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• We have developed a new computerized vision

tester.• A sine wave grating is presented on the top or

bottom of a high-resolution, calibrated computer monitor.

New Directions

• The monitor is connected to a laptop which has a “library” of sine wave gratings.

• The gratings are arranged in 5 spatial frequency sets.• Tester simply points and clicks to present a sine wave grating.• Has a number of advantages over traditional tests.

– High Testability– Calibrated– An infinite number of contrast levels can be used– Accurate

New Directions

• CS Cards Computerized Test 21.8 22

1615

11 11.5 10

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.