associated phoria
The associated phoria is the amount of prism required to reduce a fixation disparity to zero.1 A fixation disparity is the small misalignment of the eyes that occurs while single binocular vision is maintained, so the associated phoria assesses vergence under binocular conditions.2
The dissociated phoria comes from tests that break fusion completely, such as the cover test or Maddox rod. Heterophoria implies dissociation, so the International Standards Organization (1995) proposed the term aligning prism in place of associated heterophoria.3 For exophoric patients the associated phoria is generally significantly smaller than the dissociated phoria, and for esophoric patients it is often greater.1
At distance the associated phoria is measured with a Mallett unit, an American Optical vectographic chart, or a Bernell lantern. At near it is measured with the American Optical near vectographic card, the Borish card, or the near Mallett unit, and prism neutralizes any misalignment the patient reports.1 The vertical associated phoria is the method of choice for prescribing vertical prism. The lateral associated phoria used alone tends to overestimate the prism needed for esodeviations.1
More detail
The Mallett unit does not measure the fixation disparity itself, only the prism that restores the monocular markers to alignment with the X.3 An aligning prism of 1 prism diopter or more in pre-presbyopes, or 2 prism diopters or more in presbyopes, is likely to be associated with symptoms.3 In one study, prism glasses that eliminated the associated phoria were preferred by 73 percent of symptomatic exophoric patients and 90 percent of symptomatic esophoric patients, while 86 percent of asymptomatic patients rejected them.2
Sources (7)
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 15
- Care of the Patient with Accommodative and Vergence Dysfunction (AOA Optometric Clinical Practice Guideline)
- Pickwell's Binocular Vision Anomalies, ch. 4
- Frantz, K.A. (2020) 'Prescribing Prism with Prowess', Review of Optometry. Available at: https://www.reviewofoptometry.com/article/prescribing-prism-with-prowess (Accessed: 4 May 2024).
- Karania, R. and Evans, B.J.W. (2006) 'The Mallett Fixation Disparity Test: influence of test instructions and relationship with symptoms', Ophthalmic and Physiological Optics, 26(5), pp. 507–522.
- Mallet, R.F.J. (1964) 'The investigation of heterophoria at near and a new fixation disparity technique', Optician, 148, pp. 547–551.
- Press, L.J. (2020) 'The Art of Prescribing Low Amounts of Prism', Optometric Clinical Practice, 2(2). Available at: https://athenaeum.uiw.edu/optometric_clinical_practice/vol2/iss2/2 (Accessed: 4 May 2024). secondary
Reviewed October 2026