Considering the recent description of a “myopia boom” in Nature 2015,7 the question arises as to how the prevalence of myopia was in the middle of the 19th century in central Europe. Hermann Cohn provides extensive data. Of 40,000 patients visiting his hospital, 16.8% were myopic, using a conservative criterion for myopia of a refraction more negative than −1 D. He also describes that the rate of myopia was correlated with the type of school the patients were attending. In the intermediate level school (“Realschule”), the prevalence of myopia was 9, 16.7, 19.2, 25.1, 26.4, and 44% at the different classes from the age of 10 to 16 years. The highest levels of myopia were found at the high schools (“Gymnasium”), with 12.5, 18.2, 23.7, 31.0, 41.3, and 55.8% from the first to the last grade. He stated that more than half of the student in the last class of the high schools were myopic. In addition, Cohn provides 16 pages of tables about the prevalence of myopia in different schools across Germany and Switzerland, altogether showing similar prevalences. He also provides data on the rate of myopia in theology students in Tuebingen, during the time of 1861 to 1865: 81% (n = 138), 1861 to 1879: 79% (n = 534), and 1861 to 1882: 78% (n = 713).
A figure is presented in his book summarizing the prevalence of myopia in different age groups in 24 German intermediate level school and high schools (Fig. 4). The average prevalence at the time of the high school examination (“Abitur”) was 59%.
In summary, there is little doubt that the rates of myopia were already high in central Europe 150 years ago. At least at the time of the final examinations in high schools they reached similar levels as in high school students today (Gutenberg study in Mainz, Germany 20148: 53% of myopia in students, criterion for myopia SE −0.5 D or more myopic; E3 study in central Europe 20149 on 61,946 subjects, with 98% European ancestors: about 46% in people of the age group between 25 and 29 years; criterion for myopia: SE −0.75 D or more myopic). It must be kept in mind that enrollment in high schools was probably lower at times of Hermann Cohn than today where it is close to 50% in the studies by the E3 consortium. Because these numbers are not known from the time of Hermann Cohn, the prevalence of myopia over the entire population cannot be extrapolated.
Hermann Cohn also studied high myopia and the associated ocular problems. He referred to Arlt (1839, “Krankheiten des Auges,” Prague, published 1855) as the first to discover that the myopic eye is elongated compared to the normal-sighted eye and that the tissue in the posterior pole is stretched in myopia. Cohn summarizes that the emmetropic eye is about 24 mm long, but can reach up to 37 mm axial length with more than 30 diopters of myopia and a far point of only 3 cm. Choroid and sclera continue to become thinner and translucent during myopia progression and the choroid becomes an atrophic colorless thin tissue layer. He states that “choroidal atrophy is a most common feature of the myopic eye” and that “a problem is high myopia is that it remains progressive.” He refers to Donders (“Annomalien der Refraktion und der Akkommodation,” Wien, 1866) who had recognized that “… once the stretching of the fundal tissue layers has reached a certain level, their resistance is reduced and they are no longer stationary, in particular if intraocular pressure is elevated.” Cohn concludes “I am saying without hesitation that a myopic eye is a diseased eye … progressive myopia carries high risks for the future … visual function decays often as early as at 50 years of age, either due to retinal detachment, bleeding or atrophy and degeneration of the macula. To avoid this condition, in phases of progression, any deleterious external influences have to be anxiously avoided.” Today, the opinion is that at least low refractive errors are the result of minor disarrangements of the optical variables in the eye and scarcely associated with ocular diseases. He referred to data by Horner (“Ueber Brillen. 18. Neujahrsblatt zum Besten des Waisenhauses in Zürich,” 1885) who had described that from 1878 myopes that he examined in his practice between 1880 and 1883, 34% (n = 629) had severe ocular complications (9% vitreal clouding, 11% uveitis, 4% retinal detachment, and 23% cataract) and that these complications were present already at an average age of 50.3 years.
Cohn developed six theories to explain the development of myopia: (1) hereditary, (2) accommodation theory, (3) convergence theory, (4) nerve drag theory, (5) extraocular muscle theory, and (6) near work theory. He largely refuted the hereditary theory because myopia was only 10% more frequent in his patients when both parents were myopic. Rather, he assumed that the visual environment at school must be most important. He observed that students became more myopic after they were corrected with negative lenses and that they became less myopic after prolonged atropine application. He concluded that part of the myopia might be due to accommodative spasm as a result of continuous near work (today referred to near work induced transient myopia, NITM; i.e.10). He also excluded the accommodation theory based on three observations: (1) hyperopic subjects have to accommodate more and should therefore become more myopic which is not the case; (2) uncorrected myopes accommodate less and myopia should therefore be self-limiting; and (3) overcorrection, requiring more accommodation, should accelerate myopia progression which was not observed (in line with more recent studies, i.e.11).
