COMITANT STRABISMUS

Assigned Readings:

Caloroso; pp. 26-51

Griffin; pp 163-185

Press; pp 99-103; 202-221

I. Introduction: A strabismus can fall into many categories. It may be intermittent or constant; comitant or non-comitant; alternating or unilateral or cyclic type strabismus. All types share retinal confusion and the sensory adaptations that go along with it. The most common sequela of events resulting from a frank deviation of an eye are: diplopia, suppression and anomalous retinal correspondence.

A. The patient may experience diplopia which avoids the retinal confusion caused by each fovea receiving a different image.

B. The patient may suppress a small, "scotomatous" region of the deviating eye; starting at the fovea and spreading to a region of the retina approx. equal to the actual angle of deviation (the zero point).

C. The patient may, under binocular viewing conditions, redefine corresponding retinal points, so that the fovea of one eye "hooks up" to a non foveal point on the fellow eye. This is called (central) anomalous retinal correspondence.

II. True sensory fusion is a key component to normal binocular vision. There have been several classification schemes for a hierarchy of sensory fusion, but the one that has swept the country by storm is the British orthoptic classification scheme. (Some call it the Second British Invasion......but not me...

A. The "Oth stage" of fusion indicates that the patient is simultaneously aware of different targets presented to each eye.

B. The first stage of fusion indicates that the patient is able to superimpose these different targets at the objective angle of deviation.

C. The second stage of fusion indicates that the patient is able to fuse, without suppression, two very similar targets presented to each eye (suppression checks are necessary here) at the objective angle of deviation.

D. The third stage of fusion is achieved when the patient is able to respond properly to stereoscopic targets at the objective angle of deviation.

III. Motor fusion, then is the ability to use positive or negative fusional vergence to achieve or regain fusion. This can be trained or tested in free space (with prisms, vectograms and Brock strings and the like) or in instruments (synoptophores and stereoscopes).

IV. Tests to measure the magnitude of the strabismus have been developed. They not only help in diagnosis of a strabismus, but also help in monitoring a patients progress in therapy. The following is a short description of some of the more important tests to measure the magnitude and the comitancy of a strabismus (WHICH YOU WILL PRACTICE IN LAB!!).

A. The cover test, an old friend, is still the single most useful technique in diagnosing an quantifying a strabismus. It should be done at near and far working distances as well as different gaze positions. It measures the magnitude of the strabismus but does not test for fusional ability. If spontaneous diplopia is obtainable, it is the most "natural" visual environment to test under.

B. The synoptophore is a very useful and very expensive instrument. Objective and subjective angle measurements (at distance only) are but two capabilities of this instrument.It is nothing more than a very elaborate mirror stereoscope. A brief description of the the most important knobs and switches of the synoptophore will be reviewed later in this lab. Figure numbers refer to fig 5-4; Griffin, pg 138)

C. Tests for normal and abnormal sensory fusion (ie tests for sensory adaptation to a strabismus) have been developed for our learning pleasure. The most useful ones are described, IN ORDER OF "NATURALNESS" (...DUDE) .

1. Bagolini Striated Lens test: Materials needed are the lenses, a patient and a penlight. This test can be used to measure suppression as well as ARC. The streaks do not significantly interfere with normal viewing conditions. Hence, suppression and harmonious ARC are a very common result when using this test.

a. The etched glasses produce a perception of orthogonal streaks when a penlight is shown to the patient. Each eye sees streaks of only one orientation.

b. If part or all of one streak is missing, it is indicative of suppression.

c. If the perception of the light is through the middle of the crossing streaks: NRC or hARC (if an manifest strabismus is present)

d. If the perception of the light is below the middle of the crossing streaks: exotropia with NRC or unharmonious ARC (if after placing enough prism to place the light at the center of the streaks, a manifest strabismus is still present...this is rarely done!).

e. If the perception of the light is above the middle of the crossing streaks: esotropia with NRC or unharmonious ARC (if after placing enough prism to place the light at the center of the streaks, a manifest strabismus is still present...this is rarely done!).

f. If two independent streaks with two lights are reported: diplopia

g. The test should be repeated at least at two working distances (40 cm, 2m)

2. 4 Base Out Test: Materials needed are a patient , penlight and a 4Æ prism. This test is used primarily at nearpoint. It is designed to detect small areas of suppression. These small suppression scotomas are present in small angle strabismics. Sometimes, this is the only way to detect a 2-4Æ strabismus.

*It is sometimes difficult to see these minute movements, but with practice, you will become more observant. What type of eye movement should you expect?

3. The Synoptophore: As stated above, this instrument is indispencable for measuring the objective and subjective angle of deviation. It is very useful, then, for ARC analysis as well (at distance only).

4. Worth Four Dot Test: Materials need for this test are a worth four dot unit , a pair of r/g glasses and a patient. It is a flashlight with four acetate spots, oriented in a diamond shape, covering the bulb. The patient wears r/g glasses and (if the red lens is over the right eye) will see two vertical red dots with the right eye and three green dots in an inverted triangle with the left eye.

5. Hering - Bielschowsky After Image Test: This test requires a strobe and a patient. Occluder is optional. This test is quite "unnatural" because it instills afterimages to each fovea. It is reviewed below.

a. The modified strobe (altered to present a thin, long after image) is presented MONOCULARLY to each eye. the orientation of the afterimage should be horizontal to one fovea and vertical to the other.

b. The pt. must have steady central fixation O.U. for the test to work.

6. MITT and Afterimage Test: Materials needed are a strobe, an occluder and a MITT unit (also called a "Haidinger brush machine"). This test allows for simultaneous testing for eccentric fixation (E), and ARC.

PRE-LAB QUESTIONS

1. Why is the presence of a "cross" on the H-B. afterimage test indicative of NRC?

2. Why with the Bagolini test, is an exotrope likely to respond that the light is below the crossing streaks?

3. Why doesn't the H-B. afterimage test work for eccentric fixators?

4. What eye movements would you expect with both steps of a 4 base out test on a normal person?

5. What eye movements would you expect with both steps of a 4 base out test on a small angle strabismus?