19 A child with right esotropia is observed during attempted binocular fixation prior to the cover test.

20 The right eye is covered and the left eye observed for movement.There is no change in position.

Cover test for strabismus

Implicit to cover testing is the ability of each eye, in turn, to be capable of central (foveal) fixation when the fellow eye is covered. If organic disease (cataracts, cloudy media, and so on) or functional conditions (eccentric fixation) prevent central fixation with either eye, cover testing may be invalidated.

Placement of the occluder should be minimally traumatic to the child. The traditional black paddle is handy but, if rejected, the examiner’s hand or thumb dropped from above may provide a more familiar, less threatening cover.

Fixation targets should be used to provide an accommodative stimulus. Vision charts at 3 m (10 feet) and at 0.33 m (13 inches) work

well in older preschool and school-age children. Small interesting pictures are useful for obtaining near fixation in smaller children. Younger preschool children can be offered toys, movies, and the like, with accommodative detail and story content to enhance interest.

The cover test is performed by having the patient look at an accommodative target under binocular viewing conditions. The examiner places an occluder over one eye, while watching the fellow eye for a shift in fixation (19, 20). A shift is evidence that the uncovered eye was not regarding the target with its fovea while both eyes were viewing. This deviation is called heterotropia or, if the direction is specified, exotropia, esotropia, or hypertropia.

Heterotropia is a manifest deviation because it exists (is manifest) under normal or casual seeing circumstances, i.e., with both eyes viewing. Because the patient begins the test with both eyes viewing, the cover test examines a binocular circumstance.

If no shift occurs, heterotropia may still exist. If the occluder was placed in front of the deviating eye, the fellow eye would already be fixed on the target, and no shift would be expected. Obviously in most cases the examiner already knows which eye is deviating because one eye is directed at the object of regard and the other is not. However, in small-angle deviations this is often not obvious, and it is sometimes hard to be sure that the child is regarding the object intended.

To complete the test, the patient must be returned to binocular viewing for at least several seconds so that fusion (binocular vision) can be accomplished if this potential exists. The fellow eye is covered in the same manner as the first (21). The sequence is: (1) cover one eye, observing the fellow eye; (2) uncover that eye for a few seconds; and (3) cover the second eye while observing the first eye. The sequence should be repeated to make sure that a subtle, rapid switch in fixation did not take place unnoticed during the binocular interval. Many strabismus patients can readily alternate fixation, and if a switch occurs during the test, the examiner might be placing the occluder before the deviating eye each time.

ALTERNATE COVER TEST

The alternate cover test is performed by moving the occluder directly from one eye to the other without allowing an interval for binocular viewing. Fusion is suspended throughout the test. If no ocular shift occurs as the occluder is moved directly from one eye to the other, the eyes are truly aligned, i.e. orthophoric. Even with fusion suspended, the foveas are in the position to regard the target without having to shift. However, if a shift does occur, a deviation exists. If the deviation is corrected by fusion, i.e. if the deviating eye moves into alignment under binocular conditions, the deviation is said to be latent and is called a heterophoria (or phoria, for short). It should be apparent that the alternate cover test does not diagnose a phoria by itself but depends on the findings of the cover test.

ALTERNATIVE METHODS OF DETECTING STRABISMUS

When cover testing is impossible because of organic disease, eccentric fixation, or lack of cooperation, other estimates of ocular deviations can be made. In infants and exceptionally uncooperative patients, Hirschberg estimates may have to be done. A light source is directed at both eyes. The number of millimeters that the light reflex is decentered from the pupillary axis is approximated. This measurement can then be used to give an estimate of the magnitude of the misalignment that is present.

Light reflex testing

When performing light reflex testing, it is imperative to have the light in the same line as the examiner’s viewing eye. In fact, it is a good idea to hold the muscle light touching the examiner’s cheek as a reminder and to frequently repeat the measurement.

Angle kappa is the angle formed by the pupillary axis and the visual axis. The pupillary axis is a line passing through the center of the pupil perpendicular to the cornea. The optical axis is the line connecting the optical centers of the cornea and the lens. The visual axis is the line of sight, connecting the fovea and the fixation point. The angle kappa is formed at the intersection of the pupillary and visual axes at the center of the entrance pupil. When the optical axis and the visual axis do not coincide, angle kappa is present. Clinically this erroneously demonstrates a strabismus with a corneal light reflex test. In general, a positive angle kappa is found in most children. This means that the corneal light reflex is not centered but is located slightly nasal to the center of the pupil. A large positive angle kappa simulates an exotropia. Of course barring eccentric fixation, a cover test will distinguish a positive angle kappa from a manifest exotropia.6

Versions or binocular eye movement should be checked in the nine diagnostic or cardinal positions of gaze. Examination of the oblique positions can be done by gross observation as the patient follows a near target into these positions. If versions are not full, ductions (single eye movements) should be tested in all fields of action of the individual eye. Recognition and quantification of duction

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22 Example of the color plates used in Ishihara testing.

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23, 24 Child wearing polarized lenses attempts to pick up the stereo image of a fly.

deficits are important in diagnosing paretic and restrictive extraocular muscle disorders.

It is not uncommon for patients with congenital esotropia to have limited abduction. Physicians other than ophthalmologists frequently interpret this as sixth cranial nerve paralysis, even to the point of ordering unnecessary radiologic procedures. Patching of one and then the other eye usually results in improvement of ductions within hours or days at most. Even without patching, ductions improve with age in these patients. If the child does not allow manual covering of each eye, occluder patches will frequently work. Noisy targets (such as jangling keys) held at close range work best to produce fixation and following by the child.

Color vision testing

The H-R-R pseudoisochromatic plates are an excellent test series, since they examine for yellow—blue as well as red—green defects and provide a rough quantitative capability. The directions enclosed with this test are explicit and will not be reviewed here. The test is dependent on the child knowing numbers (22). There is a form of the Ishihara color test available that utilizes the shapes of a circle and square in place of numbers. These tests do not discriminate anopes (total color defectives) from anomalies (partial color defectives) but are useful for screening.

Assessment of stereoacuity

Stereoacuity is probably the most fundamental sensory test, widely used in vision screening. Stereoacuity of 60 seconds of arc or better correlates with bifoveal fusion (the ability to use both foveas simultaneously). A score below 67 seconds virtually proves bifoveal fixation that approaches the highest level of stereoacuity.7

The Titmus (or stereo Fly) test is used frequently (23, 24). The test is sensitive to light and a better score can be obtained by assuring good illumination. The test is also sensitive to the distance maintained from the eyes, which must be respected if accuracy is desired. Allowing the child to touch the test will deface