Amblyopia: Early Detection is Essential

BY ERIC LICHTENSTEIN, M.D.

Pediatric ophthalmology is concerned with one problem above all others: amblyopia. There is a critical (i.e., sensitive) period for vision development, and failure to diagnose and adequately treat amblyopia in a timely fashion leads to permanent vision impairment in one or both eyes. It is estimated that 1.6% to 3.6% of the population has amblyopia,¹ and it is the leading cause of impaired monocular vision between childhood and middle age in wealthy countries. Our understanding of the neurologic basis, critical period and management options are rooted in the Nobel prize-winning research of Hubel and Weisel in the early 1960s. Until the turn of the millennium there were few major changes in our understanding.

Over the past decade, a tremendous amount of useful and well-conducted amblyopia research has been performed by the Pediatric Eye Disease Investigator Group (PEDIG) and paradigm shifts have resulted. PEDIG (funded by the National Eye Institute) conducts prospective, multicenter studies of amblyopia, strabismus and other pathologies. Additionally, PEDIG is a collaborative effort with ophthalmologists and optometrists on the steering committees and at the clinical sites.

PEDIG's Amblyopia Treatment Study (ATS) protocols provide a wealth of solid, and sometimes surprising, data about how to treat amblyopia, the sensitivity to treatment at various ages and the risk factors for recurrence (among other findings). For example, it was once "gospel" that amblyopia could not be treated beyond the age of 7 or 8 years. However, PEDIG's ATS-3 demonstrated that just over 50% of children between the ages of 7 and younger than 13 years will improve at least two lines of acuity when treated with optimal optical correction in addition to patching or atropine (i.e., treatment). Between the ages of 13 and younger than 18, the response rate is just under 50% when treated the same way — but only if the patient had never been treated for amblyopia before.² Other interesting results concern the mode and duration of treatment.

For children younger than 7 years, if the amblyopic eye has acuity between 20/40 to 20/80 (moderate amblyopia), patching 2 hours per day is as effective as patching 6 hours per day (ATS-2).³ Given the same parameters (younger than 7 years, acuity 20/40 to 20/80), atropine in the non-amblyopic eye is as effective as patching 2 hours per day; atropine given just on Saturday and Sunday yields the same result as atropine given daily (ATS-4).⁴

It is important to remember that while we now know that the critical period is longer in some cases than previously believed and our therapeutic options are better defined, there remains no question that early detection and treatment is ideal: the earlier the better. PEDIG has a useful Web site: http://public.pedig.jaeb.org/index.html for your reference.

Two young patients wear eye patches and eyeglasses to treat amblyopia.

Vision Screening

Of course, there is no treatment of amblyopia without finding the cases and making the diagnosis. Vision screening is cheap, fast, and, when conducted properly, a high-yield method for detecting amblyopia. Ideally, vision screening is performed at multiple points during childhood. The American Association for Pediatric Ophthalmology and Strabismus and the American Academy of Ophthalmology issued a policy statement in 2007 entitled, "Vision Screening for Infants and Children."

To paraphrase, the guidelines are as follows: in the nursery infants are to have red reflex and anterior segment/adnexal evaluations — those with a family history of retinoblastoma, congenital cataract or glaucoma, or any abnormality detected on exam need to be evaluated by an ophthalmologist. Infants between 6 and 12 months should have their red reflex tested during well-baby visits. Between 2-and-a-half and 3-and-a-half-years, an attempt should be made to determine the visual acuity; a child who is uncooperative at a second attempt should be referred to an ophthalmologist. Further screening should be performed at school, and there is some evidence that photoscreening may be useful in addition to traditional screening methods (especially in pre-verbal children). Of note, routine comprehensive professional eye examination has no proven medical benefit.

To understand the "controversy" about vision screening versus mandated full exams for every child, it is useful to "follow the money." All eyecare professionals would see more children with mandated pediatric eye exams but they would be inundated with normal exams at great cost to society. Couple this with the rate of pediatric spectacle prescription in commercial optical shops (35%) versus pediatric ophthalmologists (6%)⁵, and it becomes clear why lens and frame manufacturers, and those who make a living selling these items, favor full exams at regular intervals for all children.

Essential Exam Steps

When an ophthalmologist is evaluating the visual acuity of a child young enough to develop amblyopia, it is essential to determine the vision in each eye separately, ensuring that the covered eye is truly covered; two-inch paper tape sealed against the orbital rim does quite nicely. Children with amblyopia will "cheat" by peeking through fingers or around occluders in an effort to "do a good job" with acuity testing, so make sure they do not "help out" in this regard.

For verbal children, there are many optotypes available on computerized vision testing devices. Don't be surprised to find that some 2- and 3-year olds recognize Snellen characters (the HOTV sequence tends to be easier for many). Whether using Snellen, Lea or Allen optotypes, the acuity measured simply "does not count" unless crowding is employed — use either an entire line of type, or crowding bars around a single optotype. Single, isolated optotypes provide a false over-estimation of the acuity and result in false-negative findings of "normal."

There are three causes of amblyopia: refractive, strabismic and occlusive. Strabismus sufficient to cause amblyopia is usually evident, but small angles of strabismus (microtropias) may not be so obvious. To determine the presence of a microtropia, place an 8 D prism base out in front of one eye while the patient is fixating on an object binocularly. Both eyes should shift towards the direction of the apex. If only one eye shifts, there is a microtropia with scotoma — the visual acuity is usually in the 20/40 range in these cases. Causes of occlusive (deprivational) amblyopia are usually evident as well: media opacities (such as cataract) and complete ptosis can be amblyopiagenic. In the case of ptosis, if the child is lifting her chin, amblyopia is not likely as she is trying to maintain binocularity.

The most difficult form of amblyopia to detect is refractive because refractive errors are not visible on the patient's face, as is the case with strabismus or ptosis. Anisometropia is the most common cause of amblyopia and while the details are too great to get into here, the take-home message is that only cycloplegic retinoscopy is acceptable when determining refractive errors in children. Children have tremendous accommodative amplitudes and, unlike adults, their refractive state cannot be properly determined without cycloplegia. Cyclopentolate 1% should be used for most patients; those with a lot of pigmentation (especially those from southeast Asia) often need cyclopentolate 2% and, in rare cases, atropine given prior to a follow-up visit. OM

References

1. Simons K. Amblyopia characterization, treatment and prophylaxis. Surv Ophthalmol. 2005;50:123-166.
2. PEDIG Study Group. 2005 Randomized Trial of Atropine Regimens for Treatment of Moderate Amblyopia in children aged 7 to 17 years. Arch Ophthalmol. 2005;123:437-447.
3. PEDIG Study Group. 2003 A randomized trial of patching regimens for treatment of moderate amblyopia in children. Arch Ophthalmol. 2003;121:603-611.
4. Pediatric Eye Disease Investigator Group. A randomized trial of atropine regimens for treatment of moderate amblyopia in children. Ophthalmology. 2004;111:2076-2085.
5. Donahue SP. How often are spectacles prescribed to "normal" preschool children? J AAPOS. 2004;8:224-229.

Eric A. Lichtenstein, M.D., completed residency at The New York Eye & Ear Infirmary prior to his fellowship in pediatric ophthalmology and strabismus at Indiana University. A member of AAPOS, he is in private practice in Queens, NY and is an Adjunct Professor at The New York Eye & Ear Infirmary.