"The eye axis is the same length, why is my child nearsighted?" myopia, and these reasons you don't know

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2024-05-04 13:14Posted on the official account of Zhejiang Chao News

Chao News Client Reporter Zheng Jiaying Correspondent Zhang Complex He Tingting

Staying at home during the holidays, children's use of electronic devices and excessive use of eyes also follow.

The problem of myopia is becoming more and more serious in mainland China, and the length of the eye axis, as a very important indicator in myopia screening, has been paid attention to by more and more parents.

In the optometry clinic of Zhejiang Provincial Eye Hospital Zhijiang Branch, this situation is often encountered: the axial length of the two children is the same, one is not myopia, and the other is myopia.

At this time, parents of children are often full of doubts: why is this?

Source: Visual China

Experts say that axial length is not the only factor in the development of myopia.

The reason why the eye can see things is that after the light enters the eye, it is refracted through a series of refractions such as the cornea and lens, and finally focuses on the retina to form a clear image.

And our eyes are like a camera, and myopia is equivalent to our eyes that the camera is not focused.

So what are the key factors that determine myopia?

The "focusing system" of the eyeball, which is often referred to as the refractive state, is mainly determined by the corneal curvature, the eye axis, and the refractive power of the lens. An abnormality in any one element will lead to an abnormality in the refractive state.

Corneal curvature

Corneal curvature refers to the curvature of the cornea, which is equivalent to the curvature of the hemispherical surface that protrudes forward from the lens in front of the camera, and represents the cornea's ability to bend light.

Everyone's cornea morphology is different, so the corneal curvature is also different. If the curved surface is more curved, that is, the higher the curvature, the stronger the convergence ability of light, which will converge in front of the retina in advance, forming myopia;

If the curved surface is flatter and flatter, the weaker the ability to gather light, and the later the light will converge, most of which converge behind the retina, forming farsightedness.

The normal corneal curvature is between 40D-46D, and the greater the corneal curvature, the higher the degree of corneal bulge, and the higher the probability of myopia.

The "OK lens" commonly used in the prevention and control of myopia in adolescents is also a orthokeratology lens, which is based on the principle of changing the curvature of the cornea by physically flattening and shaping the central area of the cornea, so that the imaging focus is shifted back to the retina, so as to achieve clear vision during the day.

Axial length

The eye axis refers to the distance from the cornea-lens-vitreous-retina, which is simply the length of the eyeball from front to back.

At birth, the eye axis is short, about 16.5 mm, and the human eye is farsighted, and with age, the eye axis gradually becomes longer, and it is not until adulthood that the eye axis gradually stabilizes, about 24 mm.

If the eye axis develops too quickly, myopia can develop. After myopia, every 1 mm increase in axial length corresponds to approximately 200-250 degrees of myopia.

Refractive power of the lens

The refractive power of the lens is equivalent to the zoom power of the camera lens, and a decrease in refractive power will cause the refractive state of the eye to develop towards hyperopia, and an increase in refractive power will cause the refractive state of the eye to develop towards myopia.

At present, it is not possible to directly measure the refractive power of a lens.

Source: Visual China

The refractive state of the eye is determined by the different combinations of the above three. Among them, because the refractive power of the lens changes relatively slowly, the curvature of the cornea and the length of the eye axis are the main factors that determine the refractive state.

Myopia caused by the "steep" curvature of the cornea is called curvature myopia, and generally speaking, the eye axis is not long, and the myopia caused by the elongation of the eye axis is called axial myopia, which is mainly manifested as the growth of the eye axis.

In general, patients with axial myopia have a longer axial length than patients with curvature myopia and are therefore at greater risk of fundus complications.

Therefore, people with the same degree of myopia can have very different axial lengths, and similarly, people with the same axial length can have different axial lengths.

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