Clinical Optometric Procedures: The Cover Test

Clinical Optometric Procedures: The Cover Test

CHAPTER 5 – THE COVER TEST

• This chapter will include a review of:
• Features of ocular deviation
• Cover test assessment of ocular deviation
• Von Graeffe assessment of ocular deviation
• Maddox rod assessment of ocular deviation

FEATURES OF OCULAR DEVIATION

Background Information

The cover test is used to determine whether there is any tendency of the eyes to deviate from well coordinated behavior. If a deviation is detected, the cover test will show a deviation if it is latent (heterophoria – tendency to turn under certain conditions) or manifest (heterotropia – permanent turn). The cover test can also be used to estimate or measure the direction and size of the deviation, and give some indication whether it is compensated or not.

Every patient’s deviation must be described in terms of its frequency, direction, magnitude, laterality and comitancy.

• Frequency: may be either constant or intermittent.
• Direction: may be horizontal, vertical or rotatory (cyclo). In addition, no deviation of the line of sight is

termed othortropia/orthophoria. Horizontal deviations are either inward (nasal) deviation from the line of sight (known as an eso deviation) or outward (temporal) deviation of the line of sight (known as an exo deviation). A vertical upward deviation of the line of sight of one eye is referred to as a hyper deviation while a vertical downward deviation of the line of sight and is referred to as a hypo deviation of the eye (Fig.

5.1). When there is a deviation of the eye around the antero-posterior axis of the eye, then the eye is said to have a cyclo or torsional deviation. An excyclodeviation is characterized by a temporal rotation of the superior aspect of the globe while an incyclodeviation is characterized by a nasal rotation.

• Magnitude: According to Daum in Eskridge et al (1991), “the magnitude of the deviation is the angular

measurement of the difference in direction of the lines of sight of the eyes for a specific fixation distance and direction of gaze”. The magnitude of a deviation is specified in prism dioptre () units.

• Laterality: is usually only specified when the deviation is constant. A constant strabismus may be either

unilateral or alternating. If the deviation is unilateral, then it implies that the deviation is constantly in only one eye, for example right constant exotropia. If the patient uses either eye to fixate, then the deviation is referred to as an alternating deviation and the deviating eye cannot be specified due to its alternation. This type of deviation would simply be documented as an alternating tropia.

• Comitancy: refers to the magnitude of the deviation when the eye changes to various directions of gaze. If

a deviation is comitant or concomitant then it implies that the magnitude of the deviation remains the same irrespective of the direction of gaze of the patient. If it is incomitant or inconcomitant then it implies that the magnitude of the deviation changes when the eye shifts its gaze from one direction to another. International Centre for Eyecare Education Figure 5.1 Diagrammatic representation of tropias

Binocularity Implications

The use of the information gathered from establishing the binocularity of the patient has several implications:

• The clinician may chose to perform further testing of the oculomotor system
• There may have to be modifications in the typical refraction routine. Additional specific tests may need to be

performed. For example, a patient who has a tropia is considered monocular and therefore cannot undergo binocular balancing tests.

• The binocular status may suggest the presence of eyestrain, headaches, decreased performance, Amblyopia

or reduced stereopsis.

COVER TEST ASSESSMENT OF OCULAR DEVIATION

• The cover test can be divided into 2 categories, viz. objective and subjective.

Objective Cover Test

The objective cover test is sometimes termed the ’cover-uncover’ test. It is probably one of the most important of all of the tests to determine the oculo-motor balance of a patient. It may be carried out at distance and near. It can also be carried out on patients wearing no correction or wearing a habitual prescription. If the patient has a habitual prescription then the cover test should be conducted with the prescription on at all times when tested.

According to Benjamin in Borish’s Clinical Refraction (2006), the unilateral cover test confirms the presence of a tropia or phoria and defines its directions.

• There are 2 types of cover tests:
• 1. Unilateral cover test (cover-uncover test)
• 2. Alternate cover test

1. Unilateral cover test (cover-uncover test) The unilateral cover test is performed by placing an occluder in front of one eye and then observing the movement, if any, in the fellow eye. This test is used to detect the presence of a tropia / heterotropia / strabismus.

A tropia is a deviation of the eye that is visible by simply looking at the patient. In Figure 5.2, when observing the position of the patient’s eyes, once can see that the right eye is turned inward. This eye is the deviating eye while the other eye is the fixating (dominant) eye. One would therefore classify this deviation as an esotropia.

