orthopic fusion
Orthopic fusion is fusion of two laterally separated targets with the eyes underconverged. The visual axes cross behind the card that carries the targets.1 A typical case is a patient with an eso-deviation who uses the three-cats card to train divergent fusional reserves.1
The contrasting response is chiastopic fusion. The patient overconverges, and the visual axes cross in front of the card.1 In chiastopic fusion the right eye fixates the left target and the left eye fixates the right target.2 The name comes from the Greek letter chi, which resembles an X.1
Patients with esophoria use the three-cats card orthopically, fixating a distant object just over the top of the card held about 30 cm away.1,2 The card distance is then adjusted until the middle two pictures fuse.2 The esophoric version tends to be harder at first, and it helps to cut the card between the cats and start with them very close together.2 The cats often look very blurred at first, and the patient concentrates on holding fusion with underconvergence before working on clear vision. Moving the card out to about 40 cm can make clear vision easier.2
More detail
The divergence procedure with clear Eccentric Circle cards also fuses targets with the eyes aimed beyond them.3 The patient holds the cards at reading distance, looks between them at a distant object, and slides them together until the two central sets of circles fuse into one, leaving three sets.3 The smaller inner ring of the fused set then appears closer than the larger outer ring, giving the effect of a stool.3
Sources (6)
- Pickwell's Binocular Vision Anomalies, Appendices (glossary)
- Pickwell's Binocular Vision Anomalies, ch. 10
- Scheiman and Wick, Clinical Management of Binocular Vision, Appendices
- 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.
- Von Noorden, G.K. and Campos, E.C. 2002. Binocular vision and ocular motility: theory and management of strabismus. 6th ed. St. Louis: Mosby.
Reviewed October 2026