Page 13 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

due to, for example, congenital cataract or retinoblastoma. The reduction in VA is commonly unilateral, but it can be bilateral. Amblyopia can only form during the sensitive period of visual development, which stretches over the first decade of life.
The causes for normally aligned eyes and isometropia are obscure and so also the causes for abnormalities in alignment and refraction remain obscure. In normal visual development, emmetropisation, the process of neutralization of the refractive status of the eye during childhood, takes places. Failure to emmetropise has been shown to be highly associated with development of amblyopia.9,10 It has been shown that in unilateral amblyopia, the fixating eye becomes more myopic, while the amblyopic eye remains hyperopic. Increasing astigmatism is also highly associated with an increased risk to develop amblyopia.11 Increasing hypermetropia has been shown to be a risk factor for esotropia (strabismus) and amblyopia.12 The relationship between anisometropia and amblyopia is still unclear. Barrett et al suggest three different hypotheses. The first (classic) hypothesis states that anisometropia (due to failure of emmetropisation) leads to amblyopia due to the chronic, unilateral blur during early visual development. The second hypothesis suggests that an amblyogenic factor leads to amblyopia and the amblyopia consecutively leads to anisometropia. The third hypothesis proposes a third factor that causes both the amblyopia and the anisometropia. In their extensive review they cannot find enough evidence to reject any of these hypotheses.13 Increasing anisometropia is associated with higher change of deterioration of VA after cessation of occlusion therapy.14
Natural history of untreated amblyopia and prevalence
There are no longitudinal studies aiming to study the natural history of untreated amblyopia. It would be unethical to withhold treatment from a child with detected amblyopia. Studies on the natural history of amblyopia due to non-compliance have shown that the VA of the amblyopic eye deteriorates during childhood15 as well as during adolescence.16 Studies on prevalence of amblyopia in countries without vision screening, and studies on prevalence in non-screened older age-cohorts in countries with vision screening, have shown higher prevalence than in vision screened populations.
Vision screening seems to lower the prevalence of amblyopia with VA ≤ 0.5 to between one third and half the prevalence of that in an unscreened population. The prevalence of residual amblyopia may be due to cases not attending screening, cases missed at screening, the condition having developed later, unsuccessful treatment (including
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General introduction
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