fixation disparity
Fixation disparity is a very small misalignment of the eyes during binocular viewing that is too small to cause diplopia.1 Each fovea corresponds to a small area around the fovea of the other eye, called Panum's area.1 An eye can therefore deviate slightly before the image leaves Panum's area and doubling appears.1 The deviation can occur in one eye or in both, and it is too small to see on the cover test.1
In fixation disparity, the images of a fixated object fall on points that do not exactly correspond but remain within Panum's area, and the object is still seen singly.2 Fixation disparity is very likely to occur under binocular stress, as in decompensated heterophoria, and tests that detect it help assess decompensation.1 Steep fixation disparity curves, large associated phorias and large fixation disparities are found more often in symptomatic patients.2
The clinical meaning of fixation disparity is still debated.3 One major text holds that the evidence does not show fixation disparity to be more than a physiologic variant of normal binocular vision.3
More detail
Clinically, fixation disparity is plotted against base-in and base-out prism as a forced vergence fixation disparity curve.2 An increase in base-out prism usually increases exo fixation disparity, and an increase in base-in prism increases eso fixation disparity.2 Asymptomatic patients usually have type I curves, while type II and type III curves go with large dissociated esophorias and exophorias and type IV with unstable binocularity.2
Sources (6)
- Pickwell's Binocular Vision Anomalies, ch. 4
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 15
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 9
- Rutstein, R., Daum, K. (1997). Anomalies of Binocular Vision: Diagnosis & Management, Mosby.
- Mallett, R. F. (1988). The investigation and clinical significance of the Mallett fixation disparity test. Ophthalmic and Physiological Optics.
- Sheedy, J. E., & Saladin, J. J. (1978). Association of symptoms with measures of oculomotor deficiencies. American Journal of Optometry and Physiological Optics.
Reviewed October 2026