The document summarizes research on the role of refractive correction in treating amblyopia. Key points: - Refractive correction alone can significantly improve visual acuity in amblyopic eyes over several weeks or months, termed "refractive adaptation." Studies found an average improvement of 0.1-0.5 logMAR units. - Refractive adaptation benefits all major types of amblyopia, including strabismic amblyopia. About a quarter of patients in some studies achieved normal vision with refractive correction alone. - Later studies confirmed these findings and established refractive adaptation as an important first step in amblyopia treatment protocols before other therapies like occlusion. - The neurophysiological basis is unclear but

1. REVIEW
The Optical Treatment of Amblyopia
Merrick J. Moseley*, Alistair R. Fielder†
, and Catherine E. Stewart*
ABSTRACT
The role of refractive correction has been underestimated as a distinct component of amblyopia therapy. Until relatively
recently, the extent to which it could ameliorate the amblyopic acuity deficit remained unquantified and the time course
of its effect unknown. Improvement of vision after refractive correction appears to occur in all the major types of
amblyopia, including, somewhat surprisingly, in the presence of strabismus. Although the neurophysiological basis of the
remediative effect of such “optical treatment” is unknown, some insight is now available from animal models and
psychophysical investigations in humans. An appreciation of the role that refractive correction can play in the overall
management of amblyopia has led to the formulation of new treatment guidelines, whereby a defined period of spectacle
or contact lens wear always precedes traditional therapies, such as occlusion or penalization.
(Optom Vis Sci 2009;86:629–633)
Key Words: amblyopia, optical treatment, refractive adaptation, treatment
T
he late 1980s was a most exciting period for infant vision
research. For the first time it became possible to measure—
simply and accurately—the vision of babies and young chil-
dren. Dobson was at the forefront of this activity and the vision
scientist most responsible for its incorporation into clinical prac-
tice and ophthalmic research. Much of the work we review here
draws on the rigorous approach pioneered by Dobson whose leg-
acy is that the quantitative assessment of vision, rather than obser-
vation of the eye alone, now lies at the heart of our approach to the
management of children with eye disease.
Even up to and beyond the time period referred to above, it was
firmly held that the correction of refractive error played only a
minor role in the clinical management of amblyopia. Indeed, ref-
erence to the failure of refractive correction to ameliorate amblyo-
pia often features in textbook definitions of the condition,1
which
generally fails to distinguish between immediate and gradual ef-
fects of spectacle or contact lens wear.
Sitting uneasily with this viewpoint was the observation that
amblyopic children would often not obtain their best “pre” treat-
ment visual acuity until a refractive correction had been in place for
some time. Although this intervention—commonly referred to as
spectacle adaptation—had formed a component of the treatment
for anisometropic ambyopia since at least the middle of the last
century,2
it was seen only as an adjunct to treatments such as
penalization or occlusion, rather than actively therapeutic in its
own right.
EMPIRICAL CONFIRMATION
It was not until 2002 that any attempt was made to quantify the
effect that refractive correction might have on amblyopia. Up until
this point there was no evidence as to what proportion or what type
of patient might benefit from this intervention or to what extent
and what timescale. In a short report published in Ophthalmic and
Physiological Optics,3
we analyzed the improvement in visual acuity
subsequent to the correction of refractive error in a small group
(n ϭ 13) of typically presenting amblyopic children and were
greatly surprised by the magnitude of improvement [0.1 to 0.5
logMAR (minimum angle of resolution) units], which occurred
among these mostly refractive amblyopes. Further, refractive cor-
rection was seen to benefit all the children. Time to best acuity
ranged from 4 to 24 weeks (Fig. 1). Control subjects who did not
undergo the repeated (weekly) testing of the experimental group
but who otherwise were treated identically also showed comparable
acuity gains, essentially ruling out practice or training effects as a
more parsimonious explanation of the improvements seen. Fur-
ther, the time period over which improvements occurred was not
so prolonged that visual maturation could be considered a signifi-
cantly contributing factor. Perplexing to us and others was the
finding that one of our amblyopic participants who had both an-
isometropia and esotropia with their amblyopia demonstrated a
gain in acuity (0.28 logMAR) that was on a par with the straight-
eyed study participants. Subsequently, when similar observations
*PhD
†
FRCOpth
Department of Optometry and Visual Science, City University, London,
United Kingdom.
