Amblyopia

Amblyopia results from abnormal visual stimulation during early childhood. The retina is not stimulated with a clear, focused image, or the visual axes may be misaligned, leading to suppression of the retinal image formed by one of the eyes. Research has demonstrated that there are critical periods for the development of normal vision and that amblyopia is associated with structural changes in the primary visual cortex.[2]Wiesel TN, Hubel DH. Single-cell responses in striate cortex of kittens deprived of vision in one eye. J Neurophysiol. 1963 Nov;26:1003-17. https://jn.physiology.org/cgi/reprint/26/6/1003 http://www.ncbi.nlm.nih.gov/pubmed/14084161?tool=bestpractice.com [3]Hubel DH, Wiesel TN. The period of susceptibility to the physiological effects of unilateral eye closure in kittens. J Physiol. 1970 Feb;206(2):419-36. https://onlinelibrary.wiley.com/doi/10.1113/jphysiol.1970.sp009022/abstract http://www.ncbi.nlm.nih.gov/pubmed/5498493?tool=bestpractice.com Amblyopia generally develops in children younger than 9 years, before the development of visual maturity.

• Strabismus (misalignment of the eyes) is thought to result in amblyopia because of the competitive or inhibitory interaction between the cortical inputs from the two eyes. Cortical vision centres from the fixating eye dominate, and cortical vision centres from the non-fixating eye are chronically suppressed.[1]Cruz OA, Repka MX, Hercinovic A, et al. Amblyopia preferred practice pattern. Ophthalmology. 2023 Mar;130(3):P136-78. https://www.aaojournal.org/article/S0161-6420(22)00865-X/fulltext http://www.ncbi.nlm.nih.gov/pubmed/36526450?tool=bestpractice.com ​​​

• Anisometropia (unequal refractive error in the two eyes) causes the image on one retina to be chronically defocused. Amblyopia results from the direct effect of image blur on visual development. It also partly results from inter-ocular competition (or inhibition) similar to that responsible for strabismic amblyopia.​[1]Cruz OA, Repka MX, Hercinovic A, et al. Amblyopia preferred practice pattern. Ophthalmology. 2023 Mar;130(3):P136-78. https://www.aaojournal.org/article/S0161-6420(22)00865-X/fulltext http://www.ncbi.nlm.nih.gov/pubmed/36526450?tool=bestpractice.com ​

• Ametropia (both eyes are about equally defocused) causes no cortical competition or inhibition between the two eyes. Amblyopia results from image blur alone.​[1]Cruz OA, Repka MX, Hercinovic A, et al. Amblyopia preferred practice pattern. Ophthalmology. 2023 Mar;130(3):P136-78. https://www.aaojournal.org/article/S0161-6420(22)00865-X/fulltext http://www.ncbi.nlm.nih.gov/pubmed/36526450?tool=bestpractice.com ​

• Corneal and/or lenticular astigmatism in a particular meridian may result in meridional amblyopia due to optical defocus. Amblyopia is partially due to loss of cells in the primary visual cortex with an orientation similar to the blurred meridian.​[1]Cruz OA, Repka MX, Hercinovic A, et al. Amblyopia preferred practice pattern. Ophthalmology. 2023 Mar;130(3):P136-78. https://www.aaojournal.org/article/S0161-6420(22)00865-X/fulltext http://www.ncbi.nlm.nih.gov/pubmed/36526450?tool=bestpractice.com ​

• Occlusion of the visual axis or severe image blur caused by opacities in the visual axis may result in form-deprivation amblyopia. Unilateral occlusion is worse than bilateral occlusion of similar magnitude, because inter-ocular competition adds to the direct developmental impact of image degradation.​[1]Cruz OA, Repka MX, Hercinovic A, et al. Amblyopia preferred practice pattern. Ophthalmology. 2023 Mar;130(3):P136-78. https://www.aaojournal.org/article/S0161-6420(22)00865-X/fulltext http://www.ncbi.nlm.nih.gov/pubmed/36526450?tool=bestpractice.com ​

Types according to aetiology

Strabismic amblyopia

• Strabismus (misalignment of the eyes) results in amblyopia if the child prefers to fixate with one eye instead of freely alternating fixation between two eyes.

• The non-fixating eye (or deviated eye) is suppressed as an adaptive mechanism to avoid double vision.

• Prolonged suppression during early childhood leads to amblyopia.

Refractive amblyopia

• Refractive errors defocus the retinal image and, if marked, can cause amblyopia.

• May occur with strabismus.

• Greater anisometropia or astigmatism increase the risk and severity.

• Subtypes include:

  • Anisometropic amblyopia: caused by unequal refractive error between the two eyes. Most often develops in hyperopic (long-sighted) children who focus with the less hyperopic eye, leading to persistent blurring of the more hyperopic eye.

  • Ametropic (isoametropic) amblyopia: caused by high and similar refractive errors in the two eyes. Both high hyperopia and high myopia (short-sightedness) can cause sufficient retinal image blur to induce amblyopia, but amblyopia occurs more commonly with high hyperopia.

  • Meridional amblyopia: high astigmatism in one or both eyes causes blur of a retinal image in a particular meridian.

  • Bilateral refractive (isoametropic) amblyopia: uncommon and results from the effect of bilateral blurred retinal images.

Form deprivation amblyopia

• This results from blurring of the retinal image caused by opacities in the cornea, anterior chamber, lens, vitreous chamber, or retinal surface (e.g., macular haemorrhage).

• Deprivation can also occur due to severe ptosis or prolonged occlusion of one or both eyes.

• It is the least common cause, but is often the most severe and difficult to treat.

Occlusion (reverse) amblyopia

• This is a form of deprivation amblyopia that develops after therapeutic patching or pharmacological cycloplegia of the non-amblyopic eye.

• Visual acuity usually returns to baseline without treatment.

Combined mechanism

• Different subtypes of amblyopia can co-exist in the same patient (e.g., strabismic and hyperopic anisometropic amblyopia frequently co-occur).

• Some children with moderate to high hyperopia develop accommodative esotropia (inward turning of the eye). This is a type of strabismus resulting from the yoking of convergence and accommodation (which is necessary to focus the image in the hyperopic eye). If the hyperopia differs between the two eyes, it is generally the more highly hyperopic eye that turns in and develops amblyopia.