Page 62 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS
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CHAPTER 3
Several orthoptists and strabismologists were asked as experts (i) to report the youngest age at which patients were diagnosed with strabismic, refractive, combined mechanism, and deprivation amblyopia, and (ii) to estimate the highest age at which each of the four types of amblyopia could develop. For instance they were asked whether they had ever seen a patient with refractive amblyopia diagnosed at age 5 who had good VA at age 3. By approximation of the observational RAMSES data, in conjunction with experts’ estimations and literature,6–8 incidence curves for the four types of amblyopia were estimated, with upper and lower estimated boundaries. From the literature and expert opinion we assumed that the sensitive period for amblyopia to develop starts at 3 months (tstart) and that there is no additional amblyopia development after the age of 5 years (tend).6–8 The upper limit follows a negative polynomial function (equation (1)). For the lower estimate, the progression is more conservative and stagnates in time, described by a parabolic function (equation (2)).
(1) (2)
Where I(t) is the incidence at time t and a, b, c, α and β are variables indicated in the input function:
where tstart is the age for which the first diseased cases start to develop, tend is the age for which the incidence rate reaches zero or is equal to zero, and p is the prevalence at the end of amblyopia development, that is when incidence rate is zero.
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