Psychology 4051
Vision
Screening
Vision Screening
• The duration over which one suffers from an
amblyogenic factor is critical in determining outcome
of treatment.
• The earlier we detect amblyogenic factors, the
shorter the duration of deprivation, the better the
outcome.
• Most researchers and clinicians advocate vision
screening as soon as children are capable of
completing an optotype visual acuity test.
Recommendations
• Most North American vision/pediatric organizations
recommend that visual acuity be assessed along with
alignment of the eyes.
• Vision/pediatric organizations are also starting to
realize the value of autorefraction and
photoscreening.
What Should be Screened?
Visual Acuity
• Visual Acuity provides a direct measure of
functioning and therefore, should be measured in
preschoolers.
• Optotype acuity tests are recommended.
– Resolution acuity tests (like the Teller acuity cards) are not
recommended because they generally overestimate visual
acuity in children and adults with amblyopia
– Note however, we have found that 76% of all cases of
amblyopia can be detected using the Teller acuity cards
(Drover et al., 2009).
What Should be Screened?
• HOTV, Lea Symbols, and PattiPics following logMAR format are
recommended.
• However, young children have some difficulty with linear
optotype tests.
• Completion rates tend to be higher with isolated optotype
tests in which a single optotype is shown at a time.
• But, the crowding effect is critical in detecting children with
amblyopia.
What Should be Screened?
• This can be remedied using isolated optotype tests in
which letters are presented surrounded by crowding
bars.
• The crowding bars produce a crowding effect making
the optotype difficult to detect.
What Should be Screened?
• It is also recommended that visual acuity testing be
conducted at a distance of 10 feet rather than the
traditional 20 feet.
• Preschoolers are shy
• A well-lit 20 foot room is difficult to find in a daycare
centre.
What Should be Screened?
Ocular Alignment
• Hirschberg corneal reflex and the cover test are commonly
recommended.
• In the Hirschberg corneal reflex, the patient fixates a small
penlight which is shone into the child’s eyes from a distance of
40-50 cm.
• The positions of the corneal reflections should be the same in
each eye.
What Should be Screened?
• During the cover test, the patient fixates a target
from a distance of 3m (distant cover test) or 40cm
(near cover test).
• One eye is covered repeatedly (usually three times)
with a plastic occluder (ie., a plastic paddle) for
about 3 seconds at a time.
• If the uncovered eye has to shift or move to find the
target when the other eye is covered, this is evidence
that the eyes are misaligned.
What Should be Tested?
• In the figure below, a child who has an obvious case
of exotropia is being tested.
What Should be Screened?
Stereopsis
• Assessment of ocular alignment requires expertise
either in the administration of the procedure, or the
interpretation of the results.
• Thus, these tests should only be conducted by an
optometrist or ophthalmologist.
• This poses a problem for preschool vision screening.
• Ideally, you want to use tests that can be carried out
by educated people with no specific training in vision
assessment.
What Should be Tested?
• Perhaps a more suitable alternative to the
assessment of ocular alignment is the measurement
of stereopsis.
• If a child’s eyes are misaligned, they should have
poor stereoacuity and should be detected with
stereopsis tests.
• Children can be tested with the Randot Preschool
Stereoacuity test.
• They can also be tested with the Randot E
Stereotest.
What Should be Tested?
• The subject is present with two random dot plates (see Figure
below).
• When wearing the polarized glasses, one plate will appear to
have an “E” that floats above the plate. The other is a blank.
• The subject has to point at the plate that has the “E”.
• Subjects can be tested at 50, 100, and 150 cm which
correspond to stereoacuity of 500, 250, 168 arc sec.
New Directions
Refractive Error
• Many pediatric/vision organizations now recommend
the assessment of refractive error.
• This can be done using photoscreeners and
autorefractors.
• The major advantages of these techniques are:
– Rapid: they can provide estimates of refractive error in less than 2
mins.
– Objective: no judgments have to be made on the part of the tester.
– No response is required on the part of the subject.
– No Expertise Required: these techniques are so simple no vision
training is required.
New Directions
Contrast Sensitivity
• CS: the minimum amount of contrast required to
detect sine wave gratings at different spatial
frequencies.
• Measures one’s sensitivity to size and contrast
simultaneously.
• Measured by assessing one’s contrast threshold to
sinewave gratings at different spatial frequencies.
New Directions
• Can be measured in preschoolers using contrast sensitivity
cards (see below).
• The CS cards consist of 40 cards arranged in 5 spatial
frequency sets.
• The lowest contrast sine wave grating detected at each spatial
frequency is a measure of contrast threshold.
New Directions
• Despite its promise, no screening study has
evaluated whether contrast sensitivity is effective in
detecting amblyopia or amblyogenic factors in
preschoolers.
New Directions
Vernier Acuity
• Vernier acuity may be appropriate for vision
screening because it appears to be mediated by the
visual cortex.
• Amblyopia is a disorder that is cortical in nature.
• Thus, if amblyopia exists, it should be detected with
vernier acuity.
• In fact, adults with amblyopia possess severe vernier
acuity deficits in the amblyopic eye (Levi & Klein,
1982).
New Directions
• Can be assessed using the vernier acuity cards.
• Cards covered in a square wave grating that contains a
misalignment in stripe position in the form of a flower or star.
• The smallest amount of misalignment at which the shape is
detected provides a measure of vernier acuity.
New Directions
• Can also be assessed using the Preschool Vernier Acuity
Booklet which we developed in Dallas.
Star
Flower
New Direction
• The book contains square wave gratings arranged in groups of
4.
• One grating possesses a misaligned star, another possesses a
misaligned flower.
• The other two are blank.
• The subject has to identify the star and the flower.
• The lowest level of misalignment at which the subject
identifies the star and flower provides a measure of vernier
acuity.
• No screening study has assessed whether vernier acuity is
effective in identifying cases of amblyopia.