Emmetropia

Emmetropia
Emmetropia
Specialty	Ophthalmology

Last updated June 06, 2024

Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle ^[1] 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 (such as an eye chart test).^[2]

For example, on a Snellen chart test, emmetropic eyes score at least "6/6"(m) or "20/20"(ft) vision, meaning that at a distance of 20 ft (the first number) they see as well as a "normal" eye at a distance of 20 ft (the second number). Eyes that have enough myopia (near-sighted), hyperopia (far-sighted, excluding latent and facultative hyperopia), or optical aberrations would score worse, e.g. 20/40 (visual acuity of 0.5). Typical emmetropic vision might be 20/15 to 20/10 (visual acuity of 1.3 to 2).^[3]

Emmetropes with presbyopia might use lenses for near vision.

Overview

Emmetropia is a state in which the eye is relaxed and focused on an object more than 6 meters or 20 feet away. The light rays coming from that object are essentially parallel, and the rays are focused on the retina without effort. If the gaze shifts to something closer, light rays from the source are too divergent to be focused without effort. In other words, the eye is automatically focused on things in the distance unless a conscious effort is made to focus elsewhere. For a wild animal or human prehistorical ancestors, that arrangement would be adaptive because it allows for alertness to predators or prey at a distance.

Accommodation of the lens does not occur in emmetropia, and the lens is about 3.6 mm thick at the center; in accommodation, it thickens to about 4.5 mm. A relatively thin lens and relatively dilated pupil are also associated. The lens usually stiffens with age, causing less ability to focus when the eyes are not in a state of emmetropia.^[4]

Corrective eye surgery such as LASIK and PRK aims to correct anemmetropic vision. This is accomplished by ensuring the curvature of the cornea, the shape of the lens and their distances from each other and the retina are in harmony. By shaping the cornea, emmetropic vision can be achieved without corrective lenses. The correction for only emmetropic vision is often the reason that patients are advised to keep wearing glasses to read as they age because of presbyopia.^[5]

Emmetropization

The development of an eye towards emmetropia is known as emmetropization. This process is guided by visual input, and the mechanisms that coordinate this process are not fully understood.^[6] It is assumed that emmetropization occurs via an active mechanism by which defocus drives growth of the eye^[7] and that genetic factors and emmetropization both influence the growth of the eye's axis.^[8] Newborns are typically hypermetropic and then undergo a myopic shift to become emmetropic.^[9]

There has been some research on causal factors involved in the development of myopia and of hyperopia. In particular, prolonged near work is correlated with the development of myopia.^[9] Furthermore, outdoor activity has been found to have a protective effect on myopia development in children.^[9] It has long been assumed that wearing corrective spectacles might possibly perturb the process of emmetropization in young children, with this assumption being supported in particular also by animal studies. However, undercorrection of myopia in humans has been shown to increase the rate of myopic progression.^[10] However, it is not yet fully understood for which patient groups, if any, the wearing of corrective spectacles in childhood actually impedes emmetropization.^[11]

In hyperopic children, yet more factors are to be considered: Hyperopia is known to be a significant risk factor for esotropia, therefore undercorrection may have the side effect of increasing this risk.^[12] There is widespread consensus that undercorrection is counterindicated for children with accommodative esotropia.^[10] It is still unclear for which hyperopic, non-strabismic children corrective spectacles may translate to a lower strabismus risk.^[7]^[11] There are indications that emmetropization is relevant for hyperopic children who have at most about 3.0 diopters, whereas children with stronger hyperopia seem to not change their refraction independently of whether the refractive error is corrected or not.^[13]

A Cochrane Review of three trials seeking to determine whether spectacle correction reduced the occurrence of strabismus in children^[14]^{[ needs update ]} included one study which suggested that spectacle correction perturbed emmetropization in children,^[15] while a second study reported no differences.^[16]

Etymology

"Emmetropia" is derived from Greek ἔμμετρος emmetros "well-proportioned" (from ἐν en "in" and μέτρον metron "measure") and ὤψ ōps "sight" ( GEN ὠπός ōpos). Translated literally, the term indicates the condition of an eye's having in itself (i.e., without recourse to corrective lenses or other instruments) the capability to obtain an accurate measurement of an object's physical appearance.

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.

Esotropia is a form of strabismus in which one or both eyes turn inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance. It is the opposite of exotropia and usually involves more severe axis deviation than esophoria. Esotropia is sometimes erroneously called "lazy eye", which describes the condition of amblyopia; a reduction in vision of one or both eyes that is not the result of any pathology of the eye and cannot be resolved by the use of corrective lenses. Amblyopia can, however, arise as a result of esotropia occurring in childhood: In order to relieve symptoms of diplopia or double vision, the child's brain will ignore or "suppress" the image from the esotropic eye, which when allowed to continue untreated will lead to the development of amblyopia. Treatment options for esotropia include glasses to correct refractive errors, the use of prisms, orthoptic exercises, or eye muscle surgery. The term is from Greek eso meaning "inward" and trope meaning "a turning".

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.

