Emmetropia

Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfect vision. A human eye in a state of emmetropia requires no corrective lenses; 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 "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). Either myopic (near-sighted) eyes or hyperopic (far-sighted) eyes would score worse, e.g. 20/40 (visual acuity roughly half normal). Exceptionally acute vision (excellent cornea and lens and better than standard retina) might be 20/15.

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.[1]

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.[2]

Emmetropization

Eyeball lengths: far-sighted (hyperopic), emmetropic, and near-sighted (myopic).

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.[3] It is assumed that emmetropization occurs via an active mechanism by which defocus drives growth of the eye[4] and that genetic factors and emmetropization both influence the growth of the eye's axis.[5] Newborns are typically hypermetropic and then undergo a myopic shift to become emmetropic.

There has been some research on causal factors involved in the development of myopia and of hyperopia. In particular, statistics show that prolonged near work correlates with the development of myopia, but it is still unclear whether there is a causal relation.[6] Furthermore, outdoor activity has been found to have a protective effect on myopia development in children.

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.[7] However, it is not yet fully understood for which patient groups, if any, the wearing of corrective spectacles in childhood actually impedes emmetropization.[8]

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.[9] There is widespread consensus that undercorrection is counterindicated for children with accommodative esotropia.[7] It is still unclear for which hyperopic, non-strabismic children corrective spectacles may translate to a lower strabismus risk.[4][8] There are indications that emmetropization is relevant for hyperopic children who have at most about 3.0 diopter, whereas children with stronger hyperopia seem to not change their refraction independently of whether the refractive error is corrected or not.[10]

A Cochrane Review of three trials seeking to determine whether spectacle correction reduced the occurrence of strabismus in children[11] included one study which suggested that spectacle correction perturbed emmetropization in children,[12] while a second study reported no differences.[13]

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.

References

1. Saladin, Kenneth S. "16." Anatomy & Physiology: the Unity of Form and Function. New York, NY: McGraw-Hill, 2012. Print.
2. "Photorefractive Keratectomy (PRK) Eye Surgery"
3. 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.
4. 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.
5. 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.
6. Pan, CW; Ramamurthy, D; Saw, SM (January 2012). "Worldwide prevalence and risk factors for myopia". Ophthalmic & Physiological Optics. 32 (1): 3–16. doi:10.1111/j.1475-1313.2011.00884.x. PMID 22150586.
7. 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.
8. 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. 4: CD007738. doi:10.1002/14651858.CD007738.pub3. ISSN 1469-493X. PMC 7117860. PMID 32240551.
9. "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.
10. Birgit Lorenz; Anthony Moore (31 January 2006). Pediatric Ophthalmology, Neuro-Ophthalmology, Genetics. Springer Science & Business Media. p. 15. ISBN 978-3-540-31220-8.
11. 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.
12. 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.
13. 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.

Further reading

• Bernard Gilmartin, 1998. Myopia and nearwork Pg 33-34, Elsevier Health Sciences. ISBN 0-7506-3784-6