Refractive error

Last updated
Refractive error
Other namesRefraction error
Refractive Errors of the Eye.jpg
A correctly-focused eye (top), and two showing refractive error: in the middle image, the light 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 with cornea shape, aging of the lens [3]
Diagnostic method Eye examination [3]
Treatment Eyeglasses, contact lenses, refractive surgery [3]
Frequency~1.5 billion [4]

Refractive error is a problem with focusing light accurately on the retina due to the shape of the eye and/or cornea. [3] 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. [3]

Contents

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. [3] 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. [3] 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. [3]

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. [5] Rates among adults are between 15 and 49% while rates among children are between 1.2 and 42%. [6] Far-sightedness more commonly affects young children and the elderly. [7] [8] 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. [9] Of these 9.5 million were blind due to the refractive error. [9] It is one of the most common causes of vision loss along with cataracts, macular degeneration, and vitamin A deficiency. [10]

Classification

Top: farsighted corrected using convex lens Bottom: nearsighted corrected using concave lens. Refractive error.jpg
Top: farsighted corrected using convex lens Bottom: nearsighted corrected using concave lens.

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. [11] [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. [16]

Other terminology include anisometropia, when the two eyes have unequal refractive power, [17] and aniseikonia which is when the magnification power between the eyes differ. [18]

Refractive errors are typically measured using three numbers: sphere, cylinder, and axis. [19]

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. [20]

Risk factors

Genetics

Fundus of person with retinitis pigmentosa, early stage Fundus of patient with retinitis pigmentosa, early stage.jpg
Fundus of person with retinitis pigmentosa, early stage

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. [21] 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). [22] 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); Bornholm eye disease (OMIM 310460). [22] Many genes that have been associated with refractive error are clustered into common biological networks involved in connective tissue growth and extracellular matrix organization. [22] Although a large number of chromosomal localisations have been associated with myopia (MYP1-MYP17), few specific genes have been identified. [21]

Environmental

In studies of the genetic predisposition of refractive error, there is a correlation between environmental factors and the risk of developing myopia. [23] Myopia has been observed in individuals with visually intensive occupations. [22] 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. [22] 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. [24] [25] Socioeconomic status and higher levels of education have also been reported to be a risk factor for myopia. [26] Blepharoptosis can also induce refractive errors. [27]

Normal Refraction

In order to see a clear image, the eye must focus rays of light on to the light-sensing part of the eye – the retina, which is located in the back of the eye. This focusing – called refraction – is performed mainly by the cornea and the lens, which are located at the front of the eye, the anterior segment. [28]

When an eye focuses light correctly on to the retina when viewing distant objects, this is called emmetropia or being emmetropic. 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 must increase its refractive power to bring those rays of light together on the retina. This is called accommodation, and is accomplished by the eye thickening the lens. [28]

Diagnosis

A doctor uses a trial frame and trial lenses to measure the person's refractive error. Refrarction error test.JPG
A doctor uses a trial frame and trial lenses to measure the person's refractive error.

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 an eye care professional during an eye examination using a large number of lenses of different optical powers, and often a retinoscope (a procedure entitled retinoscopy ) 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 as subjective refraction . Cycloplegic agents are frequently used to more accurately determine the amount of refractive error, particularly in children [23]

An automated refractor is an instrument that is sometimes used in place of retinoscopy to objectively estimate a person's refractive error. [29] Shack–Hartmann wavefront sensor and its inverse [30] can also be used to characterize eye aberrations in a higher level of resolution and accuracy.

Vision defects caused by refractive error can be distinguished from other problems using a pinhole occluder, which will improve vision only in the case of refractive error. [31]

Screening

When refractive errors in children are not treated, the child may be at risk of developing ambylopia, where vision may remain permanently blurry. [32] 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. [32] [33]

Management

The management of refractive error is done post-diagnosis of the condition by either optometrists, ophthalmologists, refractionists, or ophthalmic medical practitioners. [34]

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. [28] [19] [21]

Glasses

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. [35]

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. [36]

Reading glasses
These are general over-the-counter glasses which can be worn for easier reading, especially for defective vision due to aging called presbyopia.
Single vision prescription lenses
They can correct only one form of defective vision, either far-sightedness or near-sightedness.
Multifocal lenses
The multifocal lenses can correct defective vision in multiple focus, for example: near-vision as well as far-vision. This are particularly beneficial for presbyobia. [37]

Contact lenses

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. [34]

There are a number of complication associated with contact lenses. Typically the ones that are used daily.[ citation needed ]

Complications of contact lens wearDescription
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 abrasionCaused by a foreign body, dust, sand, or grit trapped under the lens.
Corneal edemaCaused 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.
NeovascularizationNew blood vessels may form in the iris region and the limbus. This may impair vision.
InfectionsVarious 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.