Laser-Assisted in Situ Keratomileusis (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 an actual cure for astigmatism, since it is in the cornea. LASIK surgery is performed by an ophthalmologist who uses a laser or microkeratome to reshape the eye's cornea in order to improve visual acuity.

The Bates method is an ineffective and potentially dangerous alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates (1860–1931) held the erroneous belief that the extraocular muscles effected changes in focus and that "mental strain" caused abnormal action of these muscles; hence he believed that relieving such "strain" would cure defective vision. In 1952, optometry professor Elwin Marg wrote of Bates, "Most of his claims and almost all of his theories have been considered false by practically all visual scientists."

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.

$Radial keratotomy Refractive surgical procedure to correct myopia (nearsightedness$

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.

$Refractive surgery Surgery to treat common vision disorders$

Refractive surgery is 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.

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.

$Refractive error Problem with focusing light accurately on the retina due to the shape of the eye$

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.

<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.

An eye examination 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.

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.

Pediatric ophthalmology is a sub-speciality of ophthalmology concerned with eye diseases, visual development, and vision care in children.

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.

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.

Cyclotropia is a form of strabismus in which, compared to the correct positioning of the eyes, there is a torsion of one eye about the eye's visual axis. Consequently, the visual fields of the two eyes appear tilted relative to each other. The corresponding latent condition – a condition in which torsion occurs only in the absence of appropriate visual stimuli – is called cyclophoria.

References

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↑ "Eye test chart". Banton Frameworks. 20 December 2022. Retrieved February 11, 2023.
↑ "We have found also that the best eyes have a visual acuity which approaches 2, and we can be almost certain that if, with a good illumination, the acuity is only equal to 1, the eye presents defects sufficiently pronounced to be easily established." Physiologic Optics: Dioptrics of the Eye, Functions of the Retina, Ocular Movements and Binocular Vision
↑ Saladin, Kenneth S. "16." Anatomy & Physiology: the Unity of Form and Function. New York, NY: McGraw-Hill, 2012. Print.
↑ "Photorefractive Keratectomy (PRK) Eye Surgery"
↑ Mutti, DO (2005). "Axial Growth and Changes in Lenticular and Corneal Power during Emmetropization in Infants". Investigative Ophthalmology & Visual Science. 46 (9): 3074–3080. doi:10.1167/iovs.04-1040. ISSN 0146-0404. PMID 16123404.
1 2 Babinsky E, Candy TR (2013). "Why do only some hyperopes become strabismic?". Investigative Ophthalmology & Visual Science (Review). 54 (7): 4941–55. doi:10.1167/iovs.12-10670. PMC 3723374 . PMID 23883788.
↑ Siegwart JT, Norton TT (March 2011). "Perspective: how might emmetropization and genetic factors produce myopia in normal eyes?". Optometry and Vision Science. 88 (3): E365–72. doi:10.1097/OPX.0b013e31820b053d. PMC 3075852 . PMID 21258261.
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1 2 Creig Simmons Hoyt; David Taylor (2012). Pediatric Ophthalmology and Strabismus, Expert Consult - Online and Print,4: Pediatric Ophthalmology and Strabismus. Elsevier Health Sciences. pp. 33–34. ISBN 978-0-7020-4691-9.
1 2 Jones-Jordan, Lisa; Wang, Xue; Scherer, Roberta W.; Mutti, Donald O. (2 April 2020). "Spectacle correction versus no spectacles for prevention of strabismus in hyperopic children". The Cochrane Database of Systematic Reviews. 2020 (4): CD007738. doi:10.1002/14651858.CD007738.pub3. ISSN 1469-493X. PMC 7117860 . PMID 32240551.
↑ "Children with a greater degree of hyperopia are at a greater erisk to become esotropic; thus, a dilemma exists in presribig convex lenses to prevent the deviation as opposed to a possible interference with the emmetropization process." Quoted from: Robert H. Duckman (2006). Visual Development, Diagnosis, and Treatment of the Pediatric Patient. Lippincott Williams & Wilkins. p. 71. ISBN 978-0-7817-5288-6.
↑ Birgit Lorenz; Anthony Moore (31 January 2006). Pediatric Ophthalmology, Neuro-Ophthalmology, Genetics. Springer Science & Business Media. p. 15. ISBN 978-3-540-31220-8.
↑ Jones-Jordan L, Wang X, Scherer RW, Mutti DO (2014). "Topical Spectacle correction versus no spectacles for prevention of strabismus in hyperopic children". Cochrane Database Syst Rev. 8 (8): CD007738. doi:10.1002/14651858.CD007738.pub2. PMC 4259577 . PMID 25133974.
↑ Ingram RM, Arnold PE, Dally S, Lucas J (1991). "Emmetropisation, squint, and reduced visual acuity after treatment". Br J Ophthalmol. 75 (7): 414–416. doi:10.1136/bjo.75.7.414. PMC 1042408 . PMID 1854694.
↑ Atkinson J, Anker S, Bobier W, Braddick O, Durden K, Nardini M, et al. (2000). "Normal emmetropization in infants with spectacle correction for hyperopia". Invest Ophthalmol Vis Sci. 41 (12): 3726–3731. PMID 11053269.