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Astigmatism

A disorder of the vision, usually due to a misshapen cornea, such that light does not focus correctly on the retina causing a blurred image.

PubMed Health Glossary
(Source: Wiktionary)

About Astigmatism

Astigmatism is a common type of refractive error. It is a condition in which the eye does not focus light evenly onto the retina, the light-sensitive tissue at the back of the eye.

What is refraction?

Refraction is the bending of light as it passes through one object to another. Vision occurs when light rays are bent (refracted) as they pass through the cornea and the lens. The light is then focused on the retina. The retina converts the light-rays into messages that are sent through the optic nerve to the brain. The brain interprets these messages into the images we see.

How does astigmatism occur?

Astigmatism occurs when light is bent differently depending on where it strikes the cornea and passes through the eyeball. The cornea of a normal eye is curved like a basketball, with the same degree of roundness in all areas. An eye with astigmatism has a cornea that is curved more like a football, with some areas that are steeper or more rounded than others. This can cause images to appear blurry and stretched out.

Read more about Astigmatism NIH - National Eye Institute

What works? Research summarized

Evidence reviews

Iris-registration in wavefront-guided LASIK versus conventional LASIK for correction of myopia and myopic astigmatism: a meta-analysis

Bibliographic details: Li Y, Cheng SM, Zhou X, Xu L.  Iris-registration in wavefront-guided LASIK versus conventional LASIK for correction of myopia and myopic astigmatism: a meta-analysis. Chinese Journal of Evidence-Based Medicine 2013; 13(3): 358-364 Available from: http://www.cjebm.org.cn/en/oa/DArticle.aspx?type=view&id=20130319

Excimer laser versus phakic intraocular lenses for the correction of moderate to high short‐sightedness

Myopia is a condition in which the focusing power (refraction) of the eye is greater than that required for clear vision of distant objects. Myopia is a common cause of visual disability throughout the world. The World Health Organization (WHO) has grouped myopia and uncorrected refractive error among the leading causes of blindness and vision impairment in the world. The overall power of the lens that would be needed to correct the myopia is expressed in diopters (D) of a sphere. Most people have some degree of astigmatism where the eye is better at focusing light in one meridian than it is at another. It is possible to combine the effect of any astigmatism with the overall focusing power of the eye as a spherical equivalent in diopters. There are two main types of surgical correction for moderate to high myopia; excimer laser and phakic intraocular lenses (IOLs). Excimer laser refractive surgery for myopia works by removing corneal stroma to lessen the refractive power of the cornea and to bring the image of a viewed object into focus onto the retina rather than in front of it. Phakic IOLs for the treatment of myopia work by diverging light rays so that the image of a viewed object is brought into focus onto the retina rather than in front of it. They can be placed either in the anterior chamber of the eye in front of the iris or in the posterior chamber of the eye between the iris and the natural lens.

Biaxial microincision cataract surgery versus conventional coaxial cataract surgery: metaanalysis of randomized controlled trials

A comprehensive literature search of Cochrane Library, PubMed, and Embase was performed to identify relevant prospective randomized controlled trials (RCTs) comparing biaxial microincision cataract surgery (MICS) and conventional coaxial phacoemulsification. A metaanalysis was performed on the following outcome measures: effective phacoemulsification time (EPT), phacoemulsification power (%), corrected distance visual acuity (CDVA), surgically induced astigmatism (SIA), laser flare photometry value, percentage of endothelial cell loss, change in central corneal thickness (CCT), and complications. Eleven RCTs describing a total of 1064 eyes were identified. There were no significant differences between the techniques in CDVA, mean percentage of endothelial cell loss, laser flare photometry value, CCT change, and intraoperative and postoperative complications. However, EPT was statistically significantly shorter and the mean phaco power was statistically significantly lower in the biaxial group than in the coaxial group, and biaxial MICS induced less SIA.

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Summaries for consumers

Excimer laser versus phakic intraocular lenses for the correction of moderate to high short‐sightedness

Myopia is a condition in which the focusing power (refraction) of the eye is greater than that required for clear vision of distant objects. Myopia is a common cause of visual disability throughout the world. The World Health Organization (WHO) has grouped myopia and uncorrected refractive error among the leading causes of blindness and vision impairment in the world. The overall power of the lens that would be needed to correct the myopia is expressed in diopters (D) of a sphere. Most people have some degree of astigmatism where the eye is better at focusing light in one meridian than it is at another. It is possible to combine the effect of any astigmatism with the overall focusing power of the eye as a spherical equivalent in diopters. There are two main types of surgical correction for moderate to high myopia; excimer laser and phakic intraocular lenses (IOLs). Excimer laser refractive surgery for myopia works by removing corneal stroma to lessen the refractive power of the cornea and to bring the image of a viewed object into focus onto the retina rather than in front of it. Phakic IOLs for the treatment of myopia work by diverging light rays so that the image of a viewed object is brought into focus onto the retina rather than in front of it. They can be placed either in the anterior chamber of the eye in front of the iris or in the posterior chamber of the eye between the iris and the natural lens.

How can presbyopia be corrected?

Our ability to see nearby objects gets worse with age. This is known as presbyopia. Although it can’t be reversed, it is easy to correct. The simplest way is to wear reading glasses. Laser treatment and other kinds of eye surgery have hardly any advantages, but are associated with a lot of risks. Presbyopia usually becomes noticeable in your mid-forties, and at first is often only a problem when reading. Wearing reading glasses is then a simple and effective solution for people who don’t already have other problems with their eyesight. Those who already had to wear glasses or contact lenses beforehand can use varifocal or multifocal glasses to avoid having to switch between different pairs. Multifocal contact lenses are an alternative to multifocal glasses. They also allow you to see both nearby and distant objects clearly. Laser treatment and other kinds of eye surgery are called refractive surgery. The aim of these procedures is to change the refractive power (focusing power) of the eye in order to correct your vision. They can’t yet effectively reverse presbyopia. What’s more, most of the procedures are still being tested. The advantages of not having to wear glasses or contact lenses have to be weighed against the possible risks associated with the procedure. For instance, your spatial vision may be worse afterwards, or you may suddenly have trouble seeing things that are further away. The costs of these procedures aren’t covered by statutory health insurers in Germany. People have to pay for them themselves.

Different‐sized incisions for cataract surgery in people with age‐related cataract

The aim of this Cochrane Review was to find out if the size of the incision (cut in the eye) during cataract surgery results in a difference in outcome of cataract surgery. We found 26 studies that answered this question.

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More about Astigmatism

Photo of an adult

Other terms to know:
Cornea, Refractive Error, Retina

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