refraction error

Refractive error, also known as refraction 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, and presbyopia 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 permanently changes the shape of the cornea.

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 15-49% while rates among children are between 1.2-42%. 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

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.

An eye that has refractive error when viewing distant objects is said to have ''ametropia'' or be ''ametropic''. This eye cannot focus parallel rays of light (light from distant objects) on the retina, or needs accommodation to do so.

The word "ametropia" can be used interchangeably with "refractive error". Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical errors and cylindrical errors:
* Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refractive error frequently have blurry vision.
** Nearsightedness: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea or crystalline lens with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can be corrected with a concave lens, which causes the divergence of light rays before they reach the cornea.
** Farsightedness: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea or crystalline lens with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). 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 would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses.
* Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if the corneal curvature tends towards a cylindrical shape. The angle between that meridian and the horizontal is known as the axis of the cylinder.
** Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose.

Other terminology include anisometropia, when the two eyes have unequal refractive power, and aniseikonia which is when the magnification power between the eyes differ.

Refractive error may be quantified as the error of a wavefront arising from a person’s far point, compared with a plane, or zero vergence, wavefront compared at an appropriate reference plane. The reference plane may be a real plane such as the spectacle plane or the corneal plane, or an imaginary plane such as the first principal plane or the entrance pupil plane. In diopters, spherical refractive errors can be expressed as K=1/k, where k is the distance in meters from the reference plane to an eye’s far point, and K is the refractive error in diopters. Thus, a person with myopia would have a negative refractive error, a person with emmetropia would have zero refractive error and a person with hyperopia would have a positive refractive error. In the case of regular astigmatism, refractive error needs to be expressed as 3 values: classically as sphere, cylinder and axis. However, it can also be expressed in vector terms, for example, M (mean sphere), J0 (With the rule/Against the rule astigmatism), J45 (oblique astigmatism). Refractive errors containing higher order aberrations (sometimes referred to as irregular astigmatism) can be expressed for a given pupil size using wavefront errors or optical path differences, often as coefficients for Zernike polynomials. A more subjective quantity visual acuity (expressed as a fraction) may be used, but there is no direct or exact conversion between the two.

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