5.2
Physical Factors in Myopia and
Potential Therapies
Wallace S. Foulds*,† and Chi D. Luu†
Introduction
As is obvious from other contributions to this book, factors leading to the development of myopia in childhood are thought to include both genetic predisposition and environmental factors.
Although it has been held that after birth the expression of the genetic program involved in refractive development may be fine-tuned by environmental factors acting through the visual system,1 the interplay between genetic factors and environmental factors remains unknown.2
Among possible environmental factors contributing to the etiology of childhood myopia are a number of physical factors, some well recognised and others not. Among these are factors contributing to the sharpness or lack of sharpness of the retinal image that include the vergence of incident light upon the eye, the curvatures and refractive indices of the transparent media in the eye that are involved in the formation of the retinal image and the nature of the incident light itself including its homogeneity, contrast, spatial and temporal frequency characteristics and its spectral composition.
The physical characteristics of the eye in terms of axial length, vitreous chamber length, corneal and lens curvatures and their refractive indices are obvious contributors to the formation of an in-focus retinal image as is the degree of accommodation being exercised. Intraocular pressure (IOP) might be a factor involved in the ocular expansion that underlies axial myopia but raised IOP is not found in lid fused eyes developing myopia,1 and pressure lowering treatment with eye drops such as timolol
*Corresponding author. E-mail: wallace.stewart@seri.com.sg
†Singapore Eye Research Institute, Singapore.
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has failed to influence myopia progression.4 Even the temperature of the eye has been invoked as a possible aetiological factor in the development of experimental myopia.4
As regards therapy, correction of myopia by spectacles, contact lenses or various forms of refractive surgery that include corneal reshaping procedures, clear lens extraction, insertion of piggyback lenses and (mainly in the past) scleral shortening procedures may correct the optical consequences of myopia but these do not address the underlying condition of the eye.
Pharmacological treatments such as atropine eye drops can beneficially influence the abnormal growth of the eye that occurs in myopia, but as abnormal growth of the eye is probably a consequence of altered retinal cell signalling in response to an abnormal retinal image, such treatment, although effective to a degree, addresses the result rather than the cause of the condition.
Genetic factors that appear to play a role in the predisposition to myopia are almost certainly polygenic rather than monogenic, thus restricting the possible application of genetic engineering. Genetic manipulation is also limited due to an incomplete knowledge of the genes involved in the genesis of myopia. Manipulation of retinal cell signaling is also restricted by an inadequate understanding of the retinal factors involved in the etiology of myopia. It has been stated that the identification of myopia susceptibility genes could provide an insight into the molecular basis of myopia and could lead to therapies to prevent the development of myopia or to slow its progression but to date such information remains elusive.6
As physical factors affecting the visual image appear to play a crucial role in the etiology of myopia, modification of these factors holds promise of future therapies but again this would be dependent upon a better understanding of the role of the various physical factors involved in the genesis of myopia. In this chapter, we explore the known roles of some of these factors and suggest hypotheses to explain others.
In 1990, Wallmann6 asked, “What visual stimuli or lack of what visual stimuli provoke myopia?” and although much more is known about the factors involved in the genesis of myopia we are still not in a position to answer Wallman’s question with any confidence.
Accommodation
In the early 20th century, it was accepted that excessive close work was the main environmental factor involved in the etiology of myopia and as
363 Physical Factors in Myopia and Potential Therapies
close work demanded extended periods of accommodation, it was believed that excessive accommodation was the major contributor not only to the development of myopia but also its progression.
Sight-saving schools, involving large print and distance learning rather than close work, were established in the belief that avoidance of prolonged accommodation would prevent or slow childhood myopia. In the event sight-saving schools were a great disappointment for no benefit was found and before long these schools were closed down.
There are many reasons why excessive accommodation is unlikely to play a direct role in the etiology of childhood myopia although, as has previously been suggested, the stimulus to accommodation from a blurred retinal image may be the same stimulus that leads to axial ocular growth and myopia.6
Accommodation undergoes constant short-term fluctuations in response to variations in the proximity of objects in the field of view, whereas if the same factors that stimulate accommodation are also those involved in eye growth, in the latter instance, they must act over a much more prolonged time-span. A greater than average degree of retinal image blur over time could be a factor influencing eye growth, carrying with it a greater than average degree of accommodation. Thus, a variably blurred retinal image could result in the short-term stimulation of accommodation and in the longer term an alteration in ocular growth, both being a consequence of excessive retinal image blur but without accommodation in itself having a direct effect on ocular growth or refractive development.
Evidence that excessive accommodation does not cause myopia includes the fact that visual deprivation myopia can be induced in the young of many animal species, including primates, when accommodation has been abolished by destruction of the ciliary ganglion or the Edinger–Westphal nucleus in the brain stem or following optic nerve section.7 Recovery from refractive errors induced by the wearing of minus or plus lenses can also occur after accommodation has been surgically abolished.8
Experimentally astigmatic errors of refraction can be induced in chicks by the wearing of cylindrical lenses, an outcome that cannot be related to accommodation.9 It has also been found that steps taken to reduce accommodative effort such as the use of bifocal glasses have had no demonstrable beneficial effect on the progress of childhood myopia.10 The administration of 1% atropine eye drops has been shown to slow the progress of childhood myopia,11–13 and may be effective even in reduced