Myopia is also called nearsightedness or shortsightedness. In simple terms, a person with myopia can not see clearly in the distance but they can see objects clearly that are nearby or a short distance away. Thus the condition is also called near-sight (nearsightedness) or short-sight (shortsightedness). For comparison, an auto-focus camera focuses its image by moving lenses back and forth. Eyes don’t change focus by similarly bulging in and out of the head, but they do change focus by changing the shape of a lens that sits behind the round, black pupil inside the eye. It happens automatically, very quickly and with no sensation of eye movement. The muscle that controls this focus has only two actions: it can constrict (to focus on objects nearby) or it can relax (to focus on objects further away). There is no opposing focus muscle to enhance the relaxation, unlike most muscle groups in your body. For example, to move one's arm downward, one set of muscles constricts while the other relaxes. The muscles change roles to move the arm upward, the constricting muscles now relaxing and the relaxing muscles now constricting. The eye, however, does not have both a "focus" and an "unfocus" set of muscles. The focus muscle can only relax and you depend on its ability to totally relax and stretch out to it full size to obtain clear distance vision. If the focus muscle relaxes all the way and the sharp focus point does not reach to the back of the eye (the eye is too long), distance objects will still be blurred. The muscle just can’t lengthen any more after it relaxes. The eye will be blurred for distance objects and the person has myopia. The image above depicts how an eye is usually shown to be "in-focus". This eye is in focus centrally and is considered to have a normal sharp focus, a condition called emmetropia. Note that the peripheral vision is also commonly shown as being in focus although this is not strictly part of the definition of emmetropia. The image above shows an eye that is out of focus because the light is brought to a sharp focus before it reaches the back of the eye. In effect, the eye is too long. As is commonly drawn, the peripheral focus is also shown as too short for the eye's length. This condition is called myopia, nearsightedness or shortsightedness.
Hyperopia is generally considered the opposite of myopia in that light in a relaxed focus hyperopic eye is focused behind the back of the eye. The eye can compensate for small amounts of hyperopia by activating the focus muscles to change the shape of the eye's internal lens, in effect an auto-focus mechanism, making the eye's focus stronger and making the eye focus internally within a shorter distance. This brings the formerly out of focus image back into focus but it requires a constant constriction or activation of the focusing muscle to maintain focus. This compensation for blurred distance vision is not available to the myopic eye because the myopic eye is already focused at too short a distance when the eye is relaxed and it can't relax any further to focus internally within a longer distance. As a person ages, they lose the ability to auto-focus their eyes due to the lens slowly hardening, considered a normal aging process of loss of elasticity in tissues. Thus the young low level hyperope may never notice any focus problems until they get older. The image above shows how an eye is usually shown to be hyperopic. The central focus is behind the eye and thus out of focus. Note that the peripheral vision is also usually depicted as focusing behind the eye although, just as in myopia, the peripheral focus does not have to match the central focus to meet the strict definition of hyperopia.
Peripheral hyperopia, or peripheral hyperopic defocus, is the condition where peripheral vision is focused behind the back of the eye's light receptors located in the retina, regardless of where central focus is located. Thus central focus might be myopic (focused in front of the retina), hyperopic (behind the retina) or emmetropic (in focus on the retina). Generally one is not aware of whether their peripheral focus is in focus or not. Because there are fewer light receptors, the acuity or sharpness of vision is less in the periphery regardless of where the focus is located. Peripheral vision in this discussion is actually very close in position to central vision, or anything beyond about 15 degrees of central vision. A rough approximation of 15 degrees is to hold your hand out at arm's length with your fingers spread. Your fingers of one hand cover roughly 15 degrees. If you use two hands and put your thumbs together, anything outside a circle drawn around your little fingers is peripheral vision. The image above shows an emmetropic eye (the central vision is in focus) and the peripheral vision shows hyperopia. One should remember that this is an eye with essentially no visual complaints. The person may even have better than 20/20 (or 6/6 if metric) vision. They see well, they are happy and they pass their vision exam acuity test for sharpness of vision with flying colors. If you compare this eye shape to the previous eye with central hyperopia, it is obvious that the eye is more "pointed" or less round. This shape is called a prolate and becomes important in discussions of what causes myopia.
