The Case for the Preventability of Myopia

by Alexander Eulenberg, Bloomington, Indiana

March 3, 1996

Thanks to the Indiana University Optometry library and Professor David Goss, for assisting me in finding various information pertaining to the subject. The author is of course wholly responsible for the content of this paper. --AE


Persons with myopia, or nearsightedness -- those whose eyes are unable to see distant objects clearly -- are usually prescribed concave lenses to compensate for their defect and allow them to see clearly. Although such lenses are incapable of relieving the underlying condition, the present treatment is felt to be justified because it relieves the most obvious symptom. In spite of this "treatment" however, the condition usually worsens for a number of years, resulting in poorer and poorer vision without glasses, and the necessity for stronger and stronger lenses to provide normal vision. As far as the public is told, the only hope for myopes is the possibility of slowing its progression through special bifocals or contact lenses, or to cure it through surgery. Never is there mention of the possibility that myopia can be prevented or cured by any means short of surgery. In this paper I will review the history of research on the nature and proposed treatments of myopia, with particular attention given to the much-neglected case for the preventability and, to some extent, the reversibility of myopia.

The Nature and Prevalence of Myopia

Myopia may be defined as the state where an eye's refraction is too strong to converge rays from points from ditant objects to a point on the retina. That is, distant objects always end up focused on some point in front of the retina, causing an indistinct image. Myopia is often compounded with astigmatism, which means that the error of refraction is increased or decreased along one meridian of the refracting surface, usually the cornea, but sometimes also the lens.

Newborns vary greatly as to how far they are able to focus (Cook & Glassock 1951), but by the age of five years, the overwhelming majority of children have developed normal or slightly farsighted refraction (Kempf et al. 1928, Mohindra & Held 1981). It is during childhood and adolescence that most myopia begins to appear. In the United States, it is estimated that between 15 and 25 percent of the adult population is myopic, most of whom had normal refraction before the age of seven years (National Research Council 1979:3-4). Among academic populations around the world, including military academies, the proportion of myopes often exceeds 50%. (p. 20)

It is generally recognized that myopia can be functional as well as structural.

Functional myopia is an inability to see at the distance stemming from a dysfunction in the muscles (ciliary) that regulate the medium of accommodation (crystalline lens). In other words, the eye is in a contracted state, perpetually focusing for near objects. This type of myopia is cured if the ciliary muscle can be made to relax long enough for the lens to regain its previous state. It has been the practice to call the type of myopia due to an overcontracted focusing muscle "pseudomyopia."

As for structural myopia, it has long been observed that myopic eyes measure on average longer than normal eyes, and thus the cause of myopia is generally agreed to be "long eyes"; the rays from distant objects being prevented from converging on the retina because the retina is too far back. A indirect measure of the lengthening of the eye in myopia can be made by reference to the evidence of expansion at the back of the eye, which is visible with an ophthalmoscope. Other evidence of a lengthened eyeball is visible to the careful observer. In an early textbook (Wells 1883), it is noted that in myopia of considerable degree, "the increase in the length of the eyeball ... can be easily recognized when the eye is turned far inwards towards the nose." Sato (1957) writes that the connection between long eyes and myopia was made as early as 1632 by examiniation of a surgically removed eye, and that the relation between long-eyedness and myopia was, according to the German ophthalmologist Carl von Hess, well-known to doctors in the 18th century. By the end of the 19th century, the idea that myopia is caused by an increase in the length of the eye had been established as fact. J. S. Wells writes in his textbook (1883) "The most frequent cause of myopia is an abnormal increase in the length of the eyeball in its antero-posterior axis."

However, there have been opponents to the idea that structural myopia is acquired by an increase in the length of the eyeball. Tikasi Sato (1957) made a valiant attempt to prove that the association between lengthened eyes and myopia was not at all causal, and that advances in myopia were entirely due to adaptive organic changes in the lens and ciliary muscles, producing a too-powerful lens. At the time, the only data on axial length was from corpses and lensless patients, as ultrasound measurement techniques were not available to him. Based on a statistical analysis of his cross-sectional data, he concluded that acquired myopia could be explained entirely as an increase in lens power. However, more recent longitudinal studies using ultrasound have confirmed (e.g. Tokoro & Suzuki 1968, Goss et al. 1990, Grosvenor & Scott 1993, Zadnik 1994) that of all the ocular components of refraction, an increase in axial length is most responsible for increases in myopic refraction, for children as well as young adults, regardless of degree of myopia.

For theoretical purposes, it is convenient to talk of "true axial-length" versus "functional" myopia, but in practice, there is no easy way to distinguish the two types. True, atropine and other chemicals known as cycloplegics have been used to paralyze the ciliary muscle and thus eliminate the "pseudomyopia" effect, but as optometrists have often pointed out, paralysis of the ciliary does not completely relax the muscle, and in fact, sometimes "after the use of a strong solution of atropine, minus lenses were prescribed by the oculist, when upon reexamination without a mydriatic it was found that plus [farsighted] lenses were required" (Raphaelson 1911). Optometrist Eugene Heard notes (1914) "an increasing number of cases coming to us, having failed to get relief from oculists who had put them under these various cycloplegics, the cramp not being revealed by their method, as it was not relaxed, but locked up." And while cycloplegia is very likely to reveal latent hypermetropia, in nonmyopes, more recent studies (Rengstorff 1966, Ludlam et al. 1972) have confirmed that the effect of a cycloplegic on manifestly myopic eyes, especially in a clinical setting, is often unpredictable, and that refractions done on drugged eyes are as likely to reveal more myopia as less, compared to the refraction performed without cycloplegia.

Cause of Functional Myopia

Close work theory

Functional myopia is usually held to be caused by excessive close use of the eyes. It is supposed by many to be only temporary and not to require treatment. Early investigators, however, observed a tendency for functional myopia to become more or less permanent. Chalmers Prentice, a physician writing in 1895, explained how excessive near work leads to the inability to see in the distance:
All evidence bears out the fact that myopes generally are people of a higher civilization, who exercise their eyes at the near point, and thus establish an abnormal impulse in the ciliary centers during such use of the eyes. The stimulus has been so constant for a long period that the impulse to the ciliary is unable to suspend itself and bring about distant vision again. The motive-impulse keeps coming. The ciliary will not relax from its contracted condition, the refraction remains high, and the patient near-sighted.

