Emmetropia

Emmetropia
Emmetropia
Specialty	Ophthalmology

Last updated August 21, 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).

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).^[2]

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.^[3]

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.^[4]

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.^[5] It is assumed that emmetropization occurs via an active mechanism by which defocus drives growth of the eye^[6] and that genetic factors and emmetropization both influence the growth of the eye's axis.^[7] Newborns are typically hypermetropic and then undergo a myopic shift to become emmetropic.^[8]

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.^[8] Furthermore, outdoor activity has been found to have a protective effect on myopia development in children.^[8] 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.^[9] However, it is not yet fully understood for which patient groups, if any, the wearing of corrective spectacles in childhood actually impedes emmetropization.^[10]

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.^[11] There is widespread consensus that undercorrection is counterindicated for children with accommodative esotropia.^[9] It is still unclear for which hyperopic, non-strabismic children corrective spectacles may translate to a lower strabismus risk.^[6]^[10] 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.^[12]

A Cochrane Review of three trials seeking to determine whether spectacle correction reduced the occurrence of strabismus in children^[13]^{[ needs update ]} included one study which suggested that spectacle correction perturbed emmetropization in children,^[14] while a second study reported no differences.^[15]

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

A corrective lens is a transmissive optical device that is worn on the eye to improve visual perception. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal but can be used for purely refractive purposes.

<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.

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.

Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that 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 reading small print which results in having to hold reading material farther away. Other symptoms associated can be headaches and eyestrain. Different people will have 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">Strabismus</span> Eyes not aligning when looking at something

Strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. The eye that is pointed at an object can alternate. The condition may be present occasionally or constantly. If present during a large part of childhood, it may result in amblyopia, or lazy eyes, and loss of depth perception. If onset is during adulthood, it is more likely to result in double vision.

Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process input from one eye and over time favors the other eye. It results in decreased vision in an eye that typically appears normal in other aspects. Amblyopia is the most common cause of decreased vision in a single eye among children and younger adults.

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 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.

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.

The human eye is a sensory organ in the visual system that reacts to visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and keeping balance.

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.

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.

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.

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

↑ Hall, John E.; Michael E. Hall (2021). Guyton and Hall textbook of medical physiology (14th ed.). Philadelphia. ISBN 978-0-323-59712-8. OCLC 1129099861.{{cite book}}: CS1 maint: location missing publisher (link)
↑ "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.
1 2 3 Medina, A (2022). "The cause of myopia development and progression: Theory, evidence, and treatment". Surv Ophthalmol. 67 (2): 488–509. doi:10.1016/j.survophthal.2021.06.005. PMID 34181975. S2CID 235672990.
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.