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THE PREVENTION OF ACQUIRED MYOPIA

   In the normal eye, the parallel light rays which come from a distant object pass through the various parts of the eye and come to a focus on the retina. The rays are bent (refracted) as they pass through the cornea and the lens, with most of the refraction taking place at the cornea. If the rays come to a focus on the retina (at E) when the lens of the eye is relaxed, as it is assumed to be when an individual is attempting to view a distant object, the eye is said to be EMMETROPIC. If, under the same set of conditions, the parallel rays would not come to a focus unless they were projected behind the retina (at H), the eye is said to be farsighted or HYPEROPIC. If, under the same set of conditions, the parallel rays come to a focus in front of the retina (at M), the eye is said to be nearsighted or MYOPIC.

   Myopia develops in a small percentage of persons because of a congenital (present at birth) or a pathological (diseased) condition, but by far the most common type is called acquired myopia and is the topic of this booklet. This acquired myopia, which is also called simple myopia or school myopia, usually increases steadily starting in about the 5th or 6th grade of school. Statistics show that less than 5% of the students are myopic at this age but at the graduate level over 50% are myopic. Acquired myopia has always been considered by most people to be an inherited condition, but studies made over the past 20 years have shown that the actual cause is the stress associated with excessive close work such as reading.

1.

   The late Tikasi Sato of Yokohama, who spent almost his entire professional life attempting to determine why myopia suddenly became widespread throughout Japan after World War I, believed that it was due to the close work stress associated with the attempt to wipe out illiteracy in Japan during those years. Japanese children must learn several thousand characters to become literate in the language, and it requires a great deal of close work to learn to read and write with these characters. He felt that if the language could be modified to a 50 character alphabet it would do much to help prevent this myopia.

   Francis Young, Director of the Primate Research Center at Washington State University in Pullman, Washington, has recorded much important data over the past several years in his studies of the causes of myopia. By using a hood to physically restrict the vision of laboratory monkeys to a distance of less than 50 cm (20 inches), it was found that 75% of them became myopic within a three-month period (l) It has also been found that monkeys in captivity, and consequently placed in a visually restricted environment, show a higher percentage of myopia than monkeys living in a normal free environment. (2)

   Dr. Young also conducted an interesting study of Eskimos in Barrow, Alaska,(3) in which he substantiated the work of previous investigators who have found that primitive people who do not read or engage in substantial close work have virtually no myopia. The study was done on 508 volunteer family members with ages ranging from 6 to 88 years. Practically no myopia was found in the oldest generation—those over 50 years of age. In the second generation, from 26 to 50 years of age, more myopia was found among the younger persons-—less than 5% of the 41-to 50-year olds being myopic, 23% of the 31-to 40-year-olds, and 43% of the 26-to 30-year-olds. However, of the 21- to 25-year-olds, 88% were myopic. It is interesting to note that most of the older generation lived the typical early Eskimo life with no reading and much outdoor activity. However, the persons under 25 years of age had received compulsory schooling.

   Furthermore, most of the homes were equipped with only one 40 watt bulb per room. Because the winter months are almost without daylight, whatever reading the students did at home was done under this extremely low level of lighting. In the tests on monkeys mentioned earlier, it was found that monkeys kept under low levels of lighting progressed faster into myopia than the animals kept under higher levels.(4)

   Dr. Young concludes that it is difficult to see how such. a great increase in myopia could be accomplished by hereditary changes within only three generations.

2.

The visual environment must play a large role in this myopia development. He also concluded, "In our studies to date on both monkeys arid humans, it appears quite clear that myopia results from a continuous level of accommodation, and if one prevents this continuous level of accommodation from occurring, very little myopia, if any, should occur." (5)

   In order to understand what accommodation is and how myopia develops, it is first necessary to know something about the structure of the eye. Fig. 1 showed how parallel rays from a distant object are brought to a focus on the retina.  The lens shape must change in order to bend the rays more and bring them to a focus on the retina. This process is called accommodation and is believed to be accomplished in the following way: When the ciliary muscle is stimulated to contract it pulls the choroid forward. This releases the tension on the suspensory ligament which surrounds the lens and, after passing through the ciliary muscle, attaches to the choroid. The lens is made of soft material contained in an elastic capsule which molds its shape, and it bulges forward (becomes more convex or thicker) when the tension on the suspensory ligament is released. The back of the lens changes very little during this action since it is supported by the solid vitreous body which presses against it due to the tightening of the choroid around the vitreous.(6) The lens in this accommodated state causes the rays to be refracted more so as to focus on the retina. This is the automatic focusing process which is initiated in the normal eye by the blurring which occurs when the point of fixation is moved closer to the eye.