Optometrist O.J. Melvin observed:

Usually [patients] express [the onset of nearsightedness] in these terms: "After doing a lot of close-work, I find that I have to focus my eyes for a little while before I can see clearly at a distance." These patients report that this period of "re-focussing" took longer and longer until finally they saw dimly at a distance all the time. (Melvin 1936)

That over-use of the eyes at a very close range can bring on functional myopia is hardly questioned today, although modern articles tend to downplay its significance. Ciuffreda & Ordonez (1995) characterize the myopia that follows from sustained near work as something that occurs in "some patients" that lasts "several seconds or even minutes".

Mental strain theory

A different opinion on the cause of functional myopia is given by William Bates (1912). First of all, he proposed that functional myopia was produced not as an after-effect of having viewed near objects, but by "an effort, usually unconscious, to see distant objects" which became habit. Using the retinoscope, Bates observed people with normal vision as they looked at "new, strange, or unfamiliar" objects, such as an unfamiliar eye chart, map, or strange handwriting on the chalkboard. He found that whenever an unsuccessful effort was made to recognize these objects (as evidenced by squinting, frowning and other contortions of the eye and face) various types of myopic refraction were produced: "myopic astigmatism, usually--compound myopic astigmatism, occasionally, or simple myopia infrequently." This temporary myopia or myopic astigmatism could be avoided, Bates found, if the person relaxed while viewing distant objects. Another difference in Bates's understanding of functional myopia was that he defined it as being produced by muscular action -- any muscular action -- not just the commonly accepted regular focusing action of the ciliary muscle on the lens. He observed functional myopia in "cases in which the accommodation is apparently paralyzed by atropine, and in aphakia after cataract extraction."

Modern support for Bates's mental strain theory of the production of functional myopia comes, among others, from the research of Bullimore & Gilmartin (1987). They have measured the resting state of accommodation before and after tasks requiring various levels of "cognitive demand" such as solving arithmetic problems. They found that tasks involving relatively high levels of cognitive demand, independent of optical stimulus to focus for near, will cause the resting state of the focusing system to move towards near-focusing.

Very little research has been done to test the idea that myopic astigmatism could be produced by the action of the extraocular muscles. However, optometrist J.W. Parker (1931) reported that he was able to relieve many cases of astigmatism through extraocular muscle therapy (he did not specify what kind), and Fairmaid (1959) reported that significant corneal changes occur whenever the eyes are pulled together to converge.

Compromise theory?

It should be noted that the two proposed causes of functional myopia -- protracted near work and a habitual strain produced when looking at distant objects -- are not incompatible. In fact, Bates himself hints at a connection in an early paper (1911):
Why did children strain their eyes when looking at distant objects? They strained because their experience had taught them that to accomplish most things an effort was required. They learned that they saw near objects more distinctly by making a voluntary effort. Naturally, most of them strained, when looking at distant objects, to improve their sight.

Cause of Structural Myopia

What causes myopia of the structural, supposedly incurable variety? Thoughout history, there have been three main explanations: the heredity theory, the close-work theory, and the nutrition theory.

Heredity theory

First, the heredity theory. This is the theory most widely accepted today. As mentioned previously, most children are slightly farsighted by age five or six; that is, they have more than enough capacity to adjust their eyes for focus on distant objects. But many of them lose at least some of their hyperopia as they grow. During this time, the back part of the eyeball lengthens. Thus, it is easy to conceive of myopia as simply an overshooting of the target of growth for the eye, a process determined completely by genetic factors. A modern textbook on refraction advises:
It is wise to explain to the parents that the glasses will have to be made stronger as the child grows bigger, because the eye also grows, and this progression of the myopia should not be a cause of undue concern. [Elkington & Frank 1991:181]

In other words, the commonly accepted and propagated opinion nowadays is that structural myopia is nothing more than the genetically preprogrammed overgrowth of the eye.

The hereditary explanation, the classic exposition of which was presented by Adolf Steiger in 1913, is based largely on the observation that there seem to be myopic and non-myopic "peoples", the ones with the longer litererary traditions, such as the Chinese and Jews, having a greater percent of their populations being myopic than others. (Modern statistical support for this age-old observation is provided by National Research Council 1989). Steiger explains the correlation thus:

In the circles of scholars, etc., who are engaged in near work, myopia is not so much an obstacle to their profession. Consequently, myopic people tend to collect in near-work circles. When this state of things continues for many generations, by a natural selection, myopic people increase in near-work circles, which results in a special increase in the predisposition for myopia in near-work circles. (Quoted in Sato 1957, pp. 29-30)

Since Steiger, genetics, or "relaxed natural selection" (Post 1962) has been held to be responsible for myopia in populations where near work is highly valuable. In primarily illiterate populations, such as Africans in Gabon (Holm 1937), or full-blooded East Greenland Eskimoes (Skeller 1954), myopia was found to be practically nonexistent. The explanation could easily be found in the supposed survival value of good distant vision in these cultures.

Modern studies have tried to isolate genetic factors, and for the most part, the authors conclude that there is a significant genetic determining factor for myopia. Teikari et al. (1991) did a study on myopia in fraternal versus identical twins aged 30-31, and found that doctors' myopic spectacle prescriptions for identical twins differ between the twins less than they do between fraternal twins. Zadnik et al. (1994), controlling for grade in school and amount of "diopter-hours" (hours watching TV + 2 x hours playing video games + 3 x hours reading) found that the eyes of nonmyopic children (ages 6-14) with two myopic parents tend to have less hyperopia (ie. are further along the way to myopia) than those with one or no myopic parents.

One problem with these studies, however, is that they are incapable of determining whether the correlations in amount of myopia are directly due to a shared genetic defect, or due to "myopia-risk-increasing" habits encouraged by myopic parents or lookalike siblings. [note] In any event, that there often are significant differences between twins' prescriptions (Teikari et al 1991) does show that genetics does not determine everything.

Several studies have shown that for at least one population, the natives of the Arctic region, myopia does not come from the parents. Francis Young et al. (1969) made a study in Barrow, Alaska, of 283 Eskimo children (6 to 25 years) and 225 adults (26 to 88 years). The sample contained family groups of parents and grandparents. The children had all gone to school, while the parents had lived a traditional Eskimo life. While less than 2% of the parents were myopic, approximately 58% of the children were myopic, and the severity of the myopia increased with number of years in school. There was no significant correlation between the parents' refraction and the refraction of the children. Since the percentage of myopes is very high, and the population sample had been all volunteers, it seemed possible that "in the younger volunteers, at any rate, there was a strong element of self-selection: they came because they had visual trouble." (Sorsby 1970). To rule out this possibility, the refractive status all children at the local school -- from age 5 to 16 -- was tested. "The results obtained on the remainder of the school population agree with the results obtained on the volunteer subjects." (Young 1970). In addition to Young's experiment, other independent surveys have been done, confirming beyond a doubt that throughout the North American Arctic, the children of the natives, brought up according to the ways of the white man, are far more likely to develop myopia than their parents were (Cass 1966; Morgan et al. 1973; Alsbirk 1979).

