One way of regarding fixation disparity is as a very small ocular misalignment, so small in fact that it is impossible to detect using conventional cover testing and the subject retains good levels of fusion and stereopsis.
It has been described it as an ocular misalignment usually less than 10 minutes of arc or approximately 0.25 prism dioptres (∆) present during fusion.
Fixation disparity can be convergent (eso-disparity or eso-slip), divergent (exo-disparity or exo-slip), vertical (hyper-disparity or hyper-slip in one eye) or torsional (cyclo-disparity or cyclo-slip). Fixation disparity is present when the fusional reserves opposing a heterophoria are inadequate. Given this scenario symptoms are very likely to result since the presence of fixation disparity is an indication that any heterophoria present is causing ‘stress’.
It is important to remember that this situation may occur irrespective of the size of the heterophoria, in fact the author has seen symptomatic patients presenting with a heterophoria that was too small to detect with cover testing which was associated with marked fixation disparity.
Furthermore, a patient presenting with medium or large heterophoria will not necessarily exhibit any fixation disparity or be suffering from symptoms. The term uncompensated has been coined to describe that part of the heterophoria that was not opposed by adequate fusional reserves and led to the presence of fixation disparity. This is not synonymous with the term decompensated although both an uncompensated and a decompensated heterophoria are likely to lead to symptoms or avoidance strategies.
Fixation disparity may be present in both eyes, resulting in slip in each eye, or it may be present in one eye only (the non-dominant eye), when the visual direction of the fused images will be that of the other (dominant) eye. The uncompensated component of the heterophoria can be removed with optimum refractive correction, modification of the subjective refraction result using spherical lenses or prisms or orthoptic exercises and any remaining imbalance between the heterophoria and the opposing fusional reserves will not result in fixation disparity.