In more than 30 years of teaching optometry interns, vision therapy (VT) residents and graduate optometrists, I’ve found few topics that concern primary care optometrists more than the diagnosis and treatment of amblyopia. Perhaps, the primary reason for this is that they are dealing with children who, potentially, could have permanent vision loss based on their diagnosis and treatment.

In this article, we will concentrate on functional amblyopia, which is the more common form of amblyopia and must be accompanied by one or more amblyogenic conditions (mentioned below), present by the ages of 6 to 8.¹ (See also “Amblyopia Masqueraders,” p.42.)

Here, I provide the steps needed to make the diagnosis and management of this condition manageable.

1 UNDERSTAND THE CONDITION

Amblyopia is the leading cause of monocular vision loss in children in the United States, with estimates of prevalence between 2% to 2.5% of the general population.² It can be defined as a unilateral (occasionally bilateral) reduction in BCVA to less than 20/20 in the absence of obvious structural or pathologic abnormalities.

Amblyogenic conditions include:

• Anisometropia
• Bilateral isometropia
• Unilateral or bilateral astigmatism
• Constant unilateral strabismus

When amblyopia is unilateral, it is most commonly associated with constant unilateral strabismus, amblyogenic anisometropia or both. When amblyopia is bilateral, it is usually from bilateral high-refractive error or bilateral form deprivation (such as in congenital cataracts).

The above listed amblyogenic conditions can lead to refractive or strabismic amblyopia or combined strabismic/refractive amblyopia:

• Form deprivation amblyopia. This is caused by conditions, such as congenital or traumatic cataracts, severe eyelid ptosis and corneal opacities that obstruct the visual axis during the critical period (the age range that visual deprivation — e.g. constant strabismus or high refractive error — can cause amblyopia).
• Refractive amblyopia. This amblyogenic condition can be divided into two areas of causation: (1) isoametropic amblyopia is induced by high, but equal, refractive error in each eye, such as astigmatism greater than 2.50 D, hyperopia greater than 5.00 D or myopia greater than 8.00 D. (2) Anisometropic amblyopia is caused by unequal, uncorrected refractive error of one eye, such as astigmatism greater than 1.50 D, hyperopia greater than 1.00 D or myopia greater than 3.00 D. Amblyogenic factors for refractive amblyopia are summarized in Table 1.

Amblyopia can be categorized by the level of the BCVA:

However, amblyopia is more than just reduced Snellen VA. It is actually a syndrome of visual deficits often characterized by the following:

• Inaccurate accommodative responses and reduced accommodative amplitude.
• Reduced contrast sensitivity and visual spatial distortion.
• Increased sensitivity to contours (“crowding effect”).
• Unsteady and inaccurate monocular fixation and poor visual tracking ability.
• Reduced stereopsis.

To identify these deficits, the following diagnostic tests may be employed:

1. Monocular accommodative amplitude testing.
2. Monocular contrast sensitivity testing.
3. Single letter vs. whole chart Snellen VA.
4. Careful observation of monocular eye movements using magnification, such as through a slit lamp.
5. Standard stereoacuity tests.

2 KNOW THE TREATMENT OPTIONS

Traditionally, the mainstay of amblyopia treatment has always been patching of the non-amblyopic eye with the general concept of full optical correction, early patching (“time is of the essence”) and long periods of patching (“the more, the better”). This encourages the brain to use visual information from the amblyopic eye as well as help reduce cortical suppression from the non-amblyopic eye over the amblyopic eye. However, clinical thinking has changed.

Starting in 1999, the Pediatric Eye Disease Investigator Group (PEDIG) embarked upon a group of clinical trials known as the Amblyopia Treatment Studies (ATS).³ These randomized, clinical trials or prospective observational studies provided analysis of amblyopia treatment options. In addition to patching, these treatment options include atropine, (which blurs the image of the non-amblyopic eye for all near visual activities), spectacles (which reduce visual blur to the amblyopic eye) and Bangerter filters. A Bangerter filter (Fresnel Prism & Lens Co.) is a translucent filter applied to the sound eye’s spectacle lens for full-time wear for amblyopia treatment. Different density filters produce different degrees of image defocus that degrade the VA of the non-amblyopic eye.

