Page 67 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

The effect of omitting an early population-based vision screen in the Netherlands: A micro-simulation model approach
DISCUSSION
This study shows that the effect of omission of components of a screening programme can be calculated with the micro-simulation model, provided that sufficient and detailed data are available. It confirms the suggestion, from a large prospective birth- cohort observation study, that part of the screening programme seemed to add little to the detection of amblyopia. Analysis of the data had shown that screening at 6–24 months, an age when VA cannot be measured yet, contributed little to the detection of refractive amblyopia, whereas strabismic amblyopia was detected outside of screening in half of cases.4 With the micro-simulation model, the screening examination at 24 months was found to be least effective. Omitting this screen reduced the total number of detected cases of amblyopia at age 5 years from 59 to 57 (3.4%).
Our model has limitations. It is not incontrovertible to adapt some of the less well determined input parameters, like the incidence curves, to fit the detailed data from the RAMSES study, but our pragmatic approach served as a good starting point of the simulation of omission of part of the programme. For micro-simulations, accurate data on the prevalence and incidence of the disease at the time of screening and on the sensitivity of the screening methods are essential, but these data are difficult to obtain from observational studies. The age-specific incidence also varies per amblyopia type. But the curves for the four types are essential for the simulation of the effectiveness of each screening examination. We had to make assumptions to estimate these incidence curves, and for the sensitivity of the tests. To calibrate the upper and lower limits of the amblyopia incidence curves, we therefore had to use indirect derivatives such as the RAMSES data, literature, and expert opinion. If the age specific incidence of amblyopia were in fact lower than estimated, the incidence curves would be closer to the curves of detected cases in the RAMSES study, and the sensitivity of the screen would be higher. If the incidence curves were higher than estimated, the sensitivity would be lower than estimated. If children were to be tested at a later age, when they would probably be easier to test, the sensitivity of the tests would increase, leading to a higher amblyopia case detection.
It is difficult to compare our incidence curves with data in current literature. Atkinson et al. identified manifest strabismus and strabismogenic and amblyogenic refractive errors in children aged 7–9 months and found a hyperopia (≥3.5D) prevalence of 5–6% and an anisometropia and manifest strabismus prevalence of<1% each. Untreated hyperopes developed strabismus in 21%.9 The children were not classified as amblyopic or not,
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