1040-5488/09/8606-0629/0 VOL. 86, NO. 6, PP. 629–633
OPTOMETRY AND VISION SCIENCE
Copyright © 2009 American Academy of Optometry
Optometry and Vision Science, Vol. 86, No. 6, June 2009

2. LogMARvisualacuity
Amblyopic Eyes
4 weeks
0.10
logunits
0.90
0.88
0.78
0.54
0.44
0.42
0.40
0.36
0.30
0.16
0.10
0.34
0.60 (0.50)
0.66 (0.48)
0.28 (0.22)
0.08 (0.04)
0.06
0.08
0.00
0.00
0.16
-0.02
0.08
0.00
FIGURE 1.
“Waterfall” plot of logMAR visual acuity as a function of time. Plot lines show individual subject data ordered by severity of amblyopia. Initial and final
corrected acuities appear, respectively, on the left and right of each plot line. Parenthetic values are best acuities attained during the study if not those
recorded at the last visit. Reprinted with permission from Ophthal Physiol Opt, 22, 296–9, 2002.
630 The Optical Treatment of Amblyopia—Moseley et al.
Optometry and Vision Science, Vol. 86, No. 6, June 2009

3. were again reported by ourselves4
and by others,5
this became less
controversial; we shall return to this issue later.
Although “refractive adaptation”—as the improvement in acu-
ity attributable to the correction of refractive error became
known—did not immediately impact on clinical practice, it soon
gained significance in the context of clinical trial design. A much-
publicized systematic review of amblyopia6
treatment had, by this
time, provided an impetus for a more evidence-based approach to
amblyopia therapy, resulting in the design of dose response and
controlled clinical trials. Crucially, it was now considered necessary
when attempting to evaluate the effectiveness of, say, a regimen of
occlusion, that the study account for the effects of prior refractive
adaptation. The first such study we conducted—the Monitored
Occlusion Treatment for Amblyopia Study (MOTAS7
)—put this
important aspect of study design into practice. It comprised three
phases. In the first (“baseline”) phase, participants were clinically
assessed and stable baseline visual performance established. In the
second, participants underwent a period of refractive adaptation
until we were reasonably certain that all improvement attributable
to this process would have occurred (the duration of this
phase—18 weeks—was based on our previous study,3
where no
gains in acuity exceeding 0.1 logMAR occurred beyond this pe-
riod). Only at this point did phase 3 begin, within which occlusion
was prescribed.
MOTAS, it could be said, put the concept of refractive adapta-
tion firmly on the map, such that we felt that the findings of the
refractive adaptation phase merited publication in their own
right.4
Sixty-five children [mean (standard deviation) age ϭ 51(1.4
years)] were enrolled, of which just under half were anisometropic
and strabismic, and the remainder either anisometropic or strabis-
mic in roughly equal proportions. LogMAR visual acuity im-
proved on an average by 0.24 log unit (range: 0.00 to 0.60) over the
18-week adaptation phase (Fig. 2). The improvement was not seen
to differ as a function of age or type of amblyopia. Fourteen chil-
dren (22%) improved to such an extent during refractive adapta-
tion that they became ineligible to proceed to the final phase of the
study in which they would have been prescribed occlusion. This
outcome led us to ponder the fact that had these children under-
gone routine clinical management as practiced at that time, they
would likely as not, have undergone a quite unwarranted period of
occlusion therapy. Where fellow eyes had significant refractive er-
rors (Ն1.75 D), their acuity was also observed to improve, on an
average, by 0.1 log unit. However, it was unclear to what extent
such gains can be interpreted as arising from the remediation of
bilateral amblyopia or from the simple optical benefits of refractive
correction.