To summarize then, the hereditary theory, while plausible and very widely accepted, accounts for only very general tendencies, and is insufficient to explain the variations that do occur in people born with the same genetic material.

Close work theory

That close workers tend to be myopic, while the opposite is true of those who use their eyes to see things at great distances, is simply common wisdom. Tscherning (1883) did a survey of vision among people of various occupations. Confirming the widely-observed correlation, he found that the occupations that demanded more close work had a higher percentage of myopes than did the occupations that demanded less close work. Today, that a large proportion of cadets enter military academies with perfect vision but become nearsighted after four years of study, is a fact sadly acknowledged by military researchers (National Research Foundataion 1979:21-22). It is only common sense to conclude that too much near vision causes the loss of far vision. An early proponent of this theory was Dr. Albrecht Haller, who, in his Primae Linae Physiologiae (1758, S531), wrote that myopia was caused by an excessive amount of working with small objects at a close range. Writes David Hosack, MD, in the Transactions of the Royal Society of London (1794), arguing that such effects are due to the fact that the extraocular muscles change the shape of the eye when it adjusts to different distances:
Again, does not the habit of long sight so remarkable in sailors and sportsmen, who are as much accustomed to view objects at a great distance, and that of short sight, as of watchmakers, seal-cutters, &c. admit of an easy solution on this principle?

Before Helmholtz's (1855) theory of accommodation took hold (that only the inside of the eye -- lens and ciliary muscles -- change during accommodation), no further explanation was thought necessary. The myopic eye had simply twisted itself more or less permanently into a strong focus. Later, those who adhered to the close-work theory, Helmholtz's (1855) theory of accommodation, and the idea that myopia is caused by an elongation of the eyeball, needed to come up with a more complex explanation.

J. Soelberg Wells (1886) identified congestion of the inner tunics of the eyeball as the main cause of myopia. This could be brought about by "continuous use of the eyes at near objects," wherein

the near approach of the object necessitates a strong convergence of the visual lines, which causes an accumulation of blood in, and congestion of, the inner tunics of the eyeball, these conditions being increased still more by the stooping position generally indulged in during such employment. We can easily understand that this congestion and augmentation in the pressure of the ocular fluids must, if long continued, necessarily lead to an extension of the tunics at the posterior pole.

In 1895, Chalmers Prentice writes that the exccessive innervation used in close work for accommodating and convergence, takes its toll on the tissues of the eye.

Thus myopia is the result of a nervous disturbance which causes an associated disarrangement in the impulses of assimilation in various parts of the eye, especially in the sclerotic coat. This disturbance in the nutrition of the sclerotic and other parts, tending to soften them and lessen their mechanical support, together with the pressure of the rectus muscles, probably brings about that elongation of the eye that we find in axial myopia.

S.D. Risley (1897), an ophthalmologist who did an extensive review of schoolchildren in Philadelphia, found evidence that the development of even low degrees of myopia is a result of injury caused by the strain of near work, as opposed to normal growth (emphasis Risley's):

A careful study reveals, during the early history of these eyes, a more or less tonic cramp in the accommodation, injected external tunics, and a great hyperaemia of the optic nerve, retina, and choroid. The subjective symptoms, together with the intra-ocular hyperaemia, subside under rest, but recur when work is resumed. If the refraction is myopic, the degree of myopia steadily advances, and is attended with certain intra-ocular changes of an unquestionably pathological character, which also steadily advance with the increasing refraction. If the eyes are hypermetropic, they have been observed, in a large group of cases to be hereafter noted, to increase their refraction; but in each instance the increase was attended with advancing pathological conditions of the intra-ocular membranes of the same nature as those observed in the myopic eyes. (p. 359)
This quotation is of special interest, since it is often alleged nowadays that pathological changes are characteristic only of a special class of very high "malignant" myopia (National Research Council 1989:90). Also, Zadnik et al. (1994:1326) say that their study showed that eyes that were assumed to be genetically destined to become myopic during school become longer "before there is any evidence of myopia" -- but it is clear that "evidence of myopia" in their survey only means having a refractive error of more than -0.75 D (those with a myopic refractive error less than 0.75 D were classed as non-myopes!). Zadnik et al. did not include an examination of the tissues of the growing eyes.

Optometrist Francis King wrote (1912:395) that the ill and the young are particularly prone to developing myopia at the strain of close work:

Personally, I have met with a considerable number of cases unquestionably due to measles or scarlet fever--doubtless the affection developed because of too great an application in study, reading or close work about the time of convalescence. Hyperemia of the ocular tissues followed. A long resisting power was inevitable. The choroid and sclera at the posterior pole were stretched, and so the eyeball was elongated. The same prolonged and continuous study by precocious children, whose tissues are not yet hardened or mature, would bring about similar results.

Further evidence that myopia is not overgrowth of the eye but injury, comes from the records of W. F. Norris and Risley (emphasis Risley's),

Adding [my] seventeen cases to the eleven observed by Norris, we have twenty-eight examples in which the change of refraction was observed through a series of years, and the static refraction of each eye repeatedly demonstrated by the rigid employment of a strong mydriatic for many days. Since the publication of these cases I have seen a much larger number presenting similar histories. My own cases, without exception, passed from the hypermetropic ball over to near-sight through the turnstile of astigmatism. In all of them the obseved changes of refraction were attended with pain and symptoms of external irritation,--e.g., blepharitis, conjunctival hyperaemia, increased lacrymation, undue sensitiveness to light, etc.,--while the ophthalmoscope revealed advancing pathological changes in the choroid. In no instance did the eyes, in passing from hypermetropia into myopia, become emmetropic at any stage of their progress. (p. 363)

Risley was of the opinion that an inherent tendency toward astigmatism was what caused the increase in refraction in those forced to do near work. It is the failing attempt to relieve astigmatism through accommodation that causes myopia, according to Risley:

The struggle to improve the sharpness of vision by accommodative effort causes undue strain upon each eye and disturbs also the the proper relation between accommodation and the binocular balance. The irritation and hyperaemia caused by these anomalous conditions, sooner or later, in a large number of individuals, set up the pathological states which lie at the foundation of progressive near-sight.... I am of the opinion that the congenital anomalies in the form of the eyeball are hereditary rather than the myopia itself or any tendency to myopia. (pp. 362-63)

Another researcher who found a link between the development of astigmatism and the development of myopia was Joseph Raphaelson. However, instead of saying that astigmatism caused myopia, he asserted that prolonged intensive near focusing during school years where young hyperopic eyes are "given no opportunity to relax and to stretch" (1958, 63) causes both myopia and astigmatism. He summarizes a report by E.W. Adams, OD, to the Optometric Research Institute:

[Adams] reports ... that in the first and second grades very little astigmatism is found, but after these two beginning grades each successive grade up to about the sixth increases the percentage of astigmatism; after the sixth the percentage remains about the same.

