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Refractive Errors, Myopia, and Presbyopia
Published in Ching-Yu Cheng, Tien Yin Wong, Ophthalmic Epidemiology, 2022
Ka Wai Kam, Chi Pui Pang, Jason C. S. Yam
Among children with hyperopia, more than two-thirds have hyperopia in both eyes. A significantly greater proportion of children with high hyperopia (≥+4.0 D) also had concurrent astigmatism compared to those who were emmetropic or had low to moderate hyperopia. Moreover, hyperopic children had significantly shorter axial lengths, flatter corneal curvatures, and hence lower corneal refractive power compared to emmetropic children.145 Hyperopes also have difficulty converging incoming lights from nearby objects. They are thus associated with accommodation abnormalities. As accommodation brings about convergence, uncorrected hyperopia may lead to excessive convergence and accommodative esotropia.
Etiology of accommodative esotropia — current concepts
Published in Jan-Tjeerd de Faber, 28th European Strabismological Association Meeting, 2020
In non-refractive accommodative esotropia, there is an esodeviation unrelated to the refractive state, but with a high AC/A ratio. Clinically, this gives a significant greater squint angle for near than for distance. Such non-refractive accommodative esotropia may principally be found both in emmetropic, hypermetropic or myopic individuals. However, most of these patients are moderately hypermetropic.
Neurological Implications
Published in John William Yee, The Neurological Treatment for Nearsightedness and Related Vision Problems, 2019
When the emmetropic eye brings day-to-day objects into focus, it activates a different neurological message. The eyeball does not change its shape to bring about near and distant focusing. The crystalline lens carries out that task. With sudden changes in focus from near to far and vice versa, only the ciliary muscle is flexible enough to facilitate the immediate changes. When immediate focusing is required, the lens acts independently. It is evident in mild myopia when the crystalline lens assumes a bulged shape first before the sclera starts to elongate to alleviate the stress incurred by the ciliary muscle.
Refractive error, risk of amblyopia and eye care services utilisation among Nunavik Inuit in Northern Canada
Published in Clinical and Experimental Optometry, 2022
Benoit Tousignant, Julie Brûlé
There are some limitations of this study. First, this report relates to a clinical population, presenting for services, rather than a randomly sampled population. Since little is known of the rest of the population, this implies that the proportions reported may either be overestimated (if care-seeking patients present more eye problems) or underestimated (if various barriers prevent the more affected patients to seek care). However, the present cohort shows a proportion of emmetropic patients at 32.6% (41.7% in children and 23.9% in adults), indicating that not all patients consulting have obvious eye disorders. Furthermore, because of the large sample size, comprising almost half of the population, some estimates remain concerning even if they were completely absent from the rest of the population. Such is the case, for example, for the high proportion of myopia (46.5%), which, if halved, would still stand higher than worldwide estimated pooled prevalence of 11.7%.26
Distribution of near Point of Convergence, near Point of Accommodation, Accommodative Facility and Refractive Errors in a Rural Population Living in Northern Iran
Published in Journal of Binocular Vision and Ocular Motility, 2021
Ali Zakian, Samira Heydarian, Ali Mirzajani, Ebrahim Jafarzadehpur, Abbasali Yekta, Mehdi Khabazkhoob
Visual acuity was measured with a Snellen E-chart at 6 m. Then, refraction was done for all subjects using the Topcon RM-8800 auto refractometer (Tokyo, Japan) and the results were used for objective and subjective refraction. The data obtained at this stage were used to measure the best-corrected visual acuity. Next, cycloplegic refraction was measured for all subjects 30 minutes after instilling cyclopentolate 1% drops twice 5 minutes apart. Refractive error was categorized according to cycloplegic refraction. Spherical equivalent (SE) was used for calculations of refractive error. Myopia was defined as an SE of at least −0.5 D and hyperopia was defined as an SE of +0.5 D or more. Emmetropia was defined if neither eye was myopic or hyperopic. Astigmatic subjects were a cylinder refraction of 0.75 D or more in at least one eye, which was recorded with a negative sign.
Optical mechanisms regulating emmetropisation and refractive errors: evidence from animal models
Published in Clinical and Experimental Optometry, 2020
Ranjay Chakraborty, Lisa A Ostrin, Alexandra Benavente‐perez, Pavan Kumar Verkicharla
Both the optical power in the anterior segment of the eye and axial length determine refractive state.2004 Emmetropisation is an active, visually guided mechanism whereby the axial length and the combined optical powers of the cornea and lens precisely match with each other to eliminate neonatal refractive errors, and bring the eye to perfect focus (also known as emmetropia). In non‐accommodating emmetropic eyes, visual images of distant objects are clearly focused at the retinal photoreceptors. Any disruption to this homeostatic mechanism of ocular growth results in the development of refractive errors. In myopia, or near‐sightedness, the eye is too long for the optical power of the cornea and lens, and images of distance objects focus in front of the photoreceptor plane. In hyperopia, or far‐sightedness, the eye is too short for the optics, and images of distant objects focus behind the photoreceptor plane.