In the subjects with low and moderate myopia, the prevalence of RA (≥ 1.0 D) was 19.9%, ACA (≥ 1.0 D) 85.5%, and ORA (≥ 1.0 D) 51.5%. The mean ACA was 1.62 D (SD 0.62 D), the median (IQR) RA was 0.49 (0.46), and the median(IQR) ORA was 1.02 (0.58). Compared with the whole categories of refractive states (including emmetropia and ametropia), the prevalence of RA ≥ 1.0 D (19.9%) was similar to those reported in central China (17.4%) [3] and Singapore (19.2%) [22] but lower than those reported in Hong Kong (28.4%) [23] and Taiwan (32.6%) [24]. However, the prevalence of ACA (85.5%) and ORA (51.5%) were significantly higher than those studies [3, 25]. Meanwhile, the magnitude of ORA was larger than that in other studies. Li et al.[3] analysed 1783 12-year-old students and reported that the mean ORA was 0.72 D. Huynh et al. [8] found that the mean ORA was 0.76 D in 6-year-old children. One possible reason for this result is that the magnitude of ACA and ORA in myopic eyes was significantly larger than that in emmetropic and hyperopic eyes. Another possibility is that the compensation effects of ORA to ACA in myopia were more significant than those in other refractive states.
The prevailing wisdom was that the ORA compensated ACA [8,9,10]. Although our recent work evaluated the contribution of ORA to ACA in 5-year-old children [21], there is still no detailed information on the compensation effects in myope children. There was a significant and moderate correlation between the magnitude of ORA and ACA (r = 0.50, P < 0.001). The ORA had compensatory effects on the ACA in 240 eyes (99.6%).In 6.7% (16/240) of eyes, the compensation values exceeded the magnitude of the ACA. The axial classification of the ACA and RA were different after the compensation effects in 15.4% (37/240) of eyes. For 235 eyes with WTR ACA, 99.6% (234/235) of the ORA worked to offset it. Both ATR and oblique ORA can counteract WTR ACA, while oblique ORA can also superimpose it. The results were similar to our recent study in 5-year-old children with significant astigmatism [21]. With regard to ATR ACA, both WTR and oblique ORA had compensatory effects on it. WTR and ATR ORA can counteract oblique ACA.
Actually, the correlation between ORA and ACA and between ORA and RA would vary according to the population being studied (such as age, sphere, cylinder, etc.). We found a moderate and positive correlation between ORA and ACA (r = 0.50, P < 0.001) in low and moderate myopic eyes. Wallerstein and colleagues [7] obtained similar outcomes (r = 0.44) by studying 21,580 myopic eyes. Nevertheless, our previous study found a much weaker correlation between ORA and ACA (r = 0.17) by studying 14 emmetropic eyes, 68 myopic eyes, and 19 hyperopic eyes [21]. Moreover, there were no significant correlations between ORA and RA (r =—0.05, P = 0.42) in this study. Piñero et al. also reported a negative but nonsignificant correlation (r = -0.01, P = 0.89) by studying 14 emmetropic eyes, 68 myopic eyes, and 19 hyperopic eyes [6]. A positive correlation was found in other studies [1, 7]. Our previous study found a significant and negative correlation between ORA and RA (r = -0.27, P = 0.001) [21]. Characteristics of the studied population, such as age, sphere, cylinder, etc., may be the reason for the different research results.
Multiple studies have demonstrated that orthokeratology causes significant changes in corneal astigmatism [18,19,20]. Mountford and Pesudovs[18] stated that 87.0% of patients with reverse geometry orthokeratology lenses had some reduction of ACA in their study. Chan [19] reported reductions in ACA of up to -2.50 D three weeks after toric orthokeratology treatment. Chen et al.[20] investigated 35 myopic children with moderate-to-high astigmatism and found a 79% reduction in ACA after one month of toric orthokeratology. As mentioned previously, orthokeratology did not change the posterior corneal radius [16, 17]. Consequently, the ORA was unchanged and exposed after orthokeratology treatment. Relatively large amounts of ORA, which is mainly against-the-rule astigmatism, existed in 12-year-old children with orthokeratology indications, which may be one of the reasons for the degraded visual quality after orthokeratology. Sorbara et al. [26] found that the proportion of subjects with spectacles reaching 6/6 or better visual acuity was higher than those wearing orthokeratology lenses ( 89% vs. 83%). Unfortunately, there have been no studies on the relationship between ORA and orthokeratology. Future research needs to be done on the correlation between ORA and residual astigmatism after orthokeratology and on the specific influence of ORA on visual quality after orthokeratology.
In conclusion, for low- and moderate-myopia eyes, we found that the prevalence of ORA (≥ 1.0 D) was relatively high, and the magnitude was large. Nearly all (99.6%) ORA compensated for ACA, the magnitude of CV/ACA exceeded 1.00 in 6.7%(16/240) of eyes, and 15.4% (37/240) of eyes had a different axial classification of ACA and RA after the compensation effects. The ACA was reduced significantly, yet the ORA was unchanged and exposed after orthokeratology treatment. Therefore, ORA was the main source of residual astigmatism after okeratoplasty. Evidence suggests that residual astigmatism might be more problematic than expected if orthokeratology was used. Measuring ORA is equivalent to evaluating residual astigmatism that is not accounted for by the treatment. Therefore, the ORA should be assessed first before the completion of a course of orthokeratology. It may be a useful indicator to decide which patient is a good candidate for orthokeratology (those with low ORA). In addition, more attention should be given to the specific influence of ORA on the effectiveness of orthokeratology.