DARWIN VS. SKEFFINGTON

No matter what type of practice, refractive conditions are a major aspect of optornetric care. The argument concerning the genesis and progression of refractive conditions has long been the subject of debate, I presume from the time refractive conditions have been recognized, measured and compensated.1 The argument that the environment somehow impacts the refractive condition has deep roots in behavioral optometry and the philosophy of the Optornetric Extension Program. 1-4 Skeffington's philosophy explained how the complex interaction of the identification and centering systems interacted to cause the organism to become myopic. There were others who argued that myopia was genetic.1 I must confess that I see both positive and negative arguments for both sides. As with most things, the cause probably is a marriage of several variables.

When I began my optometric education career at Northeastern State UniversityOklahoma College of Optoraetry (NSUOCO), a colleague, Dr. David Goss, was (and still is) seriously pursuing the study of myopia.5-8 Under the influence of Dr. Goss, I began to search the literature concerning refractive conditions. One of the most dramatic studies was with the !nuits (Eskimos) of Alaska9,11 and Greenland.12,1-1 In the Alaskan study a group of scientists went to the Arctic Circle to study an entire community of Inuits before mandatory education was initiated. It was found that pre-presbyopic Inuits were not by and large myopic. (There were 1.5 to 2.0% of myopes, 41 years of age or older.) When the children began school, a dramatic increase in myopia was measured (44.7%).10 Other studies from Greenland have shown that Inuits living a traditional lifestyle did not measure myopia. Myopia was denned as -1.5D under cycloplegia. However, those Tnuits who were in school showed similar prevalences of myopia thai were found in Alaskan lnuit children. The increase in myopia correlated with the academic experience.13

The phenomenon of the increased incidence of myopia happened in one generation. This one generation increase in the prevalence of myopia could not be explained by Darwin's theory since natural selection requires time for the "desired" characteristics to be developed. Darwin's concept is succinctly described below.

The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As iong as there is some variation between them, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are inherited, then differential reproductive success will lead to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.14

Darwin clearly argued that the change in the characteristics of a species group would be by natural selection over many generations. The myopic change with the Inuits happened instantly in evolutionary terms. These lnuit studies offered strong evidence for environmental influences.9,10,12,13

Another interesting refractive phenomenon that soon became apparent at NSUOCO was the propensity of Native Americans to measure significant amounts of with-the-rule astigmatism. Optometrists began to publish papers on the epidemiology of Native American refractive conditions.15-20 From a clinical perspective, when examining primarily Cherokees, Goss reported that the greater the documented Native American blood quantum, the more likely the occurrence of withthe-mle astigmatism.19,20 The prevalence and amounts however, did not approach the refractive amounts documented in the Western tribes. 15-18,21,22

Garber and Hughes published a paper showing that in the Navajo tribe, only those birth cohorts born after 1935 measured significant with-the-rule astigmatism.21 They state in their summary:

The relatively small incidence of this refractive anomaly m the population bom before 1936 emphasizes the importance of the question, 'Why do the Indian children have such a large percentage of high astigmatism?' ...might be an environmental change ofrecen! origin thai has altered the visual status of a race of people.

This prevalence of with-the-rule astigmatism was again verified by Hughes and myself when we evaluated over 6,000 records of Navajos examined in the ln-_ clian Health Hospital m Shiprock, New Mexico." If the with-the-rule astigmatism in Native Americans was somehow being caused by near work, as myopia appeared to be in the Inuits.then it was reasoned that the preschool Navajo would not exhibit the condition. To study this, keratometric measures were obtained on preschool Navajo children.21 The conclusion of this study was that with-the-rule astigmatism is corneal ana is preseni before entering school This finding argued that with-the-rule astigmatism in Navajos was genetic; this presumed that there had not been a significant environmental or Skeffington type near stress influence with the Head Start children.

Thus, if the change were from a Darwinian, natural selection function it would require that the predilection for with-therune astigmatism took place though natural selection and over generations. On the other hand, in the case of myopia these changes were instantaneous, one generation. Was there an environmental trauma that, in one generation, was expressed in the genes of the offspring? It is as if there was a genetic predisposition for Navajos to be susceptible to some unknown influence. However, once the change is noted in the parent it is transmitted to the offspring (preschool Navajos).

While scanning a recent issue of Time magazine, I came across an article on epigenetics.24 The concept of epigenetics might be an explanation for the "instantaneous" expression of myopia in Inuits and with-the-rule astigmatism in Native Americans. A recent definition of epigenetics is:

Epigenetics is the study of heritable changes in gene expression that do not involve changes in (he DNA sequence. Epigenetic mechanisms play important roles in our development..., topics such as DNA mefhylation, hisione modification, RNAi, cancer epigenetics, siem cells, and the environment (e.g., nuti-ition ana s,t.ress) as a potential modifier of our own eptgenome.25

