Amblyopia is a neuro-developmental disorder of the visual cortex that arises from abnormal visual experience early in life. It is commonly described as a type of “lazy eye,” when one eye isn’t able to see as well as the other despite using glasses or compensating lenses. Amblyopia can occur with or without a visible eye turn (strabismus). When amblyopia is found with an eye turn it is called strabismic amblyopia. When it is accompanied by a significant difference in compensating lens strength between the two eyes it is called refractive or anisometropic amblyopia. This usually happens when one eye is more farsighted (rarely more nearsighted) than the other. Amblyopia can also be due to deprivation of necessary visual stimulation during development. This is called deprivation amblyopia. The risk of developing amblyopia occurs when the causal factor is present during the sensitive period, which is from birth up to 8 years of age. Different functional losses in amblyopia result from different early visual experiences.
The best estimate of the prevalence of amblyopia in the general population is 2 to 4 percent. Refractive and strabismic amblyopia account for the vast majority of amblyopia. Amblyopia is the most common reason for monocular vision loss in people ages 20 to over 70, and it causes more vision loss than trauma and ocular disease in people under age 20. Although amblyopia usually happens in one eye, there are conditions that create amblyopia in both eyes. Specific risk factors for amblyopia have been identified as: prematurity, low birth weight, cerebral palsy, family history of amblyopia, and prenatal smoking, drug, or alcohol use. In cases of anisometropic amblyopia, the severity of amblyopia tends to be proportionate to the amount of asymmetry between the eyes; greater asymmetry is associated with more severe amblyopia.
Traditionally, the main consideration in amblyopia has surrounded the reduced level of eyesight (visual acuity), being less than 20/20 in the absence of structural anomalies or eye disease that would cause reduced vision. When there is an asymmetry between the two eyes in either alignment or power it causes each eye to receive a different image. To eliminate the confusion of processing dissimilar images, the visual system actively inhibits or suppresses the image from the affected eye. Over time, this causes changes in the brain’s visual cortex that result in a loss of visual acuity.
However, amblyopia is more than simply reduced visual acuity. Amblyopic eyes have difficulty with many skills and tasks. Amblyopia should be considered a collection of deficits in visual skill that can affect one or both eyes including: crowding phenomenon (surrounding detail interferes with visual identification), poor spatial localization, unsteady and inaccurate fixation, poor tracking, reduced contrast sensitivity, and poor accommodation or focusing.
A person with amblyopia affecting one eye (as in anisometropic and strabismic amblyopia) may have few reported symptoms, because they typically have normal visual acuity in their preferred eye. Although they may not notice it, people with amblyopia typically have poor 3-dimensional depth perception, which requires two eyes working together. This can cause difficulties with tasks that require accurate judgements about distance and localizing objects in space, such as driving. Visual performance is less efficient for actions involving hand-eye coordination and has implications for occupational eligibility. An additional consideration with amblyopia is that there is a greater risk of losing vision in the better seeing eye. Although it is not clear why, the risk of losing eyesight in the preferred eye is 3 times higher for an adult, and 17 times higher for a child.
Twenty-seven percent of patients with amblyopia affecting both eyes from high farsightedness may have an accompanying visual perceptual skills deficit associated with early learning problems. Perceptual deficits are three times more likely to occur when the farsightedness is not corrected for before the age of 4. Child development is dependent upon vision development and highlights the importance of providing an eye examination for children in the first year of life. Amblyopia is a preventable and a treatable condition especially when detected early. All children should have vision examinations at 1 and 3 years of age and before entering school, at the very least! Those with risk factors should be monitored yearly throughout the sensitive developmental period extending to 8 years of age. Unfortunately, the percentage of preschool children that are receiving appropriate vision care is low.
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Read more about amblyopia:
- Ciuffreda, KJ; Levi, DM; Selenow, A. Amblyopia: Basic and Clinical Aspects. Boston: Butterworth-Heinemann; 1991.
- Kiorpes L. Visual processing in amblyopia: animal studies. Strabismus. 2006; 14:3 -10. [PubMed: 16513565]
- Levi DM. Visual processing in amblyopia: human studies. Strabismus. 2006; 14: 11-9. [PubMed: 16513566]
- Wu C, Hunter DG. Amblyopia: diagnostic and therapeutic options. Am J Ophthalmol. 2006; 141: 175-84. [PubMed: 16386994]
- Von Noorden GK. New clinical aspects of stimulus deprivation amblyopia. Am J Ophthalmol. 1981; 92:416-21. [PubMed: 7294102]
- McKee SP, Levi DM, Movshon JA. The pattern of visual deficits in amblyopia. J Vis. 2003; 3: 380-405. [PubMed: 12875634]
- Levi DM, McKee SP, Movshon JA. Visual deficits in anisometropia. Vision Res. 2011; 51:48-57. [PubMed: 20932989]
- Hess RF, Thompson B, Baker DH. Binocular vision in amblyopia: structure, suppression and neuroplasticity. Ophthal,ic Physiol Opt 2014; 34:146-62.