Metric and other causes of refractive errors of the eye
Introduction
According to the World Health Organization (WHO), refractive errors are one of the most common causes of visual impairment (44%). In 2010, there were 285 million visually impaired people worldwide, of which 39 million were blind. According to the author Naidoo et al. (2016) there were almost 101 million people in the world affected by uncorrected refractive error in 2010. Blindness (visual acuity less than 3/60) affected 6.8 million people. Almost every human eye is affected by some refractive error, regardless of whether it is corrected with glasses or contact lenses (Harb 2019).
Refractive defects are the result of incorrect adjustment of the axial length of the eye and its optical power, which is manifested, among other things, by a blurred image on the retina. This inappropriate setting can already appear in newborns, but thanks to the process of emmetropization of the eye, it decreases during the life of the individual and can disappear completely (Muti et al. 2005). We most often distinguish three types of refractive errors: hypermetropia, myopia and astigmatism. From another point of view, we can state that astigmatism is only a modification of a short-sighted or far-sighted eye.
The process of emmetropization represents the reduction of the refractive error at birth to the state of emmetropia. The development of the eye includes the process of lengthening the eye, i.e. increasing the axial length and reducing the optical power of the eye. The average cycloplegic refractive error of the newborn is +2 D and approaches zero during life (Mayer et al. 2001). In several clinical studies (Kiorpes & Wallman 1995, Rabin et al. 1981) it was found that if the eye is present, for example, with cataract or ptosis, the emmetropization process fails.
Prevalence of refractive errors
According to a meta-analysis by Hashemi et al. (2018) we can state that the prevalence of refractive errors is higher in adults compared to children. In children, the prevalence of astigmatism (>0.5 D) was found in 14.9% of cases, hypermetropia (≥+2.0D) in 4.6% of cases and myopia (≤-0.5D) in 11.7% of cases. In adults, it was 40.4% for astigmatism, 30.9% for hypermetropia, and 26.5% for myopia. The prevalence of refractive errors also varies by country and is influenced by environmental factors. The increase in the prevalence of myopia from 10.4% in 1993 to 34.2% in 2016 is noteworthy.

Results

Aims: To demonstrate the causes of refractive errors in adults. To evaluate the influence of systemic, i.e. primarily curvature and on the other hand axial causes of refractive errors of the eye. We assume finding a combination of the above causes when comparing with the Gullstrand model of the eye.
Material and methods: The basic sample included 30 subjects with an average age of 22.3 ±1.1 years. There were 3 men and 27 women in the group. They were young, generally healthy individuals without serious eye pathologies. The average value of the spherical equivalent on the right eye of all individuals was -1.3 ±1.9 D and the left eye of all individuals was -1.0 ±1.9 D. We measured the axial length of the eye using the IOL MASTER 500 optical biometer (Zeiss). We measured the topographic and keratometric values of the cornea using the Pentacam HR device (Oculus). We obtained the objective refraction values by measuring them on a standard instrument refractometer. For our study, we determined the axial length of the eye (AXD), the average value of keratometry (KRT) and the spherical equivalent of the refractive error (SE) as significant variables. We performed the mutual evaluation of the dependence of the variables only for the right eyes (OD).
Results: In our study, we calculated the significance of the axial and curvature causes of ametropia in the myopic group and in the control group consisting of emmetropes and hypermetropes based on the Gullstrand eye model. The percentage expression of the ratios of the causes of ametropia in the myopic group was 72% axial and 28% curvature. In the control group, the cause of ametropia was 83% axial and 17% curvature. We did not demonstrate a statistically significant difference between the axial cause of ametropia in the myopic group and the control group (p = 0.28). On the contrary, we demonstrated a statistically significant difference between the ratios of the axial length of the eye and the average keratometry in the myopic

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