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Electronic monitoring of compliance shows that prescribing longer periods of occlusion is not always better
Amblyopia affects about 3-5% of the population. Occlusion therapy using an eye patch to cover the non-amblyopic eye for a couple of hours each day has been the principal means of treatment. The sensitive period in which vision loss can develop and be recovered is generally up to 6 years of age.1 In many European countries, population based screening and treatment by orthoptists has reduced the proportion of people with untreated or insufficiently treated amblyopia to about 1% of the population.2 3
The effectiveness of screening and treatment for amblyopia in the United Kingdom has been questioned because of insufficient evidence from randomised controlled trials,4 and an effort is now being made to assess its effectiveness and cost. In this week's BMJ, a randomised controlled trial by Stewart and colleagues compares the effect of prescribing six or 12 hours of occlusion each day in 97 children with amblyopia associated with strabismus, anisometropia, or both.5 It is the first randomised controlled trial to investigate the relation between the duration of occlusion and visual acuity, and so it greatly contributes to our understanding of the effectiveness of occlusion therapy.
The potential costs that screening and treatment could save in cases where vision in the better eye is lost can be calculated: when amblyopia is insufficiently treated, the duration of bilateral visual impairment (visual acuity 6/12 or less—not being able to read) in later life is 0.6 years longer than in people without amblyopia (average of 1.3 v 0.7 years).6 For example, if 1% of the Dutch population had insufficiently treated amblyopia, 1800 people would be at risk each year for bilateral visual impairment. If a visually impaired person costs society €5000 (£3500; $7000), a conservative estimate, a minimum of €5.4m could be saved annually in the Netherlands. Adding to this, the remaining patients with insufficiently treated amblyopia, who do not lose vision in their better eye, have slightly reduced quality of life.7
The main cause of insufficiently treated amblyopia is poor compliance.8 9 Electronic monitoring of compliance with occlusion therapy is now possible with the occlusion dose monitor.10 Previous studies using this monitor found that compliance averaged 50%,8 even though parents knew that compliance was being monitored. Median compliance is 70%, but a considerable number of children do not occlude at all. The most important non-clinical predictor for poor compliance is poor fluency in the national language. This can be remedied by giving information aimed primarily at the child.11
Few guidelines exist when prescribing occlusion therapy. Age, visual acuity, and, to a lesser extent, the cause of amblyopia (strabismus, or anisometropia, or both) seem to be important determinants when prescribing a certain number of hours. Electronic monitoring now allows a precise assessment of the relation between the duration of occlusion and the increase in visual acuity.
The study by Stewart and colleagues found no significantly different increase in visual acuity between children who were prescribed six hours of occlusion each day and those who were prescribed 12 hours. The beneficial effect of wearing glasses was analysed separately. Surprisingly, however, the six hour group had occluded on average 4 hours a day (standard deviation 1.7) and the 12 hour group on average 6 hours a day (4.1). The study confirmed that older children need more hours of occlusion each day—common knowledge among orthoptists.8 Children younger than 4 years needed less occlusion (less than three hours a day) than children older than 6 (who needed three to six hours). Occluding for six hours or more each day marginally improved acuity, even in older children. A greater number of occlusion hours hastened the response but did not improve the final outcome.
Although the study was a randomised controlled trial, it was analysed on an “as treated” basis, which relates the outcome to the occlusion time actually received. However, poor initial visual acuity is the most important clinical predictor for poor treatment outcome. One reason for this is that children are less likely to accept the patch when visual acuity is low.
So what do these results mean for clinical practice? As Stewart and colleagues suggest, when evaluating compliance or a dose-response relation, compliance should be monitored electronically. Relying on patients' reports or diaries is not good enough. Their results also show that the perceived hardship of wearing a patch for 12 hours a day, imposed on the child and his or her parents, has a negative effect on compliance. Orthoptists and ophthalmologists are becoming increasingly aware that when compliance is faltering it may be better to prescribe fewer hours of occlusion if it means that their instructions are actually carried out.
Competing interests: None declared.
Provenance and peer review: Commissioned; not externally peer reviewed.