Dear Editor,

There appear to be a number of questions with the regard to the findings reported by Hrisos et al:[1]

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 amblyopia.

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.[2]

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 correct.)

(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.[5]

(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[1]). Similar anomalies have been reported in a previous study.[7] 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,[3] 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 acuity.[3, 4]

In summary, the Hrisos et al.[1] 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

References

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.

2. Simons K. Major review. Preschool vision screening: rationale, methodology and outcome. Surv Ophthalmol 1996;41: 3-30.

3. Simons K. Amblyopia characterization, treatment, and prophylaxis. Surv Ophthalmol 2005;50: 123-66.

4. Steele AL, Bradfield YS, Kushner BJ, France TD, Struck MC, Gangnon RE. Successful treatment of anisometropic amblyopia with spectacles alone. J AAPOS 2006;10: 37-43.

5. Williams C, Harrad, RA, Harvey, I, et al. Screening for amblyopia 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.