Peripheral myopia is the condition where the peripheral vision is focused in front of the eye's light receptors located in the retina, regardless of where central focus is located. Thus central focus might be myopic (focused in front of the retina), hyperopic (behind the retina) or emmetropic (in focus on the retina). Just as in peripheral hyperopia, generally one is not aware of whether their peripheral focus is in focus or not. Just as in peripheral hyperopia, if central vision is in focus the person will have essentially no visual complaints and will have normal, sharply focused vision. The image above shows an emmetropic eye (the central vision is in focus) and the peripheral vision shows myopia. This eye is more rounded than the prolate shape of the peripheral hyperopia eye.
Peripheral myopia is thought to be a significant part of the mechanism that controls the process of emmetropization, which is how the eye determines to stop its growth and myopic progression. An eye with peripheral myopia will generally not become more myopic so that the artificial creation of peripheral myopia by use of optical devices is important in several myopia prevention techniques.
Emmetropia is the normal focus condition of the eye, neither nearsighted (shortsighted, myopia) or farsighted (hyperopia). A person with emmetropia has clear distant vision with a relaxed eye focusing muscle.
Emmetropization (em-ma-trope-ah-za-shin) is the process that a maturing eye goes through during growth and development to create this clear focus out of all the maturing components that focus light within the eye, including the front surface or cornea, the lens and the distance between the front and back of the eye. During the growth phase as a child matures, the eye must coordinate all the optical parts to achieve a sharp focus. If the front surface of the eye is a bit flatter, the eye may need to be a bit longer; if the internal lens is stronger, the eye must be shorter or the front surface must be flatter, etc. Many different parts must coordinate to obtain clear vision. The process does not work the same for every individual because not everyone ends up with clear distance vision.
Emmetropization is not strictly a grow/no-grow signal for the eye but rather is a modifcation of the normal growth rate present during maturation.
The study of myopic development is largely the study of emmetropization.
Ortho-K (short for orthokeratology) is a process of gently reshaping the front surface of the eye to give clear comfortable vision. It is a non-surgical procedure using specially designed customized contact lenses that are worn only at night and removed when awakening. The result is clear, corrected vision all day with no daytime lens wear and no glasses. The lenses are re-applied each night as the procedure is not permanent, making it safer for children. When the lenses are removed the eye slowly returns to its former shape and vision, normally taking several days or weeks to return to pre-fitting parameters. Ortho-K goes by several different names depending on the manufacturer of the lenses and the prescribing doctor. Some of these names include: Ortho-K, OrthoK, Corneal Refractive Therapy, CRT, Vision Shaping Treatment, VST, Gentle Shaping System, The Gentle Vision Shaping System, Corneal Molding (CM), Advanced Orthokeratology, Custom Accelerated Orthokeratology, Corneal Reshaping, Wave Front Corneal Molding and Gentle Molding. They are all based on the principles of orthokeratology that describe how a lens must be designed to modify an eye's prescription when the lens is removed. Ortho-K is an excellent method for slowing myopic progression. Read about this aspect in the Treatments menu. You can learn more about Ortho-K itself and find doctors who fit the lenses at OrthoKDoctors.com. The image above depicts an ortho-k lens molding the eye. The red lens is only worn at night, but when it is removed, the front surface of the eye is flatter. This illustration is exaggerated - the effect does not change the appearance of the eye. The molding creates a flattened area centrally that decreases or corrects the central myopia. It also creates a steeper ring of increased power that does the opposite: it creates a ring of increased myopia. This is shown by the following illustration that shows a prolate shaped eye that has the peripheral areas brought back into focus. The red spots on the front corneal surface represent these areas of steeper (stronger) curves. This illustration is a cross section and only shows two spots of increased power, but in reality the area of increased power is a ring of peripheral increased power that surrounds the central circle of decreased power. An instrument called a topographer allows a doctor to see these shape changes on individual corneas. It functions somewhat like a 3-D camera, converting the different curves on the eye's surface into different colors so that the shapes can be seen. The picture above is a mapping from a topographer that shows these shape changes. The picture on the left is a normal eye before the ortho-k procedure. The central yellow colors blending to the peripheral blue represent steeper central curves and flatter peripheral curves. That is normal for eyes - they are relatively more myopic (stronger curves) centrally and more hyperopic (flatter curves) peripherally. The picture on the right is taken after the molding ortho-k lens is removed. The central area is now flatter, represented by a darker green color. This corrects the person's central myopia. The most striking feature is the red ring. The red color represents a steeper curve that was shown by the two red spots of power in the cross section of the previous illustration. Here the entire ring is visible. Just as this creates a ring of increased power on the cornea, the result is a ring of increased myopia (less hyperopia) at the back of the eye.