Page 112 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

CHAPTER 6
When a VA measurement from the treating orthoptist was available, amblyopia diagnosis was based on the first VA measurement, before glasses adaptation. The orthoptists classified children into definitively, probably, probably not or no amblyopia. This classification was mainly based on a VA difference 2 logMAR lines difference between the eyes or a bilateral VA ≤0.5 snellen VA before glasses adaptation or strong fixation preference or amblyopic factor.
The research orthoptist (MT), researcher (FS) and ophthalmologist (HJS) determined the definitive presence, type and severity of amblyopia in both groups, taking all VA measurements, from both the CHC centres and the treating orthoptists, and the orthoptist’ classification into account. If amblyopia was present, the type of amblyopia was defined based on the presence of an amblyogenic factor (strabismus, deprivation or refraction). Refractive amblyopia, for all age groups, was diagnosed when spherical equivalent between the eyes differed ≥1.00 dioptres or astigmatism with oblique axis, especially with opposite direction was present. Strabismus amblyopia was diagnosed when strabismus was determined by the orthoptist.
Statistical Analysis
All analyses were performed with the statistical package for social sciences (SPSS, IBM Corp.) software, version 25.0.0.2. Statistical significance was set at the 0.05 level and all testing was two sided. Testing of categorical variables (e.g. two by two tables) was conducted with a Chi square test. We aimed to study and compare both groups, with respect to their time to referral and time to amblyopia diagnosis. The time to amblyopia analysis follows an intention to screen method (primary end-point). Secondarily, a per protocol analysis (no screening versus at least 1 screening test) was performed. These time to event analyses, including the corresponding hazard ratio’s (HR) and figures, were performed with a Cox regression model for proportional hazards. In the sensitivity analysis, a covariate was added to the model to study its influence on the results. A Mann-Whitney U-test was performed to investigate difference in depth of amblyopia between the two groups (not normally distributed data).
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