Theories (3) to (5) above are not discussed because they do not seem very convincing. However, the “near work theory” (6) is discussed in more detail. Cohn writes “most ophthalmologists believe that continuous near work, in particular with the head tilted forward and with poor illumination, stimulates myopia.” To this end, a number of new designs of desks were presented which were also patented at the time. They were supposed to optimize reading distances (Fig. 5A and B).
In addition to the list of myopia theories shown above, Hermann Cohn may have been the first to propose that light is a potentially important factor in myopia development. He compared data sets from six different authors who measured visual acuity with decreasing luminance. If small “E” hooks could just be identified at 6 m distance and at 1 cd/m2, it was tested how much closer the subjects need to move to resolve them when the light was reduced. At 0.5 cd/m2, the maximal distance was reduced to about 4 m. At 0.16 cd/m2, it declined to 1.2 to 4 m, depending on the subject. Although Cohn admitted that “results vary considerably among authors” and “we are still far from understanding how visual acuity drops with luminance,” he concluded that reading distance should vary with luminance. Because reading distance is related to myopia development (Fig. 5B; newer work12), he assumed that low luminances during reading represent a further risk factor for myopia. Accordingly, he put large efforts into a better control of luminances at the reading desks at school. An interesting problem was that there were no reliable devices available to measure luminances. Hermann Cohn used Leonard Weber’s photometer. The device uses a prism which works like a hemi-silvered mirror, so that the luminance of a real candle could be compared to the luminance of a surface. A brightness match had to be done by the observer. Weber defined a new unit, the “meter candle,” which is the luminance of a white piece of paper that is positioned at 1 m distance from a candle. It becomes possible to define how many candles are necessary to illuminate a surface to the same level as the diffuse daylight which is present in the class room. Cohn requested an absolute minimum luminance at work places of “10 meter candles” (equivalent to 10 lux), assuming that visual acuity dropped to three fourths at the most. Measuring the luminances at various schools in Wrozlaw (now Poland), he found that parts of some class rooms did not meet these requirements. He managed to shut some of these places down. He also considered the possibility to attach large mirrors in front of the windows that guide the light from the sky into the class room. As expected, he found that these mirrors could double the luminances inside. It was only discovered in recent times by experiments in animal models that inhibition of (deprivation) myopia by light occurs at much higher illuminances than what Cohn suggested (review13), far beyond the illuminances necessary to achieve best visual acuity. A description of the light concept of Hermann Cohn was recently published14 although the point was not made in this paper that light inhibits myopia only at much higher illuminances than what was requested by Cohn. Another recent publication touching the issue was by Hopday.15
Hermann Cohn also assumed that the style of writing may have a major impact on myopia development. He concluded that “titled font will cause a tilt of the head” (Fig. 6) and closer viewing distances.
Being convinced that near work at school is the major reason for myopia, he cited his colleague Giraud-Teulon who said that “the screaming of the students when they can leave school is expression of their protest against the torture of school work … but how much louder would their eyes scream if they could.” Cohn formulated a few new rules. He requested that “it is urgently important to interrupt writing and reading intermittently and to do gymnastic exercises for a few minutes.” These exercises should include stretching of the body, lifting the shoulders, pushing the shoulders backwards, and crossing the arms. He requested that “writing exercises should never be extended to more than two hours … and writing should only occur at the brightest time at noon.” He requested a “break for 15 minutes every hour where the children have to go outside.” He believed that, after 5 or 6 hours of school, it would be most destructive if the pupils have to do further work at home where the desks are most often inappropriate in terms of available luminances and their design. He requested that “Sundays and holidays are resting times for the eyes and impositions are irresponsible. Juveniles with progressive myopia should not touch a book at least during the summer vacations.” Today, there is plenty of data showing that myopia progresses less during the summer than during the winter months although it is not known whether this is due to less reading or due to more outdoor activity in bright light during summer time (i.e.16).
Hermann Cohn had collected expertise with atropine treatment of myopia already in the second half the 19th century. He referred to several other authors (Dobrowolsky 1883, Mooren, Schiess, Derby, and Schröder, all cited by Cohn without sufficient details to locate the originals) who also used atropine with good success against myopia, “although the effects were only transient.” He claimed that “the cure is absolutely free of risk but only disturbs the children by causing glare” and “ after having seen many cases of atropine treatment against myopia in 8 years of observation, absolute lack of reading and writing for 3 to 4 weeks is similarly effective and without the inconvenient side effects of atropine.”
