Underestimation of astigmatism without cycloplegic refraction in preschool children

Signi�cance: It is generally believed that cycloplegia optometry had no effect on astigmatism. The comparison was all made on the spectacle plane. There is a dearth of studies on the in�uence from corneal plane. Purpose: Study on the difference between cycloplegic and noncycloplegic measurements of astigmatism on corneal plane. Methods: This is a retrospective study. Vertexing the total ocular astigmatism to the corneal plane. To transpose astigmatism into J 0 and J 45 components by Fourier transformation. The changes of astigmatism measured with and without cycloplegic were compared from corneal plane. Data were compared by paired t test. Results: The changes of the total ocular astigmatism were signi�cantly different on corneal plane (total ocular astigmatism: t = -2.31, P < .05; J 0 components: t = -2.71, P < .01; J 45 components: t = -1.09, P(cid:0).05). The magnitude of total ocular astigmatism generally increased after cycloplegia (2.12 ± .87 D vs 2.07 ± .89 D). 37 eyes (32.17%) had a change value greater than 0.25 D. The proportion of with-the-rule total ocular astigmatism increased (from 90.43–92.17%) and that of oblique total ocular astigmatism decreased (from 9.57–7.83%). While the changes of anterior corneal astigmatism were no signi�cantly different . Conclusion: There were underestimation of total ocular astigmatism without cycloplegic refraction in preschool children. The proportion of with-the-rule astigmatism increases after cycloplegia.


Introduction
Refractive error is the main cause of low vision in children, [1][2] and astigmatism is the main refractive error.8][9] However, previous studies on the in uence of astigmatism were all carried out on the spectacle plane.The corneal plane should be taken into account when performing astigmatism treatments surgeries, cataract surgeries or matching contact lens.This study is the rst to report the in uence from corneal plane.

Materials And Methods
This study followed the tenets of the Declaration of Helsinki and was approved by our institutional review board.Informed consent was obtained from each subject' parents after explanation of the nature of the study.To analysis astigmatism in the form of positive-cylinders.

Participants Selection
We conducted a retrospective study of children diagnosed with refractive astigmatism ≥ 1.00D who presented at the outpatient ophthalmology clinics at Lianyungang Maternal and Child Health Hospital during June 2018 to December 2018.Excluding organic diseases of eyes such as cataract, glaucoma, keratoconus, irregular astigmatism, nystagmus and children with strabismus.We analyze the right eye data only.Finally, a total of 115 right eyes met the inclusion criteria (53 females and 62 males).The mean age was 4.8 ± 1.0 years.

Examination Protocol And Collect Parameters
To use 1% cyclopentolate hydrochloride eye drops to perform cycloplegia (one drop each time, once every 8 minutes, four times in total).After each time, pressed the lacrimal sac for 3 minutes, and waited for at least 30 minutes after four times until the pupillary reaction to light disappeared or only the weak light re ex remained.All measurements were performed by the same experienced ophthalmologist, checked the right eye rst, then the left eye.Used a fully automatic computer optometry/ corneal curvature meter of NIDEK ARK-1 (Japan) to measure the at curvature radius(R 1 ), steep curvature radius(R 2 ), and meridian of R 1 , the refractive error pre-and post-cycloplegic.Measured two times, took the mean value of the results with con dence ≥ 8.The NIDEK ARK-1 measured the radius of corneal curvature within 3.3 mm of the center of the cornea.

Data Analysis And Calculations
To convert all manifest refraction data from the spectacle to the corneal plane (adjusting for vertex distance), as the intended rst optical surface is always the cornea. 10The spherocylindric refraction in this example is vertexed to the corneal plane as shown below: To compute corneal front surface power, the change in media for the light rays is from air (n = 1.000) to cornea (n = 1.376), so, as Holladay and Waring 13 and Mandell 14 have recommended, the correct formula for computing the power and any change in power would be where r a is the anterior radius of curvature of the cornea(m) and K a is the front surface corneal power (D).
So the magnitude of anterior corneal astigmatism (ACA) was derived by the following formula: R 1 is anterior corneal curvature radius of the at, R 2 is anterior corneal curvature radius of the steep.
Both of the total ocular astigmatism and anterior corneal astigmatism are converted into the positivecylinders form.The with-the-rule astigmatism is de ned as axis from 75 degrees to 105 degrees, the against-the-rule astigmatism is de ned as axis from 0 degrees to 15 degrees and 165 degrees to 180 degrees, and the oblique astigmatism is de ned as axis from 16 degrees to 74 degrees and 106 degrees to 164 degrees.

Statistical Methods
This was a retrospective study.SPSS statistics software package version 17.0 for Windows (IBM, Armonk, NY, USA) was used for the statistical analysis.The magnitude of total ocular astigmatism, anterior corneal astigmatism, and their J 0 and J 45 components were normal distribution.They were expressed as mean ± standard deviation.Data were compared by paired t test.Statistical signi cance was assumed at P < .05.

