small misalignment that isn’t evident on a standard cover test. The practical boundary used is a deviation under 10 prism diopters. Because it’s so small, a routine cover test often looks normal, even though a tiny misalignment exists. To reveal it, clinicians rely on more sensitive dissociating tests, such as an alternate cover test with prisms or other prism-based measurements.

Deviations around or above 15–20 PD are typically detectable with the regular cover test, so they wouldn’t be labeled microstrabismus. While some sources might note even smaller thresholds (like under 5 PD) in other contexts, the convention reflected in this question is that microstrabismus is a deviation smaller than 10 PD.