Page 109 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

Effectiveness of routine population-wide orthoptic preschool vision screening tests at age 6-24 months in The Netherlands
Sample size calculation
We calculated the sample size for this comparative two sampled non-inferiority study, based on the assumption that an incidence of only 2.7% amblyopia could occur, the most disadvantageous incidence threshold in the RAMSES study. In the RAMSES study, 2964 children had undergone the complete 7 years follow-up and vison testing, yielding an amblyopia diagnosis in 100 children (3.4%, 95%CI: 2.7-4.0%).7 Using a type 1 error rate of 0.05 (α), a power of 80% (1-β, wherein the β (type 2 error) is 0.20) and a non-inferiority margin of 0.8%, we calculated that 5076 subjects were required per study group. We added a 5% anticipated loss to follow-up and dropout rate, yielding a minimum study population size of 10660 children.
Screening examinations
All children were invited to visit the CHC centres at 6, 7.5, 9, 11, 14, 18, 24, 36 and 45 months of age for general health screening. In the first National protocol, vision screening should be performed at 6, 9, 14 and 24 months by CHC physicians. In a later version of the National protocol, vision screening was only obligatory at 6-9 and 14-24 months.
Control (standard screening) group 6-24 months
Children born between July and December 2011 were vision screened according to the national protocol at 1-2 and 3-4 months with inspection of the eyes, pupillary reflexes, eye motility and red fundus reflex testing to rule out congenital eye disorders. At 6-24 months, the orthoptic vision screening took place at least two times: at age 6-9 and 14-24 months. The examination consisted of inspection of cornea and pupil, pupillary reflexes, eye motility, Hirschberg test, cover test and pursuit movements (table 1).
Intervention (reduced screening) group 6-24 months
Children born between January and June 2012 were eye screened at 1-2 and 3-4 months. These children attended general health screening visits at 6-24 months, but were only vision screened in case an eye abnormality was noticed or suspected by the screening physician or parent or in case of a positive family history.
Visual acuity measurements at 36-45 months in both groups
Visual acuity measurements were performed in both groups at the age of 36 and 45 months at the CHC centres with the APK and Landolt C chart, respectively. According to the national protocol, the result of the VA measurement can be sufficient or insufficient or the measurement itself fails (table 2). Insufficient and failed measurements must be
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