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Fixation disparity is a tendency of the eyes to drift in the direction of the heterophoria. While the heterophoria refers to a fusion-free vergence state, the fixation disparity refers to a small misalignment of the visual axes when both eyes are open in an observer with normal fusion and binocular vision. [1]
Heterophoria is the misalignment of the visual axis such that one or both eyes are not properly fixated to an object of interest. When the visual axis is misaligned in such a way, it is corrected by the fusional vergence system.
Strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. [2] The eye that is pointed at an object can alternate. [3] The condition may be present occasionally or constantly. [3] If present during a large part of childhood, it may result in amblyopia, or lazy eyes, and loss of depth perception. [3] If onset is during adulthood, it is ...
The prism cover test ( PCT) is an objective measurement and the gold standard in measuring strabismus, i.e. ocular misalignment, or a deviation of the eye. [1] It is used by ophthalmologists and orthoptists in order to measure the vertical and horizontal deviation and includes both manifest and latent components. [1]
The cover test is used to determine both the type of ocular deviation and measure the amount of deviation. The two primary types of ocular deviations are the tropia and the phoria. A tropia is a misalignment of the two eyes when a patient is looking with both eyes uncovered. A phoria (or latent deviation) only appears when binocular viewing is broken and the two eyes are no longer looking at ...
Prism lenses reduce vertical fusional demands by allowing the eyes to rest in their vertically misaligned state. When they are removed the patient may experience vertical diplopia they find hard to resolve due to the rested state of their eyes.