Research on the human genome is bringing us closer to an answer.
For many years the ophthalmic community has debated whether refractive myopia is genetically determined, or is the result of tissue alterations influenced by environmental factors. A third option now exists that considers both arguments. To fully understand that option, we must delve into the evolving field of genetic research. As scientists unlock the human genome, they are finding many ocular anomalies are actually multifactoral in nature. These findings also point to the involvement of multiple genetic loci in the genome related to the production of myopia.
A multifactoral genetic defect occurs when multiple genetic expressions interact to produce a condition, or environmental factors interact with these existing genes to produce an anomaly. In this description of complex disease, a single genetic locus is not responsible for production of the condition. Rather, a multifactoral inheritance pattern considers the combined effects of genetic predisposition, lifestyle, and environmental factors in evaluating the etiology of a condition.
Examples of multifactoral dis ease are evidenced in the study of neural tube defects, diabetes, heart disease, and many other conditions. Studies on myopia among people of Chinese descent also support evidence of multifactoral transmission. The incidence of myopia in main land China varies from between 37 and 55%. In Taiwan, where students often study 10 hours a day - the incidence of myopia among the young is close to 84%. In contrast, a 1949 study Chinese Hawaiians had the highest incidence of myopia at around 17%. This was still the highest incidence of myopia of any ethnic group in Hawaii, but a much lower incidence than that seen in China or Taiwan.
Variable expressivity can exist with multifactoral inheritance patterns. For example, a mother with -4.0OD could easily give birth to myopic children, but the degree of error in the children may vary greatly. The characteristics of multifactoral conditions cannot be neatly categorized, but some patterns are appearing.
Disruption and deformation
Two other factors related to genetic alteration have been identified that can be applied to myopic development: disruption and deformation.
Disruption occurs when something interferes with - or destroys - the normal developmental process of an organ, causing a morphological defect. Many of the animal studies examining the development of myopia use a disruption mechanism to produce experimental myopia. Examples of disruption in clinical, non-experimental situations include congenital cataracts, retinal coloboma, large upper-lid hemangiomas, and congenital corneal opacities. There is a tendency for the affected eye to become axially distended in each of these conditions.
Deformation is a type of structural defect characterized by the abnormal form or position of a body part caused by a mechanical force. An example of this type of defect might be the patient who develops a corneal ectasia after many years of hard contact lens wear without management. Cause and effect are clear in such situations. Environmental theories of myopia lack such clear-cut, causal evidence. There's enough circumstantial evidence of environmental mechanisms to keep theories alive. Deformation techniques are often used to counter myopia, as in orthokeratology or LASIK.
A multifactoral paradigm
If we use the broadened scope of multifactoral genetic theory, both genetic aspects and environmental concerns can be accommodated. Those exploring environmental theories of myopic development should find some encouragement in this realignment. At the same time, environmentalists should not ignore the role of genetics. Environmental theory advocates believe manipulating the environment may provide a way to alter myopic development. But, we may also someday be able to use bioengineering technology to alter the genetic role in refractive error development.
At last year's 10th International Myopia Conference, researchers presented studies that seemed to show conflicting etiologies of myopia again. Some studies confirm dramatic increases in the incidence of myopia in Singapore, Hong Kong, and Taiwan. Gene researchers suspect at least five different loci of the genome can produce high myopia. Dr. Leon Davies and associates evaluated accommodative lag response and heart rate in subjects given both simple and difficult reading materials. The results indicate a correlation between accommodative lag and increased heart rate when subjects read difficult material.
A related study by the same group showed this increased heart rate was attributable to increased sympathetic innervation. Myopes in this study showed greater heart rate increases than other refractive groups, and associated increases in tonic accommodation and overall accommodation response. Results may indicate a connection between difficult reading material and sympathetic neural triggers that could cause myopic creep.
The Polytechnic University of Hong Kong is currently involved in myopia research. According to the school's Web site, "It is believed that common myopia is a multifactoral condition caused by the interaction of multiple genes and environmental factors, just like diabetes and hypertension." There is a sense of urgency in finding a way to reverse myopia trends in East Asia. Myopic degeneration is the second leading cause of low vision in Hong Kong. If trends continue, in main land China, "700 million people will become myopic in the next 20 to 40 years."
What's it mean?
Researchers, both here and abroad, should examine the multifactoral approach to understanding myopia. Clinically, we may need to break certain habits of thinking. We tend to view conditions with a genetic basis as unalterable. Conversely, we tend to view conditions of an environmental origin as easily malleable. Neither approach is useful as we look ahead to future technologies that could unlock the secrets of myopia.
Patients are alarmed by the insidious progression of myopia. It's our responsibility to address patients' visual problems. Being a part of the myopia solution of the future will help us retain our patent's loyalty.
Gary Bell, O.D.
Dr. Bell is in private practice in Corona, Calif. He has authored numerous papers on myopia and tear film fluid dynamics, as well as a book on world views published this year.
Copyright Boucher Communications, Inc. Sep 2005
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