Amblyopia: past, present and future

Amblyopia affects around 2–2.5% of Western populations. Undetected refractive error aside, it is the most common form of childhood visual impairment that front-line eye care professionals encounter. In fact, it has recently been shown that management of amblyopia accounts for around 75% of all eye appointments in the NHS paediatric eye service. This fact, combined with a national screening programme to detect the condition early, suggests that amblyopia remains a major public health concern and considerable healthcare resources are devoted to detecting and treating it. If left untreated, amblyopia produces a range of functional deficits that include poor monocular visual acuity, reduced contrast sensitivity, positional uncertainty, marked distortions of visual space and elevated levels of visual crowding in the amblyopic eye. Binocular function is also compromised; subjects commonly display poor oculomotor control (e.g. fixation stability, saccade accuracy and timing, fusional vergence), suppression of all or part of the visual input from the affected eye and reduced stereoacuity. More recently, studies have started to quantify how the presence of amblyopia (or its treatment) impact on educational attainment, future career opportunities, self-esteem and quality of life. These studies reveal the practical and emotional impact of amblyopia and provide additional evidence in support of the need to develop effective treatment. Given the scale of the problem and the obvious functional and social costs of not treating the condition, it seems timely to review the body of work devoted to this topic that has appeared in Ophthalmic and Physiological Optics over the last decade or so. This period has seen intense debate around whether amblyopia should be treated, the quality of evidence needed to justify screening and treatment, and the development of some exciting new treatment strategies. All of these important issues are covered in this special virtual issue on amblyopia. The first paper in the collection asks a simple but important question. What are the principal causes of reduced visual acuity in children? Using a school-based paediatric survey, the authors show that in children in China (age 3–6 years), around two-thirds have reduced visual acuity from uncorrected refractive error and the remaining third from amblyopia. These figures are also likely to represent those of Asian populations in other countries. Animal models of amblyopia have been instrumental in revealing the underlying pathophysiology of the condition. Studies in both cat and monkey have shown that even brief periods of abnormal visual input, introduced early in development, have dramatic consequences on the structure and function of cortical visual pathways. Specifically, sensitivity to high spatial frequencies is reduced and binocular responses are abnormal. These studies have placed the primary site of the abnormality at the level of the visual cortex. However, many people have argued that visual deprivation, which is usually introduced via lid suture, is not a particularly representative model of the human condition. In humans, amblyopia is much more frequently associated with unequal amounts of refractive error or strabismus. This has led some to speculate that the visual deficit associated with naturally occurring human amblyopia may reside at a different location in the visual pathway. The next two papers deal with this issue. Debert et al. ask whether there are important differences in the oculometric parameters in children with amblyopia and esotropia. They found that amblyopic eyes are significantly more hyperopic and have reduced corneal power, greater lens power and shorter vitreous chamber depth and axial length compared to fellow eyes. The degree of refractive error is strongly related to the axial length/corneal radius ratio and this is similar in fellow and amblyopic eyes. This suggests that the mechanisms that govern relative growth of the ocular components are essentially normal in amblyopia. Next, Brown et al. examine neural retinal function to probe whether the primary deficit in amblyopia resides at the level of the retina. In subjects with previously identified compromised function at the level of the lateral geniculate nucleus (established using functional magnetic resonance imaging), they used multi-focal ERG, optical coherence tomography and microperimetry to evaluate responses from the central visual field. Although they found central deficits in retinal function, there were no obvious anatomical or structural problems identified. They speculate that the retinal and LGN deficits may not be linked. However, previous work in cats has indicated that early visual deprivation can introduce deficits earlier in the visual pathway. The potential