National Academies Press: OpenBook

Myopia: Prevalence and Progression (1989)

Chapter: Appendix E: Glossary

« Previous: Appendix D: The Etiology of Myopia
Suggested Citation:"Appendix E: Glossary." National Research Council. 1989. Myopia: Prevalence and Progression. Washington, DC: The National Academies Press. doi: 10.17226/1420.
Page 93
Suggested Citation:"Appendix E: Glossary." National Research Council. 1989. Myopia: Prevalence and Progression. Washington, DC: The National Academies Press. doi: 10.17226/1420.
Page 94
Suggested Citation:"Appendix E: Glossary." National Research Council. 1989. Myopia: Prevalence and Progression. Washington, DC: The National Academies Press. doi: 10.17226/1420.
Page 95
Suggested Citation:"Appendix E: Glossary." National Research Council. 1989. Myopia: Prevalence and Progression. Washington, DC: The National Academies Press. doi: 10.17226/1420.
Page 96

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Appendix E Glossary Accommodation (of the eye). rays from various distances to focus upon the retina. Againet-the-rule astigmatism. Astigmatism in which the meridian of greatest refractive power of the eye is the vertical, or 30° either side of vertical. As a consequence, the most minus (or least plus) correction is at, or within, 30° of the vertical. Ametropia. The refractive condition in which, with accommodation relaxed, parallel rays do not focus on the retina; a condition representing the manifestation of a refractive error. Changes in the ciliary muscle and the lens in bringing light Amplitude of accon~nodation. The maximum dioptric lens power increase resulting from contraction of the ciliary muscle in response to a near stimulus. Anisometropia. A condition of unequal refractive state for the two eyes, one eye requiring a different lens correction from the other. Astigmatism. A condition of refraction in which rays emanating from a single lurn~nous point are not focused at a single point by an optical system, but instead are focused as a line. In the eye, astigmatism is a refractive anomaly in which incident light is refracted unequally along different meridians. In so-called regular astigmatism, corrected with spectacles, the maximum difference in refraction occurs for meridians 90 degrees apart. . A general term used to encompass any subjective symptoms of discomfort or Asthenopia. eyestrain arising from use of the eyes. Axial elongation. A lengthening along an axis. In myopia axial elongation refers to a lengthening of the anteroposterior axis of the globe. Bifocal lenses. A pair of lenses for spectacles or contact lenses, each one having a part that corrects for distant vision and another that corrects for close vision. Ciliary Uncle. The smooth muscle of the ciliary body, involved in ocular accommodation. ciliary tonus. Refers to the degree of contraction present in the ciliary muscles when not undergoing active contraction in response to a stimulus. Contact lens. A lens consisting of a plastic shell which is placed on the cornea for the correction of refractive errors. 93

94 Cornea. The transparent most anterior tissue portion of the eyeball. Cornea curvature. Refers to the degree of curving of the corneal surface. Crystalline lens. The lens of the eye, a refractive organ of accommodation. A biconvex, transparent, elastic body lying in its capsule immediately behind the pupil of the eye' suspended from the ciliary body by the ciliary zonule. Cycloplegia. Paralysis of the ciliary muscles of the eyes, which greatly reduces the am- plitude of accommodation. Atropine cyclopedia, for example, refers to artificially induced paralysis caused by the anticholinergic agent atropine. Atropine is an example of a cyclo- plegic drug. Diopter. A unit of measurement of the refractive power of an optic lens. It is the refractive power of a lens having a focal distance of one meter. Abbreviated: d. or D. Emmetropia. The absence of refractive error. The condition in which parallel rays are focused exactly on the retina without effort of accommodation. Esophoria. The convergent position of the eyes with respect to the object of regard when binocular vision is obviated. Far point of accommodation. The conjugate focus of the retina (fovea) when the accom- modation is relaxed or at its minimum. In emmetropia, the far point is said to be infinity; in myopia, it is at some finite distance in front of the eye; in hyperopia, it is at some finite (virtual) distance behind the eye. Hyperopia. A refractive error in which the focus of parallel rays of light falls behind the retina; due typically to an abnormally short anteroposterior diameter of the eye or to subnormal refractive power. Farsightedness. People with this condition are referred to as hyperopes. Hypermetropia. Used synonomously with hyperopia. Hysteresis effect. In the absence of accommodative demand, the tendency of ciliary muscle to maintain a degree of muscle tonus appropriate to focus on a previously presented target. Incidence. Incidence rate is the number of cases of a disease appearing per unit of popu- lation within a defined time interval (typically one year). Beratometer. An instrument for measuring the curvature surments of reflected images formed by the anterior surface of the cornea. Meridian. The angular orientation on a circle. of the cornea by utilizing mea Myopia. An optical defect, usually due to too great length of the anteroposterior diameter of the globe, whereby the focal image is formed in front of the retina. Nearsightedness. People with this condition are referred to as myopes. Myopia progression. The advancement in the severity of myopia with time. Near point of accommodation. The nearest point on which the eye can focus with maxi- mum accommodation. Phakometry. Measurement of the crystalline lens power, especially accommodative changes. Presbyopia. The diminished power of accommodation commonly manifest for near work after the middle forties. It arises from impaired elasticity of the crystalline lens, which

95 begins in childhood, whereby the near point of distinct vision is removed farther from the eye so that the individual has difficulties in focusing on near objects and in reading fine print. Prevalence. Proportion of individuals having a specified condition or characteristic at a · ~ given time. Pseudon~vonia. The appearance of myopia due to incomplete relaxation of the accom ~ ,¢ Ma, ,~ ~ , ~ . · ~ · ~ . ~ ·1~ 1 modatlve mechanism or a spasm OI tne clllary muscle. Refraction. The change in curvature of a wave front which is manifest by the deviation of a ray of light from a straight line in passing obliquely from one transparent medium to another of different density. Also used to refer to the state of refractive power correction needed by the eye. Also, the act of determining the refractive error. Refractive error. A defect of the eye which prevents parallel light rays from a distinct object from being brought to a single focus precisely on the retina without any accommodation. Usually expressed in terms of the reciprocal in meters of the far point of accommodation. Stethoscopy. A method of determining the refractive error of the eye by observation of the movement of the shadow phenomena produced and observed by means of a retinoscope. Also referred to as skiascopy. Senile myopia. Myopia developing in the eye of older people. Sperical equivalent power. The spherical power equivalent to the power referenced on the circle of least confusion of a spherocyclindrical lens. Spherical lens. Also referred to as sphere. A lens in which the curved surfaces are segments of a sphere. A minus sphere has curved surfaces which produce a net refracting power which causes light from a distant object to diverge and is used in the optical correction of myopia. Subjective refraction. Strictly speaking, refers to refraction based on the subject's report but in practice has been broadened to include any determination of refractive error without the use of cyclopedia. Sometimes described as "accommodation relaxed physiologically." The most common procedure is known as "maximum plus to best visual acuity" method. Visual acuity. The measured central vision for recognition of the smallest spatial separation of objects or parts of an object (e.g., a letter) which can be seen. Dependent upon the clarity of the retinal focus, integrity of the nervous elements, and cerebral interpretation of a given stimulus at a given distance as tested with a Snellen or similar chart. In the USA using Snellen's letter chart at 20 feet, normal visual acuity is at least 20/20 for letters of high contrast with their background.

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This book considers the issues surrounding the occurrence, progression, and predictability of myopia (near-sightedness), with special emphasis on the 16- to 26-year-old population. Myopia reviews only the most pertinent published research in this area, analyzing the findings and drawing conclusions from these studies. The observations and recommendations will undoubtedly be of considerable interest to vision scientists and clinicians alike.


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