Which of the following is NOT a device used to measure fixation disparity?

Which of the following is NOT a device used to measure fixation disparity?

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

Answer: (D)

Fixation disparity is the small misalignment that can occur between the eyes when both are fixating the same target. To measure this subtle misalignment, clinicians use devices that present a dissociated, easily reportable target and allow prisms or line/space alignment tasks to quantify how much misalignment remains under binocular viewing. The Mallet unit is designed specifically for fixation disparity testing; it uses a setup that presents slightly dissociated targets and paraxial prisms so you can gauge the exact amount of prism needed to bring the eyes back into alignment at the fixation target. The Bernell lantern functions in the same spirit, providing a simple fixation target under controlled dissociation, with prism adjustments revealing the small misalignment that constitutes fixation disparity. The Wesson fixation card offers a straightforward line-based target that patients compare or align, yielding a direct readout of fixation disparity through the observed misalignment of the lines seen by each eye. The Maddox Rod, on the other hand, is primarily used to assess phorias by dissociating the eyes with a special cylindrical lens that converts a point light into a line seen by one eye. This test is about latent misalignment (phoria) under dissociation, not about the small binocular misalignment at a shared fixation target. It isn’t designed to quantify fixation disparity in the way the others are. So, the device not used to measure fixation disparity is the Maddox Rod.

Fixation disparity is the small misalignment that can occur between the eyes when both are fixating the same target. To measure this subtle misalignment, clinicians use devices that present a dissociated, easily reportable target and allow prisms or line/space alignment tasks to quantify how much misalignment remains under binocular viewing.

The Mallet unit is designed specifically for fixation disparity testing; it uses a setup that presents slightly dissociated targets and paraxial prisms so you can gauge the exact amount of prism needed to bring the eyes back into alignment at the fixation target.

The Bernell lantern functions in the same spirit, providing a simple fixation target under controlled dissociation, with prism adjustments revealing the small misalignment that constitutes fixation disparity.

The Wesson fixation card offers a straightforward line-based target that patients compare or align, yielding a direct readout of fixation disparity through the observed misalignment of the lines seen by each eye.

The Maddox Rod, on the other hand, is primarily used to assess phorias by dissociating the eyes with a special cylindrical lens that converts a point light into a line seen by one eye. This test is about latent misalignment (phoria) under dissociation, not about the small binocular misalignment at a shared fixation target. It isn’t designed to quantify fixation disparity in the way the others are.

So, the device not used to measure fixation disparity is the Maddox Rod.