Description

• Amblyopia is a reduction in visual acuity due to inadequate visual experiences during the 1st years of life. It is the most common cause of monocular visual loss in children (1.3–3.6%). The result is abnormal visual development in the absence of a structural or pathologic abnormality of the eye.
• The cerebral cortex is functionally and structurally impaired leading to the loss of vision. The visual deficit is not correctable by eyeglasses or contact lenses.
• The abnormality is in the brain’s visual pathways and interpretative areas, not in the eye itself, although the eye is the trigger for the perceptual change.
• The reduction in visual acuity is most commonly associated with a deviation in the fixation of one eye (strabismus, 2.8%), a difference in the refractive error between the two eyes (anisometropia, 2.4%), high refractive error (such as high myopia >6.00 diopters), or a physical obstruction to the visual pathway (such as cataract).
• Functional amblyopia is potentially reversible with occlusion therapy.
• Organic amblyopia is irreversible.
• The visual acuity deficit is typically unilateral; rarely, it may be bilateral.
• The reduction in vision is usually apparent in the eye with the greater refractive error or with abnormal position of gaze (strabismus).
• Identification of abnormal visual pathway development became the basis of the 1981 Nobel Prize in Physiology or Medicine awarded to David H. Hubel and Torsten Wiesel.
• System(s) affected: nervous
• Synonym(s): lazy eye, squint

Pediatric Considerations
More commonly seen in the pediatric age group. The mean age at presentation is 3 to 6 years.

Epidemiology

• Predominant age: Onset may be present from birth or can appear in early childhood. If diagnosed in adulthood, it is most likely permanent and uncorrectable. The condition often goes undiagnosed but can be detected at any age.
• Predominant gender: male > female

Incidence
Amblyopia carries a projected lifetime risk of visual loss of 1.2%.

Prevalence
~2–3% of children

Etiology and Pathophysiology

• Strabismus causes disparate retinal images whereby one eye sees the object of regard in the fovea and the other in a different part of the retina.
• Inability to fuse the two images from each eye results in the brain ignoring the less preferred image (this does not necessarily need to be the less clear image).
• Refractive errors such as anisometropia (a difference in refractive error between the two eyes) can cause the two retinal images to be of unequal clarity.
• An obstruction to the visual axis, such as cataracts, causes unequal clarity of the retinal image. Cataract in an infant or child, therefore, is a medical emergency warranting surgical intervention and prompt removal.
• The result of one eye seeing better than the other interrupts the development of fine visual perception, which can contribute to the development of amblyopia.
• Individuals with amblyopia do not have normal degrees of stereovision and often complain of not appreciating 3D images.
• Strabismic amblyopia is a loss of visual acuity due to suppression of the images from an eye that turns in or out in an individual with misalignment of the visual axis.
• Anisometropic amblyopia is present when one eye has a significantly different refractive error than the other, especially if that error is hyperopia; it leads to visual blurring and therefore suppression of the image from that eye.
• Refractive amblyopia is due to uncorrected high refractive error, resulting in visual blurring in either or both eyes.
• Deprivation amblyopia (amblyopia ex anopsia) is due to relatively complete visual deprivation in one eye, which may be caused by a congenital abnormality such as a corneal scar or cataract.
• Deficiency amblyopia is also known as nutritional optic neuropathy or tobacco–alcohol amblyopia. Deficiencies of vitamin B₁, B₁₂, or riboflavin may be responsible.
• Amblyopia can only occur early in life.
  • When the brain detects unequal images, for any reason, it is forced to ignore one.
  • The ability of the brain to suppress the unwanted image can only occur when the development of neuroadaptive responses is in a critical “plastic” period, usually the 1st several years of life.
  • If amblyopia has not developed after that period, the individual will be unable to “suppress” the unwanted image, and diplopia or double vision will result.

Genetics
Increased incidence in children with a parent or sibling with a history of amblyopia

Risk Factors

• Preexisting refractive error, such as myopia, hyperopia, or astigmatism
• More common with a preexisting occlusion of the visual pathway, such as abnormal eyelid position, hemangioma, or malposition of the eye
• Conditions that cause anisometropia (unequal refractive difference between the eyes) or obstruction to clear vision (i.e., cataract, corneal abnormalities) can lead to permanent amblyopia.

General Prevention

• All infants and children should be screened from birth for normal visual acuity and development. Refractive errors or abnormal position of the eyes should be promptly evaluated by an ophthalmologist and treated if permanent visual loss is to be avoided.
• Children do not grow out of refractive errors or strabismus and should always be evaluated and treated by a specialist in pediatric ophthalmology.

Commonly Associated Conditions

Neurologic abnormalities, Down syndrome, cerebral palsy