WordNet

(ophthalmology) the normal refractive condition of the eye in which there is clear focus of light on the retina

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/11/12 13:19:01」(JST)

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Emmetropia (from Greek emmetros, "well-proportioned" or "fitting") describes the state of vision where an object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. This 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. An eye in a state of emmetropia requires no correction.

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 (an emmetropic eye does not need corrective lenses). 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, this 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. In emmetropia, 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 why patients are still advised to wear glasses to read as they age due to presbyopia.^[2]

Newborns begin hypermetropic and then undergo a myopic shift to become emmetropic.

Etymology

"Emmetropia" is ultimately derived from Greek en/em ("in") + metron ("measure," not to be confused with metr-/meter "mother") + op[s/t]- ("eye") + -ia ("condition"). 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.

Notes

^ Saladin, Kenneth S. "16." Anatomy & Physiology: the Unity of Form and Function. New York, NY: McGraw-Hill, 2012. Print.
^ [1]

References

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

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UpToDate Contents

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1. 成人における視覚障害：屈折障害と老視 visual impairment in adults refractive disorders and presbyopia
2. 小児における屈折異常 refractive errors in children
3. レーザー屈折矯正手術 laser refractive surgery
4. 小児における白内障 cataract in children

English Journal

Comparison of Corneal Layers and Anterior Sclera in Emmetropic and Myopic Eyes.

Pekel G1, Yağc R, Acer S, Ongun GT, Çetin EN, Simavl H.
Cornea.Cornea.2015 Jul;34(7):786-90. doi: 10.1097/ICO.0000000000000422.
PURPOSE: The aim of this study was to compare the thickness of preocular tear film, corneal layers, and anterior sclera in patients with moderate to high myopia and emmetropia with anterior segment spectral domain optical coherence tomography.METHODS: This cross-sectional comparative study included
PMID 25811725

DMEK in phakic eyes: targeted therapy or highway to cataract surgery?

Gundlach E1, Maier AK, Tsangaridou MA, Riechardt AI, Brockmann T, Bertelmann E, Joussen AM, Torun N.
Graefe's archive for clinical and experimental ophthalmology = Albrecht von Graefes Archiv für klinische und experimentelle Ophthalmologie.Graefes Arch Clin Exp Ophthalmol.2015 Jun;253(6):909-14. doi: 10.1007/s00417-015-2956-8. Epub 2015 Feb 17.
PURPOSE: To investigate outcome and postoperative complications in patients undergoing isolated DMEK in phakic eyes compared to patients undergoing DMEK combined with cataract surgery (triple procedure).METHODS: A retrospective analysis of 13 eyes undergoing DMEK in phakic eyes and 54 eyes undergoin
PMID 25683875

Progressive Myopia and Lid Suture Myopia are Explained by the Same Feedback Process: a Mathematical Model of Myopia.

Medina A1, Greene PR2.
Journal of nature and science.J Nat Sci.2015 Jun;1(6). pii: e121.
OBJECTIVE: Progressive myopia in humans and lid-sutured myopia in primates have been considered to be different processes. This report seeks to establish the connection between progressive myopia in humans and lid suture myopia in macaque monkeys.METHODS: We followed the axial length of 4 lid-suture
PMID 26042228

Japanese Journal

Changes in falling risk depending on induced axis directions of astigmatism on static posture

Journal of Physical Therapy Science 27(6), 1971-1973, 2015
NAID 130005086080

Body balance under ametropic conditions induced by spherical lenses in an upright position

Journal of Physical Therapy Science 27(3), 615-618, 2015
NAID 130005061657

特殊な角膜形状眼の眼内レンズ度数決定

日本視能訓練士協会誌 43(0), 145-152, 2014
NAID 130004822898

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