   Recent research indicates that when accommodation is maintained for long periods of time (as when reading or doing any other kind of close work), a chronic spasm of the ciliary muscle can occur so that the lens will no longer fully relax. Then when a distant object is looked at, the parallel rays are bent too much and come to a focus in front of the retina. The resulting blurred vision is the first symptom of myopia. If this spasm is permitted to exist for too long a period of time, a pressure increase occurs in the vitreous chamber, causing the coats of the eye to stretch. This results in an overall enlargement of the vitreous chamber as additional fluid moves into the vitreous chamber to fill the increased volume. It is believed that the greater the amount of accommodation and convergence (the turning-in movement of the eyes) exerted, the greater the pressure. (7) Tests on monkeys indicate that this stretching will begin to occur if the spasm is permitted to exist for 2 to 4 months.(8) In other words, the eye becomes longer and the parallel rays come to a focus even farther in front of the retina. At this stage, the person has both a spastic myopia and an axial length myopia. Techniques are now available to prevent or eliminate the ciliary spasm, but there is no evidence to suggest that stretching of the eye, once it has occurred, can be reversed. For this reason, it is very important to begin treatment in the early stages of myopia, if normal vision is to be regained.

   The mechanism of myopia development described above need not be considered as something abnormal. It is felt by some researchers that this is the normal adaption which the eye attempts to make so that it can do the constant close work without having to make the effort of accommodation. According to this view, this same mechanism is at work in reducing the farsightedness of the normal juvenile eye (which must accommodate even for distance vision) to an emmetropic condition. In fact, it is believed that an eye which remains farsighted and does not elongate in this manner to reach emmetropia is for some reason not developing properly. (9)

    It should be obvious that when the myopia has progressed to the point where the limit of clear vision is no farther away than the pages of a book held at normal reading distance, the reading of the book can be done without accommodation. In theory, the spasm would then relax and the progression or the myopia would cease at this point. However, if the patient is fitted with the usual concave distance glasses, and these glasses are used for reading, the book is in effect moved closer to the eyes and the entire process is repeated because these glasses make it again necessary for the eyes to accommodate for the close work. The continuous accommodation results in a further stretching of the eye and deteriorating vision with the need for stronger and stronger glasses.

   Excessive stretching of the coats of the eye from this or other causes can lead to blindness through detachment of the retina or through other degenerative changes in the retina. One of the large organizations for the blind estimates that 1500 persons in the United States alone lose their sight annually due to the complications associated with high myopia.

   The eyes of the normal pre-school child are somewhat farsighted and it is normal for this farsightedness to diminish as the child gets older (probably by means of the stretching mechanism described above) until a point is reached where the child is neither farsighted nor nearsighted. However, if examinations indicate that the child is passing or has already passed through this point into a myopic condition, preventive measures must be undertaken. Although the various writers and researchers may offer somewhat different theoretical explanations about the mechanism of development of this environmentally caused myopia, they all recommend the same general course of action to prevent, control, or eliminate it — reduce the stress of close work. The simplest preventive methods will be described first. If the simple methods are not sufficient to control the myopia, the more strenuous methods can be used.

1) PROPER CLOSE WORK HABITS These methods are all directed toward reducing the stress to accommodation and convergence in order to prevent a spasm from occurring, without requiring the patient to wear prevention aids.

A) Reading Distance Hold the book or other material as far from the eyes as is comfortably possible. The distance from the elbow to the knuckles of the hand has been suggested as the minimum working distance. Look up and into the distance momentarily at the end of each paragraph or at least at the end of each page to relax the eyes. Ideally, the chair should be placed to enable looking out a window or across the room when looking up.

B) Posture Reading should not be permitted while lying on the floor or lying in bed. This places the eyes too close to the reading material. The elbows should not be rested on the desk or table when reading or writing since this creates a posture which usually causes the head to bend forward too close to the work.

C) Lighting Maintain proper illumination on all close work Most people tend to use too little illumination indoors. It is most unusual to find indoor illumination which even begins to approach daylight. In addition to eliminating the need to hold the book close, increased lighting causes the pupil (the opening into the lens of the eye) to become smaller so that the outer portion or periphery of the lens is not used. This reduces the amount of accommodation required. The indiscriminate use of sunglasses, particularly for close work indoors, should also be avoided for the same reason.

D) Illness When confined to bed, it is common for the sick person to be provided with books, games, etc., to relieve the boredom of confinement. In addition, illumination in the sickroom is often poor This environment, added to the body's weakened condition, can contribute to a rapid development of myopia. This is one time when watching television might be encouraged in preference to reading.

2) CONVEX OR "PLUS" LENSES If the myopia cannot be controlled by the teaching of proper close work habits, eyeglasses can be used to reduce or eliminate the accommodation. There are several variations of this technique.

A) Reading Glasses These are glasses with convex lenses which are used only for reading. By making the light rays less diverging, they reduce the amount of accommodation required and lessen the likelihood of a spasm developing. If strong enough lenses are selected, the book can actually be made to appear slightly blurred when held at the normal reading distance, indicating that all the accommodation has been eliminated. This blurred image (fogging) technique is especially useful in relaxing a spasm which is already present. The reason for this is that the ciliary muscle contains two separate sets of fibers. Some researchers believe that the circular fibers are used to tighten the muscle and increase the accommodation while the meridional fibers help in decreasing the accommodation. (10) As the eyes attempt to see the blurred image clearly, the meridional fibers are brought into play, introducing an active force to relax the spasm.