Below in Table 2 is a summary of some of the clinical guidance resulting from ATS research.

Table 2. SUMMARY GUIDANCE BASED ON FINDINGS FROM AMBLYOPIA TREATMENT STUDIES³

ATS RESEARCH TOPIC	CLINICAL IMPLICATIONS
Two hours of patching vs. six hours of patching for moderate amblyopia	In cases of moderate amblyopia, prescribing two hours of daily patching with one hour of near activities is as effective as prescribing six hours of daily patching with one hour of near activities.
Six hours of patching vs. full-time patching for severe amblyopia	In cases of severe amblyopia, prescribing six hours of daily patching and one hour of near activities is as effective as prescribing full-time daily occlusion and one hour of near activities.
Atropine treatment (1% atropine sulfate) vs. patching	Atropine penalization has a similar treatment effect as two and six hours of prescribed patching; thus, it can be considered for first-line amblyopia treatment or especially for patching failures.     Daily atropine administration is not necessary; a twice-per-week schedule and weekend dosing is effective.
Prospective observational study for spectacles only on improvement in VA in moderate anisometropic amblyopia.	There is an amblyopia treatment effect that occurs over time from wearing appropriate refractive correction distinct from the improvement due to eliminating blur.     It is reasonable to start amblyopia treatment with the refractive correction alone for young children who have anisometropic, strabismic and combined-mechanism amblyopia.     A follow-up interval of six-to-eight weeks, until improvement in the amblyopic eye VA plateaus, is a practical schedule for monitoring children for an optical treatment effect.
Prospective observational study for spectacles only on improvement in VA in moderate isometropic amblyopia.	Of the 113 participants, 74% achieved binocular VA of 20/25 or better.     Continued VA improvement was seen for up to one year in some children.     The worse the child’s VA at the start of treatment, the greater the number of lines of improvement in VA.     A majority of children also showed an improvement in near stereopsis.
Randomized clinical trial to evaluate the effectiveness of Bangerter filters in children 3 to 10 years who have moderate amblyopia (20/40 to 20/80).	Full-time wear of Bangerter filter provided VA improvement similar to two hours of daily patching.     Parents reported fewer adverse effects and better compliance with the Bangerter filters than with patching.

Some of the most exciting research in amblyopia is the possibility of binocular treatment of amblyopia or the “patchless” treatment of amblyopia.⁴ When the visual system has amblyogenic factors, such as blur or strabismus, during the visual critical period, it causes a progressive reduction of VA. The reduction of VA can be treated long after this critical period, which is what makes “patchless” treatment exciting.

How it works: In both refractive and strabismic amblyopia, the non-amblyopic eye actively inhibits the amblyopic eye at the level of the visual cortex. Rather than thinking of the amblyopic eye as a “lazy eye,” it might be more accurate to think of it as a “bullied” eye. Blur and strabismus cause binocular inhibition and visual suppression. While direct patching often improves monocular VA, it often doesn’t improve stereopsis or reduce visual suppression.

The “patchless” approach to amblyopia treatment:

1. Prescribe spectacles or contact lenses that maximize stereopsis at near.
2. Perform binocular VT to help reduce visual suppression.

The mainstay of binocular VT is dichoptic training. Dichoptic training is typically performed with red-green (or red-blue) glasses where both eyes are open, and a peripheral target is seen. In some cases, the amblyopic eye alone sees a central target (monocular fixation in a binocular field). In other cases, both eyes have a different central target, and the amblyopic eye sees a much higher contrast target than the non-amblyopic eye, thereby giving the visual advantage to the amblyopic eye. In both cases, the therapy tends to improve binocular vision and reduce suppression. The net result is often an improvement in both stereopsis and VA.

