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PURPOSE To assess the presence and magnitude of a phoria or a strabismus (tropia)．The cover test assesses the presence or absence of motor fusion．When motor fusion is present(ie，when there is no strabismus)，the cover test determines the magnitude of the demand placed on a patient’s fusional vergence system．
EQUIPMENT
·AV chart．
·Near cover test target．
·Occluder．
·Overhead lamp．
·Horizontal and vertical prism bars.
SET-UP
·The patient wears his habitual correction for the distance being tested．
·Set the target：
For distance：an isolated letter, one line larger than the visual acuity in the patient’s poorer seeing eye (with best correction)．
For near: an accommodative target, held at 16 in．Use a reduced Snellen letter one line larger than the visual acuity in the patient’s poorer seeing eye or a picture target of comparable detail．The patient may hold the target．
·The examiner holds the occluder．
·The examiner or patient holds the prism bar or bars．
·The room illumination must be sufficient to allow the examiner to observe the patient’s eye movements．The cover test may be done with full room illumination．
·The examiner must be positioned to see the patient’s eyes easily without interfering with the patient’s view of the target．
STEP-BY-STEP PROCEDUIR
ALTERNATING COVER TEST
Determines the direction and the magnitude of a phoria or tropia but does not differentiate a phoria from a tropia．
1．Instruct the patient to look at the target and to keep it clear．
2．Place the occluder in front of the patient’s right eye for 2 to 3 seconds．
3．Quickly move the occluder from the patient’s right eye to the left eye，observing the just-uncovered right eye for direction of movement．
4．Leave the occluder in front of the left eye for 2 to 3 seconds．
5．Quickly move the occluder from the patient’s left eye to the right eye, observing the just-uncovered left eye for direction of movement．
6．Repeat steps 2 through 5 several times．
7．Identify the direction of the deviation based on the direction of movement of each eye as it was uncovered (see Table 2-3)．
8．The magnitude of the deviation can be measured using a prism bar．Place the prism bar over either eye as close to the eye as possible with the base in the appropriate direction (see Table 2-4)．
Repeat the alternating cover test while increasing the amount of prism held before one eye until no movement is observed on the alternating cover test．
8．The magnitude of the deviation can be measured using a prism bar．Place the prism bar over either eye as close to the eye as possible with the base in the appropriate direction (see Table 2-4)．
Repeat the alternating cover test while increasing the amount of prism held before one eye until no movement is observed on the alternating cover test．
COVER-UNCOVER TEST
Differentiates between a phoria and a tropia and determines if a tropia is alternating or unilateral(see Figure 2-2)．The cover part of the cover-uncover test differentiates phorias from tropias by determining the position of the visual axis of each eye when both eyes are open to view the target．If the patient has a phoria， each visual axis will be aligned with the target when both eyes are open．If the patient has a tropia, one visual axis will be aligned with the target and the other visual axis will be misaligned with the target when both eyes are open．During the cover-uncover test，only the unoccluded eye is observed to determine the position of its visual axis．
The uncover part of the cover-uncover test differentiates alternating tropias from unilateral tropias (constant right or constant left tropias). A patient with an alternating tropia is able to keep either the right or left visual axis aligned with the target when both eyes are open．A patient with a unilateral tropia habitually fixates with one eye when both eyes are open，and only fixates with the troping eye when the fixating eye is occluded．The examiner observes the eye that is not covered by the occluder．
9．To test the left eye, start with both eyes open and cover the patient’s right eye．Observe the left eye for movement as soon as the right eye is covered．If there is no movement, it indicates that the left eye was fixating on the target at the start of the test when both eyes were open．Remove the occluder and allow 2 to 3 seconds for the two eyes to resume their normal relationship to one another．Repeat．
10．To test the right eye, start with both eyes open and cover the patient’s left eye．Observe the right eye for movement as soon as the left eye is covered．If there is no movement, it indicates that the right eye was fixating on the target at the start of the test when both eyes were open．Remove the occluder and allow 2 to 3 seconds for the two eyes to resume their normal relationship to one another．Repeat．
11．If there is no movement in either step 9 or 10, the patient has a phoria．Under normal binocular conditions both visual axes are aligned with the target．
12．If there is movement in either step 9 or 10, the patient has a tropia．To differentiate between an alternating tropia and a monocular (right or left)tropia．start with one eye covered and observe the uncovered eye for movement as soon as the occluder is removed：
a．If the left eye moved when the right eye was covered during step 9, uncover the right eye and observe the left eye．
1) If the left eye does not move when the right eye is uncovered，the patient has an alternating tropia．
2) If the left eye does move when the right eye is uncovered，the patient has a constant left tropia(ie，when both eyes are open，the right eye is aligned with the object and the left eye is not)．
b．If the left eye did not move during step 9 when the right eye was covered，but the right eye moved during step 10 when the left eye was covered, then uncover the left eye and observe the right eye．
1) If the right eye does not move when the left eye is uncovered，the patient has an alternating tropia．
2) If the right eye moves when the left eye is uncovered, the patient has a constant right tropia，ie, when both eyes are open，the left eye is aligned with the object and the right eye is not．
13．The alternating cover test and the cover-uncover test are done at distance and at 16 in with the patient or the examiner holding the near target at eye level in good illumination(see Figure 2—3)．
Note：Deviations found on cover test may be comitant (within five prism diopters in each of the nine cardinal positions of gaze)or noncomitant．If the deviation is noncomitant it should be measured in each of the nine cardinal positions of gaze．If only one finding is recorded，it is assumed to be a comitant deviation.
RECORDING
·write “cover test” or “CT．”
·Write “sc”(for without Rx)or “cc”(for with Rx)．
·Record separately for distance “D” and for near “N．” The near results can also be indicated by adding a prime(′)after the findings．
·Record the amount of prism that was required to neutralize the deviation．
·Record the direction of deviation, using the following abbreviations：
E for eso, X for exo
RH for right hyper, LH for left hyper
Ortho：no deviation
·Record the type of deviation(phoria or tropia)using the following abbreviations：
P for phoria
T for tropia
·If the deviation is a tropia，record “R” or “L” or “alt” for right, left, or alternating tropia．
Note：Lateral phorias reflect the relationship between the two eyes in the absence of fusion．Therefore, they do not need to be identified as right，left，or alternating．However，right or Left must be recorded for all vertical phorias and right，left，or alternating must be recorded for all tropias．The term hypo is generally used only for downward deviating unilateral tropias．The terms right hyper or left hyper are used for phorias or for alternating tropias．
·A T written in parentheses(T)indicates an intermittent tropia．If the tropia is intermittent, the percentage of time that the tropia is present should be estimated and recorded．
EXAMPLES
CT cc
at D and N
CT sc 20Δ RXT；1OΔ XP′
CT cc
at D；25Δ alt E(T)′(75%T)
CT sc 25Δ RET with 1OΔ RHT at D and N
EXPECTED FINDINGS
1Δ exophoria (±1Δ)at D; 3Δexophoria(±3Δ)at N
Note：Studies of phoria measurements by the Von Graefe method show that presbyopes have larger exophorias at near than the phorias of the nonpresbyopic population．This can also be expected on the cover test |
