Which statement about paradoxical fixation disparity is correct?

• A. It occurs when the associated and disassociated phorias are in opposite directions
• B. It occurs when the associated and disassociated phorias are in the same directions
• C. It occurs when both phorias are zero
• D. It occurs only in exotropia

Answer: (A)

Paradoxical fixation disparity comes from how the eyes’ fusion system counteracts misalignment. When you measure a phoria by dissociating the eyes, you reveal the latent drift tendency: one eye would tend to turn in or out, giving an exo- or esophoria. But once the eyes are viewing together and fusion is in play, the vergence system works to align the eyes, and sometimes this fusional effort overcompensates, moving the small residual misalignment in the opposite direction of that latent drift. In other words, the direction of the fixation disparity during normal binocular viewing is opposite to the direction suggested by the disassociated phoria (and opposite to the direction implied by the associated phoria). This mismatch is what makes it “paradoxical.” If both phorias pointed the same way or were zero, you wouldn’t see this reversal, and the phenomenon isn’t exclusive to exotropia.