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 reflex remained. All measurements were performed by the same experienced ophthalmologist, checked the right eye first, then the left eye. Used a fully automatic computer optometry/ corneal curvature meter of NIDEK ARK-1 (Japan) to measure the flat curvature radius(R1), steep curvature radius(R2), and meridian of R1, the refractive error pre- and post-cycloplegic. Measured two times, took the mean value of the results with confidence ≥ 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 first optical surface is always the cornea.10 The spherocylindric refraction in this example is vertexed to the corneal plane as shown below:
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 (REFs ) to corneal plane (REFc) 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
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 Waring13 and Mandell14 have recommended, the correct formula for computing the power and any change in power would be
where ra is the anterior radius of curvature of the cornea(m) and Ka is the front surface corneal power (D).
So the magnitude of anterior corneal astigmatism (ACA) was derived by the following formula:
R1 is anterior corneal curvature radius of the flat, R2 is anterior corneal curvature radius of the steep.
Both of the total ocular astigmatism and anterior corneal astigmatism are converted into the positive-cylinders form. The with-the-rule astigmatism is defined as axis from 75 degrees to 105 degrees, the against-the-rule astigmatism is defined as axis from 0 degrees to 15 degrees and 165 degrees to 180 degrees, and the oblique astigmatism is defined as axis from 16 degrees to 74 degrees and 106 degrees to 164 degrees.
To transpose the positive-cylinders form (S, +C × β) of refractive astigmatism into J0 and J45 components by Fourier transformation15–16:
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 J0 and J45 components were normal distribution. They were expressed as mean ± standard deviation. Data were compared by paired t test. Statistical significance was assumed at P < .05.