Under Sheard's criterion, an esophoric deviation uses which fusional vergence measure at blur/break point?

• A. BI blur/break point
• B. BO blur/break point
• C. BI and BO blur/break points
• D. None of the above

Answer: (A)

Fusional vergence reserves must be large enough to counter the phoria without causing symptoms. For an esophoric deviation, the inward drift of the eyes needs to be offset by divergent vergence, which is base-in (BI) vergence. When you evaluate at the blur or break point, you’re looking at how much BI (divergence) the patient can still produce before the image breaks up. According to Sheard’s criterion, that divergent reserve should be at least twice the phoria magnitude for comfortable single vision. So the measure you use for an esophoria is the base-in (BI) blur/break point. If it were an exophoria, you’d look at the base-out (BO) blur/break point instead.