Although the optical mechanism of myopia has long been understood, the research on the pathogenesis, natural course and effective prevention of myopia has made slow progress. On the contrary, with the development of information society and the change of human eye habits, the increase of myopia is obvious and supported by epidemiology in terms of absolute and relative number.6–9 The occurrence and development of myopia are closely related to intensity of reading and near work, which is almost the consensus of all ordinary people and clinical ophthalmologists. but how reading and near work behavior induces the occurrence and development of myopia and the mechanism of pathological changes of myopia have not yet been clarified so far.
It is well known that before presbyopia occurs in the eyes, our eyes can quickly switch between distant and near vision, and the main known physiological mechanism is to initiate accommodation, and the amplitude of this accommodation decreases with age. The amplitude of accommodation decreased from about 15D in early infancy to 1.00D at the age of 60.10 Therefore, if we can obtain the difference of ocular parameters of the eye in far and near vision, it will help to explore the impact of accommodation priming on ocular parameters of the eye. However, due to the limitations of today's measurement technology, it is not possible to obtain the instantaneous changes of all known ocular parameters in the two physiological states of far and near vision. To this end, we used the cycloplegic atropine ointment in this study, using it to simulate or even amplify the accommodation activity to study the impact of this “enlarged accommodation” on the known ocular parameters of the eye.6 The data we obtained before cycloplegia represented the state of one eye in near viewing condition, whereas the data under cycloplegia represented that of one eye in far viewing condition. So it would be reasonable for us to learn the effect of “enlarged accommodation”—the amplitude of accommodation was enlarged by the application of atropine—on ocular parameters before and after cycloplegia. However, this “enlarged accommodation” would have an influence on ocular parameters similar to that of physiological or real accommodation from the viewpoint of lens changes, because the accommodation of human eyes mainly relies on the change of the lens in shape. This was fully proven in our experiment by the significant decrease in lens thickness after cycloplegia for each group.6 As two diametrically opposed refractive states, we found that there was no significant difference in central corneal thickness, corneal curvature and central retinal thickness between myopia and hyperopia, except for the well-known difference in average AL. However, there were significant differences in anterior chamber depth and pupil diameter.
Anterior chamber depth
Anterior chamber depth is undoubtedly an important ocular biometric parameter because the anterior chamber depth is located in the relative position of the lens. In this study, we found that the mean anterior chamber depth in myopia was significantly greater than that in hyperopia both before and after cycloplegia, and the anterior chamber depth was significantly deepened after cycloplegia in both group, which was consistent with our previous research conclusion of anterior chamber depth measured by A-ultrasound.6 Zhang J et al used IOL master to compare myopia and emmetropia in children of the same age. It was found that the anterior chamber depth in myopia was greater than that in emmetropia.11 Considering that there is no significant difference in corneal curvature and central corneal thickness between the two groups in this study, we have reason to believe that the deeper anterior chamber depth in the myopia group may be closely related to the thinning of lens thickness.6 Malyugin et al studied the accommodative changes in the anterior chamber depth in patients with high myopia using anterior segment optical coherence tomography (AS-OCT). the results showed that accommodative changes in the ACD were significantly less pronounced in eyes with high myopia than in emmetropic eyes, indicating that the change of lens configuration or thickness in myopia was less than that in emmetropia,12which is also consistent with our previous study.
The difference of PD between myopic and hyperopic eyes was significant (t = 2.903, P = .0046). Whether refractive error affects pupillary diameter was controversial.13–15 Although Orr JB et al suggested that refractive error had no effect on pupil diameter,16 studies with larger samples have confirmed that adult myopes have larger pupillary diameter than hyperopes.13,14 However, our study seems to be contrary to the above conclusions. In addition to age group differences, our study did not receive the support based on children group.16 Gu Xinzu et al dynamically observed the pupil size of normal and myopia people with infrared pupil tester, and found that the pupil size of myopia was larger than that of emmetropia, and the pupil size increased with the deepening of myopia, especially in high myopia.17 How to explain this seemingly contradictory phenomenon in the conclusion between our study and the above research? We note that the above three related studies all conducted under dark adaptation conditions, which is also supported by their average pupillary diameter obtained without cycloplegia. For example, the average pupillary diameter of children's myopia was 6.78 ± 0.81mm 16 and that of adult myopia was 6.33 ± 0.82mm 13 and 6.51 ± 0.8mm.14 The average pupil diameter of the three studies is closer to that of our myopia after cycloplegia. But not 4.85 ± 0.87mm before cycloplegia. It is well known that the size of the pupil mainly determines by the intensity of light. It is regulated by the pupil sphincter and pupil dilator and through the sympathetic and parasympathetic nerves and the higher central E-W nucleus. Rosenfield et al considered that myopic eyes have weak sympathetic or strong parasympathetic innervation, which may not only explain the smaller pupil size of myopic eyes than that of hyperopic eyes in natural conditions, but also help to understand the different effects of atropine.18
Our previous studies on children's axial length were based on traditional A-ultrasound measurement, although the potential impact of corneal curvature had been considered at that time, but the potential impact of corneal curvature and central retinal thickness changes on axial length had not been considered. Therefore, in this study, we included all factors that potentially affect the measurement of axial length, although no statistical validation was obtained to support our previous conclusions. However, the change of average 30 um axial length of myopic eyes before and after cycloplegia and the corresponding statistical analysis (t=-1.976, P = .0537) may reveal a similar change rule. Moreover, the effect of cycloplegia on axial length has also been confirmed by some researchers. Raina UK et al used Lenstar LS900 to study the axial length of 56 normal children aged 5 to 15 years, and found that after 2% homatropine cycloplegia, the axial length of these children was significantly shortened compared with that before medication.19 Unfortunately, the study did not focus on the refractive status of children. Tang XP et al 20 used optical coherence biometry to measure the eye axis of children aged 4 to 11 years, and found that the eye axis of the hyperopia group was significantly shortened after atropine cycloplegic treatment, while the myopia group did not change significantly, and similar research conclusions were also supported by Wang LH et al.21 If the above studies were conducted using different cycloplegic agents, Woodman EC et al used IOL Master to measure the effect of continuous 5D accommodation near work for 30 minutes on the axial length of adult myopes and emmetropes, and found that near accommodation could significantly increase their axial length.22 Some people used LenStar to perform 3.00 and 4.50 D near stress tests on 35 adult emmetropic eyes and 37 myopic eyes, and found that the axial lengths of the eyes in both groups increased significantly.10
In a word, from the point of view of the anatomical and physiological characteristics of human eyes, the only known refractive factor that can actively change instantaneously in visual activities is the change of lens, which is caused by a series of mechanical changes involving intraocular muscles and has been confirmed by many studies. From the perspective of biological evolution, accommodation is the most primitive and basic visual control mechanism in the process of near reflex.23 As long as we live in this three-dimensional world, retinal image defocus phenomenon is inevitable, and only eye accommodation can correct this defocus. Therefore, it may be a feasible way to study the changes of ocular parameters in a short time by studying the differences of refractive components in children and using the accommodative amplification effect of atropine.