Refractive amblyopia results from consistent retinal defocus in 1 or both eyes. Anisometropia causes unilateral amblyopia; isoametropia causes bilateral amblyopia.
In anisometropic amblyopia, dissimilar refractive errors in the 2 eyes cause 1 retinal image to be chronically defocused. Considered more prevalent than strabismic amblyopia in some recent US studies, this condition is thought to result partly from the direct effect of image blur and partly from interocular competition or inhibition similar (but not identical) to that responsible for strabismic amblyopia. Levels of anisometropia that commonly lead to amblyopia are greater than 1.50 diopters (D) of anisohyperopia, 2.00 D of anisoastigmatism, and 3.00 D of anisomyopia. Higher levels are associated with greater risk. The eyes of a child with anisometropic amblyopia usually appear normal to the family and primary care physician, which may delay detection and treatment.
McKean-Cowdin R, Cotter SA, Tarczy-Hornoch K, et al; Multi-ethnic Pediatric Eye Disease Study Group. Prevalence of amblyopia or strabismus in Asian and non-Hispanic white preschool children: Multi-ethnic Pediatric Eye Disease Study. Ophthalmology. 2013;120(10): 2117–2124.
Multi-ethnic Pediatric Eye Disease Study Group. Prevalence of amblyopia and strabismus in African American and Hispanic children ages 6 to 72 months: The Multi-ethnic Pediatric Eye Disease Study. Ophthalmology. 2008;115(7):1229–1236.e1.
Isoametropic amblyopia (bilateral ametropic amblyopia) is bilaterally decreased visual acuity resulting from chronically defocused retinal images, which are due to similarly large uncorrected refractive errors in both eyes. Hyperopia exceeding 4.00–5.00 D and myopia exceeding 5.00–6.00 D are risk factors. Bilateral high astigmatism may cause loss of resolving ability specific to the chronically blurred meridians (meridional amblyopia). Most ophthalmologists recommend correction for eyes with more than 2.00–3.00 D of cylinder.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.