Spherical equivalent refraction is given in the combined effect of several ocular components—specifically, axial length, corneal power, anterior chamber depth, and lens power[7].There are limited studies on the observation of refractive power and morphology of the lens. In this paper, CASIA2 is used to compare the differences of refractive power and crystal morphology parameters before and after cycloplegia. It is well known that contraction of the ciliary muscle and relaxation of the suspension ligament will change the surface curvature of the lens, thus increasing the fundamental optical power of the lens[8].However, cycloplegia resulted in hyperopia shift and astigmatism axis change, but astigmatism power was basically unchanged[9], the record of diopter change in this paper refers specifically to a spherical mirror. For the diopter changes before and after cycloplegia, 96.2% (102 eyes) of 106 eyes showed positive changes in different degrees, while the other 3.8% (4 eyes) showed changes of -0.25 D. Due to the relaxation of accommodation, the diopter changes in the direction of correction. The conclusion of this paper is compatible with previous studies and theories[10]. It has to be mentioned that there are also very few cases with a small amount (-0.25 D) of adverse deviation, which may be due to: (1) Significant changes in high-order aberrations of eyes after pupil dilation will affect the measurement of diopter to vary degrees[11, 12].(2)Adolescents have extremely strong accommodation function, so the optometry results under the tiny pupil may not be reliable. (3)The possibility of measurement errors due to the cooperation degree of patients is not ruled out.
The change of crystal morphology before and after cycloplegia is mainly the ACL and LTH, but there is no obvious difference in PCL, LD, LT and LED. Schachar et al[13]reported that during the accommodation process, the anterior surface of the lens curvature(ACL) changes more than the posterior surface(PCL), while the lens itself has a accommodation range of about 7.8 D, and there is no movement or deviation during cycloplegia due to the lack of movement of the lens nucleus. Grzybowski et al[8]also found that the micro increase in lens thickness is related to the large change in accommodation amplitude, while the lens position has no obvious change. Changes in ocular biological parameters before and after cycloplegia have always been a hot topic in refractive research. Different mydriasis drops can relax dissimilar degrees of regulation by paralyzing ciliary muscle[14]. It can be seen that the changes of crystal morphology before and after cycloplegia are consistent with the changes of crystal morphology during dynamic accommodation of human eyes.
The change of diopter(S) before and after cycloplegia is highly related to the ACL, LTH and LED, but has nothing to do with the change of the PCL, LD and LT. Under the condition that the axial length and corneal refractive power are unvaried, the change of refractive power is mainly determined by the change of the lens refractive power. After cycloplegia, the ACL of the crystal increases and the thickness of the lens decreases. According to Lensmaker equation[15], both advancing the radius of curvature and reducing the thickness reduces the diopter of the crystal, which is corresponding with the change of diopter. The change of the diameter does not affect the change of the lens diopter, but it is related to the change of diopter. This may be that under the condition of ciliary muscle relaxation, the radius of curvature of the anterior surface of the lens increases, the thickness decreases, and the increase in diameter is an overall change process. Amy et al[16]reported that the radius of curvature of the rear surface of the crystal changes significantly during the accommodation process. But we found that the change of PCL has nothing to do with the change of diopter, and the change of PCL before and after cycloplegia has no statistical significance. There may be for two reasons: One is the case that the PCL is small and the change value is not obvious when relaxing; Second, the circular fibers of cycloplegia and relaxation mainly act on the anterior surface of the lens and have little influence on the posterior surface. The change of human eye diopter is also related to the use of cycloplegia drugs, and the most suitable cycloplegia drugs should be comprehensively selected according to the patient's age, refractive state and other basic conditions[17, 18].
The change of ACD before and after cycloplegia is highly related to the ACL and LTH, but has nothing to do with the PCL, LD, LT and LED. Chen, Z et al[19]found that the lens became thinner and moved backward after cycloplegia. The increase of ACD was primarily due to the backward movement of the lens. These results are worthwhile to clarify the effect of the lens changes during accommodation.
The changes of various parameters of crystal morphology have nothing to do with age. Richdale et al’s [20] quantitative accommodation study found that the change of the lens equatorial diameter and ciliary muscle thickness per diopter has nothing to do with the age of the subject, and even if the total accommodation amplitude decreases, ciliary muscle contraction per diopter has nothing to do with age. In addition, it is possible that the main object of this study is mainly concentrated in adolescent myopia patients, which have certain age limitations. For patients who have myopia, their accommodation is often lagging behind and insufficient[21], there is no difference in the changes of multiple lens parameters before and after cycloplegia.
CASIA2 can provide a preliminary measurement change of crystal biological parameters and can quantitatively and objectively evaluate relevant biological parameters[22], which can helps us to better understand the physical changes of various anterior segment biological parameters such as lens biometry[23].The change rule of crystal morphology is the basis for research on ametropia, regulation and other related aspects, providing reference for further research on myopia and regulation mechanism. Based on the comparison and analysis of diopter and crystal morphology before and after cycloplegia, it is found that for children and adolescents, the change of diopter after cycloplegia is highly correlated with the change of crystal morphology and age. Additional research is needed on whether the same conclusion can be reached for all age groups and different refractive states (myopia, hyperopia and emmetropia).