In 2006, our MOTAS findings were corroborated and extended
by the Pediatric Eye Disease Investigator Group (PEDIG).8
In
their study, subjects with anisometropic amblyopia (n ϭ 84) ini-
tially underwent between 5 and 30 weeks of refractive correction
during which their mean minimum angle of resolution reduced by
almost half (0.29 logMAR gain). This corresponded to an im-
provement of Ն0.2 logMAR and Ն0.3 logMAR in, respectively,
77 and 60% of participants. Resolution of amblyopia occurred in
23 (27%) children. There was no apparent effect of age on the
improvement of acuity seen but poorer initial acuity and greater
degrees of anisometropia decreased the likelihood of resolution of
amblyopia. Of note is that this study used a more stringent inclu-
sion criterion for the definition of amblyopia (Ն0.2 log unit intra-
ocular difference) compared with that of MOTAS (Ն0.1 log unit
intra-ocular difference). Given the outcome of the PEDIG study,
it now seems unlikely that the findings of MOTAS could have
been accounted for by the inclusion of some non-amblyopic
ametropes, whose acuity gains arose solely from the optical benefits
of refractive correction.
Our most recently conducted trial of occlusion therapy
(ROTAS9
) provided further confirmation that refractive adapta-
tion in itself constitutes a robust treatment for amblyopia.a
Forty-
four children undergoing 18 weeks of spectacle wear before scheduled
occlusion gained, on an average, 0.22 logMAR unit of acuity.
Statistical power constraints did not permit a secondary analysis by
age and amblyopia type.
REFRACTIVE CORRECTION IN BILATERAL
REFRACTIVE AMBLYOPIA
Where the refractive error is symmetrical and bilateral, refractive
correction is an established and uncontroversial treatment. The
improvements in acuity seen are of the greatest magnitude reported
for all types of amblyopia. For example, in this category of patients,
PEDIG5
observed a mean (95% confidence interval) improvement
in the binocular logMAR acuity of 113, 3 to 10-year-old children
of 0.39 (0.35 to 0.41) log unit.
REFRACTIVE CORRECTION IN
NON–STRAIGHT-EYED AMBLYOPIA
That refractive correction has now been convincingly estab-
lished to be beneficial,10
where the primary clinical association of
the amblyopia is refractive error had, as already mentioned in the
Introduction section, is highlighted in the clinical literature. How-
ever, our findings in 2004 (and to a limited extent in 2002) that
even in the absence of significant anisometropia and in the pres-
ence of a constant strabismus, refractive correction still appeared to
exert an ameliorating effect on the amblyopia3,4
was not a readily
a
Indeed PEDIG had already adopted the term “optical treatment of amblyopia.”8
FIGURE 2.
Change in mean (SD) logMAR visual acuity of the amblyopic eye during
refractive adaptation. Reprinted with permission from Br J Ophthalmol,
88, 1552–6, 2004.
The Optical Treatment of Amblyopia—Moseley et al. 631
Optometry and Vision Science, Vol. 86, No. 6, June 2009

4. predictable finding, or as one comment in the literature put it,
“. . . somewhat surprising . . . .”5
However, an analysis of the
changes in acuity occurring during a refractive correction “run-in”
phase to a randomized trial of occlusion therapy5
again revealed
improvements entirely comparable (i.e., Ն0.2 logMAR) with
those seen among straight-eyed amblyopes. In the case of MOTAS,
although a small amount of this improvement could be accounted
for by some subjects (n ϭ 7, 20%) having small angle strabismus
and rudimentary binocular vision, or strabismus of a refractive
nature, the majority of those categorized as strabismic amblyopes
had large angle strabismus without demonstrable binocular vision
at the start or end of the treatment.
NEUROPHYSIOLOGICAL BASIS OF
TREATMENT EFFECT
To date, most research that has examined the effect of refractive
correction on amblyopia has predominantly concerned itself with
establishing the magnitude and time course of the treatment gains,
and to a lesser extent with the categories of amblyopic patients who
might benefit. It does not appear to be an artifact of repeated
testing,3
and clearly, the time course of the improvement in vision
is incongruent with any explanation based on simple optics. Un-
fortunately, we know of no experimental models that mimic or
manipulate the effects of refractive adaptation. However, it might
prove insightful to search for clues among the considerable body of
evidence arising from experimentally induced ocular deprivation?