• Figure 5.2 Schematic of unilateral cover test

2. Alternate cover test

The alternate cover test is performed by alternating the occluder from one eye to the other while the patient fixates a target and observing the movement, if any, in the eye that has just been uncovered (Fig.5.3).

• Figure 5.3 Schematic of unilateral cover test showing an esophoria.
• Performing the Cover Test

Aim:

• The cover test is an objective method of evaluating the presence, direction and magnitude of a strabismus
• (tropia) or phoria
• www.icee.org 4
• Equipment:
• VA chart
• Occluder
• Near point target
• Overhead lamp (only if required)

Set-up:

• Patient wears the habitual Rx for the distance being tested, i.e. either distance Rx or near Rx as in the case

of presbyopes.

• Choose a single letter as a target, on a line above the patient’s best VA in the worse eye, (preferably a letter

at the edge of a line to avoid distractions). Either you or the patient can hold the target (whichever makes you comfortable) at 40cm to stimulate accommodation. If the habitual working distance is not 40cm, then the chart for near cover test must be held at the patient’s habitual working distance

• If the patient’s VA is 6/18 (20/60) or less, then the target must be a spot of light, however, if the patient’s VA

is better, then a spot of light is not a preferred target since it will not stimulate an under corrected hyperope’s accommodation

• The room must be well illuminated for eye movement observation. A lamp placed directly above patient may

be used if needs be

• The examiner must be positioned so that observation of the eyes is possible without interfering with the
• patient's view of the target

Procedure: Unilateral / Cover-uncover 1. Start off with both eyes open, and initially hold the occluder at the patient’s nose (Fig. 5.4A). To test the left eye, cover the patient's right eye, observe the left eye for movement as soon as the right eye is covered (Fig.

5.4B). Remove the occluder and wait for two seconds before covering the right eye again, thereby allowing both eyes to fixate the target. This step is performed to confirm the presence of a slight deviation in the left eye which may not have been imperceptible the first time the right eye was covered.

2. To test the right eye, start with both eyes open (Fig. 5.4C) and cover the patient's left eye (Fig. 5.4D).

Observe the right eye as soon as the left eye is covered. Remove the occluder and wait for two seconds before repeating (Fig. 5.4).

Figure 5.4 Unilateral cover test to diagnosing a left exotropia International Centre for Eyecare Education www.icee.org 5

3. If no movement is observed then there's no tropia, in which case the patient is said to have an ORTHOTROPIA (however, do not discount the presence of a microtropia – this will be discussed further in the Orthoptics course)

4. If there is an outward movement, then it implies that the eye was occupying an inward position and therefore the patient is said to have an ESOTROPIA 5. If there is an inward movement, then it implies that the eye was occupying an outward position. Therefore the patient is said to have an EXOTROPIA (Fig. 5.4 illustrates the findings in a patient with a left exotropia) 6. If there is an upward movement, then it implies that the eye was in the downward position. Therefore the patient is said to have a HYPOTROPIA 7. If there is an downward movement, then it implies that the eye was in the upward position and therefore the patient is said to have a HYPERTROPIA Recording:

• Orthotropia: (no horizontal or vertical deviations)
• Exotropia = XOT or XT
• Esotropia = SOT or ET
• Hypotropia = hypotropia
• Hypertropia = hypertropia
• Interpretation of findings:

An alternating strabismus

• If upon covering the left eye, the right eye makes a movement to take up fixation and versional movement is

made by both eyes.

• If upon removing the cover the right eye remains fixating, there will be no versional eye movement.
• When the cover is placed over the right eye, the left moves to take up fixation, similarly there will be a

versional movement of both eyes. Upon removing the cover from the right eye, the left eye remains fixating and there is no versional movement of the eyes, this confirms the presence of an alternating strabismus.

• In this case the deviating eye cannot be specified as with a unilateral strabismus.
• Procedure: Alternating Cover test
• Start off with both eyes open (Fig. 5.5).
• Place the occluder over one and then move the occluder over the patient’s other eye so that at no time both

eyes are allowed to fixate the target together (Fig. 5.5 B and C).