Surgery

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 after clear 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. [38] [39]

Medical treatment

Atropine has believed to slow the progression of near-sightedness and is administered in combination with multifocal lenses. These, however, need further research. [40] [41]

Prevention

Strategies being studied to slow worsening include adjusting working conditions, increasing the time children spend outdoors, [22] and special types of contact lenses. [42] In children special contact lenses appear to slow worsening of nearsightedness. [42] [43]

A number of questionnaires exist to determine quality of life impact of refractive errors and their correction. [44] [45]

Epidemiology

DALYs per 100,000 people due to refractive errors in 2004. .mw-parser-output .div-col{margin-top:0.3em;column-width:30em}.mw-parser-output .div-col-small{font-size:90%}.mw-parser-output .div-col-rules{column-rule:1px solid #aaa}.mw-parser-output .div-col dl,.mw-parser-output .div-col ol,.mw-parser-output .div-col ul{margin-top:0}.mw-parser-output .div-col li,.mw-parser-output .div-col dd{page-break-inside:avoid;break-inside:avoid-column} .mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{} No data Less than 100 100-170 170-240 240-310 310-380 380-450 450-520 520-590 590-660 660-730 730-800 More than 800 Refractive errors world map - DALY - WHO2004.svg
DALYs per 100,000 people due to refractive errors in 2004.
  No data
  Less than 100
  100–170
  170–240
  240–310
  310–380
  380–450
  450–520
  520–590
  590–660
  660–730
  730–800
  More than 800

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. [9]

Refractive Errors are the first common cause of Visual Impairment and second most common cause of visual loss . [47] The assessment of Refractive Error is now done in DALY (Disability Adjusted Life Years) which showed an 8% increase from 1990 to 2019. [48] [ 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. [5] Rates among adults are between 15 and 49% while rates among children are between 1.2 and 42%. [6] 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 their accommodation system. [7] [8] 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. [9] It is one of the most common causes of vision loss along with cataracts, macular degeneration, and vitamin A deficiency. [10]

Cost

The yearly cost of correcting refractive errors is estimated at 3.9 to 7.2 billion dollars in the United States. [49]

Related Research Articles

<span class="mw-page-title-main">Myopia</span> Problem with distance vision

Myopia, also known as near-sightedness and short-sightedness, is an eye disease where light from distant objects focuses in front of, instead of on, the retina. As a result, distant objects appear blurry while close objects appear normal. Other symptoms may include headaches and eye strain. Severe myopia is associated with an increased risk of macular degeneration, retinal detachment, cataracts, and glaucoma.

<span class="mw-page-title-main">Farsightedness</span> Eye condition in which light is focused behind instead of on the retina

Far-sightedness, also known as long-sightedness, hypermetropia, and hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blur is due to incoming light being focused behind, instead of on, the retina due to insufficient accommodation by the lens. Minor hypermetropia in young patients is usually corrected by their accommodation, without any defects in vision. But, due to this accommodative effort for distant vision, people may complain of eye strain during prolonged reading. If the hypermetropia is high, there will be defective vision for both distance and near. People may also experience accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus. Newborns are almost invariably hypermetropic, but it gradually decreases as the newborn gets older.

<span class="mw-page-title-main">LASIK</span> Corrective ophthalmological surgery

LASIK or Lasik, commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and astigmatism. LASIK surgery is performed by an ophthalmologist who uses a femtosecond laser or a microkeratome to create a corneal flap to expose the corneal stroma and then an excimer laser to reshape the corneal stroma in order to improve visual acuity.

<span class="mw-page-title-main">Presbyopia</span> Medical condition associated with aging of the eye

Presbyopia is physiological insufficiency of optical accommodation associated with the aging of the eye; it results in progressively worsening ability to focus clearly on close objects. Also known as age-related farsightedness, it affects many adults over the age of 40. A common sign of presbyopia is difficulty in reading small print, which results in having to hold reading material farther away. Other symptoms associated can be headaches and eyestrain. Different people experience different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia. This condition is similar to hypermetropia or far-sightedness which starts in childhood and exhibits similar symptoms of blur in the vision for close objects.