After all, refractive correction can, at one level, simply be viewed as
a more subtle alteration of spatial visual input in comparison with
the typically gross manipulation (i.e., total provision or total elim-
ination) used in ocular deprivation studies. Consider, for example,
recent findings that have highlighted the role of (non-competitive)
binocular experience in reversing the effects of monocular depri-
vation. The initial experimental manipulation undertaken by
Mitchell and Gingrass11
involved classical monocular deprivation
of a cat eye by eyelid suture. After 6 days, the eye was opened with
no attempt to eliminate the competitive advantage of the previ-
ously non-deprived eye (no reverse occlusion). Subsequent record-
ing of grating acuity (jumping stand paradigm) for up to 6 weeks
showed an orderly recovery in acuity to around 5 c/deg in the
formally deprived eye with the acuity of the non-deprived eye
reaching 7.05 c/deg (within normal range). Indeed, analogous
findings have been reported in human infants, where the restora-
tion of binocular visual input on removal of a congenital cataract
facilitates a rapid improvement in visual acuity.12
However, at this
juncture it is important to compare the experimental model and
human research. In the former, it seems that restoration of acuity
occurs only when the binocular visual input is correlated.13
This is
very different from the human situation in which refractive correc-
tion improves acuity in amblyopic children without and with stra-
bismus: the latter being a de facto clinical example of uncorrelated
visual input. However, recent studies14,15
have shown that, con-
trary to our previous understanding, subjects with strabismic am-
blyopia have mechanisms that use and combine information from
the affected and fellow eye, and they can both drive binocular cells
in visual cortical area V1. Although this was observed under exact-
ing experimental conditions with stimulus presentation at corre-
sponding retinal locations, it at least hints at the possibility that
fellow eye suppression may not result in complete inhibition of
visual input modified by refractive correction.
A TREATMENT PROTOCOL
The evidence base suggests that the majority of children with
refractive error and amblyopia will benefit from a period of refrac-
tive adaptation, negating the need for occlusion in around one
quarter to one third of patients.4,8
However, what should the
guidelines for refractive adaptation be and what is the present
uptake of clinicians worldwide to this evidence base? Possible
guidelines for refractive adaptation would include a minimum pe-
riod of full-time spectacle wear of 12 weeks for all children with
amblyopia and significant refractive error. Follow-up should be 6
to 8 weeks until substantive gains in visual acuity cease or visual
acuity becomes good and equal. In the event that little or no gains
in visual acuity are seen after 12 weeks, refractive error should be
reassessed and refractive adaptation restarted if significant differ-
ences are seen. Subjects showing no improvement in visual acuity
or difference in refractions should commence occlusion.4,8
Informal observation and feedback suggest that many orthop-
tists in the United Kingdom and Europe have implemented
protocols for prescribing optical treatment to children with ambly-
opia,16
and it is our understanding that likewise in the United
States, clinical practice is now moving in a similar direction.
CONCLUSIONS
In this review, we have examined the emerging evidence that
refractive correction can, over a period of time, significantly reduce
the acuity deficit in the most commonly presenting types of am-
blyopia. Such findings directly impinge on the way in which we
should manage our patients and the expectations of outcome we
can offer to carers. Although the neural mechanisms underpinning
optical treatment remain obscure (particularly so in the case of
strabismic amblyopia), we have highlighted some areas of research
that we consider insightful. Hopefully, the increasing implemen-
tation of optical treatment as a component of clinical management
should provide an impetus to elucidating the basis of this impor-
tant, but hitherto neglected, treatment modality.
ACKNOWLEDGMENTS
This work was supported by the Guide Dogs for the Blind Association and
Fight for Sight, UK.
Received November 6, 2008; revision received January 28, 2009.
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Merrick Moseley
Department of Optometry and Visual Science
City University
London EC1V 0HB, United Kingdom
e-mail: m.j.moseley@city.ac.uk
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