YAMOUT OPTICAL CENTER

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Strabismus Testing Techniques

Strabismus exists when both visual axes do not intersect the point of visual attention. The visual axis can be regarded as a line joining the object or fixation point, the center of the pupil, and the fovea centralis. Strabismus is convergent when one visual axis is truned medially (inward, towards the nose). It is divergent if the visual axis is turned laterally (outward, towards the temple). The direction of vergence is particularly important because convergence of the visual axes must occur in order to obtain binocular fusion. This is especially true for close work, as the eyes must converge inward as near objects move closer to the face. Likewise, the eyes must diverge outward as objects move farther away. Vergence deficiencies, thus, can interfere with proper stereoscopic vision (depth perception) and lead to diplopia (double-vision). In children, vergence deficiencies can result in amblyopia (lazy-eye). If not caught early, amblyopia can lead to permanent loss of vision in one or both eyes affected by the deviation.

Cover Test

The cover/uncover test is used to detect eye misalignment. A tropia can be detected as you cover one eye and the strabismic eye then moves to pick up fixation. A phoria can be detected as you remove the cover from one eye and watch it move to pick up fixation, having been strabismic only when covered.

Technique

Cover testing should be performed with a small target to control accommodation. The eye should be occluded for a minimum of 2 seconds to elicit any existing deviation. During unilateral testing, the clinician should pay careful attention to the movement of the fellow eye and, upon alternate cover testing, to the movement of the uncovered eye. Both the extent of the deviation and the quality of fusion should be noted. Any significant deviation seen upon alternate cover testing should be neutralized with prisms. When the patient has poor fixation, a muscle light (penlight or transilluminator) can be substituted for an accommodative target. A child's attention span is short; use lots of toys. For older children a plastic stick with pictures or letters is especially helpful in controlling fixation.

The cover test for tropia works as follows: the clinician tells the patient to stare at an object (the fixation target), then covers and uncovers first one eye, then the other eye, with a paddle. If both eyes are looking directly at the object (no strabismus), neither eye will move when the other is covered. If one of the eyes is turned (strabismus), it will move to look at the object when the other eye is covered.

1. Hold the toy or object 1-2 feet in front of the patient at eye level just in front of your face.

2. Quickly cover the patient's left eye with your thumb or paddle, resting your hand, if tolerated, on the patient's head. You may use an occluder, or a piece of paper to cover the eye, but often the patient's attention is directed to the occluder rather than the toy or object of regard.

3. As you cover the left eye, the right eye is the one to watch. Does it move to pick up fixation on the toy? If the right eye moves outward then the right eye was originally directed inward. This is right esotropia .If the uncovered eye moves inward, you have detected an exotropia. Watch for small, or "flick", movements of the eye. A small angle strabismus is often not cosmetically visible and is difficult to detect with the corneal light reflex test. This condition, microstrabismus" or "monofixation syndrome" can be associated with a mild to moderate amblyopia.

4. Next, uncover the left eye. Now watch the left eye for any movement. If there was no movement of the right eye when covering the left, and the left eye moves out to refixate when uncovered, you have detected a tendency to cross (esophoria) . With a "phoria" the eyes are aligned all the time and only by covering one eye will you detect the crossing orwaundering tendency. With a "tropia" the eyes are constantly or intermittently deviated.

5. Now, move your thumb to cover the right eye. Does the left eye move? Remove the cover. Does the right eye move to refixate on the toy? The cover-uncover test helps detect both "tropias" and "phorias". It is important to now alternately cover one eye then the other. The alternate cover test allows you to detect subtle strabismic movements.

The cover test is used to measure the phoria as follows: the clinician tells the person to stare at an object while covering one eye. The covered eye is allowed enough time to drift to its resting position. The paddle is then quickly moved to cover the other eye, and the first eye must swing into position to pick up fixation. The doctor watches this movement to see if the eye moved in (indicating an exophoria) or out (indicating an esophoria). If there is no movement there is no phoria (orthophoria). The amount of movement can be measured using prisms.

When the near cover test is completed, perform the cover test with the patient fixating on a distance object, toy, or reading chart at least 6-10 feet away. This will help determine whether a distance deviation is also present.

Corneal Light Reflex Test

The corneal reflex test can help you tell the difference between a "true" and "pseudo"strabismus.

Technique

1. Hold penlight or small flashlight 2-3 feet from patient, at eye level, aimed at the bridge of the nose.

2. Illuminate both corneas simultaneously.

3. Observe the bright corneal light reflex in each eye. Is the reflex symmetric and centered? Normally the light reflex is slightly nasal to the exact center of the pupil. This is the visual axis which differs from the pupillary axis.

4. If the patient will not fixate on the light, have them focus on another object aligned directly in their center of vision (ie. use a small toy or picture). Move the beam to center the reflexes and then observe.

Assymetric Reflex: If one reflex is located outside of the pupil on the iris or even the sclera, this indicates a significant misalignment (strabismus) is present.

Symmetric Reflex: This is normal. Some children with "pseudostrabismus" will appear crossed due to the wide bridge of the nose which covers the nasal sclera, (especially when the child is looking to the side) making the infant look esotropic.

Bruchner Test

The Bruchner test is performed by observing, simultaneously, both red reflexes with the ophthalmoscope. This simple test is helpful in detecting unequal refractive errors as well as misaligment. The optical reason for this, is that light reflecting back out of the eye into the ophthalmoscope will not be precisely aligned or focused and, thus, will appear brighter and whiter. Since the reflex differences may be subtle, the darker the room, the easier it is to detect small assymetries. Dark specks or spots in the red reflex can be due to corneal abnormalities, cataracts, or opacities in the vitreous gel.

Technique

1. As you face the patient from 3 feet away, use your ophthalmoscope light (set at 0 or +1) to illuminate the face. Turn down the room lights. Complete darkness works well.

2. Look at both eyes simultaneously through the ophthalmoscope, and observe the normal red reflexes.

3. Are the two reflexes symmetric or assymetric? If one is whiter than the other, it is assymetric. The brighter, whiter reflex is the abnormal one.

Frequently, glasses, patching, prisms, eye exercises, or surgery may be indicated to correct the strabismus misalignment. Some forms of strabismus may be secondary to intraocular disease, tumors, and optic nerve dysfunction. Therefore, it is extremely important that all strabismus patients have a dilated examination with a complete fundus evaluation.