The cover test is the gold standard examination for objectively evaluating the presence, type, and degree of ocular misalignment (strabismus). It is widely used together with corneal light reflex and Brückner test for assessing binocular alignment 1). It is usually performed by ophthalmologists, orthoptists, or ophthalmic technicians.
The cover-uncover test is used to detect manifest strabismus (tropia), and the alternate cover test is used to measure total deviation including latent strabismus1).
The only items needed for the test are an occluder and a fixation target; no special equipment is required.
Occluder: Any light-blocking material can be used, not necessarily a dedicated instrument. Thick paper, the examiner’s palm, or thumb can be substituted. A translucent occluder allows observation of the covered eye and is suitable for evaluating conditions such as alternating hyperphoria. Glossy materials are unsuitable because they reflect the patient’s eye.
Fixation target: There are two types: light target and accommodative target. The light target uses a penlight; accommodation is less likely to be stimulated, making it less likely to induce esotropia, but corneal reflection can be observed. The accommodative target uses letters or numbers, allowing observation of eye position with controlled accommodation and convergence.
The test is performed both at near (accommodative target held at 33 cm) and at distance (fixating on a 6 m target)1). Adequate visual acuity and patient cooperation with fixation are required1). It is important to perform the test with the head in a correct position for all eye position tests; avoid testing with an abnormal head posture.
QWhat types of cover tests are there?
A
Broadly, there are three types: cover test (CT), cover-uncover test (CUT), and alternate cover test (ACT). Combining these with prisms (prism cover test: PCT, SPCT, APCT) quantifies the deviation. See the section “Diagnosis and Examination Methods” for details.
The cover test is performed when the following symptoms are present.
Diplopia: Perceived by adults and older children
Abnormal head posture: Compensatory head posture to maintain binocular vision as ocular torticollis
Closing one eye or photophobia: Seen in intermittent exotropia when going outdoors in bright light
Aversion reaction: In infants, turning the face away or pushing away the hand when one eye is occluded. This suggests reduced vision in the eye that does not resist occlusion.
The magnitude of deviation is quantified in prism diopters (PD).
QWhat is the difference between manifest strabismus (tropia) and latent strabismus (phoria)?
A
Manifest strabismus is a condition in which the eye deviation persists even when fusion is active, and it is detected by the cover test. Latent strabismus is normally kept aligned by fusion but appears when fusion is disrupted by covering one eye. The alternate cover test measures the total deviation including both components.
The cover test is generally performed in the following order: cover test → cover-uncover test → alternate cover test. Before testing, it is important to observe the patient with both eyes open while fixating on a target to determine whether the condition is phoria or tropia, or which is more prevalent.
This test detects the maximum eye misalignment (total deviation) when fusion is eliminated. It measures the total deviation including manifest and latent strabismus1).
Cover each eye for 2 seconds, and quickly alternate the cover repeatedly.
Observe the movement of the eye after the cover is removed.
To thoroughly eliminate fusion, repeat the alternate covering to prevent binocular vision.
If neither eye moves during the alternate cover test, it can be judged as orthophoria.
The faster the eye recovers when switching the cover, the better the control state.
This test quantifies the strabismic angle in prism diopters (PD) by combining a cover test with prisms. The prism bar is held parallel to the frontal plane (frontal plane position), and the prism power is increased repeatedly until the deviation is neutralized.
Prism cover test (PCT): A method combining the cover-uncover test with prisms. Quantifies the manifest strabismic angle.
Simultaneous prism cover test (SPCT): A method in which a prism is placed over the deviating eye while simultaneously covering the fixating eye. Quantifies manifest deviation under conditions close to everyday vision.
Alternate prism cover test (APCT): A method combining alternate cover test with prisms. Quantifies the total strabismic angle (manifest + latent). The relationship APCT = SPCT + latent deviation holds.
Horizontal and vertical prisms can be used together, but two horizontal prisms cannot be stacked. If the deviation is large, prisms can be split between both eyes. The larger the angle, the greater the error caused by prism placement, so caution is needed. Note that cyclotropia cannot be quantified using prisms.
The following are representative ocular alignment tests used in conjunction with the cover test.
Corneal light reflex test (Hirschberg test): A simple method to estimate ocular alignment by shining a penlight on both eyes and observing the position of the corneal light reflex. A reflex at the pupillary margin indicates 15° (30 PD), on the iris 30°, and at the corneal limbus 45°. A 1 mm displacement from the center of the pupil corresponds to 12.7°. This test can be performed even in infants, but it cannot rule out ocular misalignment on its own.
Krimsky prism test: A prism is placed in front of the fixating eye, and the prism power that centers the corneal light reflex of the non-fixating eye in the pupil is determined. Used in infants who cannot cooperate or when the strabismic eye cannot fixate.
4-prism base-out test: A test to detect the presence of a small central suppression scotoma (2–4 degrees).
Differentiation from pseudostrabismus: In East Asian infants, the nasal bulbar conjunctiva is less exposed due to epicanthal folds, making it difficult to differentiate esotropia from pseudostrabismus using only the corneal reflex test. Even if pseudostrabismus is suspected based on facial appearance, a cover test must be performed to evaluate the aversion response.
Differentiation of pseudoptosis: When unilateral ptosis is suspected, performing an alternate cover test can differentiate pseudoptosis associated with hypotropia.
Evaluation of paralytic strabismus: In paralytic strabismus, the deviation angle is larger when fixating with the paretic eye (secondary deviation) than with the healthy eye (primary deviation). In a case of superior oblique palsy, the cover test detected 20 PD left hypotropia at distance and intermittent 15 PD left hypotropia at near 2).
Aversion response in infants: If an infant shows aversion to occlusion of one eye (aversion reflex), the vision in the eye that is not occluded may be reduced. In infantile sensory strabismus, conditions requiring early diagnosis such as retinoblastoma may be involved, so caution is needed.
QWhich should be performed first: the cover test or the alternate cover test?
A
Perform in the order: cover test → cover-uncover test → alternate cover test. Since the alternate cover test is the most dissociative test that eliminates fusion, performing it first would disrupt fusion and affect subsequent test results.
QWhat should be noted when esotropia is suspected in East Asian infants?
A
In East Asian infants, the nasal bulbar conjunctiva is less exposed due to epicanthal folds, making it difficult to differentiate esotropia from pseudostrabismus using only the corneal reflex test. Always perform a cover test to evaluate eye movements and the aversion response.