<span class="mw-page-title-main">Astigmatism (optical systems)</span> Optical aberration

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".

<span class="mw-page-title-main">Visual acuity</span> Clarity of vision

Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye influence the sharpness of an image on its retina. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain.

<span class="mw-page-title-main">Radial keratotomy</span> Refractive surgical procedure to correct myopia (nearsightedness

Radial keratotomy (RK) is a refractive surgical procedure to correct myopia (nearsightedness). It was developed in 1974 by Svyatoslav Fyodorov, a Russian ophthalmologist. It has been largely supplanted by newer, more accurate operations, such as photorefractive keratectomy, LASIK, Epi-LASIK and the phakic intraocular lens.

<span class="mw-page-title-main">Refractive surgery</span> Surgery to treat common vision disorders

Refractive surgery is an optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement. The most common methods today use excimer lasers to reshape the curvature of the cornea. Refractive eye surgeries are used to treat common vision disorders such as myopia, hyperopia, presbyopia and astigmatism.

<span class="mw-page-title-main">Intraocular lens</span> Lens implanted in the eye to treat cataracts or myopia

An intraocular lens (IOL) is a lens implanted in the eye usually as part of a treatment for cataracts or for correcting other vision problems such as short sightedness and long sightedness; a form of refractive surgery. If the natural lens is left in the eye, the IOL is known as phakic, otherwise it is a pseudophakic lens. Both kinds of IOLs are designed to provide the same light-focusing function as the natural crystalline lens. This can be an alternative to LASIK, but LASIK is not an alternative to an IOL for treatment of cataracts.

Anisometropia is a condition in which a person's eyes have substantially differing refractive power. Generally, a difference in power of one diopter (1D) is the threshold for diagnosis of the condition. Patients may have up to 3D of anisometropia before the condition becomes clinically significant due to headache, eye strain, double vision or photophobia.

<span class="mw-page-title-main">Phakic intraocular lens</span> Lens implanted in eye in addition to the natural lens

A phakic intraocular lens (PIOL) is an intraocular lens that is implanted surgically into the eye to correct refractive errors without removing the natural lens. Intraocular lenses that are implanted into eyes after the eye's natural lens has been removed during cataract surgery are known as pseudophakic.

<span class="mw-page-title-main">Eye examination</span> Series of tests assessing vision and pertaining to the eyes

An eye examination, commonly known as an eye test, is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ophthalmologist, or an orthoptist. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.

<span class="mw-page-title-main">Accommodation (vertebrate eye)</span> Focusing ability of eye

Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image. Accommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled.

Aphakia is the absence of the lens of the eye, due to surgical removal, such as in cataract surgery, a perforating wound or ulcer, or congenital anomaly. It causes a loss of ability to maintain focus (accommodation), high degree of farsightedness (hyperopia), and a deep anterior chamber. Complications include detachment of the vitreous or retina, and glaucoma.

<span class="mw-page-title-main">Astigmatism</span> Type of eye defect

Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at night. Astigmatism often occurs at birth and can change or develop later in life. If it occurs in early life and is left untreated, it may result in amblyopia.

<span class="mw-page-title-main">Emmetropia</span> State of vision

Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle in a relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfectly sharp distance vision. A human eye in a state of emmetropia requires no corrective lenses for distance; the vision scores well on a visual acuity test.

<span class="mw-page-title-main">Blurred vision</span> Medical condition

Blurred vision is an ocular symptom where vision becomes less precise and there is added difficulty to resolve fine details.

Vision of humans and other organisms depends on several organs such as the lens of the eye, and any vision correcting devices, which use optics to focus the image.

Laser blended vision is a laser eye treatment which is used to treat presbyopia or other age-related eye conditions. It can be used to help people that simply need reading glasses, and also those who have started to need bifocal or varifocal spectacle correction due to ageing changes in the eye. It can be used for people who are also short-sighted (myopia) or long-sighted (hyperopia) and who also may have astigmatism.

The eye, like any other optical system, suffers from a number of specific optical aberrations. The optical quality of the eye is limited by optical aberrations, diffraction and scatter. Correction of spherocylindrical refractive errors has been possible for nearly two centuries following Airy's development of methods to measure and correct ocular astigmatism. It has only recently become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism.

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