Recall also that Bates (1912), using the retinoscope, observed myopic astigmatism produced in children as a direct result of their straining to see.

One theory on how chronic close focusing ultimately leads to permanent structural myopia was proposed by British optometrist O.D. Rasmussen (1956). It is perhaps the most simple.

Failure of the crystalline [lens] to recover from its highly convexed state may create secondary tensions by pressure on the irides and thus the angle of the anterior chamber. Such pressures, by reducing the flow of lymph may in due course affect the whole eyeball and by swelling slightly, automacially separate the foci from the retina. (p. 85)

Modern research on animals leaves no room for doubt that the ultimate shape of the eye can be severely influenced by environmental factors, although most of it has been looking at changes in the shape of the eye as "axial length growth" and has not dealt with the issue of whether astigmatism also develops as a result of a near viewing environment, or whether the development astigmatism is an important factor in the development of myopia. A representative anthology of the modern work in this area has been collected in Ciba Foundation 1990. To summarize, axial length myopia can be consistently induced in laboratory animals by sewing the eyelids shut, by raising them in low-light conditions, or by restricting their visual input to near or virtually-near (through minus-diopter contact lenses) objects.

Other experiments are worthy of mention. Francis Young (1981) conducted a series of experiments. In these experiments, monkeys were restricted to close-viewing conditions, sitting within translucent hoods. The experimental monkeys, both young and mature, consistently developed significant degrees of myopia. His studies showed that a slight increase (~ +6mm Hg) in vitreous chamber (area behind the lens) pressure occurs during accommodation, accompanied by an decrease in anterior chamber (in front of the lens) pressure. The higher the accommodation, the greater the change in pressure. Under the assumption that a slight increase in pressure in the vitreous chamber will, over time, cause it to elongate, Young proposed that "as myopia develops there may be a temporary change in the thickness of the lens which is followed within less than a year by an increase in the size of the vitreous chamber." In other words, chronic over-accommodation could bring about a change in vitreous chamber depth through its effect on the lens. The effect on the lens is temporary; the effect on the outer coats of the eye is permanent. Young found support for this hypothesis by comparing the measurements of lens power (phacometry) of subjects that did near work and developed myopia, to normal subjects. An increase lens power not found in emmetropes was determined to be responsible for the myopic shift that occurs subsequent to near work (in normal eyes, the lens decreases during maturation). Only later was there an increase in eyeball length in the myopes.

Wallman et al. (1995) showed that the choroid in chicks changes thickness, moving the retina forward or backward. When the chicks wore minus lenses, and thus their visual input was restricted to near viewing, their choroid thinned; when the lenses were removed, it thickened. Thus, even axial length may be affected by visual input, at least during growth periods. In fact, there is evidence that axial length responds to refraction throughout life; one study showed that adults who acquired myopia late in life after months or years doing near work in a textile factory evidenced longer average axial lengths. (Simensen & Thorud 1994). More importantly, axial length appears to reduce to compensate for the lens's increase in power (Grosvenor 1987); it is possible that this mechanism could be accelerated in the face of a near-work reduced environment.

To summarize, early close-work theories, as well as Young's theory of myopia tended to treat myopia as a pathological phenomenon, while most modern close-work theories conceive of myopia as a matter of adaptive growth. Neither type of close work theory can explain everything about myopia, since different people growing up in the same close-work environment contract different amounts of -- or no -- myopia. If close-work induced myopia is pathological, then strength or health of the tissues of the eye may mitigate its effects. As noted by Risely, genetic factors influencing the shape of the eye may alter the intensity of the stress that close work entails. If, on the other hand, close-work induced myopia is a matter of adaptive growth, then differences in growth hormones, also genetically or perhaps nutritionally determined, may produce different reactions to the retinal stimulus.

Nutrition theory

Another possible factor in the etiology of structural myopia is nutrition. Emanuel Josephson (1939), a New York ophthalmologist, believed that the real cause of myopia is the "lengthening of the eyeball caused by increased volume of fluid in the eye," which had its roots in the adrenal cortex. To Josephson, the use of the eyes had nothing to do with whether or not they would become myopic. Rather, myopia was a result of a lack of salts in the body fluids, ultimately due to a malfunctioning of the adrenal cortex:

The adrenal cortex influences the water exchange of the body by causing retention of salt in the blood. The determining force in the exchange of water between the blood and the organs is their relative salt content. When the salt of the blood is reduced in quantity, water seeps, or osmoses, out of the blood into the organs. Insufficient secretion of the adrenal cortex causes such a disturbance and results in the increased flow of fluid into the eye. Thus is near-sightedness caused. (p. 26)

The way to prevent nearsightedness, Josephson argued, was to assure a properly functioning adrenal cortex. This could be done by assuring proper nutrition:

Malnutrition and defective diets play a large role in causing glandular disorders and the other disturbances which give rise to nearsightedness. A diet which is high in carbohydrates, starches and sugars, and low in proteins and fats, favors the development of near-sightedness. It is probable that such diets are apt to be deficient in vitamins; and that vitamin deficiency aggravates their effects. (p. 28)

He noted that the percent of schoolchildren with myopia rose and fell with the severity of the depression, and linked that to nutrition:

In 1925, it was reported that 25% of the schoolchildren attending a group of clinics in New York were afflicted with near-sightedness. With the advent of the depression, the figure rose steadily from over 40% in 1932 and to 72% in 1935. Reflecting re-employment and improved nutrition in 1936, the percentage incidence of near-sightedness dropped to about 51%. In 1937, the figure dropped to 42%.

