Visual development, amblyopia and vision screening in childhood

Overview & Definition

Picture a four-year-old who sailed through her newborn and infant checks and sees perfectly with both eyes open, but who, at her school-entry vision test, can read only the top line with her left eye when the right is covered. The eye looks completely normal, there is no turn and no white pupil, yet acuity in that eye is poor and does not improve with a pinhole. This is amblyopia, and the key idea is that the problem is in the brain, not the eye: during the sensitive period of visual development the cortex received a blurred or competing image from that eye and learned to suppress it, so the neural machinery for sharp vision never developed. [2]

Normal visual development is a race against a closing window. A newborn sees at roughly 6/60 and refines acuity steadily through infancy and early childhood toward the adult 6/6, building the cortical circuits and binocular connections that depend on two clear, well-aligned images. That construction is most plastic in the first two to three years and tapers over the sensitive period to about seven to nine years of age, after which the cortex is much harder to remodel. The same plasticity is the reason amblyopia can be treated at all — forcing the weaker eye to work during the window lets the cortex recover — and the reason that, once the window closes, untreated amblyopia is largely permanent. [1] [11]

Amblyopia is defined as a unilateral, or occasionally bilateral, reduction in best-corrected visual acuity caused by abnormal visual experience during the sensitive period, in the absence of an organic lesion of the eye or visual pathway that would fully explain the deficit. It is common, affecting about one to three per cent of children, which makes it the leading cause of unilateral vision loss in this age group. Crucially, it is invisible to a both-eyes-open check, because the better eye hides the poorer one, so detection depends on testing each eye separately. [2]