Myopia, hyperopia and presbyopia are indistinguishable

Abstract: the important characteristic of refractive system is refractive power. The greater the refractive power, the stronger the ability to focus the line. The cornea has the largest refractive power in the refractive system, about more than 4000 degrees. But the cornea is a “fixed focus lens”, and the refractive power cannot be changed. The refractive power of the lens is slightly inferior to that of the cornea, about less than 2000 degrees.

eye is a camera.

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actually compare the eye to the camera. The eye is more precise than the camera, but a comparison between the two can make it easier for us to understand the problems of myopia, hyperopia and presbyopia. Like the camera, the eye can also be divided into two parts according to its function. The front cornea, aqueous humor, lens and vitreous are equivalent to the lens group of the camera and belong to the refractive system; The back retina is equivalent to the film or CCD of the camera and belongs to the photosensitive system. The refractive system is actually like a lens, which focuses the images of the wide world on the small retina. The important characteristic of

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refractive system is refractive power. The greater the refractive power, the stronger the ability to focus the line. The cornea has the largest refractive power in the refractive system, about more than 4000 degrees. But the cornea is a “fixed focus lens”, and the refractive power cannot be changed. The refractive power of the lens is slightly inferior to that of the cornea, about less than 2000 degrees. However, by changing the shape of the lens, the refractive power can be adjusted by more than 1000 degrees, just like the optical zoom function of a digital camera. The combination of cornea and lens constitutes almost all the refractive power of the eye. Through them, the eyes can completely shrink a building dozens of meters high and project it onto the retina of several square centimeters.

lens regulates refractive power in a very subtle way. There is a circle of ciliary muscle around the lens, which is connected with the lens through small ligaments. The ciliary muscle and lens compete with each other at ordinary times. When the ciliary muscle is relaxed, it will pull around and flatten and thin the lens. In this way, the degree of the lens will be reduced by hundreds of degrees, and you can have a panoramic view of things in the distance; When the ciliary muscle is tense, the lens will return to its natural state, become round, thicker and have greater refractive power. In this way, you can improve the resolution by looking at things near you. In other words, the tighter the ciliary muscle, the looser the lens and the greater the refractive power. The more suitable it is to see small objects and see more clearly; The more relaxed the ciliary muscle is, the more tense the lens is, and the smaller the refractive power is. The more suitable it is to see a large scene and the larger the range it can see. The eyes can zoom, and the refractive ability can be adjusted according to the distance of the object, which is realized by the competition between the two brothers of ciliary muscle and lens.

why can you get myopia?

if you read books and computers for a long time, your ciliary muscles will often be in a state of tension and gradually adapt to this state. If you look at the distance at this time, the ciliary muscles will not relax when they are used to tension, and the lens can’t become flat and tense. The excess positive degree of this part will make normal people feel like wearing a pair of reading glasses and can’t see things far away. You need to look closer. It is equivalent to that the camera is temporarily locked in “close range mode”, and the distant things are blurred. We call it “myopia”.

in this case, the eyedrops of atropine can paralyze the ciliary muscle, force it to relax, make the lens tense and thin, which is equivalent to removing this part of the excess positive degree, and you can see things in the distance. This kind of myopia that can return to normal is called “pseudomyopia”. When you go to the hospital with glasses, the doctor will first give you “mydriatic optometry” in order to eliminate the reversible degree of pseudomyopia. “Mydriasis” is a drug that paralyzes the ciliary muscle. At the same time, it can also paralyze the pupil muscle and dilate the pupil.

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and “pseudomyopia” are reversible and can be eliminated. What about “true myopia”? “True myopia” is caused by abnormal eye structure. When the eyes are in close working state, the eyeballs will converge inward under the action of extraocular muscles and become mild “eye to eye”, otherwise there will be ghosting when looking at near objects. In this state for a long time, the extraocular muscles will compress around the eyeball, lengthen the anterior and posterior diameter of the eyeball, and the ball becomes an ellipsoid. This change cannot be reversed. When the anterior posterior diameter of the eyeball becomes longer, less refractive power is required for imaging. When looking at the distance, no matter how relaxed the ciliary muscle is, it can not thin the lens and remove the excess reading glasses. Therefore, the image that should have been projected to the retina is projected to the front of the retina and cannot be focused on the retina. This kind of myopia that can no longer be reversed through self-regulation is called “true myopia”. If the degree of myopia is too high, the eyeball is too “elliptical”, the front and rear are sharp, and the retina is not firmly attached to it. If it is forced too hard or hit, it is easy to have retinal detachment, resulting in blindness.

pseudomyopia does not need to be corrected with glasses, but we should pay more attention to eye hygiene at ordinary times. True myopia should be corrected with concave lens of corresponding degree. Concave lens has the function of divergent light, so the degree is negative, which can remove the excess refractive power of the eye. What about

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hyperopia?

hyperopia is usually caused by abnormal eye structure. Because the eyeball of young children is not fully developed, the anterior and posterior diameter of the eyeball is short, and the object reaches the retina before imaging, it can not become a clear image. Therefore, under 7 years old is the high incidence age of hyperopia. In order to see close objects, the ciliary muscle needs to be in a state of tension frequently to make the lens relax and achieve sufficient refractive power. Therefore, hyperopia eyes are prone to eye fatigue. When the degree of hyperopia is large, due to the excessive tension of ciliary muscle and the convergence of binocular reflex, it is easy to develop into esotropia for a long time, that is, “cockfighting eye”. Mild hyperopia below

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300 degrees generally does not need correction, but those with visual fatigue or esotropia should be corrected with convex lens as soon as possible. Convex lens has the function of converging light, so the degree is positive, which can make up for the diopter lacking in hyperopia. Preschoolers with high degree of hyperopia should also have optometry and glasses as soon as possible. However, with the growth of age and the development of eyeballs, the light should be re examined once a year to reduce the power of lenses in time to meet the needs.

old flower, like hyperopia, but far from that,