Amblyopia is the most common cause of unilateral vision loss in childhood, with an incidence of 3–5% (5,8). It is susceptible to diagnosis and treatment until 10 years of age (9), and the earlier the diagnosis, the better the outcome (10), hence the importance of investigating the causative indices leading to amblyopia in kindergarten.
Larsson et al. (11) found a mean spherical equivalent (SE) of 0.64 D ± 0.70 for the right eye and 0.67 D ± 0.80 for the left in 10-year-old Swedish children, similar to our data of 0.56 D ± 0.66 for the right eye and 0.59 D ± 0.67 for the left. Sandfeld et al. (12) studied 445 Danish children from 4.5 to 7 years old, finding 1.75 D ± 0.97 and 1.72 D ± 0.96 for right and left eye, respectively. Mayer et al. (13) found a mean SE of 1.13 D ± 0.85 for both eyes in 2-year-old US children. The difference of these two studies compared to our findings could be due to the non-standardised lighting conditions, and the fact that we used non-cycloplegic refraction, so we could not eliminate accommodation as a confounding variable. Hence, our results likely underestimated the refraction, mainly in hyperopes. A previous study from our group (7) found the differences between cycloplegic refraction and PlusoptiX photoscreening to be 0.68 D ± 2.63 and 0.25 D ± 1.31 respectively for mean SE.
Larsson et al. (11) reported a mean cylinder value of -0.43 D ± 0.26 for the right eye and − 0.44 D ± 0.28 for the left eye, similar to our findings of -0.53 D ± 0.48 for the right eye and − 0.53 D ± 0.47 for the left eye. They found with-the-rule astigmatism to equal against-the-rule, whereas Sandfeld et al. (12) found mostly with-the-rule and Mayer et al. (13) found mostly against-the-rule. The different instruments used for collecting data could be responsible for these differences, as discussed in our previous study , that using non-cycloplegic refraction like the PlusoptiX finds a mean cylinder of -0.66 ± 0.77, whereas cycloplegic refraction finds − 0.61 ± 0.74. Another cause of the discrepancy could be the differences in lighting conditions (14).
Few reports exist on normative data concerning pupil size, anisocoria and laterality in children. Using an infrared pupillometer, Silbert and colleagues (15) studied 1,306 children, finding an average pupil size of 5.92 mm for 4- to 7-year-old children, with no significant difference between left and right pupillary sizes. 219 children (16.77%) had anisocoria of 0.5–0.9 mm, and 30 children (2.3%) had anisocoria of > 1.0 mm. This is similar to our findings of 5.83mm ± 0.87 for the right pupil and 5.82mm ± 0.88 for the left. 13.26% (13,448 children) were found to have anisocoria ≥ 0.4mm; of these, 6.23% (6,314 subjects) had anisocoria of 0.5-0.9mm, and 0.66% (669 subjects) had anisocoria of > 1.0mm. Boev and colleagues (16) studied 90 children, finding average pupillary resting diameter in ambient lighting conditions to be 4.11 mm for 2- to 6-year-olds. The difference could be due to the fact that Silbert and colleagues  used a PlusoptiX device as we did, whereas Boev and colleagues (16) used a NeurOptic device. Discrepancies could also be due to the fact that we studied over 100,000 children between only 4 and 5 years old, whereas Silbert and colleagues studied children up to the age of 17 years, and Boev and colleagues up to 18 years.
MacLachlan C and Howland HC (17) studied interpupillary distance (IPD) of 155 males with average age 5.48 years and 144 females with average age 5.48 years, reporting a mean of 52.02 mm ± 2.55 for males and 51.03 mm ± 2.65 for females. Our results were lower, with a mean of 49.58 mm ± 3.50 for both sexes, possibly due to the measuring technique. Discrepancies could also be due to differences in the populations; little is known regarding the influence of ethnicity.
We found no statistical correlation between pupil size and SE, which had a Pearson correlation for the right eye of 0.053, and left eye of 0.033. There was also no significant correlation between pupil size and IPD, which had a Pearson correlation for the right eye of 0.192, and left eye of 0.189.
We based our amblyopia risk factor thresholds for referral on the AAPOS revised screening criteria (18); for children over two years, the threshold for hyperopia is > 3.5 D, myopia is > -1.5 D, astigmatism is > 1.5 D, and anisometropia is > 1.5 D. We had a total of 15,362 referrals (15.15%) out of 101,417 children. 807 children (0.80%) had hyperopia and 171 children (0.17%) had myopia. 6,307 children (6.22%) had astigmatism and 11,008 children (10.85%) had anisometropia. According to Dan Huang et al. (19) who studied Chinese children aged 3 to 4 years also using a PlusoptiX device, only 14 children (3.90%) out of 359 had refractive amblyopia risk factors; 2 (0.56%) had hyperopia, 10 (2.79%) had astigmatism, 3 (0.84%) had anisometropia. Their significantly lower referrals are possibly due to population differences; different ethnicities were studied, although of a similar age group. Li Deng and Jane E. Gwiazda (20) investigated anisometropia in 1,827 US Children by non-cycloplegic retinoscopy. They found a prevalence of 1.96% at 6 months, 1.27% at 5 years, and 5.77% from 12 to 15 years. Surprising, they found a lower prevalence of anisometropia despite using a lower threshold for referral of 1.0 D.
The effects of the potentially low sensitivity of the PlusoptiX S12, resulting in false negatives, may have resulted in a falsely lower number of referrals. Noelle S. et al. (21) studied amblyopia risk factors with the PlusoptiX S04 and found a sensitivity of 98% and a false negative rate of 1.4% based on the old AAPOS screening criteria. For further studying, the sensitivity of the PlusoptiX S12 could be investigated against the current AAPOS screening guidelines.
After searching the literature on amblyopia, paediatric normal ocular indices, refractive error in paediatric population, this is the largest study population for this topic to our knowledge, which is one of the major strengths of this study. However, this study only represents Israeli children, not the general paediatric population. We tested children of a specific age, 4 to 5 years old, denying us the opportunity to study the dynamics of changes throughout development. Data was not collected on comorbidities and drug history, potential confounding variables affecting pupil size. Although examination rooms were standardised, we were not able to provide conditions for dark adaptation. However, although pupil size is affected by varying ambient lighting, Ettinger and colleagues (14) showed that a tenfold change in luminance caused no more than a 1mm change in pupil size, so variability should not significantly affect our study.
For further studying, we propose that more data from other geographic areas and among other age groups should be done to further establish normative data, producing data that could be used worldwide.