| Refractive error | |
|---|---|
| Other names | Refraction error |
 | |
| A correctly-focused eye (top), and two showing refractive error: In the middle image, thelight is focused too far forward; in the bottom image, the focal point is behind the eye. | |
| Specialty | Ophthalmology,optometry |
| Symptoms | Blurry vision,double vision,headaches,eye strain |
| Complications | Blindness,amblyopia[1][2] |
| Types | Near-sightedness,far-sightedness,astigmatism,presbyopia[3] |
| Causes | Eyeball length, problems withcornea shape, aging of thelens[3] |
| Diagnostic method | Eye examination[3] |
| Treatment | Eyeglasses,contact lenses,refractive surgery[3] |
| Frequency | ~1.5 billion[4] |
Refractive error is a problem withfocusing light accurately on theretina due to the shape of theeye and/orcornea.[3] The most common types of refractive error arenear-sightedness,far-sightedness,astigmatism, andpresbyopia. Near-sightedness results in far away objects beingblurry, far-sightedness and presbyopia result in close objects being blurry, and astigmatism causes objects to appear stretched out or blurry. Other symptoms may includedouble vision,headaches, andeye strain.[3]
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 thelens of the eye such that it cannot change shape sufficiently.[3] Some refractive errors occur more often among those whose parents are affected. Diagnosis is byeye examination.
Refractive errors are corrected witheyeglasses,contact lenses, orsurgery.[3] Eyeglasses are the easiest and safest method of correction. Contact lenses can provide a widerfield 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, implantingintraocular lenses.[3][5]
The number of people globally with refractive errors has been estimated at one to two billion.[4] Rates vary between regions of the world with about 25% of Europeans and 80% of Asians affected.[4] Near-sightedness is the most common disorder.[6] Rates among adults are between 15 and 49% while rates among children are between 1.2 and 42%.[7] Far-sightedness more commonly affects young children and the elderly.[8][9] Presbyopia affects most people over the age of 35.[3]
The number of people with refractive errors that have not been corrected was estimated at 660 million (10 per 100 people) in 2013.[10] Of these 9.5 million wereblind due to the refractive error.[10] It is one of the most common causes ofvision loss along withcataracts,macular degeneration, andvitamin A deficiency.[11]
Refractive error – sometimes called "ametropia" – is when 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.[12]
Types of refractive error include myopia, hyperopia, presbyopia, and astigmatism.
Children are typically born hyperopic and shift toward emmetropia or myopia as their eyes lengthen through childhood.[17]
Other terminology includeanisometropia, when the two eyes have unequalrefractive power,[18] andaniseikonia which is when themagnification power between the eyes differ.[19]
Refractive errors are typically measured using three numbers: sphere, cylinder, and axis.[20]
An eye that has no refractive error when viewing distant objects is said to haveemmetropia or beemmetropic 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.[21]
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.[3]
The Online Mendelian Inheritance in Man (OMIM) database has listed 261 genetic disorders in which myopia is one of the symptoms.[22] Myopia may be present in heritable connective tissue disorders such as:Knobloch syndrome (OMIM 267750);Marfan syndrome (OMIM 154700); andStickler syndrome (type 1, OMIM 108300; type 2, OMIM 604841).[23] Myopia has also been reported inX-linked disorders caused by mutations inloci involved in retinalphotoreceptor function (NYX,RP2, MYP1) such as: autosomal recessivecongenital stationary night blindness (CSNB; OMIM 310500);retinitis pigmentosa 2 (RP2; OMIM 312600); Bornholm eye disease (OMIM 310460).[23]Many genes that have been associated with refractive error are clustered into common biological networks involved in connective tissue growth and extracellular matrix organization.[23] Although a large number of chromosomal localisations have been associated with myopia (MYP1-MYP17), few specific genes have been identified.[22]
In studies of the genetic predisposition of refractive error, there is a correlation between environmental factors and the risk of developing myopia.[24] Myopia has been observed in individuals with visually intensive occupations.[23]Reading has also been found to be a predictor of myopia in children. It has been reported that children with myopia spent significantly more time reading than non-myopic children who spent more time playing outdoors.[23] Additionally, focusing on near objects for long periods of time - such as when reading, looking at close screens, or writing - has been associated with myopia.[25][26]Socioeconomic status and higher levels of education have also been reported to be a risk factor for myopia.[27]Blepharoptosis can also induce refractive errors.[28]
In order to see a clear image, the eye must focus rays of light on to the light-sensing part of the eye – theretina, which is located in the back of the eye. This focusing – calledrefraction – is performed mainly by thecornea and thelens, which are located at the front of the eye, theanterior segment.[29]
When an eye focuses light correctly on to the retina when viewing distant objects, this is calledemmetropia or beingemmetropic. This means that the refractive power of the eye matches what is needed to focus parallel rays of light onto the retina. A distant object is defined as an object located beyond 6 meters (20 feet) from the eye.[citation needed]
When an object is located close to the eye, the rays of light from this object no longer approach the eye parallel to each other. Consequently, the eye mustincrease its refractive power to bring those rays of light together on theretina. This is calledaccommodation, and is accomplished by the eye thickening the lens.[29]
Blurry vision may result from any number of conditions not necessarily related to refractive errors. The diagnosis of a refractive error is usually confirmed by aneye care professional during aneye examination using a large number of lenses of different optical powers, and often aretinoscope (a procedure entitledretinoscopy) to measure objectively in which the person views a distant spot while the clinician changes the lenses held before the person's eye and watches the pattern of reflection of a small light shone on the eye. Following that "objective refraction" the clinician typically shows the person lenses of progressively higher or weaker powers in a process known assubjective refraction.Cycloplegic agents are frequently used to more accurately determine the amount of refractive error, particularly in children[24]
Anautomated refractor is an instrument that is sometimes used in place of retinoscopy to objectively estimate a person's refractive error.[30]Shack–Hartmann wavefront sensor and its inverse[31] can also be used to characterizeeye aberrations in a higher level of resolution and accuracy.
