Which device is used to measure fixation disparity?

Which device is used to measure fixation disparity?

• A. Mallet unit
• B. Bernell lantern
• C. Wesson fixation card
• D. Maddox Rod

Answer: (C)

Fixation disparity is the small misalignment of the eyes that persists when a person tries to fuse two images. To measure it, you need a setup that dissociates the eyes just enough to reveal this tiny misalignment while fusion is still active. The Wesson fixation disparity card is designed for this purpose: it presents a binocularly viewed target with perceptual cues that the patient can judge as aligned or offset, and the examiner adjusts prisms until the perceived alignment is neutral. The amount of prism required provides a direct readout of the fixation disparity, giving a practical measure of binocular alignment under fusion. Other tools like the Maddox rod are useful for dissociating the eyes to assess phorias and direction of deviation, but they aren’t specialty instruments for obtaining a straightforward fixation disparity readout with a simple, patient-reported alignment cue. Thus, the Wesson fixation disparity card is the best fit for measuring fixation disparity in routine practice.

Fixation disparity is the small misalignment of the eyes that persists when a person tries to fuse two images. To measure it, you need a setup that dissociates the eyes just enough to reveal this tiny misalignment while fusion is still active. The Wesson fixation disparity card is designed for this purpose: it presents a binocularly viewed target with perceptual cues that the patient can judge as aligned or offset, and the examiner adjusts prisms until the perceived alignment is neutral. The amount of prism required provides a direct readout of the fixation disparity, giving a practical measure of binocular alignment under fusion.

Other tools like the Maddox rod are useful for dissociating the eyes to assess phorias and direction of deviation, but they aren’t specialty instruments for obtaining a straightforward fixation disparity readout with a simple, patient-reported alignment cue. Thus, the Wesson fixation disparity card is the best fit for measuring fixation disparity in routine practice.