• Upon moving the occluder to the fellow eye, the examiner will observe an inward movement of the eye that
• has just been uncovered. International Centre for Eyecare Education

www.icee.org 6

Figure 5.5 Alternate cover test diagnosing exophoria (observe position of eye under occluder)

• If no movement observed then there's no phoria, in which case the patient is said to have ORTHOPHORIA
• If there is an outward movement, then it implies that the eye was occupying an inward position. Therefore the

patient is said to have an ESOPHORIA

• If there is an inward movement, then it implies that the eye was occupying an outward position. Therefore the

patient is said to have an EXOPHORIA

• If there is an upward movement, then it implies that the eye was in the downward position. Therefore the

patient is said to have a HYPOPHORIA

• If there is a downward movement, then it implies that the eye was in the upward position. Therefore the
• patient is said to have a HYPERPHORIA
• Recording:
• Orthophoria: (no horizontal or vertical deviations)
• Exophoria = XOP or XP
• Esophoria = SOP or EP
• Hypophoria = hypophoria
• Hyperphoria = hyperphoria

The practitioner can through clinical experience estimate the deviation or measure the deviation using prisms.

This will be further explained in your binocular vision/orthorptics module. If you determine a measurement, then it should be recorded. For example, you measure a constant XOT of 4 prism dioptres in the right eye, then this result is recorded as: 4RXOT

• Expected values:

Distance phoria: 1

XOP ± 1

Near: 3

XOP ± 3

Note: Patients with binocular vision anomalies may present with diplopia, abnormal head posture, reports of eyestrain/asthenopia and various other complaints based on the type of deviation that exists and its magnitude, frequency, etc. All of these factors will be explored further in the binocular vision module. International Centre for Eyecare Education www.icee.org 7

Subjective Cover Test

The subjective cover test was introduced by Duane in 1925. He called it the parallax test. This test is performed to assess a phoria. It is sometimes referred to as the Phi phenomenon.

Figure 5.6 Ray diagram showing the subjective cover test for an esotrope.

In Figure 5.6, a case of esophoria with the right eye being covered and then the cover is moved to the left eye.

The image in the right will initially fall on the nasal retina (B’R) and the image will be projected temporally (i.e. to the right of the eye: BR).

Therefore the individual will perceive the fixation object apparently jumping to the right (BR). Consequently the right eye will rotate rapidly to return the image to the fovea (M’R). Therefore in a case of esophoria, the apparent movement of the target is in the opposite direction to the movement of the cover, while in exophoria it is perceived in the same direction as the movement of the cover. Similarly, in R hyperphoria, the object appears to move downwards and upwards in a L hyperphoria when the cover is moved from the right to the left. It must be noted that the subjective direction of movement is in the same direction as that of refixation.

OTHER METHODS TO DETERMINE THE PRESENCE OF A

DEVIATION AND ITS MAGNITUDE

Von Graefe Assessment of Ocular Deviation The Von Graefe method measures the phoria subjectively. It uses dissociating prisms and measuring prisms. It has the same prerequisites as that of the objective determination of the deviation. The target still has to be one of detail that would stabilize accommodation and provide good target alignment. It is performed at distance and near and can be performed with or without an Rx.

Purpose:

The von Graefe phoria test is a subjective method of evaluating the presence, direction and magnitude of a phoria at distance or near International Centre for Eyecare Education www.icee.org 8

• Equipment:
• Phoropter with Risley prisms (Fig. 5.7)
• VA chart at distance
• Near VA chart with an isolated letter or line of 20/30 size

12∆ BI 6∆ BU

Measuring prisms Dissociating prisms Figure 5.7 Detailed view of Risley prism orientation when conducting Von Graefe technique.

Procedure:

1. Place phoropter with the patient’s distance Rx before the patient. Ensure that the pupillary distance is appropriate for the testing distance, i.e. distance PD for distance Von Graefe and near PD for near Von Graefe. If conducting the near Von Graefe test, a near point card must be attached to the phoropter at 40cm.

2. When conducting the distance test, an isolated letter one line above the best acuity at distance must be provided as the target. This target may differ slightly based on the equipment available for testing. For example, if it is not possible to isolate a single letter, the practitioner may have to use a vertical column of letters.

3. The practitioner instructs the patient to close their eye’s while the Risley prisms are set at 12 BI RE and

6

BU LE (some texts mention that the practitioner may use 10BI RE and 6BU LE) as in Fig. 5.7.

