Module 5: Ocular Motility - Cover Tests http://www.eyetec.net/ce/M5START.htm
OVERVIEW Estimated study time: 3/4 hour. Target Audience: This module covers basic concepts and is appropriate appropriate for assistant and and technician level personnel. Pre-requisites: A basic knowledge of extraocular muscle actions. Date of original release: February 2000
Section 1:
The Cover Test
detect a tropia. The procedure Description of content: This section discusses how the Cover Test is used to detect is reviewed step-by-step. Animated examples are included. The difference between a tropia and a phoria phoria is discussed, and types types of deviations deviations are reviewed. reviewed. Instructional objectives: Upon completion completion of this section, the student should be be able to: Explain the difference between a tropia and a phoria y Describe the Cover Test procedure y Identify types of deviations revealed by the Cover Test y Section 2:
The Uncover Test
phoria. The Description of content: This section discusses how the Uncover Test is used to detect a phoria. procedure is reviewed step-by-step. An animated animated demonstration is included. included. Instructional objectives: Upon completion of this section, the student should be able to: Explain how the Uncover Test works to reveal a phoria y Describe the Uncover Test procedure y Identify types of deviations revealed by the Uncover Test y Section 3:
The Cover-Uncover Test
Description of content: This section discusses how the Cover Test and the Uncover Test are combined into one procedure; the Cover-Uncover Test. Test. The procedure is reviewed step-by-step, step-by-step, and animated examples examples are given. Instructional objectives: Upon completion of this section, the student should should be able able to: y Explain the functions of the steps involved in the Cover-Unco ver Test Describe the Cover-Uncover Test procedure y Identify types of deviations revealed by the Cover-Uncover Test y Section 4:
The Alternate Cover Test
Description of content: This section discusses the difference between the Cover-Uncover Test and the Alternate Cover Cover Test. Advantages and disadvantages of the Alternate Alternate Cover test test are explained. Th e procedure is reviewed step-by-step, and an animated animated demonstration is given. Instructional objectives: Upon completion of this section, the student should should be able able to: y Explain the difference between the Cover -Uncover Test and the Alternate Cover Test Describe the Alternate Cover Test procedure y Identify types of deviations revealed by the Alternate Cover Test y
Section 1:
The Cover Test Introduction
This module uses animations extensively to demonstrate important concepts. The material is best viewed online in a browser window. Viewing the material on a printed page may not give sufficient information for a good understanding of the material. The cover test is a simple procedure, using only the occluder, that is used to detect the presence of an eye muscle imbalance known as a tropia. Ortho
A patient who does not demonstrate a deviation of eye alignment is termed "ortho" or "orthophoric".
Tropia
A tropia is an eye turn or deviation that the patient has no, or very little, control over. The patient is unable to keep the eye straight with the power of fusion. Even though there is such a thing as an intermittent tropia, a tropia is a manifest deviation, meaning it is evident upon inspection and is not hidden. Phoria
A phoria is a muscle imblance that is hidden by fusion. The eyes remain straight as long as fusion is present. The phoric deviation only becomes evident when fusion is disrupted. Identifying deviations by direction and by deviating eye
While the fellow eye fixes on a visual target, the tropic eye deviates either toward the nose (esotropia), temporally (exotropia), superiorly (hypertropia), inferiorly (hypotropia), or a combination of a horizontal and a vertical deviation.
