Which fixation disparity curve is seen in an eso deviation?

• A. Type 1
• B. Type 2
• C. Type 3
• D. Type 4

Answer: (B)

Fixation disparity curves show how a small, residual misalignment at fixation changes as you add horizontal prism. They reveal how the vergence system compensates for a phoria. In an eso deviation, the way the disparity responds to prism fits a pattern clinicians label as Type 2. This type is characterized by a monotonic, relatively predictable change in fixation disparity as prism is increased, with the misalignment moving toward zero (and crossing it at a modest prism value) in a way that matches the typical fusional vergence behavior seen with inward-turning eyes. Other patterns reflect different underlying responses of the vergence system, which don’t align with the way an eso deviation behaves.