P.A. Gardiner, a London ophthalmologist, also found a connection between poor nutrition and myopia. In particular, he found that myopic children are more likely to refuse to eat foods high in animal protein, this tendency being more marked the more rapid the progression of myopia (1956).

Further support for the idea that myopia is caused by a lack of protein in the diet, comes from a remark by Elizabeth Cass, who examined the eyes of 2,124 Eskimo (1966), some of whom lived in a traditional village setting, and some of whom lived "in settlement or hostels, using White man's diet." (1966b).

Myopia is unknown among pure-blooded adult Eskimos. The majority have negligible refractive errors and a small number have low hypermetropia. Children whose parents have no refractive errors, after living on the white man's food for some years, develop myopia and carious teeth, i.e., they change from a high protein to a carbohydrate diet. (1966:1051)

Although it is possible to embrace the nutritional theory while rejecting the close work theory (Josephson [1939:22] writes "the theory ... that close use of the eyes determines the stretching of the eyeball and the development of near-sightedness .... is glaringly absurd from the physical point of view"), they are in fact compatible, if one grants the connection between near-seeing and general health, as observed by Prentice (1895, 1905) and McCormick (1906). The body's organs could be weakened by the poor blood supply caused by prolonged near-seeing. Raphaelson (1959:90), drawing on the work of these two physicians, notes:

Indirectly, prolonged near-seeing may affect the blood circulation by the excessive drain on the oxygen in the blood. When an excessive amount of nerve energy is used up by the visual centers, a likewise amount of oxygen in the blood which feeds the brain to create energy is used up. The loss of oxygen in the blood causes a derangement in the blood circulating system of our eyes and body.
In other words, proper nutrition depends on many things, one of which is a healthy nervous system. If close application of the eyes exhausts the nervous system, then not only is the blood in the immediate vicinity of the eyes affected, but also other organs may be affected, including the organs of digestion.

Treatment of Myopia--Optical Solutions

Concave lenses

The prevailing school of eye doctors has always held that myopia can only get worse, and the best thing to do is to prescribe concave lenses, to let the myope see clearly. In the beginning of the century, doctors claimed that myopia would get worse if the myope were not "fully corrected." For example, in 1906 (New York Medical Journal, Nov. 17, 1906) Dr. Wendell Weber commented that myopia would be diminished if the physicians were "careful to use full correction in young children." And even now, one finds comments in an ophthalmology textbook (Elkington & Frank 1991:181) such as "myopia of sufficient degree to prevent the child from seeing what is written on the blackboard should be corrected before the child starts formal schooling."

However, adherents of the close-work theory argue that myopia is caused and made worse by an excess of near focusing; thus, as minus diopter lenses intensify the dioptric demand on the eye, especially for near vision, they make myopia worse. They say that the gift of being able to see small words written on the chalkboard clearly at 20 feet during kindergarten is more than offset by the risk of floaters, flashes and retinal detachment, due to a severely elongated eye, at a later age.

Convex Lenses

While strict adherents to the genetic and nutritional theories of myopia see no harm and only good in the prescription of concave lenses, according to the close work theory, the opposite is recommended. Physician Chalmers Prentice (1895) advises the use of lenses which reduce the demand on the ciliary (convex lenses) for all study (p.151):

If the eyes are to be used at a distance of ten inches, aid them artificially by a ten inch magnifying glass; then the nerve-impulses to the ciliary muscle will be no more than if the patient were leading an outdoor life and viewing objects at twenty feet or more. The nerve-centers are not called upon for so excessive an impulse, and they become habituated to sending the same amount of nerve-force as if an outdoor life were led. ... If the little student at school or any other person using the eyes at the near point, were to be supplied with such glasses during the hours of study, on leaving the school room they could be taken off and the natural use of the eye at all other times would be quite sufficient to cultivate and establish the habit of accommodation.

Many clinicians in the past have used this technique for curing functional myopia. Usually it is for very low degrees. Writes ophthalmologist Walter Lancaster (1944):

A young man who had been wearing concave lenses asked if there was any way he could pass the test for 20/20 vision. Vision was 20/15 with glasses but was 20/30 without glasses. He was given a +1.00D. sphere for each eye to wear constantly for three days. His visual acuity was 20/15 without glasses and 20/15 with a +0.50 D sphere, and he read some letters of the 20/20 line with a +1.00 D sphere. Was his [structural] myopia cured? No, because he did not have [structural] myopia to begin with. He learned to relax his accommodation.
However, there have been reports of plus lenses being used to cure functional myopia of a significant degree. Physician Chalmers Prentice (1895:148-49) relates:
Age forty-three; myopia; had been wearing over the right eye -1.25 D, left eye -1 D, with little or no change for the space of two years; eyes in use more or less at the near point. I recommended the removal of the concave glasses for distant vision and prescribed +3.50D for reading, writing and other office work. After reading in these glasses for several days, the patient was able to read print twelve inches from the eyes. This patient was of more than ordinary intelligence and understood the aim of the effort. In six months I changed the glasses for reading and writing to a +4 D without seeing the patient. After using the +4 D glasses for several months he again came under my care for an examination, when the left eye gave twenty-twentieths of vision, while the right eye was very nearly the same, but the acuity was just perceptibly less. ... Similar results have been attained in thirty-four like cases; but the process is very tedious for the patients, and unless their understanding is clear on the subject, it is almost impossible to induce them to undergo the trial.

In younger patients, faster results have been reported seen with plus lenses. C. P. Rakusen (1937) did his work in Shanghai, China. He took ten children "whose age and history indicated recently acquired myopia." They all had a manifest refraction of one to two diopters of myopia. Their naked vision ranged from 20/100 to 20/200. He had the school physician verify the children's naked eye vision, after which he prescribed convex lenses, with base-in prisms, to be worn constantly for close work, and as much as poible for indoor activities. After only one week, every single child had improved naked-eye vision, and three of them had improved to 20/20. Rakusen followed the cases for two years, and found that there wsa further imrpvement in all cases and no retrogression as long as the special lenses were worn for close work. The one child who discarded the glasses became nearsighted again.

Rakusen also cites a case of a girl who was prescribed concave lenses by an oculist who used cycloplegics. Rakusen had seen this girl for three years since the age of seven, who had been suffering headaches, but could read the distant eye chart with normal acuity. As she had a tendency to hold the book close to her face, Rakusen prescribed weak plus lenses, through which the girl maintained a vision of 20/40. The girl left the country for three years. When she came back she was wearing myopic glasses of 2 diopters, which were prescribed by an ophthalmologist using cycloplegic drops. Even though these drops are supposed to entirely eliminate functional myopia, Rakusen found that much of the myopia was indeed functional, since he was, after one year, able to reduce it "by more than a half" using plus lenses for reading.