The deviation is further identified by the eye that is deviating. Possibilities are: right esotropia (R ET) right exotropia (RXT) right hypertropia (RHT) right hypotropia left esotropia (LET) left exotropia (LXT) left hypertropia (LHT) left hypotropia alternating esotropia (ALT ET) alternating exotropia (ALT XT) A hypotropia is a bit confusing because they may be labeled as a hypertropia of the other eye. For example, a left hypotropia may be labeled as a right hypertropia, even though the left eye is deviating and the right eye is fixing. To avoid confusion only use the RHT to indicate a right eye that is deviating upward. If the left eye is deviating downward you can simply write out "left hypotropia". An alternating tropia occurs when a patient always has a deviating eye, but can and does fixate with either eye. For example, the patient may be fixing with the right eye and the left eye may be deviating outward (XT). A moment later the patient may switch fixation to the left eye and the right eye will move to an outward deviation. A combination left exotropia and left hypertropia might be labeled: LXT with LHT. Procedure
When performing the cover test, our job as technicians is to answer the question: Is there a tropia present? Yes or no. If the answer is yes, we should identify (name) the tropia. The Uncover Test (Sections 2 and 3) and the Alt ernate Cover Test (Section 4) will be used to detect a phoria. y y
Have the patient view a distant target (the test is also performed at near). While you observe the right eye, use the occluder to cover the left eye. One of two things will happen:
1) If the right eye does not move and remains fixed on the visual target, then the right eye is not tropic. 2) If the right eye is observed to move in order to take up fixation, then the eye is tropic and the tropia is identified according to the direction the eye moved from, and which eye it is that moved. Look at the t wo demonstrations below. In the top demo a right esotropia (RET) is demonstrated. The patient is viewing a distant target. It is obvious in this example that the left eye is fixing on the target and the right eye is turning in. Small angle tropias are not this obvious. As the demo starts, an occluder moves over the left eye, forcing the right eye to search for the target. The right eye moves to take up fixation. The right eye is moving outward from an inward position, so the
devi ti
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i termed a r i
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t esotropia.
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The demo below ill strates a i
EX T
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XT revealed by the cover test.
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We have checked the r i ht eye with the cover test. Now we complete the cover test exami ation by testing the lef t eye. R emove the occl der from the lef t eye and allow the patient's gaze to stabilize on the target. Now cover the r ight eye with the occl der while you observe what happens to the lef t eye. As discussed above, the lef t eye will either remain straight, or it will move from a deviated position to take up f ixation.
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Ex
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t is possi ble that an obviously tropic eye will not move to take up f ixation. This typically occurs when the vision is poor in the deviating eye. The eye cannot see the f ixation target, so it doesn't pick up f ixation. n these cases the tropia is usually so obvious that a cover test is not needed. i i . This patient may A confusing situation for beginners is the patient with an l f ixate f irst with the r ight eye and a moment later shif t f ixation to the lef t eye, and may do so back and-for th. As f ixation is shif ting, the fellow eye moves to a deviated position. You may not even need an occluder for this patient. The tr ick is to recognize that the patient is shif ting f ixation from one eye to the other and to observe which way the deviating eye is moving.
Section 2:
The Uncover Test Introduction
When a phoria is present it means that there is a muscle imbalance, but the eyes are kept in the aligned position by the power of fusion. The goal of the testing procedure is to disrupt fusion (cover) and observe which direction the covered eye moves to take up fixation when double vision occurs (uncover). Procedure
Let¶s look at what happens in the procedure step-by-step. We have already determined by the Cover Test that no tropia is present (see Module 5, Section 1). Both eyes appear to be fixing (or straight) when not covered.
The patient is instructed to look at the distant target. We cover the left eye and leave it covered for about 10 seconds. We have disrupted fusion.
In the Cover Test we were interested in what was happening with the uncovered eye. This time we are interested in what is happening with the covered eye. By way of illustration let¶s say that the left eye, which is covered and is not fixing, now drifts to it¶s resting position, which happens to be outward. We now uncover the left eye.
At this moment (which indeed may be a very short moment) the right eye is fixing but the left eye is not fixing and has drifted outward. At this moment the patient has.......you guessed it, double vision. This is an emergency bell to th e brain that some realignment needs to be done. An electro-mechanical process now goes into effect which realigns the eyes, restores binocular fixation, and regains single vision. The physiological action that you will observe is that the left eye will move from it¶s outward position in an inward direc tion to again take up fixation.
