Myopia

Synonyms in a broader sense

Medical: Myopia astigmatism, astigmatism, farsightedness

Definition Nearsightedness

Nearsightedness (myopia) refers to a type of ametropia in which the relationship between refractive power and the length of the eyeball is not correct. Strictly speaking, the eyeball is too long (axial myopia) or the refractive power is too strong (refractive myopia). The focal point of parallel incident rays is therefore in front of the retina. The near-sighted person can see objects close by well, but objects further away are only perceived blurred or blurred.

Cause

  • Axis myopia (axial nearsightedness) is more common than refractive myopia (refractive nearsightedness), is partially inherited and is usually congenital. It is much more common in premature babies than in those born at maturity. This type of short-sightedness develops mainly in the first 30 years of life as a result of overly intensive growth of the eyeball in length.

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For a point to be seen sharply, it must be imaged exactly on the retina. This means that the focal point of parallel incident rays must fall exactly on the retina. In people with myopia, the focal point is generally shifted forward.

Either because the longitudinal diameter of the eye is too large (common) or the refractive power of the optical apparatus is too strong (rather rare). As a result, objects at a distance cannot be imaged sharply. Closer objects, on the other hand, can be.

Even points that are so close to the eye that a person with normal vision can no longer image them sharply can still be seen well by myopic people. In order to understand why a person is nearsighted, one must know how the eye can sharply image objects at short and long distances. This is done with the help of the so-called optical apparatus (cornea and lens).

The lens is elastically formed and suspended behind the iris by a holding device. With the help of a ring muscle (ciliary muscle), the focus can be adjusted to near or distant objects. As the muscle tightens, the ligaments from which the lens is suspended slacken and the lens collapses slightly.

This results in an increase in refractive power and the focal length is thus reduced, i.e. the focal point is shifted forward. This allows objects that are relatively close to the eye to be brought into focus. When focusing distant objects, the ciliary muscle relaxes and the refractive power decreases, or the focal point is shifted further back.

The strength of myopia is indicated in diopters (dpt). This is the reciprocal of the focal length. The values always refer to the far point, i.e. the point at which the eye is somewhat sharp without accommodation (changing the focus of the view to distant or near objects).

In the case of normal vision (emmetropics), this is at infinity. A nearsighted person with a myopia of -2.0 dpt has his far point at a distance of 50cm. Objects that are at a greater distance from the eye can only be seen blurred.

In contrast to farsightedness, a person with myopia cannot compensate for his or her nearsightedness with the help of accommodation because the ciliary muscle, the muscle responsible for focusing, cannot be relaxed any further than it already is. Near-sighted people try to reduce the size of the diverging circles on the retina by blinking. This improves the sharpness of the image (stenopeic vision).