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magnitude of the deviation and to get some idea about the applicant’s fusional ability. The test can be done at
distance and near although for most screening examinations a distance measurement is all that is required.
Cover testing is often poorly done because the following points are not understood:
a) If a spectacle correction or contact lenses are required for the applicant to see properly at the
test distance, this correction must be worn during the test.
b) Accommodation must be controlled by having the applicant read symbols at a known distance
(generally 6 m, or 20 ft). The ordinary visual acuity charts are used. It is incorrect and may be
misleading to do cover testing by asking the applicant to look at a light because
accommodation is then not controlled.
c) When checking for horizontal deviations the applicant is asked to read vertical columns of
symbols, and when checking for vertical deviations the applicant should read horizontal rows
of symbols. Cover testing cannot be used to evaluate cyclo-deviations.
d) The test should be done in such a way that the examiner can observe both of the applicant’s
eyes.
11.7.26 For screening examinations it is generally sufficient to do the cover testing in the primary position
i.e. with the applicant looking straight ahead into the distance with his head straight. For more detailed
evaluation of strabismus, the testing is also done in the eight cardinal positions of gaze — left, right, up, down
and into each of the four corners.
11.7.27 There are two parts to the cover test — alternate cover and cover/uncover.
Alternate cover test
11.7.28 With the distance spectacle correction (if any) in place, the applicant is asked to read the Snellen
letters (or other suitable optotype) in columns, vertically. A cover, which can be the examiner’s hand or a
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-45
suitably shaped piece of cardboard or plastic, is placed in front of the applicant’s right eye, held there for a few
seconds then rapidly moved across to cover the left eye. After another few seconds the cover is shifted back to
the right eye. The cover is moved back and forth several times until the examiner is satisfied with his
observations.
11.7.29 If the eyes are straight (orthophoria) there will be no movement of either eye other than the slight
vertical movement as the applicant looks from one symbol down to the next. Repeat the test with the applicant
reading the letters horizontally. If the eyes are straight there will be no vertical movement of either eye. No shift
of the eyes on alternate cover testing indicates orthophoria.
11.7.30 If the applicant is not orthophoric, there will be movement of the eyes during the alternate cover
test. If the eye behind the cover abducts when uncovered it must have been turned inwards indicating an
esodeviation. If it adducts when uncovered it must have been turned outwards indicating an exodeviation. If the
eye makes a downward movement when uncovered it must have been hyperdeviated and if it makes an
upward movement when uncovered it must have been hypodeviated. These correcting movements are made
to “take up fixation”, and this is why it is essential to have the applicant reading symbols.
11.7.31 The alternate cover test prevents fusion and tells the examiner if the applicant is orthophoric or if
there is a deviation. It indicates the direction of the misalignment but it does not distinguish between a phoria
and a tropia. For this, the following test is needed.
Cover/uncover test
11.7.32 In this test the applicant does exactly the same thing as in the alternate cover test but this time the
examiner simply covers then uncovers each eye in turn. The cover is held in place for a few seconds so as to
prevent fusion while the eye position is observed. When the cover is removed fusion is permitted and again the
movement of the eyes is observed. The test is repeated several times until the examiner is satisfied that he has
observed what happens to each eye when it is covered and when it is uncovered.
11.7.33 If there is a tropia, covering the fixating eye (the one which is not deviated) will make the applicant
look with the deviated eye, which will have to move to see the letters on the chart. If the eye must abduct there
is an esotropia, if it must adduct there is an exotropia, if it must move downward there is a hypertropia and
if it must move upwards there is a hypotropia. The examiner will be able to tell if the tropia is left, right or
alternating.
11.7.34 When the cover/uncover test is done on the deviating eye there will be no shift of either eye
because the non-deviating eye is already properly aligned and reading the letters.
　
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