I certainly do not suggest that 1 fully understand this proposed mechanism. Neither do 1 know if epigenetics is a "player" in the development of myopia in lnuits or with-the-rule astigmatism in Native Americans. My limited understanding of epigenetics is that chemicals (epigenetic markers) outside the gene, but in close proximity to the gene, are able to influence the gene. These epigenetic markers are presumably influenced by the environment and one's lifestyle. Once environmental factors have impacted, epigenetic factors influence the continued manifestation of the characteristic. Epigenetics could have a dramatic influence on health care; not just for our understanding of refractive conditions but for a host of conditions, loosely defined as "lifestyle" diseases or "diseases of civilization."2'' Diet, dtttgs, tobacco, alcohol, lack of exercise and, yes, stress, are suggested to be factors that work together in a complex way to impact epigenetic markers for an individual. Some conditions that have been linked to lifestyle are: Alzheimer's Disease, atherosclerosis, asthma, cancer, cirrhosis, diabetes, heart disease, acne, stroke, depression, obesity, the list goes on.23 The future will reveal if epigenitics is a viable explanation for refractive changes, and if it offers a mechanism to help explain how stress may actually impact human genetics.

References

References

1. Birnbaum MH. Optometric Management of Nearpoint Vision Disorders. Santa Ana, CA: Optometric Extension Program Foundation, 2008.

2. Skef'fmglon AM. The myope. In: Hendrickson H, ed. Practical Applied Oplornetry. Santa Ana, CA: Optometric Extension Program Foundation, 1991.

3. Manas L. Visual Analysis, 4'1' ed. Santa Ana, CA: Optometric Extension Program Foundation, 2009.

4. Slierman A, Press LJ. Myopia control: Taming the refractive beast. Jn: Press LJ, ed. Applied Concepts of Vision Therapy.Santa Ana, CA: Optometric Extension Program Foundation, 2009.

5. Goss DA, Criswell MI-I. Myopia development in experimental animals: A literature review. Am J Optom Physio Opt 1981;58;859-69.

6. Criswell MH, Goss DA. Myopia development in nonhuman primates: A literature review. Am J Optom Physio Opt 1983:60:250-58.

7. Goss DA, Winkler RL. Progression of myopia in youth: Age of cessation Am J Optom Physio Opt 1983:60:650-58.

8. Goss DA, "Rainey BB. Control of myopia with nearpoint plus as a function of near phoria: Literature review and additional prospective dala. J Behav Optom 2009:20:! 15-22.

9. Young FA, Baldwin WR, Leary GA, West DC. et al. The transmission of refractive errors within Eskimo tamilies. Am J Optom Arch Am Acad Oplorn 1969;46:676-85.

10. Young FA, Baldwin WR. Leary GA. WestUC. el al. Refractive errors, reading performance; and school achievement among Eskimo children. Am J Optom Arch Am Acad Oplom 1970;47:384-90.

11. Young FA, Baldwin WR1 Leary GA, West DC, el al. Comparison of cycloplegic and non-cycloplegic refractions of Eskimos. Am J Optom Arch Am Acad Optom I97];48:8I4~25.

12.Alshirk PH. Retraction in adult West Greenland Eskimos. Acta Ophthalmologica l979;57:84-95.

13. Norn M. Myopia among the Inuil population of East Greenland- Longitudinal study 1950-1994. Acta Ophthalmological 1997:75:723-25.

14. http://in.wikipedia.org /w iki/Natural_ se1ection#Darwin27s_theory. Last accessed March 30, 2010

15.LyIe WM. GrosvenorT. Dean ICL. Corneal astigmatism in Amerind children. Am J Opiom Arch Am Acad Optom 1972;49:517-24.

16. Hamilton JE. Vision anomalies of Indian school children: The Lame Deer study. J Am Optom Assn 1976:47:479-87.

17.Mohindra I, Subbarao N. Astigmatism in Zuni and Navajo Indians. Am J Optom Physio Opt 1977:54:121-24.

18.Goss DA. Meridional analysis of with-the-rule astigmatism in Oklahoma Indians. Optom Vis Sci 1989:66:281-87.

19. Goss DA. Astigmatism in American Indians: Prevalence, descriptive analysis and management issues. In: Goss DA, Edmondson LL, eds. Eye and Vision Conditions of the American Indian. Yukon OK, Pueblo Press, 1 990:61 -70.

20. Cook DT. Results from vision screenings ol" northeastern Oklahoma school children relractivc errors. In: Goss DA, Edmondson LL, eds. Eye and Vision Conditions of the American Endian. Yukon OK, Pueblo Press, 1990:101-16.

21. Garner JM, Hughes J. High corneal astigmatism in the adult Navajo population. .1 Am Oplom Assn 1983:54:815-18.

22. Maples WC, Highes J. Analysis of 6000 Navajo, optometric examinations. Unpublished

23. Maples WC, Harrmann M, Hughes J. Corneal astigmatism in preschool Native Americans. J Am Optom Assn ?997;68:87-94

24. Cloud J. Why genes aren't destiny. Time January 18,2010:49-53.

25.hppt//exlension. berkeley.edu/cat/course2 181. htmr?gclid=cl2wv$Urt8CFQgOswodlV.ULOw Last Accessed March 30, 20 1 0.

26. http://en.wikipedia.Org/wiki/L i festyle_diseases Lasi accessed March 30, 2010.