Refractive error
Structure of the eye
The eyeball is a globe about 24 mm in diameter that consists of the wall and contents (Figure 1).
The retina covers the innermost wall.
Light from the exterior of the eye reaches the retina and produces stimulation, which is transmitted to the optic nerve and the brain. Every scene, which is a unit of light, passes through and is refracted in the cornea, lens, and then focuses on the retina. A clear image is created only when the scene is focused correctly on the retina.
The refracted light can be focused as a point when the cornea curves regularly. However, in cases in which the cornea curves irregularly, the light cannot be focused correctly, which creates a blurred image. This condition is called astigmatism.
The refractive status is divided according to the relation between the focal point and the retina (Figure 2).
When the eye sees a distant landscape, the condition in which its image is correctly focused on the retina is called emmetropia. Otherwise the eye has a refractive error.
Refractive error is divided into myopia and hyperopia, both of which may or may not be associated with astigmatism.
When the eye sees a distant landscape, if the focal point is in front of the retina, it is called myopia, and when it is at the back of the retina it is called hyperopia. When the eye is not astigmatic, the light is focused as a point even in these eyes with a refractive error. However, when astigmatism is present, the focus is not a point but a plane.
Treatment for eyes with a refractive error
Eyes with a refractive error create a blurred image because the focal point is not on the retina. To correct this, the focus point should be shifted to the retina.
If the eye has severe astigmatism, some additional techniques are required to create a focus point.
In eyes with myopia, the focal point is in front of the retina, because in many cases the refraction is too powerful. However, in some cases, the eyeball is enlarged, which results in a focal point in front of the retina.
In the very early stage of myopia, training in distance viewing or application of eye drops to shift the focal point posteriorly may be effective. However, after the condition becomes stable with an enlarged eyeball, it is impossible to achieve emmetropia with training or eye drops and the eye needs to be corrected with glasses or contact lenses. In addition to these tools, refractive surgeries are available that shift the focal point backward to the retina by flattening the surface of the cornea.
In myopia, for example, eye conditions develop in the following stages:
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