Another optometrist, O.D. Rasmussen, from England, also advocated using plus lenses to deter the development of myopia. In 1954 he published an article wherein he shares the following case histories as presented to him by a colleague optician, who, instead of filling the minus-lens prescription that the patients came with, gave them plus lenses for reading. Here are a few of the case histories:

Miss H. Typist. Had few changes of lenses, gradually increasing in power, the last one being:

R.E. s -4.50 cyl +0.75 x 90
L.E. s -4.75 cyl +1.25 x 90

Her v/a for distance without lenses 6/24; with lenses 6/9. I refused to give such a prescription, and prescribed R.E. +1.50; L.E. +1.25, to be used for all close work. After three months her v/a without lenses for distance is 6/12. Change of lenses now due and expect 6/6 within 6 months.

Miss P.; Teacher. Brought Rx for O.U. -1.50, which I refused to give. Have prescribed for near +1.25 with suitable exercises. In three months her distance vision without lenses was normal.

Miss E.; Schoolgirl. I would not give prescription for R.E. -3.00; L.E. -2.75. Gave her O.U. +3.50. After three months her v/a for distance without lenses is normal.

American optometrist Jacob Raphaelson wrote and published a series of books detailing the advantages of the universal use of +1.00 glasses for everyone who does close work, as a way to prevent myopia and cure it in its early stages. One of the points that he brought out is that children start using their eyes at an extremely close distance even when their distance vision is normal. In 1934, he visited various schools and noticed that there was a marked tendency for children to bring their eyes closer and closer to their books or papers after only a minute or two. That year, in order to get some hard data, he did a survey of elementary schoolchildren in rural Clermont County (near Cincinnati). He measured how far they placed themselves from their close work (reading, writing) when they began, and at what distance they ended. He found that of the total of 503 children he observed, 85% used their eyes at a distance of six inches or less from their work. The following table is from Raphaelson 1956, p.84:

Distance    # Started at    (%)  # Finished at    (%)

10-12 inches    287         57%       2         00.2%

6-10 inches     205         41%      76         15%

3-6 inches       11          2%     331         66%

3 or less         0          0%      94         19%

Total           503        100%     503        100%

Raphaelson also found that while practically all the pupils were using their eyes at six inches or less from their work, only 5 out of the 523, or 1% of the students had myopic refraction. Furthermore, 84% of the students had 20/20 unaided distance acuity. He then had the children with poor vision (20/30 or less) wear plus lenses for five or six minutes. He found that of the 77 children with poor vision, 43, or 55%, had their vision improved and 22, or 29%, had their vision restored to 20/20 vision.

Raphaelson concluded from his experiments that near-seeing requires much effort and energy for young, hyperopic eyes. The strain on the eyes due to the continued close focusing causes a reduction in vision. Unfortunately, the solution to the optical problem causes an aggravation of the physical problem:

At the start, normally, the eyes use primary or central vision, but the eyes soon tire. When the children bring their eyes closer and closer to their work, they get assistance from secondary vision (peripheral) which functions better closer to the eyes, thus making it easier for them to see. This habitual use of secondary vision which requires more optical contraction will, sooner or later, make the child near-sighted, poor-sighted and astigmatic. [1961, p. 123]

He argued that it is difficult to maintain focus at 12 inches:

The near-vision survey also indicated that prolonged near-seeing affects our children adversely in many other ways. For, many of them not only began to bring their eyes closer and closer to the book or paper, but also began to frown and wrinkle their foreheads or turn their heads sideways. This should be proof enough to any unbiased person that prolonged near-vision is not effortless for small children but a real task.

Raphaelson, like Rakusen, was convinced that much supposedly structural myopia is actually functional, inasmuch as it can be cured by the use of convex lenses.

Although Raphaelson could not prove that near-seeing causes permanent myopia, his study and that of Rakusen are revealing in that he showed that a large percentage of schoolchildren suffer from subnormal vision that could be relieved by the use of plus lenses occasionally, or for periods of intense close work. Given the most widely accepted theory of how the eye focuses, such actions could apparently only handle the "ciliary cramp" type of myopia -- functional myopia. However, if functional myopia leads to structural myopia, as the close work theory proposes, then such lenses, used for all close work, would also prevent structural myopia.

Distant vision practice

Another method of preventing and eliminating functional myopia worth mentioning is daily reading of a distant eye chart, as advocated by William Bates (1911, 1913, 1920). Bates found that if students would refer to a distant, familiar Snellen visual acuity chart whenever distant objects became indistinct, they would regain control of their ability to focus. He first introduced this technique in a school in Grand Forks, North Dakota in 1903. According to his account, he came upon the technique when testing the eyes of children at a school. Sometimes, children who failed the eye test at first would ask to be retested, and pass. Children who were known to be nearsighted would be able to read not only the test card, but writing on the chalkboard and distant clocks, which they had been unable to see clearly before. On the basis of this observation, eye charts were installed in the classrooms, so that there would be a familiar object on which to practice distance vision. In the first school system in which this technique was used, myopia -- as determined by examination "during a study period while [the children were] sitting in their seats" -- was reduced from 6 percent to 1 percent (1911). In 1912, he brought the technique to the New York City public schools.

It is important to note that Bates did not claim that myopia is caused by too much close work. Rather, he identified the "effort to see distant objects" as the sole cause of myopia (1913). "Near use of the eyes is not a cause of myopia," he insisted. This unique stance is based on his observations of people making efforts to see under adverse conditions:

It has been repeatedly demonstrated with the aid of the retinoscope that all school children with normal eyes when regarding the unfamiliar writing or figures on the blackboard, distant maps, diagrams, or pictures had myopic refraction. It was quite otherwise when they regarded a familiar distant object. The retinoscope used at the same time indicated no myopic refraction. (1913)

In other words, faulty distance focusing is a habit encouraged by trying to see distant objects. This "myopic response" to difficult viewing situations was widely recognized in Bates's day. In 1914, Walter B. Lancaster wrote: "when the patient strains to see, he exerts his accommodation and so sees better with a lens which permits or encourages this accommodation." However, Bates attaches unusual significance to this response and made it the cornerstone of his anti-myopia strategy, which was to encourage schoolchlidren to focus properly for the distance by having them practice daily with a familiar distant object -- the Snellen test card. The method was the following:

A Snellen test card was placed permanently where all the pupils could see it from their seats. Daily the teachers recommended all the children to silently read the card with each eye separately, covering the other eye with the palm of the hand in such a way as to avoid pressure on the eyeball.

