Fixation disparity

Fixation disparity exists when there is a small misalignment of the eyes when viewing with binocular vision. The misaligment may be vertical, horizontal or both. The misalignment (a few minutes of arc) is much smaller than that of strabismus, which prevents binocular vision, although it may reduce a patient's level of stereopsis. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near.

Measurement of fixation disparity
There are several methods to quantify fixation disparity. The Mallet card, the Bernell lantern slide, the Wessen Card and the Disparometer may be used. A patient's associated phoria is the amount of prism needed to reduce their fixation disparity to zero minutes of arc.

Reference

 * Eskridge, JB, Amos, JF, Bartlett, JD. Clinical procedures in Optometry. Lippincott Co. New York 1991.