Should we be screening for and treating amblyopia?

In 1981 the award of the Nobel prize for medicine for the discovery of the pathophysiology of amblyopia marked a turning point in the management of children with this condition.¹ Recognition that early visual experience is essential for the development of the visual brain has fundamentally changed the way we manage disorders that interfere with image formation in the eye during early life. For example, very early screening, detection, and intervention for sight threatening congenital cataract² has practically eliminated this condition as a cause of long term visual impairment in the developed world.

People looking after children with amblyopia often see improvement of vision after patching of the good eye and no improvement (or even deterioration) in children whose patching is not carried out as recommended,³ but the lack of controlled trials led to the recommendation that a randomised controlled trial should be carried out in which the control group would not be treated.⁴ The trial reported by Clarke et al in this week's BMJ (p 1251)⁵ is a response to this challenge and affirms that treatment of unilateral amblyopia is effective in improving poor vision, but not in improving vision in children with a starting visual acuity of 6/9 or 6/12 vision. A visual acuity of 6/9 can fall within the age norms for the logMAR crowded test used in this study,⁶ so it is possible that not all children with this level of vision were amblyopic and that the small changes observed represented normal visual maturation. The lack of a statistically significant loss of vision in the no treatment group undermines the argument that children with 6/9 or 6/12 vision in the affected eye need to be identified and treated to prevent their vision from deteriorating.

The optimal time for visual screening has also been the subject of debate. Preschool screening in the community has been advocated to ensure timely treatment, but high default rates hamper the efficacy of detection and screening for amblyopia.⁷ The current finding that treatment efficacy is not diminished if treatment is deferred⁵ potentially allows for screening and treatment to be conducted during the first year at school.⁸ This could improve both coverage and compliance with treatment. It is reassuring to learn from this paper that deferring treatment for 12 months in children with acuities of 6/36 or better does not have an adverse outcome, but we do not know whether this applies to children whose vision is worse than this at the outset.

The results described by Clarke et al⁵ should not lead to the conclusion that screening and treating children with 6/9 or 6/12 vision is not worth while, because the treatment applied was based on accepted tradition. An alternative approach is to question whether we can improve our treatment protocols to attain better results in this group. The traditional model of patching for amblyopia tends to be one in which the closer vision is to normal the less treatment is given. However, recent work has indicated that more intensive treatment may be needed in children with better levels of vision in order to re-establish bifoveal fixation.⁹ Psychosocial analysis is also necessary to determine whether the benefits of treatment outweigh the strains imposed by patching.¹⁰

Nevertheless, good evidence exists that screening and treatment for amblyopia in young children is warranted. For example, a large study from Israel found that 8 year old children previously screened and treated for amblyopia had a 1% prevalence of the disorder compared with 2.6% for a matched untreated population.¹¹

It could be questioned whether it is worth while treating amblyopia in one eye. After all the other eye can act as a spare. In many countries driving standards for professional drivers, the armed forces, and emergency services require good visual acuities in both eyes, so amblyopia can be a bar to entering these professions. Moreover, sizeable numbers of people go blind every year because of new disease in one eye and prior amblyopia in the other.¹² The projected lifetime risk of vision loss for an individual with amblyopia was found to be at least 1.2%.¹²

The study by Clarke et al confirms that in at least some children detecting and treating amblyopia is beneficial. We now need to turn our attention to optimising strategies to get the best results.