We have been using this approach with great success in our practice. Given the learning curve and comfort level associated with dichoptic training, a primary care optometrist might feel more comfortable co-managing such patients with a VT optometrist experienced with this approach.

3 INSTITUTE A PLAN

I defer to the abundant findings of the ATS studies, which I encourage you to read for yourself, as there is a great deal of information there. Here is a clinical plan for primary care optometrists who would like to use these evidence-based clinical trials to guide their initial treatment of functional amblyopia.

1. Start by prescribing the spectacle (or contact lens) prescription that gives BCVA, while also maximizing binocularity.
2. Schedule appropriate follow-up visits to determine and maximize the effect of optical correction before the initiation of direct patching.
3. Identify appropriate patching time, based on the ATS studies mentioned above and the patient’s level of amblyopia. Another potential option: Bangerter filters. Recommend the appropriate therapy based on your clinical judgement, in concert with the patient’s age, social and lifestyle considerations.
4. Prescribe a regimen of near vision activities. This includes coloring, tracing, dot-to-dot drawing, beading, puzzles, Legos, reading, handheld games and/or distance vision activities, such as computer games projected on a TV, and “flashlight tag,” with the parent and the patient taking turns trying to keep up with the flashlight making random patterns on a wall). This requires a bit of trial and error to see what activities the patient will be compliant with.
5. Employ atropine penalization, should you find patching fails and/or poor patching compliance.
6. Consider providing VT or referring out for VT services.

Amblyopia Masqueraders

According to the AOA practice guidelines, while functional amblyopia occurs before age 8 and is attributable to form deprivation, strabismus and anisometropia, other forms of vision loss may masquerade as amblyopia.¹ These include:

  Psychogenic vision loss, defined as a substitution of physical signs or symptoms for anxiety or emotional repression.

 

Organic vision loss attributed to ocular disease and anatomical anomalies that are not included in the definition of functional amblyopia. 

PREVENT VISION LOSS

Amblyopia can be a challenging condition to diagnose and manage, but if you familiarize yourself with the current research, clinical trials and related therapies, you can institute a plan to prevent and/or minimize vision loss in these patients. OM

REFERENCES

1. American Optometric Association. Optometric Clinical Practice Guideline: Care of the Patient with amblyopia. https://www.aoa.org/AOA/Documents/Practice%20Management/Clinical%20Guidelines/Consensus-based%20guidelines/Care%20of%20Patient%20with%20Amblyopia.pdf . Reviewed 2004. Accessed Aug. 3, 2021.
2. American Optometric Association. Evidence-Based Clinical Practice Guideline: Comprehensive Pediatric Eye and Vision Examiniation. aoa.uberflip.com/i/807465-cpg-pediatric-eye-and-vision-examination/0?m4= American Optometric Association website. Published Feb. 12, 2017. Accessed Aug. 4, 2021.
3. Chen, AM, Cotter, SA, The Amblyopia Treatment Studies: Implications for Clinical Practice. Adv Ophthalmol Optom. 2016;1(1):287-305. doi: 10.1016/j.yaoo.2016.03.007.
4. Kelly KR, Jost RM, Wang YZ, et al. Improved Binocular Outcomes Following Binocular Treatment for Childhood Amblyopia. Invest Ophthalmol Vis Sci. 2018 Mar 1; 59(3): 1221-1228. doi: 10.1167/iovs.17-23235.

DR. TANNEN is in private group optometric practice at EyeCare Professionals, PC in Hamilton, N.J., where he focuses on visual deficits related to concussion, acquired brain injury, strabismus, amblyopia and learning-related vision problems. He is a Fellow in the American Academy of Optometry and the College of Optometrists in Vision Development (COVD), past president of COVD and an associate clinical professor Emeritus of the SUNY/State College of Optometry in New York.