What is amblyopia?

Amblyopia may be defined as a potentially reversible loss in visual function (e.g., acuity, contrast sensitivity, motion perception, binocularity), in one or both eyes, that results from inadequate or abnormal stimulation of the visual system during a critical period of early visual development.

Explain the concept of the “critical” or “sensitive” period.

This period is central to the concept of amblyopia. It refers to a developmental time frame early in life during which there is robust plasticity within the visual system, particularly the visual cortex. Although not precisely defined, this period extends from birth to approximately 8 to 10 years of age. During this period the visual system is profoundly affected by the quality of visual stimulation it receives. Abnormal visual experience can lead to developmental abnormalities at both the structural and the functional level. If amblyopia occurs, it must be detected and treated during the critical period for vision to develop normally.

How is amblyopia classified?

Amblyopia is classified according to the underlying mechanism: strabismic, optical defocus, pattern or form deprivation, and organic.

Strabismus can lead to amblyopia if one eye becomes dominant, which causes the afferent input from the deviating, nondominant eye to be chronically suppressed. Optical defocus encompasses anisometropia as well as bilateral severe ametropia. Pattern or form deprivation amblyopia is caused by lesions that physically obstruct the visual axis, such as a congenital cataract, corneal opacity, vitreous hemorrhage, or ptosis. Organic amblyopia occurs secondary to a defined lesion of the visual pathways, such as a macular scar or coloboma. It is fundamentally different from the other types, because some or all of the vision loss is irreversible, and not simply a secondary effect on receptive fields in the lateral geniculate nuclei and visual cortex.

How does strabismus cause amblyopia?

Manifest strabismus disrupts sensory fusion. As a result, the vision from one eye must be suppressed to avoid diplopia and visual confusion. If a child with strabismus develops a strong preference for the use of one eye over the other, the nondominant eye may become amblyopic because of chronic suppression.

How prevalent is amblyopia?

The incidence of amblyopia is 1% to 3.5% in developed countries, and it is the most common cause of unilateral vision loss in children and young adults.

What factors place children at increased risk for amblyopia?

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  Developmental delay

  
  
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  Positive family history of amblyopia

  
  
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  Prematurity

These factors lead to a two- to sixfold increase in a child’s chance of developing amblyopia.

What anatomic changes have been shown to occur in amblyopia?

extensive animal studies have shown several neuroanatomic alterations in amblyopia. The primary abnormality appears to be the atrophy of cells in the layers of the lateral geniculate nucleus and visual cortex serving the amblyopic eye. These changes can be partially or wholly reversed if the amblyopia is successfully treated.

How early should children be screened for amblyopia?

The American Academy of Ophthalmology, American Academy of Pediatrics, and American Association of Pediatric Ophthalmology and Strabismus recommend routine vision screening in children by a pediatrician or properly trained health care provider as follows:

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  In newborns in the newborn nursery

  
  
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  At each routine well visit from 1 month to 4 years of age

  
  
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  A formal visual acuity should be documented by 5 years of age, or earlier if possible

The optimal time to diagnose and treat amblyopia is as soon as it occurs, but it is critical to do so before the close of the critical period (ideally before the child is 5 years of age).

What are some clinical techniques to check for amblyopia in nonverbal children?

Fixation preference testing is especially useful. In strabismic patients, a lack of spontaneous alternation in visual fixation between the two eyes suggests amblyopia in the nonpreferred eye ( Fig. 24-1 ). In patients with straight eyes or small-angle strabismus the vertical prism test is used to determine fixation preference. A child who consistently objects to occlusion of one eye but not the other can be assumed to have decreased vision in the eye that he or she will allow to be covered. Visual evoked potentials and preferential looking (e.g., Teller acuity cards) tests can be used to measure visual acuity. The Bruckner test, comparing the quality and symmetry of the red reflex between the two eyes using a direct ophthalmoscope, can help detect small-angle strabismus or anisometropia.

Child with esotropia showing spontaneous alternation in fixation. A, The left eye is used for fixation. B, The right eye is used for fixation. Alternating fixation is good evidence against the presence of amblyopia in children with strabismus.

Describe photoscreening and its role in detecting amblyopia.

A photoscreener is a device used by pediatricians or other individuals to screen for amblyogenic risk factors in children. The photoscreener is a camera that takes multiple images of a child’s undilated eyes to detect amblyogenic risk factors including high refractive errors, anisometropia, anisocoria, and the presence of strabismus ( Fig. 24-2 ). Children who are identified as having risk factors for amblyopia by the photoscreener should be referred to a pediatric ophthalmologist for a complete examination. Photoscreeners may have significant advantages over traditional eye chart acuity screening, especially in younger children who are preverbal or may not be able or willing to participate in the eye chart acuity test.

Image taken from a commercially available photoscreening device. Similar to the Bruckner test, the red reflex is evaluated. Based on the shape, size, and location of the bright crescents in the pupillary light reflexes of the undilated pupils, a determination can be made as to whether the child has significant refractive error, anisometropia, or strabismus. Digital analysis software available in many of the commercially available devices can analyze the images and provide referral recommendations to the tester.

What is the usual presenting complaint of a child with anisometropic amblyopia and at what age does it occur?

Similar to other forms of amblyopia, anisometropic amblyopia is generally asymptomatic. Detection in children depends on effective screening programs. Because of the lack of an overt external sign, such as strabismus or ptosis, the average age at presentation for anisometropic amblyopia is approximately 5 to 6 years, when school-initiated screening programs begin.

How does anisometropia cause amblyopia?

In anisometropia the retinal image in one eye is always defocused. If fixation is not alternated, the chronically defocused eye becomes incapable of processing high-resolution images. In addition, the binocular rivalry between the blurred image in one eye and the clear image in the other eye leads to foveal suppression of the blurred image as a way to avoid visual confusion. In the absence of strabismus, the suppression affects the foveal region, where high-grade visual acuity is processed and binocular rivalry is poorly tolerated. As a result, such patients often display peripheral sensory fusion and gross stereopsis (monofixation syndrome) and maintain good ocular alignment.

In addition to visual acuity, what other aspects of visual function may be affected in amblyopia?

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  Binocular vision and stereoacuity

  
  
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  Contrast sensitivity

  
  
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  Motion perception and processing

  
  
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  Spatial localization

Which is more likely to produce amblyopia—unilateral or bilateral ptosis? Why?

Unilateral ocular abnormalities are much more likely to lead to amblyopia than binocular ones. If one eye has a competitive advantage over the other, its afferent connections become stabilized and more numerous while those of the other eye atrophy and retract. This competition also forms the basis for treating amblyopia. The amblyopic eye, by one means or another, must be given a temporary competitive advantage over the dominant eye.