Refractive Errors, Myopia, and Presbyopia
Ching-Yu Cheng, Tien Yin Wong in Ophthalmic Epidemiology, 2022
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.
Treating Astigmatism
John William Yee in 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.”
Sensory organs
Aida Lai in Essential Concepts in Anatomy and Pathology for Undergraduate Revision, 2018
Astigmatism Uneven curvature of cornea/lens in vertical and horizontal planesManagement: – cylindrical lens
Cataract surgery in Keratoconus revisited – An update on preoperative and intraoperative considerations and postoperative outcomes
Published in Seminars in Ophthalmology, 2023
Charul Singh, Vineet P Joshi
Keratoconus (KC) is a bilateral ectasia of the cornea due to noninflammatory thinning.1 This corneal thinning causes mild to severe visual impairment by inducing myopia or astigmatism (Regular or irregular).2 About 1 in every 2000 people are estimated to develop keratoconus,2 with its onset in early adolescence, and the possibility of progression up to about 30 or 40 years of age, after which it generally stabilizes.3 It is commonly associated with allergic eye disease. Keratoconus patients tend to have a cataract in the younger age group and the most common type of cataract seen in these patients is nuclear sclerosis(83%)4 The surgical planning of cataracts in eyes with keratoconus brings into perspective several unique considerations such as determination of progression of the disease, preoperative planning, biometry, choice of the intraocular lens (IOL), intra-operative surgical challenges due to poor visibility, image distortion and tackling the postoperative refractive surprises and visual outcomes. In this context, we aim to review the recent practice patterns and literature to help plan cataract surgery in keratoconus more effectively.
Effects of Superficial Keratectomy in Peripheral Hypertrophic Subepithelial Corneal Opacification on Front and Back Corneal Astigmatism
Published in Current Eye Research, 2021
Jana C Riedl, Alexander K Schuster, Aytan Musayeva, Joanna Wasielica-Poslednik, Susanne Marx-Gross, Adrian Gericke
Kheirkhak et al. used Scheimpflug imaging to evaluate the effect of pterygium surgery on the anterior and posterior corneal astigmatism.16 They also described a decrease in anterior corneal surface astigmatism. In contrast, the absolute amount of astigmatism of the posterior corneal surface remained almost stable, though, the type of the astigmatism shifted after surgery (before surgery: 43.9% with-the-rule, 24.6% oblique, 31.5% against-the-rule; after surgery: 87.7% with-the-rule, 8.2% oblique).16 In our study, a shift of the axis could also be observed, whereby there was a greater shift of the anterior axis (55° vs. 18° shift of the posterior axis). Before surgery, the type of back astigmatism included with-the-rule and oblique in 66.7% and 33.3%, respectively. However, there was no significant change postoperatively (73.3% versus 26.7%). In normal eyes, the posterior corneal back is steeper in the vertical meridian.17,18 Therefore, it seems, that PHSCO causes an increase in posterior astigmatism without changes of the axis. Since the posterior astigmatism decreased after surgery in this study, the topographic changes are very likely caused by changes of the corneal shape, which are reversed after surgery.
Co-occurrence of incontinentia pigmenti and down syndrome: examining patients’ potential susceptibility to autoimmune disease, autoinflammatory disease, cancer, and significant ocular disease
Published in Ophthalmic Genetics, 2021
David C. Gibson, Natario L. Couser, Kayla B. King
Physical examination showed a well-nourished infant. Facial features included epicanthal folds, up-slanted palpebral fissures, flat nasal bridge, and a large protruding tongue. Ocular examination revealed bilateral astigmatism. There were no lid, conjunctival, scleral, or corneal lesions. Fundus examination was unremarkable and showed normal vasculature. There was no strabismus or nystagmus present. Neurological examination revealed no global or focal deficits; however, musculoskeletal examination revealed mild hypotonia. A large umbilical hernia was noted upon abdominal examination, but there were no other masses or hepatosplenomegaly present. The rest of the physical examination was normal besides upper airway stridor and a soft 1–2/6 ejection murmur upon auscultation of the neck and thorax.
Related Knowledge Centers
- Amblyopia
- Blurred Vision
- Cornea
- Eye
- Nyctalopia
- Refractive Error
- Headache
- Rotational Symmetry
- Eye Strain
- Genetics