Binocular Vision and Ocular Motility Testing
Cover testing
The cover test is the backbone of strabismus evaluation. With the patient fixating a distance target, cover one eye and watch the uncovered eye: movement from a deviated position toward fixation indicates a tropia. The alternate cover test breaks fusion and reveals the total deviation, phoria plus tropia. Neutralize with prisms placed before the deviated eye, base opposite the deviation, increasing until movement disappears. Record distance and near, with and without correction, in prism diopters with the direction (eso, exo, hyper, hypo).
Adjunct tests
Hirschberg estimates deviation from the corneal light reflex, roughly 1 mm equals 15 prism diopters (about 7 degrees). Krimsky combines prisms with the Hirschberg. Versions test both eyes together and ductions test one eye; an incomitant deviation suggests paresis or restriction. Near point of convergence with an accommodative target: normal break under 5 cm and recovery under 7 cm. Stereoacuity (Randot, Titmus fly) confirms binocularity. Worth 4-dot and Maddox rod assess suppression and phorias.
Worked example
Alternate cover testing at near shows the uncovered eye swinging inward to fixate each time, and neutralization requires 10 PD base-out before the right eye. That is a 10 PD esophoria at near. If no deviation appears at distance, document both values; the difference reflects the accommodative convergence relationship.
Traps
Using a non-accommodative target for near testing. Testing only at distance. Holding the prism tilted rather than parallel to the face. Neutralizing before deciding whether the deviation is a phoria or a tropia.