Identi i
ti n
i . The lef t eye has moved How do we name or descr i be what we have observed? t is anex from it s outward ( ex ) position in an inward direction to take up f ixation. f the eye had moved from an inward (eso) resting position in an outward direction to take up f ixation it would be an esophori , as pictured below.
f the eye had moved from an upward ( hyper) resting position to take up f ixation it would be a left hyperphori .
f the eye had moved from a downward ( hypo) resting position it would be a ri ht hyperphori . ven though it seems logical to refer to it as a left hypophori this condition is usually identif ied by the eye that is most upward. Please note that nothing has been mentioned about a lef t or r ight esophor ia, or a lef t or r ight exophor ia. That is because this fur ther identif ication is unnecessary when referr ing to a hor izontal phor ia. f there is a lef t exophor ia present then there is automatically a r ight exophor ia present. A phor ia descr i bes the resting, non-f ixating relationshi p between the two eyes. The terms lef t or r ight exotropia are used because lef t or r ight descr i bes which eye is deviating when both eyes are not covered. n the case of a phor ia, both eyes f ixate (do not deviate) when the eyes are not covered. emonstration
The r ight eye is covered and uncovered f irst, and then the lef t eye, to demonstrate the presence of an esophoria. When cover ing, be sure to pause long enough with the eye covered to allow the covered eye time to dr if t to it's resting position.
UNCOVE
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Section 3:
The Cover-Uncover Test Introduction
It is useful for the beginner to think of the Cover-Uncover Test as two separate tests: The Cover Test and the Uncover Test. The Cover Test, which tests for the presence of a tropia, was presented in Section 1. The Uncover Test, which tests for the presence of a phoria, was presented in Section 2. In clinical practice the two tests are performed together as the Cover-Uncover Test. Let s go through the procedure step-by-step. Procedure
Our patient is instructed to view a distance or near fixation target.
We then cover the left eye while simultaneously watching the right eye for any movement. This is the "Cover" part of the test. If the right eye does not move, and it appears to be fixing, then there is no tropia of the right eye.
If the right eye does move to pick up fixation, then we identify the deviation by the direction from which it moved to pick up fixation. For example, if it moved from a temporal position inward to take up fixation, then it is a right exotropia (RXT). If we detect a tropia, then we don t have to pay attention to the "Uncover" part of the test because we have already identified the deviation . If no tropia is detected, then we proceed with Uncover Testing. After leaving the occluder in place for several moments, we then remove the occluder from the left eye while watching the left eye as the occluder is removed.
If there is no movement of the left eye, we can usually assume that there is no phoria. Movement of the left eye indicates the presence of a phoria, which is identified according the position from which the eye moved. For example, if the eye moved from an inward, or nasal position, it would be termed an esophoria. A phoria is confirmed by observing the same type of movement after the right eye is uncovered. We now allow the patient a few moments to regain stable fixation with the eyes not covered.
We proceed with testing by cover ing the r ight eye while observing the lef t eye (Cover Test) for any movement which would indicate the presence of a tropia in the lef t eye.
f a tropia is not detected in the lef t eye, then we observe the r ight eye as it is being uncovered. Any movement of the r ight eye to take up f ixation would indicate the presence of a phor ia.
LXT
example
llustrated below is a demonstration of a lef t exotropia revealed by cover-uncover testing. R ead an explanation of the procedure under the illustration.
LXT
DE O COVE UNCOVE
TE TING
1) We cover the lef t eye and observe the r ight eye. f a tropia exists in the r ight eye, the eye will move from the deviated position to take up f ixation (assuming the eye has good enough v ision). The r ight eye does not move and appears to be f ixing, so we conclude there is not a tropia involving the r ight eye. 2) Our attention now shif ts to the lef t eye as we uncover the lef t eye. f a phor ia exists, the lef t eye will shif t to take up f ixation again as the cover is removed. The lef t does not shif t to take up f ixation as the cover is removed, so we assume that a phor ia does not exist in the lef t eye. 3) We now cover the r ight eye and observe the lef t eye for any movemen t, which would indicate the presence of a tropia. We observe that the lef t eye moves inward from a temporal position to take up f ixation, indicating the presence of a lef t exotropia (LXT). 4) As the cover is removed from r ight eye we observe the r ight eye return to f ixation and the lef t eye move back to exotropia. f there was an alternating exotropia, the r ight eye would stay in the exo position with the lef t eye f ixating. Exophoria example
llustrated below is a demonstration of an exophor ia revealed by cover-uncover testing. R ead an explanation of the procedure under the illustration.
