Before you can appreciate the various treatment approaches to myopia, hyperopia, and/orastigmatism you must appreciate the basic optics of the problem.
Optical difficulties which can be corrected by adding additional lenses to the patient's own optical system are called refractive errors.
The measurement of such errors is called a refraction.
Spherical refractive conditions
- Emmetropia
Figure 1. Emmetropia
In the physiologically normal eye, parallel rays converge to form a principle focus upon the retina; when these ideal optical conditions occur with the eye in a state of rest — the condition is termed emmetropia (Figure 1).
Since the principle focus is of a size approximating a point and lies in one plane — such a system is also called stigmatic (from the Greek meaning point).
It would be strange if this were a common state of affairs, for its attainment depends on an almost perfect correlation of such measurements as the length of the eye and the shape of the cornea and the lens.
Such regularity and conformity to optical perfection necessitates a mathematical accuracy which is nowhere realized in the constitution of living organisms.
Emmetropia may be optically normal, but it is no more biologically normal than would be the universal attainment of a uniform height of 5 feet 6 inches.
The opposite condition of ametropia, when parallel rays of light are not focused exactly upon the retina with the eye in a state of rest, is therefore much the more common.
The three basic refractive errors are Hyperopia (farsightedness), Myopia (nearsightedness), and Astigmatism.
- Hyperopia (hypermetropia, hyperope)
Figure 2. Hyperopia (hypermetropia, hyperope)
In hyperopia (commonly called farsightedness since the patient can see clearly far away but not up close), parallel light rays come to a focus behind the retina.
In this case, the "focusing power” of the eye is not great enough for an eye of that length (Figure 2) .
The best example of this condition is aphakia, a condition that exists when the lens is removed from the eye as in cataract surgery.
When a lens is implanted in place of the cataract, the eye is restored to a more emmetropic state. Without such an implantation, thick, heavy spectacles are required to make up the difference.
- Myopia (myope)
Figure 3. Myopia (myope)
In myopia (commonly known as nearsightedness since the patient can see clearly at near but cannot see clearly at a distance), parallel light rays come to focus in front of the retina.
In this case the “focusing power” is too great for an eye of its given length (Figure 3).
The “focusing power” of the eye is determined mostly by the curvature of the cornea.
The cornea of the eye is the clear window through which the eye sees and through which others can see your blue eyes (or brown or what ever).
The greater the curvature of the cornea, the greater its focusing power.
An eye can be of normal length but have an increased degree of corneal curvature and be myopic.
An eye can also have a normal corneal curvature but be longer than normal and thus still be myopic.
In most cases, however, it is the combination of the corneal curvature and the length of the eye which causes myopia. This is less true in hyperopic eyes.
Astigmatic refractive conditions
- Simple Myopic Astigmatism
Figure 4a. Simple Myopic Astigmatism
In astigmatism, the curvature of the cornea (and, therefore, its “focusing power") is not the same in all directions (Figure 4a).
In a normal eye, the corneal curvature is like the inside of a bowl, that is, ball-shaped, or spherical.
In an eye with astigmatism, the corneal curvature is like the inside of a kitchen spoon, that is, it has a given curvature on one meridian while at right angles to this meridian it has a different curvature.
Astigmatism rarely exists by itself but rather is usually associated with hyperopia or myopia.
In simple myopic astigmatism, one focus is on the retina and the other is in front. (Figure 4a)
- Simple Hyperopic Astigmatism
Figure 4b. Simple Hyperopic Astigmatism
Note that one point of focus is on the retina and the other is behind the retina. These eyes are typically somewhat shorter than normal eyes. (Figure 4b)
- Compound Myopic Astigmatism
Figure 4c. Compound Myopic Astigmatism
Compound differs from simple myopic astigmatism in that both principle foci lie in front of the retina.
These eyes are often longer than normal. (Figure 4c).
- Compound Hyperopic Astigmatism
Figure 4d. Compound Hyperopic Astigmatism
In compound hyperopic astigmatism, both foci liebehind the retina. These eyes are often shorter than normal eyes. (Figure 4d)
- Mixed Astigmatism
Figure 4e. Mixed Astigmatism
When one principle focus lies in front of the retina and the other lies behind, the astigmatism is called - mixed. (Figure 4e).
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