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fusional convergence

Fusional convergence is the reflex convergence that maintains binocular single vision by keeping similar images on corresponding retinal areas of the two eyes.1 Bitemporal retinal image disparity initiates fusional convergence.1 Fusional convergence is one of four components of reflex convergence, along with tonic, proximal and accommodative convergence.1

Fusional convergence helps control an exophoria, and fusional divergence helps control an esophoria.1 Fatigue or illness can weaken the fusional vergence mechanism and turn a phoria into a tropia.1

Clinicians measure fusional convergence as the positive fusional reserve, the largest amount of convergence the eyes can produce and still keep binocular single vision.2 Accommodation normally changes by a linked amount as vergence changes.2 The relative fusional reserve is the amount of vergence change possible without any change in accommodation.2

More detail

During measurement of convergence amplitudes the target stays single and clear up to a point, then blurs but stays single, and finally doubles at the break point.3 The blur point marks the limit at which accommodation can still clear the target despite the added convergence.3 Vergence movements are slow, and the prism must be increased slowly.3 Changing it too quickly gives falsely low amplitudes.3

Normal convergence amplitudes are about 15 to 20 prism diopters at distance and 25 prism diopters at near.1 Orthoptic exercises can improve the amplitude, particularly near fusional convergence for the relief of convergence insufficiency.1

Sources (8)
  1. Kanski, Clinical Ophthalmology (2024), ch. 18
  2. Pickwell's Binocular Vision Anomalies, Glossary
  3. von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 12
  4. Caloroso, E.E. and Rouse, M.W. (1993) Clinical management of strabismus. Boston: Butterworth-Heinemann.
  5. Noorden, G.K. von and Campos, E.C. (2002) Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th edn. St. Louis: Mosby.
  6. Rowe, F.J. (2012) Clinical Orthoptics. 3rd edn. Chichester, West Sussex: Wiley-Blackwell.
  7. Scheiman, M. and Wick, B. (2020) Clinical management of binocular vision: heterophoric, accommodative, and eye movement disorders. 5th edn. Philadelphia: Wolters Kluwer.
  8. Wright, K.W. and Strube, Y.N.J. (2012) Pediatric Ophthalmology and Strabismus. 3rd edn. Oxford: Oxford University Press.

Reviewed October 2026

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