Results
The changes of the total ocular astigmatism and J 0 components were signi cantly different pre-and post-cycloplegic on corneal plane (total ocular astigmatism: t = -2.31,P < .05;J 0 components: t = -2.71,P < .01).The other changes were no statistically signi cant (Table 1).The comparison of total ocular astigmatism between with and without cycloplegia refraction: The magnitude total ocular generally increased after cycloplegia (from 2.07 ± 0.89 D to 2.12 ± 0.87 D).Speci cally, the magnitude of total ocular astigmatism increased in 87 eyes (75.65%) after cycloplegia, 28 eyes (24.35%) increased larger than 0.25 D, with the largest change from 2.22 D to 2.81 D.
Meanwhile, the magnitude of total ocular astigmatism decreased in 25 eyes (21.74%), 9 eyes (7.83%) decreased more than 0.25 D, with the largest change from 3.59 D to 2.72 D. The value of total ocular astigmatism remained unchanged in 3 eyes (2.61%).In conclusion, 37 (32.17%)eyes had astigmatism value changes of more than 0.25 D. The proportion of with-the-rule total ocular astigmatism increased (from 90.43-92.17%),and the proportion of oblique total ocular astigmatism decreased (from 9.57-7.83%).In 82 eyes (71.30%), the positive value of J 0 components of total ocular astigmatism increased, and 33 eyes (28.70%) decreased.The distribution of the total ocular astigmatism with and without cycloplegic refraction on double angle vector diagram were shown in Fig. 1.

Discussion
To our knowledge, this is the rst clinical study of the difference of astigmatism refractive errors with and without cycloplegic refraction from the corneal plane.In this study, we analyzed the astigmatism changes between pre-and post-cycloplegic refraction on the corneal plane.We found that the changes of total ocular astigmatism were statistically signi cant.Speci cally, the positive value of J 0 components of total ocular astigmatism generally increased, it means that after cycloplegia, the with-the-rule component of total ocular astigmatism was increased, which mainly caused by the change of lenticular The lenticular astigmatism are great majority against-the-rule astigmatism.8][19][20] Duke-Elder 21 considers that lenticular astigmatism can counteract with-the-rule corneal astigmatism and superimpose against-the-rule corneal astigmatism, however, the oblique astigmatism rule is not clear.The ciliary muscle relaxes under cycloplegia condition occurscausing the tighten of the zonular bers allowing the crystalline lens to decrease curvature and thickness, and leading to an decrease in the crystalline lens refractive power.As a result, the ability of the crystalline lens to counteract the with-the-rule anterior corneal astigmatism is weakened.It results in the increasing of the proportion of with-the-rule total ocular astigmatism and the magnitude of total ocular astigmatism.On the contrary, the value of astigmatism, which was characterized by superposition, become smaller.
Astigmatism is a common optical disorder and exists in most human eyes in subtle amounts.Astigmatism affects visual acuity and contrast sensitivity and compared to other refractive errors, is more di cult to treat. 224][25] Due to the strong accommodating power of 3-6-year-old children, it is necessary to obtain accurate refractive power with cycloplegia refraction, and it remains the gold standard for measuring refractive status in children.The possibility of glaucoma induced by cycloplegia in adults is greatly increased.Therefore, it is usually to perform optometry for adults without cycloplegia.However, before astigmatism treatments surgeries and cataract surgeries on adults, we need to analyze the astigmatism refractive errors from the corneal plane.Therefore, it is necessary to analyze the changes of astigmatism before and after cycloplegia on corneal plane.This study found that there was statistical signi cance in the changes of total ocular astigmatism with and without cycloplegic refraction from corneal plane in preschool children.Although the conclusion cannot be directly applied to adults, but it reminds us that this problem should be considered before laser surgery and cataract surgery in adults.
Whether the difference of total ocular astigmatism before and after cycloplegia is one of the reasons for the poor effect of surgeries remains to be further studied.
As we known that cycloplegia can lead to signi cant changes in the sphere, i.e. increase in hyperopic sphere,decrease in myopia sphere.Nevertheless, astigmatism is the difference between the maximum and the minimum power, which is actually the difference between the utmost and the minimal sphere.We observed that the change in the amount of astigmatism appeared statistical signi cance on corneal plane.Therefore, on the corneal plane, not only does the sphere changes post-cycloplegic, but also the difference between the largest and smallest power, that is, astigmatism changes.
Previous studies had shown that the astigmatism were affected by many factors, such as extraocular muscles and accommodation.Astigmatism can causes near and far blurred vision in children.Prevention of astigmatic amblyopia, we mainly focus on distance vision.The accommodation was relaxed after cycloplegia.Therefore, the correction of astigmatism should be based on the refractive status after cycloplegia. Figures Spherocylindric form: +1.75-3.25 × 180 Cross cylinder form @ spectacle: +1.75 × 90 and − 1.50 × 180 Vertex: 12.00 mm (ARK-1 sets the vertex to 12 mm) Vertex formula from spectacle plane (REF s ) to corneal plane (REF c ) 11,12 : Substituting the values from the example Cross cylinder form @ cornea: 1.79 × 90 and − 1.47 × 180 Vertex: 0 mm Minus cylinder form: 1.79-3.26× 180 Plus cylinder form: -1.47 + 3.26 × 90

Figure 1 Double
Figure 1

Table 1
Astigmatism comparison pre-and post-cycloplegic on corneal plane