Which measurement is used to assess esophoric deviations under Sheard's criterion?

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

Answer: (A)

Sheard's criterion looks at how much fusional vergence a person can pull together to neutralize a phoria, and it uses the direction opposite the phoria to judge the necessary reserve. For an esophoric deviation, the needed compensatory vergence is outward (divergence). So you test divergent vergence with base-in prisms and note how far you can go before the patient loses single vision—the blur and break points. The base-in blur/break point reflects the divergent fusional vergence reserve the patient can muster. If that BI value is large enough—typically compared to the phoria magnitude (about twice the phoria)—fusion should feel comfortable. Thus, the measurement used here is the base-in blur/break point.