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There appear to be a number of questions with the regard to the
findings reported by Hrisos et al:
1. The study could not distinguish between amblyopia and other forms
of “unilateral visual impairment,” but groups all cases together. This
limits the study’s utility in demonstrating the functional effects of
2. The paper describes its acuity testing only as a "Snell...
2. The paper describes its acuity testing only as a "Snellen based
vision test." The citation given is Sheridan's STYCAR test (reference #
11). If that test was in fact used by Hrisos et al., the accuracy of the
study's acuity measurements in amblyopes is open to question, in view of
that test’s lack of crowding, which is critical to accurate acuity
measurement in amblyopes.
3. My major concern with this paper, however, is the question of
whether its sample of nominally visually impaired children in fact had
visual acuity significantly worse than that of the normal sample. Thus:
(a) The Results section states that four children with normal acuity
in both eyes were included in the impaired vision sample. The abstract
and Results section state that the unilaterally impaired children had
acuity as good as 6/6 in their worse eye. (The Methods section states that
their acuity was as good as 6/9, but if visually normal children were
included in the impaired vision sample, the 6/6 figure would appear
(b) Only straight-eyed patients were used, which means that, to the
extent amblyopes were included in the sample, they were anisometropic
amblyopes. Of the 30 children in the “unilaterally impaired” group, 10
had been in glasses for up to 6 weeks, and 5 for more than 12 weeks, at
the time of the study. In other words, half the sample had been so
treated. Since glasses alone will improve acuity in some anisometropic
amblyopes, [3, 4] if any of these refractively corrected “impaired”
children had been amblyopic, the amblyopia may have been mitigated.
(c) The median acuity of the unilaterally impaired sample’s worse
eyes, uncorrected, was 6/12, which would be considered normal acuity for
the younger children in the sample by some standards.
(d) The authors report that poorer amblyopic eye acuities were
significantly under-represented in their sample.
(e) Interpretation is complicated by the fact that the children
wearing glasses were classified on the basis of acuity testing done
without correction which, as the authors themselves note, may have
produced a treatment effect.
4. Another difficulty with this study lies in the stereotest used,
the Randot 2 Circles test.
(a) Stereo tests such as the Randot 2, with visible contours (i.e.
with notable low spatial frequency content and of parafoveal or larger
diameter), are able in many cases to be passed by anisometropic
amblyopes.[2, 6] Here again, as in the case of the visual acuity
measure, to the extent this occurred it would reduce the apparent
difference between the nominally visually impaired and normal subjects.
(b) Further complicating the matter is the finding of anomalous
responses to the Randot test in some subjects in both the impaired and
normal study groups (see Table 2). Similar anomalies have been
reported in a previous study. Three of the five such subjects had 6/6
to 6/12 acuity but failed the stereotest and so were assigned a 600 arc-
second “notional” threshold, which would, again, tend to reduce the
apparent difference between the impaired and normal groups.
(c) Anisometropic amblyopia is a condition apparently more benign
than strabismic amblyopia, and thus more likely to achieve normal or
near-normal stereopsis than would strabismic amblyopes. If the patients in
the "visually impaired" sample were such amblyopes, those fit with glasses
may have had their stereoacuity improved as well as their visual
In summary, the Hrisos et al. study’s failure to find a “visuo-
motor integration” function difference between its nominally visually
impaired and normal samples may have been due to the two samples’ acuities
not in fact being significantly different. On the other hand, the
association that was found between stereoacuity and “visuo-motor
integration” functions may have underestimated what would have been a
stronger association if the stereoacuity difference between the two groups
had not been reduced by the artifacts noted.
Kurt Simons, Ph.D.
Krieger Children’s Eye Center
The Wilmer Institute
Johns Hopkins Hospital
Baltimore, MD 21287
1. Hrisos S, Clarke MP, Kelly T, Henderson J, Wright CM. Unilateral
visual impairment and neuro-developmental performance in pre-school
children. Br J Ophthalmol, 2006.
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RE. Successful treatment of anisometropic amblyopia with spectacles alone.
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in preschool children: Results of a population-based, randomised control
trial. Ophthal Epidemiol 2001;8: 279-295.
6. Fawcett SL. An evaluation of the agreement between contour-based
circles and random dot-based near stereoacuity tests. J AAPOS 2005;9: 572-8.
7. Lam SR, LaRoche R, DeBecker I, Macpherson H. The range and
variability of ophthalmological parameters in normal children aged 4 1/2
to 5 1/2 years. J Ped Ophthalmol Strab 1996;33: 251-56.