INTRODUCTION

Amblyopia is the most common cause of monocular vision loss in children, with an estimated prevalence of approximately 2% in the United States.1–3 The exploration of the effectiveness of various amblyopia treatments, along with their clinical challenges, remains an ongoing area of investigation. In this point-counterpoint article, clinicians discuss 2 contrasting approaches to amblyopia treatment: the monocular approach versus the binocular approach.

POINT: MONOCULAR APPROACH TO AMBLYOPIA TREATMENT

Timothy Hug, OD, FAAO; Jolie LeGate, OD

Origin

Amblyopia is a neurodevelopmental disorder of the visual cortex that arises from abnormal visual experience early in life.4 Abnormal visual experiences can include strabismus, monocular blur due to anisometropia, and physical deprivation (eg, cataract). Animal models have been used to study the effects of monocular deprivation on visual development at the cortical level. These studies indicate a loss of cellular activity and cellular development in the visual cortex of a deprived eye. A study of the ocular dominance columns reveals a significantly lower number of cells in the columns responding to a deprived eye and in the columns shared by the deprived and sound eye.5

The origin of monocular treatment for amblyopia is based on the theory of removing cortical competition from the sound eye during the critical period of visual development to allow the deprived eye an opportunity to promote cortical development at the cellular level. This leads to better visual acuity in the deprived eye and improved binocularity. Treatments that penalize the sound eye, such as patching, atropine, and Bangerter filters, are considered monocular treatments for amblyopia.

Evidence-Based Treatments

The Pediatric Eye Disease Investigator Group has published multiple studies demonstrating improvement in amblyopic eye visual acuity with monocular treatment for amblyopia.6–9 Patching is widely considered the most intense monocular treatment for amblyopia. These studies specifically investigated 2 hours, 6 hours, and full-time daily patching of the sound eye, over 4 to 6 months, in children aged 3 to 6 years.6,7 The studies included moderate (20/40-20/80) and severe (20/100-20/400) levels of amblyopia secondary to either strabismus and/or anisometropia. These studies consistently showed improvement in visual acuity of the amblyopic eye of multiple lines, with some patients even reaching resolution of amblyopia. Atropine and Bangerter filters are considered less intense monocular treatment options. However, both methods of sound eye penalization have shown similar levels of treatment success to those of patching.9,10

Improved stereopsis has also been reported in patients receiving monocular treatment for amblyopia. In a study by Lee and Isenberg, over 50% of patients with anisometropic amblyopia showed improved stereopsis, including some achieving a stereopsis value as low as 40 seconds of arc. Additionally, 100% of patients who initially reported negative responses (no stereopsis) before monocular treatment for amblyopia achieved some level of stereoacuity after undergoing occlusion therapy.11

Cost

Atropine and patching are low-cost treatment options for amblyopia. In the Pediatric Eye Disease Investigator Group study that compared atropine and patching, the cost of each treatment modality was reported as $100 for 6 months of patching and $10 for 6 months of atropine. These costs excluded initial spectacle lens correction or change of spectacle lens to plano if indicated.10 Although the cost information from the study may be outdated, the findings remain relevant, as both treatments continue to be viewed as affordable options for managing amblyopia. There have been no studies identifying the exact cost of Bangerter filters over a specified period for treatment of amblyopia. However, it has been considered a low-cost option.12

Universal Applications

Monocular treatments can be used in patients younger than those included in the Pediatric Eye Disease Investigator Group studies, including infants. Visual insults in infants include anisometropia, strabismus, and congenital cataracts. The younger the child is when visual interference is treated, the better the visual outcome tends to be.13 This makes monocular amblyopia treatment an option for infants with unilateral amblyopia.

Summary

Treatment of amblyopia is critical to allow neurodevelopment of the visual cortex. By removing competition from the sound eye through monocular penalization, the deprived (amblyopic) eye is given the opportunity to promote visual development at a cellular level. Monocular treatment of amblyopia has been shown to be effective for improving visual acuity and restoring variable levels of stereopsis, is cost effective, and can be used across all age groups. Monocular treatment should be considered the first-line option for unilateral amblyopia. As with any amblyopia treatment, close monitoring, family education and compliance, and follow-up are important in achieving optimal outcomes for unilateral amblyopia.

COUNTERPOINT: BINOCULAR APPROACH TO AMBLYOPIA TREATMENT

Dan L. Fortenbacher, OD, FOVDR; Leonard J. Press, OD, FAAO, FOVDR

Limitations of the Monocular Approach to Amblyopia Treatment

Amblyopia has traditionally been recognized as a monocular decrease in visual acuity due to a delay in visual development in infancy or early toddlerhood most commonly caused by strabismus or significant anisometropia or both, without an associated neurological or ocular disease. Contemporary research has identified that the etiology of visual impairment in amblyopia is the early onset of binocular dysfunction leading to suppression of the affected eye and defective stereoacuity.14,15 Therefore, it is without question that the root cause of the multiple layers of the visual impairment of amblyopia including visual acuity reduction is the onset of binocular dysfunction in the critical period of infant and toddler development.

