eMedicine Specialties > Ophthalmology > Neurologic Disorders

Amblyopia

Author: Kimberly G Yen, MD, Assistant Professor of Ophthalmology, Department of Ophthalmology, Cullen Eye Institute, Baylor College of Medicine
Contributor Information and Disclosures

Updated: Dec 31, 2008

Introduction

Background

Amblyopia refers to a decrease of vision, either unilaterally or bilaterally, for which no cause can be found by physical examination of the eye. The term functional amblyopia often is used to describe amblyopia, which is potentially reversible by occlusion therapy. Organic amblyopia refers to irreversible amblyopia.^1,2,3

Most vision loss from amblyopia is preventable or reversible with the right kind of intervention. The recovery of vision depends on how mature the visual connections are, the length of deprivation, and at what age the therapy is begun. It is important to rule out any organic cause of decreased vision because many diseases may not be detectable on routine examination.

Pathophysiology

Although many types of amblyopia exist, it is believed that their basic mechanisms are the same even though each factor may contribute different amounts to each specific type of amblyopia. In general, amblyopia is believed to result from disuse from inadequate foveal or peripheral retinal stimulation and/or abnormal binocular interaction that causes different visual input from the foveae.⁴

Three critical periods of human visual acuity development have been determined.^5,6 During these time periods, vision can be affected by the various mechanisms to cause or reverse amblyopia. These periods are as follows:

• The development of visual acuity from the 20/200 range to 20/20, which occurs from birth to age 3-5 years.
• The period of the highest risk of deprivation amblyopia, from a few months to 7 or 8 years.
• The period during which recovery from amblyopia can be obtained, from the time of deprivation up to the teenage years or even sometimes the adult years.

Whether different visual functions (eg, contrast sensitivity, stereopsis) have different critical periods is not known. In the future, determination of these time frames may help modify treatment of amblyopia.

Frequency

United States

Prevalence of amblyopia is difficult to assess and varies in the literature, ranging from 1-3.5% in healthy children to 4-5.3% in children with ophthalmic problems. Most data show that about 2% of the general population has amblyopia.

Amblyopia was shown in the Visual Acuity Impairment Survey sponsored by the National Eye Institute (NEI) to be the leading cause of monocular vision loss in adults aged 20-70 years or older. Prevalence of amblyopia has not changed much over the years.

Mortality/Morbidity

Amblyopia is an important socioeconomic problem. Studies have shown that it is the number one cause of monocular vision loss in adults. Furthermore, persons with amblyopia have a higher risk of becoming blind because of potential loss to the sound eye from other causes.

Race

No racial preference is known.

Sex

No gender preference is known.

Age

Amblyopia occurs during the critical periods of visual development. An increased risk exists in those children who are developmentally delayed, were premature, and/or have a positive family history.

Clinical

History

• Ocular history
  • Elicit any previous history of patching or eye drops as well as past compliance with these therapies.
  • Document previous ocular surgery or disease.
• In addition to the routine information, obtaining a family history of strabismus or other ocular problems is important because the presence of these ocular problems may predispose a child to amblyopia.

Physical

• Visual acuity
  • Diagnosis of amblyopia usually requires a 2-line difference of visual acuity between the eyes; however, this definition is somewhat arbitrary and a smaller difference is common.
  • Crowding phenomenon: A common characteristic of amblyopic eyes is difficulty in distinguishing optotypes that are close together. Visual acuity often is better when the patient is presented with single letters rather than a line of letters.
• Diagnosis is not an issue in children old enough to read or with use of the tumbling E.
• Testing in preverbal children
  • If the child protests with covering of the sound eye, amblyopia can be diagnosed if it is dense.
  • Fixation preference may be assessed, especially when strabismus is present.
  • Induced tropia test may be performed by holding a 10-prism diopter before one eye in cases of an orthophoria or a microtropia.
  • In infants who cross-fixate, pay attention to when the fixation switch occurs; if it occurs near primary position, then visual acuity is equal in both eyes.
  • Caution should be used when obtaining Teller acuity in children, as grating acuity may be less reduced than Snellen acuity, especially in strabismic amblyopia.
• Contrast sensitivity: Strabismic and anisometropic amblyopic eyes have marked losses of threshold contrast sensitivity, especially at higher spatial frequencies; this loss increases with the severity of amblyopia.
• Neutral density filters: Patients with strabismic amblyopia may have better visual acuity or less of a decline of visual acuity when tested with neutral density filters compared to the normal eye. This was not found to be true in patients with anisometropic amblyopia or organic disease.
• Binocular function: Amblyopia usually is associated with changes in binocular function or stereopsis.
• Eccentric fixation: Some patients with amblyopia may consistently fixate with a nonfoveal area of the retina under monocular use of the amblyopic eye, the mechanism of which is unknown. This can be diagnosed by holding a fixation light in the midline in front of the patient and asking them to fixate on it while the normal eye is covered. The reflection of the light will not be centered.
• Refraction: Cycloplegic refraction must be performed on all patients, using retinoscopy to obtain an objective refraction. In most cases, the more hyperopic eye or the eye with more astigmatism will be the amblyopic eye. If this is not true, one needs to investigate further for ocular pathology.
• Rest of examination: Perform a full eye examination to rule out ocular pathology.

Causes

Many causes of amblyopia exist; the most important causes are as follows^2,1 :

• Anisometropia
  • Inhibition of the fovea occurs to eliminate the abnormal binocular interaction caused by one defocused image and one focused image.
  • This type of amblyopia is more common in patients with anisohypermetropia than anisomyopia. Small amounts of hyperopic anisometropia, such as 1-2 diopters, can induce amblyopia. In myopia, mild myopic anisometropia up to -3.00 diopters usually does not cause amblyopia.
  • Hypermetropic anisometropia of 1.50 diopters or greater is a long-term risk factor for deterioration of visual acuity after occlusion therapy.
• Strabismus
  • The patient favors fixation strongly with one eye and does not alternate fixation. This leads to inhibition of visual input to the retinocortical pathways.
  • Incidence of amblyopia is greater in esotropic patients than in exotropic patients.
• Strabismic anisometropia: These patients have strabismus associated with anisometropia.
• Visual deprivation: Amblyopia results from disuse or understimulation of the retina. This condition may be unilateral or bilateral. Examples include cataract, corneal opacities, ptosis, and surgical lid closure.⁷
• Organic: Structural abnormalities of the retina or the optic nerve may be present. Functional amblyopia may be superimposed on the organic visual loss.

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