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Refractive errors

Definition

In order to allow us clear vision, light rays reflected from objects towards which we are looking cross four successive eye regions: the cornea, the aqueous humour, the crystalline lens, and the vitreous body. Every time a light ray crosses one of these regions, its path deviates at a certain angle in a phenomenon called refraction. These combined refractions result in the convergence of light rays on the retina, allowing a clear view of the object.1

When light rays converge in front of or behind the retina, instead of directly on it, the patient is said to have refractive errors, also called ametropia. As a consequence of this, images are blurred. Possible manifestations of this condition are :

Myopia: If you suffer from myopia (near-sightedness), you are not able to clearly see distant objects, though closer objects can still be clearly seen. This condition occurs when the eye is elongated, which is detected using an examination called biometry, or the cornea is too convex, causing the light rays to converge in front of the retina.

Hyperopia: The opposite of myopia, if you suffer from hyperopia (far-sightedness), you are not able to clearly perceive close objects. This condition occurs when the eye is too short, detected by biometry, or the cornea is too flat, causing the light rays to converge behind the retina.

Astigmatism: In the case of astigmatism, the cornea has an abnormal, ovular shape closer to a rugby ball than a football. Also, the crystalline lens may be abnormally tilted. Light rays crossing the eye focus on different points, both behind and in front of the retina, resulting in a distorted image. If you suffer from astigmatism, your vision is imprecise and blurry at any distance.

Presbyopia: The crystalline lens hardens with age, reducing the eye’s ability to focus on close objects. Because of this, if you suffer from this condition, your near vision becomes blurred. Accommodation refers to the process which ensures, thanks to changes in the crystalline lens’s shape, that objects at different distances can be seen clearly.
  

Normal accommodation


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Epidemiology

The number of people worldwide suffering from refractive errors is estimated to range from 800 million to 2.3 billion.2 Of these, around 153 million cases have not been corrected with contact lenses or glasses.3 In France, 39% of the population are estimated to suffer from myopia, 15% from astigmatism, 9% from hyperopia, and 26% to 30% from presbyopia.4

Myopia seems to be less common in Europe than in Asia, where 84% of the population in Hong Kong and Taiwan appear to be affected. Australian aborigines represent the other extreme, with less than 5% experiencing myopia.5 Hypermetropia affects 9.9% of Americans, 11.6% of Western Europeans, and 5.8% of Australians aged 40 years or older.5

While presbyopia most often begins after 40 years of age in Western countries, studies have shown that it starts earlier and is more severe in Africa compared to Europe and North America.6 These studies have shown that in developing countries, more than 50% of adults older than 30 years suffer from presbyopia.6 Furthermore, presbyopia is more common and more severe in women than in men.6

1 Albou-Ganem C, Saragoussi JJ. Qu’est-ce que la réfraction. Dans : Chirurgie réfractive. Bash 2008, p. 32-3.
2 Titcomb L. Laser surgery for refractive errors. The pharmaceutical journal 2006;276:511-4.
3 Organisation Mondiale de la Santé. What are refractive errors? October 2006. http://www.who.int/features/qa/45/en/index.html
4 Syndicat National des Ophtalmologistes de France. http://www.snof.org/accueil/epidemio.html
5 Jobke S, Kasten E, Vorwerk C. The prevalence rates of refractive errors among children, adolescents and adults in Germany. Clinical Ophthalmology 2008;2:601-7.
6 Patel I, West SK. Presbytie : prévalence, impact et interventions. Revue de santé oculaire communautaire 2008;5:4-5.

 

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