Subthreshold Amblyopia

Here is your assignment today, class. Define amblyopia in one sentence. How about this: A binocular neurodevelopmental disorder characterized by reduced visual acuity (BVA) in one eye greater than the fellow eye and accompanied by deficits in stereopsis and other visual abilities.

Not bad, considering what we know about amblyopia these days, the basis for which much can be found in our VisionHelp Amblyopia Library. In a nice article in Vision Research a couple of years ago, Dennis Levi reviewed the contemporary view of amblyopia in an article titled “Rethinking Amblyopia 2020“. It was a clever title, and a reminder that we really need to move away from the narrow view about acuity. It was certainly catchier for the year in which it was published than a title of “Rethinking Logmar Zero Point Zero” (here is a nice logMAR/Snellen converter if you need one).

Way back in 1994, the American Optometric Association came out with a Clinical Practice Guideline on Care of the Patient with Amblyopia. It was a nice compilation that our VisionHelp colleague Dr. Barry Tannen and I co-authored, along with Drs. Mike Rouse, Jeff Cooper, and Sue Cotter. We made the point that any reduction in BVA below 20/20 was abnormal, and that Amblyopia represents a syndrome of compromising deficits, rather than simply reduced visual acuity, including:

• Increased sensitivity to contour interaction effects
• Abnormal spatial distortions and uncertainty
• Unsteady and inaccurate monocular fixation
• Poor eye tracking ability
• Reduced contrast sensitivity
• Inaccurate accommodative response

These concepts were heavily influenced at the time by Levi, Ciuffreda, and Selenow’s 1991 textbook on Amblyopia, and a seminal paper that came out a year later in the Transactions of the American Ophthalmological Society titled “The Classification of Amblyopia on the Basis of Visual and Oculomotor Performance” (cool to look back and see that its co-authors included Dennis Levi, Clif Schor, and Mert Flom). A very important point made in that paper, to which we’re return shortly, is the concept of at risk patients who no longer meet the conventional criterion of having reduced BVA because they were “successfully treated”, yet retain many of the visual deficits associated with amblyopia beyond resolving letters on a visual acuity chart. In other words, we don’t say that they have amblyopia because their BVA is considered normal, but their visual abilities substandard and they are as much at risk for compromised visual function as patients with overt amblyopia based on reduced interocular BVA differences. The paper went as far as to suggest that ‘”there may be a rationale for diagnosing amblyopia on the basis of functional visual loss instead of the traditional classification scheme based on associated condition (eg, strabismus, anisometropia, deprivation)”.

Given what we know now, nearly 30 years later, we would emphasize that it is a binocular neuro-developmental syndrome, and that its compromising deficits often involves a reduction in binocular stereo-acuity that parallels the monocular reduction in visual acuity. This has deep implications in how one approaches caring for the condition clinically (as our VisionHelp colleague Dr. Fortenbacher reviewed here), beginning with prescribing the ophthalmic Rx that provides the best binocular response rather than the best monocular visual acuity (as Sanet, Vergara-Giménez and I discussed here). The problem is that much of the ophthalmic community still conceptualizes amblyopia only on the basis of a reduction in BVA in one eye to 20/40, or an interocular difference of two or more lines in visual acuity.

That is why I was excited about a paper that was just published in the American Journal of Ophthalmology titled Subthreshold Amblyopia: Characterization of a new cohort. It essentially modernizes to some degree the at-risk population alluded to in the Transactions paper from 1992 referenced above. It is co-authored by David Hunter and his colleagues who formed the Boston
Amblyopia Study, and includes all patients diagnosed with amblyopia at Boston Children’s Hospital (BCH) over a 5-year period. For many years I have been referring to this population of patients with reasonably good BVA in both eyes who don’t meet the conventional definition of amblyopia as having “subclinical amblyopia”. (That is the term I used when lecturing to SUNY Residents on the subject.) Frankly I like the term “subthreshold” much better than “subclincial”, and will begin using it regularly now to refer to these patients who need help but fly under the radar (thank you Dr. Hunter and colleagues).

As this new paper notes, subthreshold amblyopia not only encompasses patients who have previously escaped detection or who have not been offered treatment, but those who have been previously treated and have what used to be called residual amblyopia. That brings up an important issue about what the endpoint of amblyopia treatment can or should be. The focus shouldn’t be on attaining “normal” visual acuity in both eyes. It should be on maximizing visual integration between the eyes that results in optimized (or least significantly improved) visual function. An overt metric of binocular function is stereopsis, with stereo-acuity as analogous to visual acuity. Yet that is only one of many available sets of metrics and observations, and different doctors may place more emphasis on some sets than others.