   One drawback of reading glasses is due to the fact that there is a strong linkage in the human visual system between accommodation and convergence which normally work together and stimulate each other. Consequently, the convergence of the eyes on the book tends to stimulate the accommodation, even though the reading glasses have been chosen to eliminate the accommodation. This upsetting of the normal accommodation-convergence relationship can cause fatigue, double vision, or other problems in some people. For this reason, the book should be held as far as possible from the eyes to reduce the amount of convergence needed. This problem can also be partially overcome by prescribing a prismatic component (base-in) in the reading glasses to reduce the amount of convergence required. However, using prisms to totally eliminate convergence at the usual reading distance can introduce other problems such as distortion, due to the thickness and shape of the lenses, and is not normally done. Reading glasses can also be made in a half-eye form in which it is possible to look over the top of the lenses for distance vision rather than having to remove the glasses.

B) Bifocals

    Reading glasses, described above, can be provided in a bifocal form, with the upper segment used for distance and the lower segment used for reading. The lower segment must not be made so small that the patient can avoid it by looking over it. The statements made above regarding the accommodation-convergence relationship also apply here when looking through the lower segment.

C) TV Glasses

    These are similar to reading glasses except that the lenses are selected to make the television screen appear slightly blurred when sitting a specified distance away. This aids in relaxing the spasm. Since the patient should not sit too close to the screen, little convergence will be required and the accommodation-convergence relationship is not upset to any appreciable extent.

3) THE MYOPTER* VIEWER

    One researcher has developed a viewing instrument which he claims provides a more strenuous technique for controlling and improving myopia than the above methods and it is recommended especially if the simpler techniques prove inadequate in preventing myopia. The Myopter viewer consists of a plastic housing containing lenses and mirrors, and is worn instead of glasses for close work. Light coming from the viewed object enters the single opening at the front and is split into two identical parts which are reflected into the eyes parallel so that no convergence is required. The stereoscopic effect which gives us a three-dimensional image of close objects is also eliminated. The accommodation is eliminated by selecting the proper Myopter lenses for the distance usually used for reading. By eliminating the three factors of accommodation, convergence and stereoscopic effect, all the conditions for distance vision are fulfilled. Even though the eyes are being used for reading, they are in a completely natural and relaxed condition as if viewing a far distant object. When the lenses are selected to make use of the blurred image technique, the maximum possible relaxing force is applied to the ciliary muscles.

   It has been found that about one half hour's use of the instrument in the evening is often sufficient to counteract the harmful effects of the day's schoolwork, and even bring about a permanent improvement in the vision, particularly in children. However, for maximum benefit it should be used for as much close work as possible, including classroom use. one drawback of the instrument is its rather limited field of view. It is adequate, however, for reading a book without needing to turn the head from side to side.

 The individual eye practitioner may prefer to use one, several, or all of these techniques to control myopia. However, the IMPA strongly discourages the use of concave lenses to correct myopic vision while the real cause of the problem is left untouched. If the cause is not removed, the vision will continue to deteriorate and the concave lenses will hasten the deterioration. Success in preventing myopia also depends a great deal on how well parents make sure their children follow the doctor's instructions. In conjunction with these preventive methods, certain practitioners may use vision training techniques or exercises to improve the stressed accommodation-convergence relationship which often develops in persons who do much close work, whether they have actually become myopic or not. If myopia can be prevented during the school years, there is good reason to believe that no appreciable amount of myopia will ever develop, and the preventive measures can then probably be relaxed or eliminated entirely. The reason is that in adulthood the eyes seem to become more resistant to the effects of excessive accommodation. On the other hand, it also seems to be more difficult to obtain vision improvement in myopic adults than in myopic children, although the above methods can still be used effectively to prevent these older, established myopes from progressing further into myopia. The proper time to deal with myopia, however, is when it starts.


REFERENCES

1) Young, Francis A., The Development of Myopia, Contacto, June, 1971

2) Young, Francis A., Visual Refractive Errors of Wild and Laboratory Monkeys, EENT Digest, August, 1965

3) Young, Francis A., et al., The Transmission of Refractive Errors Within Eskimo Families, American Journal of Optometry and Archives of the American Academy of Optometry, 46(9), 1969

4) Young, Francis A., The Effect of Nearwork Illumination Level on Monkey Refraction, American Journal of Optometry and Archives of the American Academy of Optometry, 39(2), 1962

5) Young, Francis A., The Development of Myopia, Contacto, June, 1971

6) Coleman, D. Jackson, Unified Model for Accommodative Mechanism, American Journal of Ophthalmology, June, 1970

7) Young, Francis A., Unpublished Study

8) Young, Francis A., The Development of Myopia, Contacto, June, 1971

9) Rehm, Donald S.. The Myopter Viewer, An Instrument for Treating and Preventing Myopia, American Journal of Optometry and Physiological Optics, May, 1975

10) Duke-Elder, Stewart, The Practice of Refraction, J & A Churchill, Ltd., London, 1969

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