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What is ametropia? What types of Ametropia are there?
In the following article:
We want to discuss ametropia and the difference between emmetropia and ametropia. We will also dig deeper and discuss the different types of ametropia: spherical ametropia (axial ametropia and refractive ametropia) and non-spherical ametropia (or cylindrical ametropia).
Finally, we will discover what causes ametropia and how it can be corrected.
What is Ametropia?
Ametropia is an abnormality in the refraction of the eye. It refers to any eye defect that causes a blurring of the image on the eye’s retina and a resulting reduction in visual acuity.
Ametropia is measured in diopters and is an abnormality in the eye’s refraction in which the image appears in front of or behind the retina or in which multiple focal points are created.
There are different types of Ametropia, including myopia, farsightedness, and astigmatism, which can be combined. It is possible, for example, to have an Ametropia of myopia and astigmatism in the same eye.
What is Emmetropia and Ametropia?
Emmetropia refers to the normal state of refraction of the eye. The person can see perfectly without straining and without wearing glasses or contact lenses because the eye can focus images correctly on the retina.
Ametropia, on the other hand, refers to any refractive error that prevents a person from seeing clearly without glasses or contact lenses because the image is not projected correctly onto the eye’s retina.
How many types of Ametropia are there?
There are two types of Ametropia. Depending on the eye’s curvature, we will distinguish between spherical Ametropia and non-spherical Ametropia.
What are spherical ametropias?
What is axial Ametropia?
Axial ametropia is ametropia in which the length of the eye is abnormal. In myopia, the eye is longer, and in farsightedness, it is shorter.
What is Refractive Ametropia?
Refractive ametropia refers to whether or not the eye is very powerful. That is, whether the eye has too much or too little power to converge.
In the case of myopia, the power of the eye is too much, and in the case of hyperopia, less.
In turn, refractive ametropia can be curvature myopia or index myopia.
In curvature myopia, there is an excess in the cornea’s curvature, unlike hypermetropia, where the curvature is lower, and the cornea is more flattened.
Myopia and hypermetropia result from a change in some refractive index; in hypermetropia, the refractive power is lower due to a lowering of the index of the crystalline lens.
Therefore, axial Ametropia refers to a deviation in the length of the eye, and refractive Ametropia refers to a deviation in the strength of the eye.
Spherical ametropia: The curvature of the eye is uniform. There are two types of spherical ametropia: myopia and hyperopia.
In myopia: The focal point where the image is formed is in front of the retina, while in farsightedness, the image is formed behind the retina.
In turn, spherical ametropia can be of the following types: axial ametropia or refractive ametropia.
What are non-spherical or cylindrical Ametropias?
Non-spherical or cylindrical ametropia occurs when the eye’s curvature is not uniform and causes the refractive power of the eye to be different in all degrees: this is known as astigmatism.
In cylindrical ametropia, there are two focal points, two planes with different angles, causing a difference in the convergence of the two points.
What causes Ametropia?
Ametropia is caused by the eye being unable to form an image on the retina because there is a mismatch between the strength of the lens and the length of the eye. As a result, the image becomes blurred.
Unlike emmetropia, where there is no refractive error, the image does form on the retina. The image is sharp.
How is Ametropia corrected?
To correct ametropia, we need to reduce or add diopters to our eyes, depending on the type of ametropia we have. This is achieved through glasses, contact lenses, or refractive surgery.
In the case of myopia, the ametropia can be axial or refractive and corrected with divergent negative spherical lenses.
Converging positive spherical lenses corrects hyperopia, which can be axial or refractive.
Cylindrical lenses correct astigmatic ametropia.