He also provided advice regarding the choices of the profession: “if you are myopic, you should not become a physician, a professor, or an administrational worker. Instead, become a gardener, farmer, baker, beer brewer and run a restaurant.” From all his studies he concluded “… it follows that myopia increases in the whole civilized world in all nations with the requirements of the schools, and with the number of classes.” Therefore, one of the puzzles of the current time is to find ways to maintain high educational standards while devising ways to prevent or limit the amount of myopia in the world.
Taken together, many of Cohn’s ideas were partially correct, but his hypotheses did not survive him for very long because no data were generated at that time to prove or disprove them.
So What is New and What is Old? (according to Hermann Cohn’s Ideas).
- It is definitely NEW to understand that eye growth is controlled by local image processing in the retina and that different eye growth can be triggered selectively in parts of the visual field (reviewed by Wallman and Winawer17). Also, all the work about biochemical features of the signaling cascades for the visual control of the growth of the sclera (reviews17,18) is new. However, it is NOT NEW that the visual environment has a major effect on myopia development (reviewed by Morgan and Rose19). It is interesting that Hermann Cohn’s conclusions were largely forgotten after his death and that later Sorsby and others (1933)4 were convinced that myopia it largely inherited. Sorsby stated that “the optimism of those who regarded myopia as a product of controllable environmental conditions has not proved altogether justified.”4
- The question of the role of accommodation in myopia development is still not fully resolved. Even though the retina can determine the position of the plane of focus and control the growth of the underlying sclera by itself, suggesting the cues from accommodation are not needed,20 it is clear that the tonus of accommodation determines the relative position of the focal plane relative to the photoreceptor layer and that the residual defocus is used as an error signal for emmetropization. Without involving accommodation, it is difficult to explain why human myopia is not self-limiting (imposed myopic refractive errors inhibit eye growth in animal models21) and why undercorrection does not reduce myopia progression22; but see also.23 It is NEW that defocus in the eye varies over the visual field24 and that a combination of the shift in the image shell with the (foveally controlled) accommodation and the local retinal defocus signals must determine the overall growth pattern of the eye.25
- It is NOT NEW that atropine can reduce myopia progression, but it is NEW that this is, at least not only, due to paralysis of accommodation26 but rather due a direct biochemical effect of atropine on axial eye growth.27
- It is NOT NEW that light interacts with myopia development, but it is NEW that this has little to do with shorter reading distances in dim light. The inhibitory effects of bright light on myopia occur in an illuminance range far beyond the level where reading ability is limited by low light.28 It is also NEW that the spectral composition of light may generate unexpected effects that still need to be explored, i.e. that long wavelength light, generating more myopia in chickens as expected from chromatic aberration,29 in fact causes hyperopia in monkeys.30
- It is NOT NEW that the prevalence of myopia in central Europe increases with the years of education—similar findings are in the current literature on myopia (the Gutenberg study in Germany8; the E3 study in Europe9).
- A number of suggestions made by Hermann Cohn were re-considered in recent times like regular interruptions during reading (proposed for instance by Winawer and Wallman,31 based on experiments in chickens), a mandatory additional 40 minutes outside added to each days’ school hours (i.e. schoolchildren in Guangzhou, China32) or spending the time of the school breaks outside,33 and controlling reading distance12 (similar regulations were recently implemented by the Ministry of Education in Taiwan; Prof. Pei-Chang Wu, presented at the IMC 2015). In fact, Taiwan has had a myopia prevention regime based on Cohn’s ideas for some years. A recent revision was motivated by the fact that the prevalence of myopia was increasing, despite this program, and led to the incorporation of time outdoors as an important element. And although it is still early days, this does seem to be turning around the epidemic.
- It is NOT so NEW, obviously, that there is a “myopia boom” in many countries (exceptions may be Denmark34 or Australia35). There was a “boom” already in the 19th century, at least in those children who attended high schools. Although there are no data on myopia prevalence in central Europe during the time during and after the wars, it is likely that myopia was less frequent because education was less organized and less demanding, and because people were forced to be more outside. A reduction in myopia was also seen during the cultural revolution in China.36 Therefore, it is likely that the prevalence of myopia fluctuates with the economical status of the countries under consideration.
University of Tuebingen
Ophthalmic Research Institute
Section Neurobiology of the Eye
The author thanks Prof. William K. Stell, Calgary, Alberta, Canada, for collecting manuscripts on the material presented at the 15th IMC, and Prof. Martin Rohrbach for discussions and for providing a PDF file of the German version of the book by Hermann Cohn on the “Hygiene of the Eye.” The Sek Jin Chew lecture 2015 was sponsored by CooperVision, USA.
Received January 4, 2016; accepted March 10, 2016.
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Keywords:© 2016 American Academy of Optometry
myopia; epidemiology; history; Europe; Hermann Cohn