Sheard’s criterion
Sheard’s criterion holds that a heterophoria is likely to stay comfortable when the opposing fusional reserve, measured to the blur point, is at least twice the phoria.1,2 For exophoria the opposing reserve is base-out positive fusional vergence, and for esophoria it is base-in negative fusional vergence.2
A patient with 10 prism diopters of exophoria therefore needs at least 20 prism diopters of base-out to blur, and a finding of 8 prism diopters fails the criterion.3 When the criterion is not met, prism can be prescribed.2 A patient with 10 prism diopters of exophoria and 10 prism diopters of base-out to blur would need about 3 prism diopters base-in.2
Sheard’s criterion is an intrasubject method, comparing the patient's own fusional reserve with that patient's own phoria.1 Sheard offered several versions of the rule, with the required opposing reserve ranging from two to four times the phoria.1 The criterion can be applied to any heterophoria, and research suggests it works best with exophores.2
More detail
Among 100 patients who failed Sheard’s criterion at near, 83% had symptoms.1 A double-masked study supported prescribing prism by the criterion for distance esophores and near presbyopic exophores, but not for other near exophores or near esophores.1 A later randomized controlled trial did not support prescribing prisms based on Sheard’s criterion.1
Sources (9)
- Pickwell's Binocular Vision Anomalies, ch. 4
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 3
- AOA Optometric Clinical Practice Guideline, Care of the Patient with Accommodative and Vergence Dysfunction
- Bade, A., Boas, M., Gallaway, M., Mitchell, G.L., Scheiman, M., Kulp, M.T., Cotter, S. and Rouse, M. 2013. Relationship between clinical signs and symptoms of convergence insufficiency. Optometry and Vision Science, 90(9), pp.988-995.
- Evans, B.J. 2007. Pickwell's binocular vision anomalies. 5th ed. Edinburgh: Butterworth-Heinemann Elsevier.
- Melville, A.C. and Firth, A.Y. 2002. Is there a relationship between prism fusion range and vergence facility? British Orthoptic Journal, 59, pp.38-44.
- Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.
- Scheiman, M. and Wick, B. 2014. Clinical management of binocular vision: heterophoric, accommodative, and eye movement disorders. 4th ed. Philadelphia: Lippincott Williams & Wilkins.
- Sheedy, J.E. and Saladin, J.J. 1983. Validity of diagnostic criteria and case analysis in binocular vision disorders. American Journal of Optometry and Physiological Optics, 60(7), pp.527-536.
Reviewed October 2026