Records were made with the same card or with an unfamiliar card for testing the vision. (1913, 410-11)

Bates felt it was important that the test card be constantly in view and memorized, so as to make it as easy for the children to focus on as possible, and encourage them to focus properly:

It was only when the eyes were properly adjusted for distant vision that the small letters were read. With other distant objects children had greater difficulty in knowing when the focus was adjusted accurately. Many persons with normal eyes believed erroneously that they saw better at the distance by partly closing the eyelids or by otherwise straining the eyes; but, when they looked at the Snellen card, they at once discovered that the effort made the letters indistinct (1911).

Often it was most convenient to test the vision with the very same memorized card. Understandably, the value of a memorized Snellen card was met with skepticism by the authorities at the schools where he tested the technique. To assure them that the memorized card could be an accurate test the vision of the children, in January of 1912, the principal of one school had the vision of 1,500 pupils tested first with the memorized card, and then with an unfamiliar card. Bates does not report the exact difference in results the two tests, but says that the principal, in a letter to the Superintendent, noted that Bates had "to a certain extent proved his point." Bates reports that in June 1913, the test was repeated, and,

the memorized Snellen card was again found satisfactory for testing the vision. Objective tests were conclusive, and demonstrated the interesting fact that school children did not deceive themselves or others, when their vision was tested with a memorized Snellen card. When a pupil said he was reading the memorized Snellen card with normal vision, the retinoscope used at the same time, indicated no manifest error of refraction; the eye was adjusted for normal vision. (1913: 411)

Other controls were also made from time to time, to verify that the training method worked:

Comparative tests were made with and without cards. In one case pupils with defective sight were examined daily for one week without the use of the test card. No improvement took place. The card was then restored to its place, and the group was instructed to read it every day. At the end of a week all had improved and five were cured. In the case of another group of defectives the results were similar. During the week that the card was not used no improvement was noted; but after a week of exercises in distant vision with the card all showed marked improvement, and at the end of a month all were cured. In order that there might be no question as to the reliability of the records of the teachers some of the principals asked the Board of Health to send an inspector to test the vision of the pupils, and whenever this was done the records were found to be correct. (1920:264-65)

Bates summarized the data from the New York City schools in two tables, reproduced below. PS = Public School # T = Number of pupils tested twice D= Number of pupils with defective vision at first test. I= Number of D in whom one or both eyes improved at second test. N= Number of D in whom both eyes were normal at second test. W= Number of T whose vision was worse at second test Te= Number of teachers Test Dates= Months in which the first and second tests, respectively, were administered during the school year of 1912-13.

Table 1 from Bates 1913. Summary of the records of the vision of the pupils made by the teachers of five New York City schools.

PS     T       D   D/T     I    I/D     N    N/D    W     W/T  Test Dates   Te
  6    925    474  .51    390   .82    303   .64    83   .09    Dec         37
183    635    333  .52    250   .75    168   .50    38   .06    Jan, Jun    21
186   1939   1223  .63    669   .55    220   .18    69   .04    Oct, Jan    49
186   2007   1139  .57    620   .54    276   .24   164   .08    Feb, Jun    57
 43    131     85  .65     61   .72     30   .30     5   .04    Mar, Jun     4
 46     63     45  .71     36   .80     26   .58     0   .00    Mar, Jun     2
Tot   5700   3200        2026         1023         359
Avg .              .58          .61           .31          .06

Table 2 from Bates 1913. Summary of the records of the vision of the pupils made by those teachers of five New York city schools who recorded that the vision of no pupil became worse.

PS      T      D    D/T    I    I/D     N    N/D    W          Test Dates   Te
6      244    135    55   129    95    111    82    0           Oct, Jun    11
183    198     85    43    81    95     62    73    0           Jan, Jun     7
186    424    318    77   170    53     52    16    0           Oct, Jan    10
43      67     49    73    37    75     19    39    0           Mar, Jun     2
46      63     45    71    36    80     26    58    0           Mar, Jun     2
Total 1351    845   605         334                 0
Avg.                       .62         .71   .40

If, then, we can trust these figures, it appears that poor distance vision can be overcome in the classroom. How much of this improvement can be attributed to a decrease in myopia and how much can be attributed to an increase in acuity or perception for other reasons, is not clear. The extent of improvement for any given student, in terms of either refraction or acuity, is also not clear at all from these statistics. However, some of the qualitative reports from the teachers give one reason to suspect that practicing with the eye chart daily helps keeps the eyes in working order. For example:

June 27, 1913, Miss Dillon was asked her opinion of the method.... She described in detail the results obtained. Some pupils, even with glasses, were unable to see the writing on the blackboard from their seats. In a short time their vision improved without glasses, so that they had no further difficulty with their sight. Others complained of eye pain or had trouble in seeing to read. They held their books close, about six inches from the face. The use of the distant Snellen card gave them relief and they later read without effort or discomfort at a comfortable distance, about twelve inches. She discarded glasses and relieved her own eyes by the use of the Snellen card.

More recently, a preliminary experiment has been done (Leber & Wilson 1994) with a computerized eye chart training system. After only five hours of training, six of seven myopes had "confirmed myopia diminution of -0.25D in one eye, two had improvement in both eyes." Furthermore, one subject "displaying pre-training 20/100 distant acuity in both eyes, recognized 20/80 letters after five hours of training and 20/50 letters after another five hours' training."

Many authors have tried to explain such successes as "purely psychological" or as the result of "blur interpretation" (e.g. Gibson 1953), since such improvement in acuity is not always attended with significant differences in refractive error as determined by a retinoscopic test of refraction at 20 feet, under cycloplegia. However, even if such practice cannot reduce existing refractive errors, there is the possibility that constant practice at focusing accurately for the distance reduces the spasm of the ciliary muscles and increases the circulation of blood in and out of the eye, improving acuity (and reducing the need to bring work materials close to the face) and, by preventing congestion and nervous disturbances, preventing structural myopia.

