The relationship between anisometropia and amblyopia

Abstract

This review aims to disentangle cause and effect in the relationship between anisometropia and amblyopia. Specifically, we examine the literature for evidence to support different possible developmental sequences that could ultimately lead to the presentation of both conditions. The prevalence of anisometropia is around 20% for an inter-ocular difference of 0.5D or greater in spherical equivalent refraction, falling to 2-3%, for an inter-ocular difference of 3D or above. Anisometropia prevalence is relatively high in the weeks following birth, in the teenage years coinciding with the onset of myopia and, most notably, in older adults starting after the onset of presbyopia. It has about one-third the prevalence of bilateral refractive errors of the same magnitude. Importantly, the prevalence of anisometropia is higher in highly ametropic groups, suggesting that emmetropization failures underlying ametropia and anisometropia may be similar. Amblyopia is present in 1-3% of humans and around one-half to two-thirds of amblyopes have anisometropia either alone or in combination with strabismus. The frequent co-existence of amblyopia and anisometropia at a child’s first clinical examination promotes the belief that the anisometropia has caused the amblyopia, as has been demonstrated in animal models of the condition. In reviewing the human and monkey literature however it is clear that there are additional paths beyond this classic hypothesis to the co-occurrence of anisometropia and amblyopia. For example, after amblyopia secondary to either deprivation or strabismus has emerged, anisometropia often follows. In cases of anisometropia with no apparent deprivation or strabismus, questions remain about the failure of the emmetropization mechanism that routinely eliminates infantile anisometropia. Also, the chronology of amblyopia development is poorly documented in cases of ‘pure’ anisometropic amblyopia. Although indirect, the therapeutic impact of refractive correction on anisometropic amblyopia provides strong support for the hypothesis that the anisometropia caused the amblyopia. Direct evidence for the aetiology of anisometropic amblyopia will require longitudinal tracking of at-risk infants, which poses numerous methodological challenges. However, if we are to prevent this condition, we must understand the factors that cause it to develop.

Stereopsis and amblyopia: A mini-review

“…There is no shortage of reviews of various aspects of amblyopia over the last decade (Birch, 2013; Hess, Babu, et al, 2014; Hess, Thompson, et al, 2014; Kiorpes, 2006; Levi, 2006; Webber & Wood, 2005; Wong, 2012: Barrett, Bradley, & Candy, 2013; Grant & Moseley, 2011; Levi, 2012; Levi, 2013; Levi & Li, 2009; Repka & Holmes, 2012). The focus of this review is on stereopsis and its potential for recovery in persons with amblyopia, specifically, we address the following issues:

How is stereopsis compromised in amblyopia?

Why does stereopsis matter?

Can stereopsis be recovered in children and adults with amblyopia?

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Amblyopia: neural basis and therapeutic approaches

“…Strabismus, one of the principal ocular deficits in low-income children (29,30) , is characterized by dysfunction of the extraocular muscles, generating binocular misalignment. The strabismic eye may fail to receive visual stimuli onto the macular area (31) , thus affecting various developmental stages of cortical processing (16) .…”

“…As a form of treatment independent of occlusion, some studies have reported the use of atropine (Table 2), a parasympatholytic muscarinic antagonist. Atropine may help in treating moderate amblyopia (29) , as it interferes with visual accommodation of the fellow normal eye, thus indirectly forcing use of the amblyopic one (19) . However, atropine treatment is not always effective (48) .…”

Sekundärerkrankungen bei hoher Myopie

“…Mit höherer Kurzsichtigkeit ist auch das häufigere Auftreten einer Amblyopie verbunden [7]. Das hängt nicht zuletzt damit zusammen, dass die Rate höherer Anisometropien auf weit über 10 % in der Gruppe mit hohen Ametropien anwächst [56].…”