4. The practitioner then has the patient open both eyes. The patient is asked to report how many targets he can observe and where they are located relative to each other. The practitioner should verify that the patient indeed sees two images by occluding one eye and asking the patient to verify how many targets he now sees. The two images seen by the patient are positioned such that one image is up to the right and the other one down and to the left (Fig. 5.8).

5. If the patient reports seeing only 1 of the targets, the practitioner must occlude one eye and assist the patient in locating the targets out in space. Alternatively, the practitioner can alter the position of the dissociating prisms before the eyes from the BU to the BD position or the practitioner may increase the amount of measuring prism. International Centre for Eyecare Education www.icee.org 9

• Figure 5.8 Resulting images viewed when using the dissociating and measuring prism
• Measuring the horizontal phoria:
• 1. When measuring the horizontal phoria, the 12
• BI represents the measuring prism while the 6BU represents

the dissociating prism.

2. The patient’s fixation is directed to the lower target and he is instructed to keep it clear (Fig. 5.8).

• 3. While the patient is observing the lower target, the 12

BI measuring Risley prism before the RE is reduced at a rate of about 2 per second until the patient reports that the targets are aligned vertically one above another or sometimes describe to the patient as “buttons on a shirt” (Fig. 5.9).

4. The practitioner must record the amount and direction of prism to achieve alignment of the targets.

5. To confirm the final result, the practitioner may overshoot the point of alignment and move the prism back in the direction toward the value that was originally obtained for alignment. If there is a difference in the amount of prism that produces alignment of the targets between the original and rechecked values, then the practitioner may take the average of the 2 measurements, however, it is advised that if the 2 values differ by more than 3∆, the practitioner should repeat the measurement.

• 1.5∆ BI 6∆ BU

Measuring prisms Dissociating prisms Figure 5.9 Scenario of position of Risley prisms when conducting Von Graefe technique when measuring the horizontal deviation (end point) International Centre for Eyecare Education www.icee.org 10

• Measurement outcomes:
• If on alignment, the 12BI has been reduced to zero  no horizontal phoria
• If remaining prism is BI  Patient has XOP
• If remaining prism is BO  Patient has SOP

E.g. in the case above (Fig. 5.9), the final result is 1.5∆ BI. This result indicates that the patient has an XOP of 1.5∆ in magnitude.

Implications of findings: It is important to note that a patient may not always end up with a result of zero. The eyes have a tolerance to overcome a small amount of latent deviation of the eyes, however, if the magnitude of the deviation lies outside of the tolerance or the patient is unable to compensate for the deviation with their inherent fusional ability, it is likely that the patient will have symptoms of asthenopia and double vision. A management plan which may include active vision therapy or a prism prescription is crucial in these patients.

• Measuring the Vertical Phoria
• 1. When measuring the vertical phoria, the 12BI is the dissociating prism and 6
• BU is the measuring prism.

2. The 6

BU is reduced before the LE until the two images appear one next to the other, also described to a patient as “headlights on a car” or “side by side”. In this case it implies that the 6 BU is the measuring prism

• and 12

BI is the dissociating prism. (Fig. 5.10) 3. The practitioner must record the amount and direction of prism to achieve alignment of the targets.

12∆ BI 3∆ BU

Dissociating prisms Measuring prisms Figure 5.10 Scenario of position of Risley prisms when conducting Von Graefe technique when measuring the vertical deviation and the resultant position of the targets Measurement outcomes:

• If on alignment, the 6
• BU has been reduced to zero  no vertical phoria
• If aligning prism is BU  Left hypophoria or Right hyperphoria
• If aligning prism is BD  left hyperphoria or Right hypophoria

E.g.: In the case above (Fig. 5.10), the practitioner is left with a measurement of 3∆ BU which indicates that the patient has either a Left hypophoria or Right hyperphoria. International Centre for Eyecare Education www.icee.org 11

• Expected values
• (Scheiman and Wick, 2008)
• Distance:
• Horizontal/lateral phoria:

Children and young adults: 1

XOP ± 1

For Presbyopes: 1

SOP ± 1

• Vertical phoria: no deviation
• Near:
• Horizontal/lateral phoria:
• Children and young adults: 3 XOP ± 3
• For Presbyopes: 8 XOP ± 3
• Vertical phoria: no deviation

Maddox Rod Assessment of Ocular Deviation The Maddox rod test is a method of detecting and measuring a tropia or phoria subjectively. The Maddox Rod is a trial lens composed of a series of powerful planoconvex cylinders in red or white plastic. Dissociation with this lens is produced by the distortion of a spot of light into a line target. When the rod is orientated with the cylinders in the horizontal direction, it produces a line target that is vertical. When orientated vertically, it produces a line target that is horizontal.

