Module 29 

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Module 29:

EOM Related Skills

Section 1:

The Worth four dot test is a subjective test of visual alignment, suppression, dominance, and diplopia. It is not typically done routinely. In other words, it is usually done only when the doctor requests it. A more precise method of testing fusional ability can be done with one of the stereo tests. If performing more than one test, the stereo test should be performed first. Cover testing can be performed after the W4D test, and visual acuity testing should be performed last. The idea is that you start with the least disruptive test in terms of binocularity.

Binocularity refers to the ability of both eyes to track the same object at the same time. This means that the image of the object falls on the fovea of each eye at the same time. This is termed bifoveal fixation. Fusion, which is the superimposition of the two images in the brain, results from bifoveal fixation. The highest degree of binocularity is stereopsis, which is three dimensional depth perception made possible by the fusion of the binocular images of an object viewed at slightly different angles.

If a person has stereopsis, you know that he/she has fusional ability. If a person has fusion, you know that she has binocular vision, at least part of the time. Thus, the usefulness of a positive stereo test. It the patient "flunks" the stereo test, the W4D test can give you more information about the patient's binocular status.

If a person has two functioning eyes but does not have stereopsis, then three conditions are possible. 

1. Either the patient will have double vision, 

2. or the brain will "suppress" the image from one of the eyes, thus avoiding double vision,

3. or the patient may have fusion with undeveloped stereopsis.

The W4D tests for the above conditions. The W4D test is subjective, meaning the validity of the test is dependent upon accurate feedback from the patient.  The W4D works best if the patient is able to distinguish between the color red and the color green, and the patient must be able to count (it doesn‘t work with very young children).

The W4D test is somewhat dissociative. This means that each eye sees a different object, making fusion more difficult.  If there is a deviation from orthophoria, it usually becomes manifest with this test.  The W4D test will tell you if there is a deviation, e.g. eso or exo, but it will not tell you if the deviation is a phoria or a tropia.  The cover tests can differentiate a phoria and a tropia.  However, the W4D is more sensitive to small deviations than the cover tests are.

The test has two components:

1 The first component is a target with four lights (dots). Two of the lights are green, one is red, and one is white. The target can be projected, it can be mounted on a wall, or it can be a flashlight with the target mounted over the glass plate of the flashlight.

2 The second component is a pair of glasses that the patient wears. One lens is red, and the other lens is green.

The eye with the green lens in front of it can potentially see the two green lights and the white light (it is seen as green through the green lens). The green lens filters out the red light so that it cannot be seen.

The eye with the red lens in front of it can see the one red light and the white light (it is seen as red through the red lens). The red lens blocks the green lights.  

If the glasses are reversible, it is best to be consistent about which eye has which color in front of it. For example, always test with the right eye looking through the red lens.

The possibilities are as follows, based upon the right eye looking through the red lens and the left eye looking through the green lens:

1. Four lights are seen: This indicates that orthophoria (bifoveal fixation) is present. 

If two lights are red and two are green, then the patient is right eye dominant. If three lights are green and one is red, then the patient is left eye dominant. If the white light seems to switch between red and green, then neither eye is dominant.

2. Two red lights are seen: The left eye is suppressing.

3. Three green lights are seen: The right eye is suppressing.

4. Five lights are seen (two red and three green): The patient either has double vision or has rapidly alternating suppression. To differentiate between the two, ask if all five lights are seen at the same time (double vision), or if the green lights disappear when the red lights appear (alternating suppression).

 

If all five lights are seen at the same time, representing double vision, you will need to determine what type of diplopia is being experienced. The following discussion assumes that the red lens is in front of the right eye.

Two red lights to the right of the green lights indicates an eso deviation (tropia or phoria).

If the right eye is turned in, the image will fall on the nasal retina, which will make the image appear to be to the right.

Two red lights to the left of three green lights indicates an exo deviation.

There is an easy way to keep these concepts  straight in your mind.  If the right lens is before the right eye, and the red dots appear to the right of the green dots, then the deviation is eso, or uncrossed.  If the red dots appear to the left of the green dots, then it is an eXo, or crossed deviation.

Two red lights above three green lights indicates a left hyper deviation.

Two red lights below three green lights indicates a right hyper deviation.

 

When trying to find out what the patient is seeing, ask specific questions instead of asking the patient to describe what she sees. For example, ask:

How many lights do you see?

How many red ones?

How many green ones?

When evaluating double vision (five lights seen), if the patient is right/left confused, ask the patient to indicate which side the lights are on by holding up the corresponding hand, or tap the child on the shoulder and ask "are the green lights on this side".

The W4D test can be performed at distance and at near. Young children can be asked to touch which lights they see if the test is performed at near. Perform the test with the room lights out or very dim.