Vision defects caused by refractive error can be distinguished from other problems using apinhole occluder, which will improve vision only in the case of refractive error.[32]
When refractive errors in children are not treated, the child may be at risk of developingambylopia, where vision may remain permanently blurry.[33] Because young children typically do not complain of blurry vision, the American Academy of Pediatrics recommends that children have yearly vision screening starting at three years old so that unknown refractive errors or other ophthalmic conditions can be found and treated if deemed necessary by healthcare professionals.[33][34]
The management of refractive error is done post-diagnosis of the condition by either optometrists, ophthalmologists, refractionists, or ophthalmic medical practitioners.[35]
How refractive errors are treated or managed depends upon the amount and severity of the condition. Those who possess mild amounts of refractive error may elect to leave the condition uncorrected, particularly if the person is asymptomatic. For those who are symptomatic,glasses,contact lenses,refractive surgery, or a combination are typically used.[29][20][22]
These are the most effective ways of correcting the refractive error. However, the availability and affordability of eyeglasses can present a difficulty for people in many low income settings of the world. Glasses also pose a challenge to children to whom they are prescribed to, due to children's tendency to not wear them as consistently as recommended.[36]
As mentioned earlier refractive errors are because of the improper focusing of the light in the retina. Eyeglasses work as an added lens of the eye serving to bend the light to bring it to focus on the retina. Depending on the eyeglasses, they serve many functions.[37]
Alternatively, many people choose to wear contact lenses. One style is hard contact lenses, which can distort the shape of the cornea to a desired shape. Another style, soft contact lenses, are made of silicone or hydrogel. Depending on the duration they are designed for, they may be worn daily or may be worn for an extended period of time, such as for weeks.[35]
There are a number of complication associated with contact lenses. Typically the ones that are used daily.[citation needed]
| Complications of contact lens wear | Description |
|---|---|
| Conjunctivitis (giant papillary form) | Caused in response to the allergen present in the material from which the contact lens is made from. There is often discomfort in the eye after wearing and vision may be affected. Choosing the right lens material and changing it regularly might prevent conjunctivitis. |
| Corneal abrasion | Caused by a foreign body, dust, sand, or grit trapped under the lens. |
| Corneal edema | Caused by decreased oxygen delivery to the tissue compressed by the lens. Usually resolved after the removal of the lenses. Discomfort upon lens removal may be seen. |
| Neovascularization | New blood vessels may form in the iris region and the limbus. This may impair vision. |
| Infections | Various viral, bacterial, and fungal infection may be seen in the eye post-contact-lens wear, if proper lens hygiene is not maintained. Acanthamoeba are the most common infections in the people using contact lenses. |
If redness, itching, and difficulty in vision develops, the use of the lenses should be stopped immediately and the consultation of ophthalmologists may be sought.
Laser in situ keratomileusis (LASIK) and photo-refractive keratectomy (PRK) are popular procedures; while use of laser epithelial keratomileusis (LASEK) is increasing. Other surgical treatments for severe myopia include insertion of implants afterclear lens extraction (refractive lens exchange). Full thickness corneal graft may be a final option for patients with advanced kerataconus although currently there is interest in new techniques that involve collagen crosslinking. As with any surgical procedure complications may arise post-operatively Post-operative monitoring is normally undertaken by the specialist ophthalmic surgical clinic and optometry services. Patients are usually informed pre-operatively about what to expect and where to go if they suspect complications. Any patient reporting pain and redness after surgery should be referred urgently to their ophthalmic surgeon.[39][40]
Atropine has believed to slow the progression of near-sightedness and is administered in combination with multifocal lenses. These, however, need further research.[41][42]
Strategies being studied to slow worsening include adjusting working conditions, increasing the time children spend outdoors,[23] and special types of contact lenses.[43] In children special contact lenses appear to slow worsening of nearsightedness.[43][44]
A number of questionnaires exist to determinequality of life impact of refractive errors and their correction.[45][46]
It is estimated that at least 2 billion people in the world have refractive errors.[4] The number of people globally with refractive errors that have not been corrected was estimated at 660 million (10 per 100 people) in 2013.[10]
Refractive errors are the first common cause of visual impairment and second most common cause of visual loss .[48] The assessment of refractive error is now done in DALY (Disability Adjusted Life Years) which showed an 8% increase from 1990 to 2019.[49][unreliable source]
The number of people globally with significant refractive errors has been estimated at one to two billion.[4] Rates vary between regions of the world with about 25% of Europeans and 80% of Asians affected.[4] Near-sightedness is one of the most prevalent disorders of the eye.[6] Rates among adults are between 15 and 49% while rates among children are between 1.2 and 42%.[7] Far-sightedness more commonly affects young children, whose eyes have yet to grow to their full length, and the elderly, who have lost the ability to compensate with theiraccommodation system.[8][9] Presbyopia affects most people over the age of 35, and nearly 100% of people by the ages of 55–65.[3] Uncorrected refractive error is responsible for visual impairment and disability for many people worldwide.[10] It is one of the most common causes ofvision loss along withcataracts,macular degeneration, andvitamin A deficiency.[11]
The yearly cost of correcting refractive errors is estimated at 3.9 to 7.2 billion dollars in the United States.[50]
