Refractive error

Refractive error is a problem with focusing light accurately on the retina due to the shape of the eye and/or cornea. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, and astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain. Near-sightedness is due to the length of the eyeball being too long; far-sightedness the eyeball too short; astigmatism the cornea being the wrong shape, while presbyopia results from aging of the lens of the eye such that it cannot change shape sufficiently. Some refractive errors occur more often among those whose parents are affected. Diagnosis is by eye examination. Refractive errors are corrected with eyeglasses, contact lenses, or surgery. Eyeglasses are the easiest and safest method of correction. Contact lenses can provide a wider field of vision; however they are associated with a risk of infection. Refractive surgery may consist of either permanently changing the shape of the cornea or, alternatively, implanting intraocular lenses. The number of people globally with refractive errors has been estimated at one to two billion. Rates vary between regions of the world with about 25% of Europeans and 80% of Asians affected. Near-sightedness is the most common disorder. Rates among adults are between 15 and 49% while rates among children are between 1.2 and 42%. Far-sightedness more commonly affects young children and the elderly. Presbyopia affects most people over the age of 35. The number of people with refractive errors that have not been corrected was estimated at 660 million (10 per 100 people) in 2013. Of these 9.5 million were blind due to the refractive error. It is one of the most common causes of vision loss along with cataracts, macular degeneration, and vitamin A deficiency.

Classification

Refractive error – sometimes called "ametropia" – refers to a condition in which the refractive power of an eye does not match the length of the eye, so the image is focused away from the central retina, instead of directly on it. Types of refractive error include myopia, hyperopia, presbyopia, and astigmatism.

Myopia or nearsightedness: When the refractive power is too strong for the length of the eyeball, this is called myopia or nearsightedness. People with myopia typically have blurry vision when viewing distant objects because the eye is refracting more than necessary. Myopia can be corrected with a concave lens, which causes the divergence of light rays before they reach the cornea. Hyperopia or farsightedness: When the refractive power is too weak for the length of the eyeball, one has hyperopia or farsightedness. People with hyperopia have blurry vision when viewing near objects because the eye is unable to focus the light sufficiently. This can be corrected with convex lenses, which cause light rays to converge prior to hitting the cornea. Presbyopia: When the flexibility of the lens declines, typically due to age, the individual experiences difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses. Astigmatism occurs when the refractive power of the eye is not uniform across the surface of the cornea because of asymmetry. In other words, the eye focuses light more strongly in one direction than another, leading to distortion of the image. Children are typically born hyperopic and shift toward emmetropia or myopia as their eyes lengthen through childhood. Other terminology include anisometropia, when the two eyes have unequal refractive power, and aniseikonia, when the magnification power between the eyes differ. Refractive errors are typically measured using three numbers: sphere, cylinder, and axis.

Sphere: This number denotes the strength of the lens needed to correct your vision. A "–" indicates nearsightedness while a "+" indicates farsightedness. Higher numbers indicate more power in either direction. Cylinder: This number denotes the amount of astigmatism, if any. Axis: This number notes the direction of the astigmatism and is written in degrees between 1 and 180. An eye that has no refractive error when viewing distant objects is said to have emmetropia or be emmetropic meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object, in this case, is defined as an object located beyond 6 meters, or 20 feet, from the eye, since the light from those objects arrives as essentially parallel rays when considering the limitations of human perception.

Risk factors

Genetics

There is evidence to suggest genetic predilection for refractive error. Individuals that have parents with certain refractive errors are more likely to have similar refractive errors. The Online Mendelian Inheritance in Man (OMIM) database has listed 261 genetic disorders in which myopia is one of the symptoms. Myopia may be present in heritable connective tissue disorders such as: Knobloch syndrome (OMIM 267750); Marfan syndrome (OMIM 154700); and Stickler syndrome (type 1, OMIM 108300; type 2, OMIM 604841). Myopia has also been reported in X-linked disorders caused by mutations in loci involved in retinal photoreceptor function (NYX, RP2, MYP1) such as: autosomal recessive congenital stationary night blindness (CSNB; OMIM 310500); retinitis pigmentosa 2 (RP2; OMIM 312600); and Bornholm eye disease (OMIM 310460). Many genes that have been associated with refractive error are clustered into common biological networks involved in connective tissue growth and extracellular matrix organization. Although a large number of chromosomal localisations have been associated with myopia (MYP1-MYP17), few specific genes have been identified.

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