5.2 Cover test & strabismus measurement (prism cover test)

Cover Testing and Strabismus Measurement

Cover tests are the clinical gold standard for detecting and classifying strabismus. The patient fixates an accommodative target while the examiner occludes and uncovers each eye and watches for movement. Two distinct tests answer two different questions.

Cover/uncover test — detecting a tropia

The cover/uncover test detects a tropia, a manifest deviation present under binocular (both-eyes-open) viewing. The examiner covers one eye and watches the OTHER, uncovered eye:

• If the uncovered eye moves to take up fixation, a tropia is present in that eye.
• The direction of the refixation movement names the deviation: an eye moving from a temporal (out) position inward indicates esotropia; moving from a nasal (in) position outward indicates exotropia; a downward move to pick up fixation indicates hypertropia.

Next, watch the eye that was just covered as the cover is REMOVED. Movement of that eye as it is uncovered indicates a phoria (latent deviation): the eye drifted under cover and refixates once binocular fusion is restored.

Alternate cover test — detecting the total deviation

The alternate cover test fully dissociates the eyes by rapidly moving the occluder back and forth so the patient never fuses. It reveals the total deviation = tropia + phoria but cannot by itself separate the manifest from the latent portion. This is why the cover/uncover test is performed first (to establish whether a tropia exists), followed by the alternate cover test (to measure the full size).

Prism-and-alternate-cover test (PACT) — quantifying the angle

To measure the deviation, a prism is held before one eye with its apex pointed toward the deviation: base-out for esotropia, base-in for exotropia, base-down over a hypertropic eye. While alternately covering, the examiner increases prism power until the refixation movement is neutralized — no shift is seen when switching the cover. The prism power at neutralization equals the angle in prism diopters (Δ); one prism diopter deflects light 1 cm at 1 meter.

Deviations are named by type and direction:

• Esotropia (ET) / esophoria (E) — inward.
• Exotropia (XT) / exophoria (X) — outward.
• Hypertropia / hypotropia — vertical, named for the higher or lower eye.

Comitant versus incomitant

• A comitant (concomitant) deviation measures the SAME in all directions of gaze; the angle does not change with gaze position. Most childhood strabismus is comitant.
• An incomitant deviation changes with gaze direction and typically signals a paretic or restrictive muscle. A sixth-nerve palsy, for example, produces an esotropia that worsens in the field of the weak lateral rectus. Incomitance must be measured with prisms in the affected gaze positions.

Corneal light reflex tests

When a patient cannot cooperate with cover testing (infants, poor fixation, dense amblyopia), corneal light reflex tests estimate the angle:

• Hirschberg test: a penlight is shone at the eyes and the reflex position on each cornea is compared. Each 1 mm of decentration ≈ 7° (≈ 15Δ). A temporally displaced reflex indicates esotropia; a nasally displaced reflex indicates exotropia.
• Krimsky test: prisms are placed before the FIXATING eye and increased until the corneal reflexes are symmetric; the prism power that centers the reflexes estimates the angle. Krimsky is more quantitative than Hirschberg for a non-fixating or amblyopic eye.

Controlling accommodation and testing at distance and near

Accurate cover testing requires steady fixation on an accommodative target — a letter appropriate to the patient's acuity, NOT a bare muscle light, because a blank light lets accommodation and its linked convergence fluctuate and distort the angle. Measure at both distance (6 m / 20 ft) and near (about 33–40 cm), since many deviations differ between the two. An esotropia much larger at near than distance suggests a high AC/A ratio (accommodative component), while an intermittent exotropia is often larger at distance. If the patient wears a refractive correction, testing is done with the glasses in place, because uncorrected hyperopia can drive an accommodative esotropia.

Simultaneous prism cover test (SPCT)

In a patient with a manifest tropia who also fuses part of the deviation, the standard prism-and-alternate-cover test measures the total angle, which can overstate what the patient shows with both eyes open. The simultaneous prism cover test places the neutralizing prism before the deviating eye at the SAME moment the fixating eye is covered, capturing only the manifest (tropic) portion. Comparing the SPCT with the full prism-and-alternate-cover measurement helps the surgeon plan how much deviation to correct.

Angle kappa and pseudostrabismus

The visual axis and pupillary axis do not perfectly coincide; the angle between them is angle kappa. A large positive kappa throws the reflex slightly nasal in both eyes and can mimic exotropia even in straight eyes. Epicanthal folds that cover the nasal sclera create pseudoesotropia in infants — the eyes look crossed, but the Hirschberg reflexes are symmetric and the cover test shows no movement. Recognizing pseudostrabismus prevents needless referral; the cover test remains the arbiter.