myopia(redirected from chromic myopia)
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defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it.
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(nearsightedness), one of the errors of refraction of the eyes causing people suffering from it to see distant objects poorly. The word “nearsightedness” describes the way myopic people usually hold objects close to their eyes.
Myopia occurs when parallel rays coming from distant objects are not focused on the retina after they are refracted in the eyes, as happens in normal refraction, but are focused in front of the retina. As a result, the retina does not receive a clear image of the object, and myopic people see poorly at a distance. Only rays with a certain degree of divergence— that is, rays coming from objects close to the eye—can be focused on the retina.
The point from which rays after refraction are focused precisely on the retina is called the farthest point of clear vision. Its position determines the degree of nearsightedness: the closer the point is to the eye, the greater the degree of myopia, and vice versa. If the farthest point of clear vision lies 1 m in front of the eye, myopia will be 1.0 diopter; 50 cm, 2.0 diopters; 25 cm, 4 diopters.
The closest point from which rays can be brought to focus precisely on the retina at a maximum tension (accommodation) of the eye is called the nearest point of clear vision. The distance between the farthest and nearest points of clear vision determines the space within which the nearsighted eye can see clearly. Three degrees of myopia are distinguished: weak, up to 3.0 diopters; moderate, 3.0 to 6.0 diopters; and strong, or high, 6.0 diopters or more. Strong myopia is to be distinguished from severe progressive, or malignant, myopia wherein organic changes occur in the vascular tunic and retina (for example, hemorrhages, atrophic phenomena, or retinal detachment) and result in varying degrees of loss of vision.
Myopia is a function of the greater optical strength of the refracting media of the eyes (cornea, crystalline lens, and vitreous body) in relation to the length of the anteroposterior axis of the eye (so-called refraction myopia) and the greater length of the axis of the eye in relation to the strength of its refracting apparatus (so-called axial myopia). Generally, so-called combination myopia occurs; both factors that determine refraction are within the average values needed for normal refraction, but these elements are so combined that myopia results.
Since nearsighted people must hold objects close to the eyes, the eyes must be made to converge by increased activity of the internal rectus muscles. If these muscles are inadequate, strabismus may develop.
Myopia often develops in schoolchildren as a result of the long periods of time they spend doing close work (reading and writing), especially in insufficient light and under poor hygienic conditions. It may also develop among workers in various fields (engravers, watchmakers, and typesetters, for example). Nearsightedness is diagnosed in clinics with the aid of special instruments (refractometers) and methods (retinoscopy, or shadow test) or by the selection of suitable lenses.
Prevention of myopia entails providing proper illumination in classrooms and alternating periods of work and rest. Developing myopia can be checked by the prompt prescription of corrective glasses, with the individual’s age and job being taken into consideration. Accompanying diseases that weaken the body and promote the development of nearsightedness should be treated. Systematic treatment and a suitable regimen are needed in the case of progressive myopia. Persons with this disorder should be kept under clinical observation. Retinal detachment requires surgical treatment.
REFERENCESMnogotomnoe rukovodstvopo glaznym bolezniam, vol. 1, book 1. Moscow, 1962. Pages 279–94.
Beletskaia, V. I., and N. V. Shubina. Gigiena zreniia. Moscow, 1968.
M. L. KRASNOV