Yet, for nearly 300 years, the generally recognized “gold standard” for amblyopia therapy has been monocular occlusion/penalization of the fellow eye with patching or atropinization in conjunction with optical correction because it targets the obvious monocular visual loss with a seemingly practical and direct intervention. Although randomized clinical trials have found occlusion/penalization therapy can be effective at improving visual acuity, it has significant limitations in efficacy for the overall management of amblyopia, as this monocular treatment does not address the etiology of amblyopia, which is a dysfunction in binocular vision. Despite the potential for improvement in visual acuity from occlusion therapy, it is now understood that the application of patching or penalization for amblyopia has limitations in outcomes and numerous negative side effects. Principal among these limitations is poor compliance, with an average rate reportedly around 50%.16 It is also important to bear in mind that occlusion therapy is rarely undertaken beyond age 10, as the visual benefits are considered to be marginal.17 Therefore, age of treatment is a significant limitation for occlusion therapy.

Furthermore, occlusion therapy is linked to psychosocial problems leading to reduced quality of life and self-esteem. On the surface, it seems that the psychosocial problems associated with patching could be easily overcome, but they are not. In a review centering on the extent of problems associated with occlusion, Wang assessed psychosocial factors that weaken adherence to a patching regimen.18 Principally, they are the physical discomfort associated with being patched, difficulty with performing schoolwork and other tasks, bullying by other children, emotional disruption within the family, and lowered self-esteem.

It is not just children that have difficulties with occlusion, but parents have problems implementing a punitive approach. Consider the following analogy: A child appears to hear normally because she relies on the input through her better ear. The parent takes her child to an audiologist and is advised that in order to improve the auditory abilities through the weaker ear and force it to process better, the child will have to put an earplug in the better ear to block out sound. That might seem archaic to a parent, who would wonder why they should put their child at a disadvantage, rather than using a treatment that boosts the ability of the weaker ear and helps the brain integrate and synchronize both channels better together.

As it turns out, parental intuition is correct. That is because patching is based on the outmoded assumption that amblyopia is a monocular problem, whereas it is now conceptualized to be a binocular problem with both monocular and binocular consequences. Although occlusion can improve monocular visual acuity and skills through the amblyopic eye, interocular suppression mechanisms are still operative when the patch is removed.19 That means that as soon as the patch is removed, the brain will resort to tuning in to the preferred eye and detuning the amblyopic eye. In order to counteract the binocular conflict between the 2 eyes, integrative rather than punitive therapy is required so that normal binocular summation can be attained.

Advantages of the Binocular Approach to Amblyopia Therapy

The paradigm shift toward a binocular approach to amblyopia therapy began with the concept of monocular fixation in a binocular field. This redefined the goal as boosting the central vision of the amblyopic eye while taking advantage of intact peripheral fusion in both eyes. It effectively provides an advantage to the brain in using the amblyopic eye while being engaged in binocular tasks that will improve visual acuity. It was initially done with inexpensive, low tech, polarized or anaglyphic materials. The dichoptic approach to binocular therapy successfully replicated and extended the monocular fixation in a binocular field to higher tech formats such as video games, providing even greater binocular interaction.

The binocular approach remedies nearly all the limitations of the monocular approach. The activities are enjoyable rather than punitive, and self-esteem is boosted rather than hindered. This approach enhances binocular vision and stereopsis and can be successful into adulthood.20 Therefore, age is no longer a barrier to treatment of amblyopia.

Research showing that the binocular approach is engaging for children in game format and results in long-lasting acuity improvement began appearing last decade.21 Virtual reality applications have been a boon to binocular therapy and have boosted compliance with the procedures.22 Most recently, research has shown that the dichoptic approach to binocular therapy can be successfully adapted to any streamed content on the internet.23

A subtle but noteworthy difference between the monocular and binocular approach is pertinent to anisometropic amblyopia. In the monocular approach, emphasis is placed on the refractive correction that provides the best monocular visual acuity through the amblyopic eye. The binocular approach emphasizes deriving a refractive correction that results in the best binocular balance, encouraging greater sensory fusion and stereopsis at the outset.24

Considerations in the Binocular Approach to Amblyopia Therapy

Evidence now points toward patching as a potential second-line treatment for amblyopia, with lens prescription and binocular therapies as the front-line treatment.25 One may wonder how practical it is for a young child to engage in binocular therapy. Studies have shown efficacy in children as young as 4 years of age, with treatment efficiencies considerably higher than occlusion.26 We acknowledge that for infants and toddlers unable to engage in active vision therapy, occlusion or other passive treatment approaches may be indicated. Lastly, although treatment costs may make the monocular approach to therapy more appealing, the role of the professional is to offer the most efficacious intervention and leave the value judgements to the patient or caretakers.

SUMMARY

A binocular approach to amblyopia therapy has distinct advantages over a monocular approach. It facilitates binocular development resulting in improved stereopsis. Regression or recidivism is less likely.27 It is engaging and rewarding relative to the monocular approach, particularly when taking advantage of newer technologies. It streamlines therapy due to improved adherence or compliance. It boosts self-esteem as the patient is not being penalized on a daily basis at the outset by having compromised visual function. Although future research will shed further light on individualizing treatment to achieve optimal outcomes, the superiority of a binocular approach to amblyopia therapy is already apparent.