Ocular Motility Examination — Cover Tests

Cover-Uncover Test — Tropia vs. Phoria

The cover-uncover test isolates one eye at a time and is the basic bedside tool for telling a manifest deviation from a latent one. The examiner covers one eye while watching the other. If the uncovered eye moves to take up fixation, a manifest deviation, a tropia, is present. The direction of that movement is diagnostic: an eye moving inward to fixate was deviated outward (exotropia), one moving outward was deviated inward (esotropia), and vertical movement points to a hypertropia or hypotropia.

What happens when the cover is removed is what actually separates a tropia from a phoria. A previously covered eye that shows a refixation movement on uncovering has a phoria: a latent deviation that drifted while the cover interrupted binocular fusion, then is held in check by fusional reserves once both eyes are open again. In a tropia the uncovered eye moves the moment its fellow is covered, because the deviation was already manifest with both eyes open.

Alternate Cover Test — Total Deviation

The alternate cover test (cross-cover test) switches an occluder rapidly and repeatedly between the two eyes so binocular fusion never gets a chance to reestablish. Because both eyes are never open together, it dissociates fusional control completely and reveals the total deviation: the manifest (tropic) and latent (phoric) components combined, rather than separated the way cover-uncover testing separates them. A deviation that only the alternate cover test uncovers, and the cover-uncover test does not, is by definition a pure phoria.

Prism Cover Test

The prism cover test (PCT) is the gold standard for quantifying strabismic angle. Prisms of increasing power go in front of the deviating eye, and alternate cover testing is repeated at each power until neither eye moves at all. That endpoint means the prism has fully neutralized the deviation. Its power, in prism diopters, equals the angle of strabismus.

A related variant, the simultaneous prism cover test, never breaks fusion at all. The neutralizing prism goes in front of the deviating eye while the occluder covers the fixing eye at the same instant, briefly enough that no dissociation occurs. A prism power that matches the deviation leaves the covered eye still when the occluder comes off. Because it isolates the manifest tropia without unmasking any coexisting phoria, this variant is preferred for measuring a small-angle deviation such as microtropia, where the alternate-cover-based prism test would instead reveal a combined tropia-plus-phoria angle.

Hirschberg and Krimsky Tests

Both tests estimate the strabismic angle from the position of the corneal light reflex rather than requiring the sustained fixation and cooperation that repeated cover testing demands. That makes them useful in infants, young children, and poorly cooperative patients generally.

The Hirschberg test reads deviation straight from how far the corneal light reflex sits off pupil center in the deviating eye. Each millimeter of displacement corresponds to roughly 15 prism diopters, about 7° of deviation. A reflex at the pupillary margin, roughly 2 mm off-center, corresponds to about 30∆; one at the limbus, 4–5 mm out, to roughly 60–75∆. That gives a fast bedside estimate before formal prism neutralization.

The Krimsky test modifies this for a deviating eye that cannot fixate steadily. Instead of measuring the reflex in the deviating eye directly, prisms go in front of the fixing (non-deviating) eye and increase in power until the corneal reflex centers in the strabismic eye. The examiner quantifies the deviation without ever needing the deviating eye itself to fixate.

Cardinal Positions of Gaze / EOM Testing

Extraocular motility is assessed across the six cardinal positions of gaze. Each position isolates the primary action of one yoke pair, a muscle from each eye working together to move both eyes the same direction: right gaze pairs the right lateral rectus with the left medial rectus, left gaze the left lateral rectus with the right medial rectus, up-right the right superior rectus with the left inferior oblique, up-left the left superior rectus with the right inferior oblique, down-right the right inferior rectus with the left superior oblique, and down-left the left inferior rectus with the right superior oblique. Testing all six lets the examiner localize weakness or restriction to a specific muscle instead of describing motility loosely as reduced.

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