Review of the Recent Advances in the Management of Amblyopia

Linda Nakanishi, MD, MPH

Introduction

Amblyopia is a common cause of decreased vision in children and young adults.^1-4 The prevalence of amblyopia varies from 0.2% to 5.3% depending on the population that is evaluated.¹ Amblyopia is often unilateral, but may also be bilateral. No pathology is seen on examination of the eye or visual pathway. If ocular abnormalities are detected, then the visual loss cannot be fully explained by the structural complications.

Classification

The visual system develops throughout early childhood. Visual deprivation, strabismus, and anisometropia adversely affect the development of the visual system.

Visual deprivation from a corneal scar, cataract, or ptosis may cause amblyopia. The amblyopia is worse if the complication is unilateral. A study was performed in which the eyelids of kittens were sewn together on one side. The results of this experiment demonstrated that cell bodies in the lateral geniculate nucleus that received input from the deprived eye were smaller.^2,5 The experiment showed that changes occurred in the cortex with visual deprivation.

Strabismus, specifically esotropia,^6,7 may result in amblyopia (amblyopia may also cause strabismus). When ocular misalignment is present, the visual cortex does not receive the same image from the two foveas. The visual cortex suppresses the nondominant eye so that one image is perceived.⁸

Anisometropia, a difference in refractive errors between the eyes, is another cause of amblyopia. Commonly, the more hyperopic eye is affected. The exception is axial myopia, in which the more myopic eye is affected.⁸ The severity of amblyopia is greater with increasing amounts of anisometropia.

Diagnosis

Amblyopia is diagnosed in a preverbal child when a difference in fixation behavior exists between the eyes. Fixation is assessed by holding objects in front of an infant and observing fixation of each eye on the object. If the child fixates with either eye and objects equally to occlusion of either eye, then vision is fairly equal.

If a child does not fixate with an eye or does not object to that eye being covered, then a difference in visual acuity may exist. In a preverbal child with strabismic amblyopia, the child may not maintain fixation with the strabismic eye once the dominant eye is uncovered (Video).

Another way to assess fixation behavior in a preverbal child is with the preferential looking test. In this test, a child chooses between a card with progressively finer black stripes or a card with a gray background. A physician can estimate vision with this test (grating acuity).

In a verbal/literate child, amblyopia is diagnosed when a difference of two or more lines on vision testing exists. The difference in acuity between the two eyes remains after refractive errors are corrected. If ocular anomalies are present, they do not fully explain the decrease in acuity.

Treatment

Occlusion treatment
A patient's normal eye is occluded with a patch so that the amblyopic eye is used and recovers vision. Variability exists in the number of hours prescribed to amblyopic patients.^2,6,9-12 Patients may be patched full-time, full-time except for 1 hour a day,⁹ or part-time. Many types of patches are available.

Pharmacologic or optical penalization
Pharmacologic therapy has traditionally been used for patients who have been noncompliant with patching, are sensitive to the adhesive, or have complications with the cosmetic appearance of the patch.^14-17

Recent Studies

Several questions regarding amblyopia therapy remain. For example, guidelines regarding the amount of patching needed for improvement in visual acuity are based largely on anecdotal information or small, retrospective studies. ^9-11,18-21

Another area of interest involves pharmacologic penalization. Although pharmacologic penalization has been shown to be an effective treatment for moderate amblyopia, ^14,15,17,19 the relative efficacy of occlusion therapy vs. pharmacologic penalization has not been studied in large numbers.²⁰

A multicentered National Institutes of Health-sponsored group, the Pediatric Eye Disease Investigator Group (PEDIG) is attempting to answer these and other questions by conducting a series of prospective, randomized clinical trials.

A paper published by the PEDIG⁵ looked at the characteristics of 409 patients with moderate amblyopia (20/40 to 20/100) who were 3 to 6 years of age. The group found equal numbers of patients with strabismic and anisometropic amblyopia. The level of visual deficit was the same regardless of etiology. The number of hours of prescribed patching varied among the eye care providers.

ATS1
In the Amblyopia Treatment Study 1 (ATS 1), the PEDIG⁶ looked at this same group with moderate amblyopia and compared atropine to patching (6 hours to all waking hours). The results indicated that vision improved faster in the group that patched but that the difference in visual acuity between the two groups after 6 months was not clinically significant. Researchers concluded that atropine and patching were acceptable treatments for children with amblyopia between 3 and 7 years of age.

ATS2
In the Amblyopia Treatment Study 2,^22,23 the PEDIG conducted three randomized, controlled trials. The first trial looked at 3 to 6 year olds with moderate amblyopia (20/40 to 20/100). They compared 2 hours vs. 6 hours of patching. Patients performed 1 hour of near activities while patching. The results at 4 months showed that the vision improved to a similar extent in both groups.

A second study was conducted in patients between 3 and 6 years of age who had severe amblyopia (20/100 to 20/400). The group was randomized to patch 6 hours a day or full-time (all hours of the day or all hours except for 1). Each patient participated in 1 hour of near visual activities while patching. The visual acuity in both groups at 4 months was compared. The vision improved about 4.8 lines in the group that patched for 6 hours and 4.7 lines in the group that patched full-time.

The third trial ²⁴ looked at 10 to 17 year olds with visual acuity ranging from 20/40 to 20/160. These patients were treated with patching for 2 or more hours. They participated in some type of activity at near for 1 hour. The visual acuity improved 2 or more lines in 27% (18 of 66 patients).

References

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