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Alaska Blind Child Discovery

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Acuity Test Issues-proposed

ABCD History
AAPOS Preschool Visual Acuity Test Recommendations DRAFT v 2.0 01/06/2006
Kurt Simons, PhD
Johns Hopkins Hospital

1. Test should use logarithmic scaling, with one logMAR unit between acuity levels. Currently available tests with this format include:
(a) Full chart (ETDRS/Bailey-Lovie, HOTV, or Lea tests),
(b) Single surrounded-line (logMar Crowded Test[2] (originally the Glasgow Acuity Cards), Lea, HOTV tests) or
(c) single surrounded-optotype (HOTV, Lea tests).
2. Optotypes in chart or line should be half-optotype-width (= 2.5 stroke widths) apart.[3,9] Surround bars single optotypes or lines should be half-optotype spacing from optotypes(s) surrounded (= 2.5 stroke widths) and equal in thickness to optotype stroke width. Surround bar stroke width and spacing should scale with optotype size.
3. Test should be administered in peek-proof monocular form, from use of adhesive patch on non-tested eye. If child refuses patch, opaque paddle or palm of child's hand should be used instead.
3. Response should be by means of verbal identification or matching card. When card is used, it is hidden while child is viewing given optotype, so as not to distract the child, then re-exposed for response to that optotype.[6]
4. Testing distance should be 3 meters (10 feet).
5. There should be a binocular pre-test to determine child's understanding of test protocol, using large (e.g. > 20/100) optotypes. Pre-test should include full set of optotypes to be used in the test.
6. If child has difficulty responding in sequence when line or chart format is used, examiner should try pointing to each letter on test plate to direct the child's attention.
7. Threshold definition:
(a) Passing a given acuity level is defined as > 75% correct of letters in a chart line or of a set of sequentially presented single surrounded-optotype presentations. With a four-alternative test (such as the HOTV or Lea), passing at a given line or single optotype size:
Passing Criterion , 4-alternative test Probability of passing by chance
3 of 4 correct p <.05
4 of 5 correct p <.02
4 of 4 correct p <.004

For interocular measure, failure criteria should be > 2 logMAR lines difference between eyes
(b) Using smaller increments in an acuity grading scale produces narrower confidence limits and reduces test-retest variability.[10.1] One application of this fact is to use letter-by-letter rather than line-by-line acuity grading. Thus, for instance, in a 4-letter line, each letter is worth 0.025 log units. Threshold is defined as the number of letters correct (or the letter count of the first failure).[4, 7, 8]
a) EVA type staircase protocol would be more efficient but appears too complicated for this setting.
1. All tests referred to are commercially available.
2. It is necessary to use surround bars even with full lines of optotypes because non-surrounded single lines produce indicated visual acuity half-way between the level of full chart visual acuity and single-optotype visual acuity.[5]
1. Bailey I, Bullmore,MA, Raasch,TW, Taylor: Clinical grading and the effects of scaling. Invest Ophthalmol Vis Sci 32: 422-32, 1991
2. Cleary M: Efficacy of occlusion for strabismic amblyopia: can an optimal duration be identified? Br J Ophthalmol 84: 572-8, 2000
3. Fern KD, Manny RE, Davis JR, et al.: Contour interaction in the preschool child. Am J Optom & Physiol Optics 63: 313-8, 1986
4. McGraw P, Winn B, Gray L, et al.: Improving the reliability of visual acuity measures in young children. Opthalm Physiol Opt 20: 173-84, 2000
5. Morad Y, Werker,E, Nemet,P,: Visual acuity tests using chart, line, and single optotype in healthy and amblyopic children. J AAPOS 3: 94-7, 1999
6. Sharkey J, Sellar, PW: Acquired central fusion disruption following cataract extraction. J Ped Ophthalmol Strab 31: 391-393, 1994
7. Simmers A, Gray L, Spowart K: Screening for amblyopia: A comparison of paediatric letter tests. Br J Ophthalmol 81: 465-9, 1997
8. Simmers A, Gray, LS, Winn, B: Visual function thresholds in children. Curr Eye Res 21: 616-626, 2000
9. Stager DR, Everett ME, Birch EE: Comparison of crowding bar and linear optotype acuity in amblyopia. Am Orthop J 40: 51-56, 1990
10. Vanden Bosch ME, Wall M: Visual acuity scored by the letter-by-letter or probit methods has lower retest variability than the line assignment method. Eye 11 ( Pt 3): 411-7, 1997
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