• Equipment:
• Phoropter or
• Loose Maddox Rod (it may be a clear lens or red lens) (Fig. 5.11) with trial frame and loose prisms
• Penlight torch or transilluminator
• Figure 5.11 Maddox rod

Procedure:

1. The patient is seated comfortably and wearing their best distance refractive correction.

2. The test may be performed at distance or near.

3. The Px fixates a spot of light from penlight or transilluminator. Ideally, this is not the best target for accommodative stabilization.

4. The room lights should be dimmed to allow a better view of the streak produced by the Maddox rod (red lens in this example). International Centre for Eyecare Education www.icee.org 12

Measuring the horizontal phoria:

1. Place the Maddox Rod before one eye (usually the deviating eye) with the cylinder axis orientated horizontally. This produces a streak that is vertical in orientation.

2. The other eye remains fixated on a spot of light.

3. The eyes are now dissociated with one eye seeing the streak of light and the other seeing the spot of light. If the patient is unable to see both the streak and the spot of light then suppression is indicated and the test cannot be performed.

The perception of the direction of the images in the Maddox rod test is based on retinal projection as depicted in figure 5.13. For example, an object that is perceived in the temporal visual field is traced back to the nasal retina in the eye. Therefore, a patient who experiences crossed diplopia indicates the presence of an exophoria (Fig 5.12a) while uncrossed diplopia is indicative of esophoria (Fig. 5.12c). Coincidence of the streak and spot of light depicts a case of orthophoria (Figure 5.12b).

Figure 5.12 Ray diagrams showing principles of crossed and uncrossed diplopia in (a) exophoria; (b) orthophoria and (c) esophoria.

4. If the patient is able to perceive both targets, then he is asked to report if the spot of light appears to the right, left or on top of the streak (Fig. 5.13).

Measurement outcomes (Maddox rod before RE):

Figure 5.13 Patient’s perceptions of the targets in cases of (a) exophoria; (b) orthophoria and (c) esophoria International Centre for Eyecare Education www.icee.org 13

Measuring the Vertical Phoria:

1. The Maddox Rod is placed before one eye (usually the deviating eye) with the cylinder axis orientated vertically. It will then produce a streak that is horizontal in orientation.

2. The other eye remains fixated on the spot of light.

3. The eyes are now dissociated with one eye seeing the streak of light and the other seeing the spot of light.

4. The patient is asked to report if the spot of light appears to the above, below or on top of the streak (Fig.

• 5.14).
• Measurement outcomes (Maddox rod before RE):
• Figure 5.14 Patient’s perceptions of the targets when measuring the vertical deviation

Point to note:

The Maddox rod test is not able to differentiate existence of a tropia or phoria. The practitioner must deduce this from the cover test or from the patient’s subjective complaints of diplopia.

Maddox Wing Assessment of Ocular Deviation The Maddox wing test was introduced in 1912. It is a convenient hand held quick and efficient test for near phoria using the Maddox wing instrument (Fig. 5.15).

• The scales are mounted at a fixed viewing distance of 33cms. This is generally much closer than the

standard working distance for most patients and the findings are therefore questionable.

• A septum on the instrument divides the visual field into two sections, thereby allowing the right eye to see

only the white and red arrows, whilst the left eye sees only the horizontal and vertical rows of figures.

• The horizontal deviations are indicated by the white arrow pointing to the white figures.
• The vertical deviation is indicated by the red arrow pointing to the red figures.
• Cyclophoria is assessed by sliding the arrow which overlaps the edge of the chart until it appears parallel

with the white line above. Incyclophoria is indicated when the arrow points above the zero while excyclophoria is indicated when the arrow points below the zero. International Centre for Eyecare Education www.icee.org 14

• Figure 5.15 Maddox wingInternational Centre for Eyecare Education

Section A – PRELIMINARY EXAMINATION