Treatment of Myopia -- Nutritional solution

Gardiner's observations on the connection between a low protein diet and myopia (1956) led him to use an increased protein diet as a treatment for myopia (1958). He performed a one-year experiment. The experimental group consisted of children of varying ages -- from 5 years to over 13 years -- and varying initial refractive errors -- from 0.25 to over 2.00 diopters of myopia. Their present diets were evaluated and then they were advised to change their diet so that the animal protein content was raised to 10%, leaving the caloric intake the same. The control group, composed of a similar population, was given no dietary advice. Gardiner found that, controlling for both age and initial refractive error, myopia in the experimental group progressed more slowly than in the control group in all cases except for the oldest children when the initial amount of myopia is small. The results are summarized in the table below, with change (D) indicating the average progression of myopia in diopters.

Table: increases in myopia with and without dietary changes.

Initial Myopia (D)      .25 to 0.9  -1.0 to -1.9   -2.0 or more

AGE N incr. (D) N incr .(D) N incr. (D) 5-7 Control 10 0.60 8 0.72 9 1.12 Experimental 5 0.32 1 0.60 3 0.23 8-9 Control 24 0.44 18 0.54 18 0.53 Experimental 3 0.00 6 0.05 11 0.22 10-11 Control 46 0.45 26 0.58 32 0.51 Experimental 5 0.22 11 0.19 12 0.17 12-13 Control 40 0.24 36 0.40 28 0.40 Experimental 5 0.40 2 0.15 14 0.16 13- Control 19 0.23 10 0.33 22 0.46 Experimental 2 0.25 6 0.02 5 0.1

Treatment of Myopia Today

The idea of using plus lenses or distant vision practice to stop functional myopia before it turns into structural myopia, and the idea of diet or general health as a factor in myopia, are absent from the characteristically pessimistic modern literature reviews on myopia control (Goss 1983, Grosvenor 1987, Sivak 1991, Goldschmidt 1991). Methods currently being studied to control myopia are primarily biofeedback and bifocals. In the biofeedback technique, an expensive piece of equipment known as the Accommotrac (developed by J. Trachtman) is used to help the patient gain voluntary control of accommodation. This device measures the refraction of an eye and converts it into a tone. It is thought that by gaining conscious control of accommodation, one can thereby expand one's range of accommodation and see farther. Some experiments (Roscoe 1987, Randle 1988) show that myopia can be reduced by about a quarter of a diopter with biofeedback, while other experiments testing the Accommotrac in particular (e.g. Koslowe et al, 1991) reveal no significant difference between the control and experimental subjects in degree of myopia reduction.

Bifocals, which allow the wearer to do close work without looking through a full distance correction, are supposed to slow the progression of myopia in children. This technique has been around since at least the 1930s (Grossman 1949). Such glasses are designed so that every object looked at is comfortably within the child's range of accommodation. They differ from regular glasses in that they allow the child to use less accommodation to look at close objects, when looking through the lower half of the glasses, as when reading. The purpose of bifocals is to reduce the amount of accommodation required for close work, and not to force a reduction in accommodation for distance work, curing functional myopia, which is what a pure convex lens and distance vision practice approach is designed to do. Bifocals are prescribed on the assumption that the myopia diagnosed is completely structural and incurable; however, it is thought that the myopia can be kept from getting worse by reducing accommodation for near work. As David Goss (1994) notes in a review of experiments with bifocals, the effects are hard to generalize. Children who wear bifocals as opposed to single vision lenses still tend to progress at the rate of a quarter to a half a diopter per year. Goss notes that the bifocals only have a consistent effect on nearpoint-esophoric patients, that is, those whose reflex to cross their eyes when focusing on a near object has been exaggerated (their eyes over-cross in response to close focus, and extra energy is needed to keep them apart.) Esophores, Goss notes, seem to have a higher rate of myopia than other phoria types with single vision lenses, and a lower rate of myopia progression with bifocals. However the differences are statistically insignificant, and the average for all categories of myopes, no matter which kind of corrective spectacles they get, falls between 0.25 and 0.55 diopters a year.

Summary and Conclusion

A range of explanations for myopia has been given throughout the years. Before the middle of the twentieth century, myopia in all its forms was often recognized by health authorities as a sign of ill health. Now it is coming to be recognized as a fact of life, and only high myopia is considered a health problem. But even though most myopes don't consider their myopia as something "wrong" with them, given a choice, they would choose to enjoy the prime of their life without dependence on glasses or contact lenses. Early research was based on the idea that myopia is largely a result of choices: of what to do with the eyes, or of what to eat. Modern research is based on the presupposition that myopia is a matter of genetic destiny, and nothing can be done to prevent it, but with a dim hope for some pharmaceutical breakthrough, and minor consolation in the prospect for surgical correction. The modern message is, "myopes, you have no choice but to be myopic and to grow ever more so." However, a few promising past theories on how to prevent myopia, especially ones that link structural and functional myopia, remain untested.


The Orinda Study

The study of Zadnik et al. is especially inadequate, and since it is today so widely cited in support of the genetic hypothesis, a discussion of its weakness is in order.

First of all, although Zadnik et al. made an attempt to account for close work in their model (they concluded that close work was a "modest" component), there are many types of close work not included in the "diopter hours" variable of Zadnik et al. for example, practicing piano, building model airplanes, solving jigsaw puzzles. Secondly, Zadnik et al. did not look at the time spent doing far vision activites, and did not consider diet. Thirdly, the exact refraction of parental myopia was not considered at all, but determined as a categorical variable (neither, one, or both parents myopic) on the basis of whether or not the parents were prescribed glasses for general or distance use before the age of 16. Thus, not only were a certain number of myopic parents who hid their myopia until driving age counted as nonmyopic, but those with glasses for hyperopic astigmatism and accommodative esotropia were classified as myopic. Precise correlations between refractive errors of the children to their parents could thus not be made. Fourthly, years in school was used as a proxy for age (even though the exact age of each child was known), making it impossible to differentiate between the "years of schooling" factor and the "growth" factor. Moreover, it was treated as a categorical, not continuous variable. Fifthly, for degree of refractive error, only three numbers were given, one covariance-adjusted mean for each of the three "parental history of myopia" (number of glasses-wearing parents) categories. Thus the "average" child of two myopes is 0.11D hyperopic, the average child with one myopic parent is 0.51D hyperopic, and the average child with neither parent myopic is 0.62D, where the age is calculated according to an unspecified linear model that adjusts for grade in school and diopter hours. In other words, the averages are not subclassified by grade and diopter-hour range, so it is impossible to tell at which age the difference becomes significant. Furthermore, no indication of variance or distribution was given for these figures. [back to text]


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Revised February 13, 2008 (Typo corrected)