EXOPHORIA DE O COVER-UNCOVER TE TING
1) The lef t eye is covered as we observe the r ight eye for any movement to take up f ixation, which would indicate the presence of a tropia. There is no movement of the r ight eye and it appears to be f ixing on the target. 2) We now shif t our attention to the lef t eye as it is being uncovered. Any movement to take up f ixation would indicate the presence of a phor ia. We observe the lef t eye move inward from a temporal position to take up f ixation. This would indicate an exophor ia. 3) We proceed to cover the r ight eye and observe the lef t eye for conf irmation that it is an exophor ia that we are observing. The lef t eye does not move and appears to be f ixing on the target, conf irming that a tropia does not exist. 4) f an exophor ia does exist, we would expect that the lef t eye would also exhi bit movement inward from an outward position as we uncover the eye. We observe that this is the case.
Section 4:
The Alternate Cover (Cross-Cover) Test Introduction
The Alternate Cover Test can be used to quickly tell whether a patient is ortho or i f the patient has a deviation. It is particularly useful in uncovering phorias because the technique does not allow the patient to establish binocular fixation. It is not always easy for the beginner to differentiate between a phoria and a tropia with the alternate cover test. The cover test can be used for confirmation. What is the difference between the Cover-Uncover Test and the Alternate Cover Test?
The Cover-Uncover test allows the patient to establish binocular fixation if possible. Of course, if there is binocular fixation, then there is no tropia. The cover test is used to detect binocular fixation and rule out a tropia. The uncover part of the test is used to disrupt fusion (binocular fixation) and hopefully reveal a phoria. The problem is that once fusion is established a covered eye will not always immediately drift off to its resting position when fusion is disrupted. Sometimes you have to leave the cover in place for an extended period of time until the eye "relaxes" into th e phoric position. This problem is solved by using the Alternate Cover (cross-cover) Test. Instead of allowing the eyes to be simultaneously uncovered for a period of time as with the cover-uncover test, in the alternate cover test the cover is moved from one eye to the other, preventing fusion. As a result, the eyes move to their "resting" positions relative to one another and the appearance of movement is accentuated during the test. Procedure
1) Either eye can be covered with an occluder to start the test.
Pause for a while with the cover in place over the eye to allow time for the eye to "drift". 2) Quickly move the cover to the other eye.
Pause again to allow the uncovered eye time to pick up fixation.
3) Quickly move the cover back to the other eye.
Pause again to allow the uncovered eye time to pick up fixation.
Repeat the procedure in a swinging fashion, observing the eye that is being uncovered. Any movement of the eyes to pick up fixation indicates that the patient has a muscle imbalance. As discussed in the previous sections, the direction of movement indicates the type of deviation (eso, exo, or hyper). Final step: After you have been able to observe any eye movement present, the final step is to remove the cover, allowing binocular fixation to re-establish, if possible. This step helps you to tell the difference between a phoria and a tropia. The following discussion only applies if you have observed eye movement during the alternate cover test. If both eyes remained fixed on the target during the alternate cover test, then the patient was ortho. If the eye being uncovered moves to take up fixation and the fellow eye remains fixing, then you have been observing a phoria. If the eye being uncovered does not move to take up fixation (it remains in the deviated position), or if the fellow eye moves to a deviated position, then you have been observing a tropia. The topia is further identified by which eye it is that is deviating (right, left, or alternating). Confirmation can be made by performing the cover test. E ample: Esophoria as revealed by the alternate cover test.
We begin by covering either eye. Allow a few moments for the eye under the cover to "drift".
Now move the cover back and forth between the two eyes. Do not allow the patient to gain binocular fixation. Repeat the process as necessary to observe any movement present. You may need to pause in the covered position to allow the uncovered eye to pick up fixation. Move your
mouse over the illustration below to activate the demonstration.
We observe that each eye is moving outward from a nasal position, indicating the presence of an eso deviation. We do not know yet if it is a phoria or a tropia. In the final step (4) we remove the cover and observe any movement of each eye. Move your mouse over the illustration to activate the demonstration. If you want to repeat the demo, move your mouse off the illustration, then move it back over the illustration.
We observe that when the cover is removed, the left eye moves from a nasal position to pick up fixation. The right eye remains stationary, fixed on the target. Since both eyes gain fixation, this would indicate that an esophoria is present.
