China
Africa
Explore chapters and articles related to this topic
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
Another global health burden, astigmatism is the second most common refractive error.124 It occurs when light coming in from two different meridians is bent differently and focuses on to two points rather than a single point, resulting in blurred images. One dioptric difference in refractive powers at two meridians constitutes 1 D of astigmatism, which is sufficient to cause meridional amblyopia in children.125 Although other thresholds such as –0.5 D and –0.75 D exist in defining astigmatism, –1.0 D remains clinically relevant as a guide for prescribing spectacle corrections.126 Astigmatism can be categorized into simple myopic, compound myopic, hyperopic, or mixed astigmatism. Astigmatism in an eye is essentially determined by the curvatures of the cornea. Corneal astigmatism refers to the difference in dioptric refraction between the flattest and steepest curvatures of the cornea, whereas overall astigmatism of the eye is further influenced by the shapes of the lens, vitreous humor, and retina. When the axis is located near the horizontal meridian, it is classified as with-the-rule astigmatism, which constitutes the majority of astigmatism phenotypes observed in children. In people aged above 50 years, the axis shifts to around 90°, and this condition is known as against-the-rule astigmatism.
Binocular vision after refractive surgery in hyperopic patients
Published in Jan-Tjeerd de Faber, 28th European Strabismological Association Meeting, 2020
R Gómez-de-Liaño, E. Piedrahita, A. Arias-Puente
This is a prospective study of patients with hyperopia or hyperopic astigmatism, operated of refractive surgery between May 1999 and February 2002. All patients were operated by the same author (AAP). Refractive technique was Lasik with the MEL-70 G-Scan Laser (Asclepion-Meditec).
Treating Astigmatism
Published in John William Yee, The Neurological Treatment for Nearsightedness and Related Vision Problems, 2019
In most cases, astigmatism is mainly due to the distortion of the cornea, and it is referred to as “corneal astigmatism.” In some cases, it is mainly due to the distortion of the crystalline lens, and it is termed “lenticular astigmatism.” Astigmatism can exist by itself, or it can be combined with myopia or hyperopia. The former is called “simple astigmatism,” and the latter is called “compound astigmatism.” Simple astigmatism is further broken down into “simple myopic astigmatism” and “simple hyperopic astigmatism.”
Vecto-keratometry: determination of anterior corneal astigmatism in manual keratometers using power vectors
Published in Expert Review of Medical Devices, 2023
Raquel Salvador-Roger, Rosa Vila-Andrés, Vicente Micó, José J. Esteve-Taboada
Characterization of the anterior corneal surface is an important aspect in the clinical management of refractive data. Since 1619 when the first attempts were made to measure corneal curvature using glass spheres [1], the study of anterior corneal curvature has evolved from some rudimentary methods to today’s modern measurement [2]. Included in this cornea characterization, corneal astigmatism (understood as dioptric power difference between principal meridians of the anterior surface of the cornea) plays a determinant role for many optometric and ophthalmic procedures and methodologies such as total astigmatism determination in a patient’s eye by using Javal’s or Grosvenor’s rules [3,4], intraocular lens power determination, and contact lens fitting processes [5,6]. Thus, measurement of complete and accurate corneal astigmatism is of capital importance in optometry and ophthalmology practice.
Cataract surgery in Keratoconus revisited – An update on preoperative and intraoperative considerations and postoperative outcomes
Published in Seminars in Ophthalmology, 2023
Spectacle correction can provide acceptable vision in patients with mild residual error.56,57 Contact lens covers the anterior corneal abnormalities and hence should be opted in patients with irregular astigmatism in whom spectacles are not of much benefits. Since most of the keratoconus patients are already using rigid contact lenses they are very comfortable going back to that. The type of contact lens will depend on the refractive error of the patient.58 Soft lenses can be used in patients with low refractive error, regular astigmatism, or mild irregular astigmatism. Soft toric or custom toric can also be tried in these patients. RGP lenses or different exclusive lenses like scleral, piggyback or hybrid lenses can be tried in patients with a higher value of error.58,59,60
A Novel Use of Retinoscopy for the Evaluation of Binocular Balance of Spherical Refractive Errors
Published in Seminars in Ophthalmology, 2022
Ebubekir Durmus, Veysel Aykut, Isil Karagoz, Mustafa Hepokur, Ozlem Turkyilmaz, Emine Savran Elibol, Fehim Esen, Halit Oguz
In this test, we first perform a monocular examination and fully correct astigmatism. After fully correcting astigmatism, this novel binocular balance test is performed with retinoscopy. In this test, we allow the patient to look far binocularly and examine both eyes at the same time with retinoscopy in the horizontal meridian. If there is asymmetry in the light reflex, we correct the refractive imbalance and repeat the test until we reach a symmetrical light reflex. This test is objective, and it can be performed in patients with limited ability of cooperation, such as children or patients with mental retardation. The current study demonstrated that the outcome of retinoscopy-assisted binocular balance test was as good as the currently existing test and it correlated with the interocular spherical equivalent difference in cycloplegic autorefractometry better than the other available tests.