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RESEARCH PRODUCT

The Prevalence of Amblyopia in Germany

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Amblyopia is a condition in which reduced visual acuity is not directly caused by an organic defect (1). Amblyopia can develop when an infant or small child suffers from strabismus. Although both eyes are healthy, different images are formed on each fovea centralis (where vision is sharpest) and on other corresponding locations on the retina; this is in contrast to individuals without strabismus. To prevent double vision the child’s brain suppresses the image produced by one eye. Complete visual ability cannot develop in this eye during the sensitive phase of visual development (2). Approximately half of amblyopia cases (3) are caused by uncorrected higher refractive errors, which are frequently different in each eye: only vague visual outlines are formed on the retina, and the stimulus required for optimal development of visual acuity is absent (4). Deprivation in which the optical axis is displaced by, for example, congenital ptosis (drooping eyelid) or a cataract can also cause amblyopia (5). Relative amblyopia develops when there are organic defects affecting visual acuity, such as an infantile cataract. The poor visual information provided by the diseased eye is, in addition, suppressed by the child’s brain, rendering visual acuity significantly worse than would be expected from organic findings (6). If there is no apparent strabismus or visible organic defect such as ptosis, such amblyopia very often cannot be detected by those around the child. Unilaterally reduced visual acuity has almost no effect on bilateral visual acuity. Even bilaterally reduced visual acuity must be severe in order to be noticeable in the child’s day-to-day life. Treatment for amblyopia must be started early. The older the child and the more advanced visual maturation when treatment begins, the lower the chance of successful treatment (7– 9). A large meta-analysis has revealed that treatment begun before the age of seven years yields significantly greater increases in visual acuity—a mean of up to four visual lines—than treatment begun later (a mean of up to two visual lines) (10). Treatment for amblyopia in adulthood is unlikely to be successful (11). Depending on the underlying cause of amblyopia, treatment consists of glasses and/or occlusion therapy (using a patch to cover the better eye). Less common in Germany but confirmed as effective in a Cochrane review is the use of atropine (which paralyzes accommodation, leading to worse near vision) in the better eye (12, 13). Amblyopia remains a lifelong problem if treated too late or left untreated. The risk of bilateral visual impairment is two to three times higher in patients with unilateral amblyopia than in those without amblyopia. In a population-based Dutch study, the cumulative lifetime risk of bilateral visual impairment was 18% in those with unilateral amblyopia and 10% in those without amblyopia (14). In a Finnish study the frequency of loss of sight in the better eye before retirement was 1.75‰ in those with unilateral amblyopia; the population frequency of blindness was 0.79 ‰ (15). According to large, population-based studies, the prevalence of amblyopia in Australia is approximately 3% among adults (3, 16) and less than 2% in preschool children (17– 19). Other studies, conducted in Cameroon for example, report a 10% prevalence of amblyopia in eye clinic patients aged between 5 and 15 years (20). The prevalence of amblyopia among schoolchildren in China is very low, at 1% (21); it is significantly higher (5.5%) among Turkish schoolchildren (22) and lies between these two levels, at 3.1%, in Polish children (23). A selection of population-based studies on the prevalence of amblyopia is provided in Table 1. However, the degree to which different studies on the prevalence of amblyopia can be compared is limited, as participant age in the investigated populations and study quality vary. In addition, the visual criteria used to define amblyopia often differ substantially from each other, as there is no internationally recognized standard. Table 1 The prevalence of amblyopia in various studies There are few figures on the frequency of amblyopia in Germany, and there is a lack of epidemiological data from population-based studies. In 1979 a study involving 830 children beginning school found amblyopia in 1.9% of children without strabismus and 44.2% of those with strabismus (24). Another study of 1030 preschool children found manifest strabismus in 3.7% (25) but provided no figures on amblyopia prevalence. This population-based study aims to obtain more precise figures on the prevalence of amblyopia in Germany and to learn more about the frequency of its causes. This research is based on data from a young population (aged 35 to 44 years).