The College of Optometrists in Vision Development (COVD) hosted an eye opening conference in 2021 (no pun intended). Among many great lectures, there was one that particularly stood out; it highlighted a paradigm shift in Amblyopia treatment. In an era of Virtual Visual Fields and OCT Angiography it seems archaic that many of us are still treating amblyopia with the use of a store bought eyepatch. It begs the question, is there new evidence out there that upgrades this treatment into the 21st century? The answer is yes. Successful treatment for amblyopia does exist which does NOT involve patching.
An epic lecture by Dr. Robert Sanet and Dr. Pilar Vergara at COVD 2021 blew the lid open on modernizing the treatment protocol for amblyopia to reflect current research. For the past decade, research has demonstrated that true gains in amblyopia management are made by treating this condition binocularly. The definition of amblyopia has long been changed from a reduction in 2 lines of acuity or more. Amblyopia is now understood to occur as a result of poor binocular function which leads to suppression and neural inhibition. One of the results of this binocular inhibition is reduced visual acuity. Dr. Arnold Sherman once wrote, quite skillfully, “reduced visual acuity in amblyopia is a monocular symptom of a binocular problem.”
At this point, many of you may have questions…If the problem is a binocular function, why are we only fixated on treating the symptom (visual acuity)? What can we do if the visual function plateaus, which has been the indicator to patch in conventional amblyopia management? What is the optimal refractive correction for a patient with presumed amblyopia and how do we figure it out? Most importantly: what do I do with the bulk supply of patches I recently ordered?
Remember those rumors about people with amblyopia who played video games that improved their overall visual function? Well, it’s true, and they were actually clinical studies. In 2011, a pilot study conducted by vision researchers at UC Berkeley found that the rate of visual acuity improvement was 5-fold faster with video gameplay vs. standard occlusion therapy. In 2013, the Tetris study demonstrated that adult brains were capable of changing their ways if the goal of amblyopia treatment was to break down suppression. This important study exposed the critical period for what it is, a myth. In 2021 COVD outlined the critical period ranging from slightly after birth to slightly before death. Meaning, visual gains can be made at any stage of life, if you treat the problem rather than the symptom.
Treating the Problem:
Step 1: Stereopsis, stereopsis, stereopsis.
I have said this, written about it, and will continue to stress it over and over again that stereopsis is a high yield test that gives you great insight into visual function. Reduced stereopsis is a well-known finding in patients with amblyopia which supports the theory that amblyopia occurs as a result of binocular inhibition. When the two eyes work together you can accurately judge where an object is located in space and consequently where you are located with respect to that object. This is known as egocentric processing. When suppression occurs early on in life, perception of visual space is altered and there is a shift in visual dominance within the brain which can lead to inhibition.
There are many ways to measure global and local stereopsis, but the preferred method is using the Randot Stereo Test book which can evaluate both. Wirt circles are an excellent measure of local stereopsis and will serve as a useful indicator for designing your initial prescription. After performing your refractive analysis use a lens combination that delivers the most improvement in near stereoacuity. There is a trial and error component to this technique, but the theory is to maximize plus over the non-amblyopic eye and actually underplus the amblyopic eye. More on this next.
Step 2: Retinoscopy
In an age of autorefractors and auto-phoropters is retinoscopy still relevant? The answer:
Retinoscopy continues to be an extremely useful tool in determining the best initial prescription in patients with (or without) amblyopia. The major components of retinoscopy used here are the brightness of the reflex and its stability. According to Dr. Vergara these two indices can help determine which initial lens will have a successful final outcome. After determining the refractive status, toggle between plus lenses over the more hyperopic eye. You want to find a lens that gives you the brightness reflex as this lens will be your best initial lens for a prescription. Keep in mind that the lenses which provide the brightest reflex may not be the ones that provide the best initial visual acuity and that is OK! The non-amblyopic eye may see 20/25 with a lens that otherwise provides favorable binocular results. This is a happy alternative to patching where a person is forced to see with an eye that could be 20/80 or more. Remember, our goal is to treat the problem and not the symptom. Acuity will improve over time as the binocular inhibition reduces.
Step 3: Identify Suppression
Measuring the degree of suppression is pivotal in amblyopia evaluation and management. Suppression leads to inhibition which causes an overall reduction in the function of the amblyopic eye while both eyes are open. Theoretically, if there was a breakdown in inhibition and suppression by working on binocular skills, function should improve and as a consequence visual acuity.
A user-friendly method of evaluating suppression is with a worth 4 dot attachment on a transilluminator. Measurements should be taken from near to far to account for the size of the suppression zone. The further the distance that binocularity is maintained, the smaller the suppression zone. Additionally, suppression should be measured in dim and light conditions. If suppression is maintained in dim lighting where the contrast of the lights is enhanced, then it is labeled as deep suppression. This assessment is another useful tool in determining your initial lens prescription. The lens combination which provides the smallest suppression zone, i.e. greatest distance of binocularity, should be your initial prescription.
Step 4: Visual Acuity & Initial Prescription
Measuring visual acuity is useful for gathering data when treating amblyopia, however, it should not be the only measure of treatment success. Uncorrected and corrected visual acuity should be measured linearly and with single symbol analysis to assess interference from crowding. Additionally, a 2.5x telescope can be used during monocular testing to isolate functional amblyopia from organic amblyopia. If the visual acuity of the suspected eye improves by a factor larger than 2.5 then you can assume there is a non-organic functional component to the amblyopia.
If anisometropia exists, the prescription need not aim to fully correct the anisometropic difference as it was previously taught. In fact, under correcting the hyperopic eye and maximizing plus over the less hyperopic eye can uncover better binocular function (improved stereo acuity) even if the distance visual acuity is mildly reduced. The cycloplegic analysis is only a data point and should not directly translate to the final prescription if it means compromising binocularity. Finally, if and when possible, using contact lenses to minimize the image size difference is preferred, however, spectacle correction is also used in clinical practice.
Step 5: Follow-ups
Follow-ups should be every 4-6 weeks as vision is subject to change rapidly. The initial prescription is not necessarily your final prescription. At the follow-up, if further improvement of stereoacuity and suppression is demonstrated with a different combination of lenses then the prescription should be altered. Parents and patients should be educated about the dynamic nature of the prescription during the initial visit. This is another reason why contact lenses are preferred to glasses, however, glasses should typically be able to be re-done within 90 days.
In severe cases, where the visual function plateaus with lens therapy, a vision therapy consult should be recommended to improve binocular skills and provide long-lasting visual success. Activities in therapy are performed with both eyes open while stressing fixation with the amblyopic eye known as MFBF (monocular fixation in a binocular field).
Takeaways:
In summary, the initial prescription for a patient with amblyopia is one that provides best stereopsis, least degree of suppression, and the brightest retinoscopic reflex. It is not one that solely improves visual acuity or fully corrects the anisometropic difference. Although much of the current research is aimed at hyperopic anisometropia, this treatment protocol should also hold true for strabismic amblyopia. In cases of strabismus, prism could be added to further enhance binocularity. Dr. Vergara reports that through this new treatment protocol “the majority of patients under six years of age achieve 20/20 visual acuity and 20 seconds of arc within three months of beginning therapy. Rapid gains are made within the first month.” So, the next time you have a patient with presumed amblyopia, consider this protocol and save your patients the psycho-social trauma of being patched.
https://cdn.ymaws.com/www.covd.org/resource/resmgr/vdr/vdr_5_3/vdr5-3_perspective_press_web.pdf
https://www.cell.com/current-biology/fulltext/S0960-9822(13)00094-8
https://journals.plos.org/plosbiology/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001135
