Myopia induction in animals following alteration of the visual input during development: A review

In this review the effects of changes in the quality of the visual environment on the development of myopia during eye growth in various mammalian and avian species are described. The effect of changes in the light/dark cycle on myopia development has been studied only in the avian eye, mainly that of the domestic fowl. In the eyes of chicks reared from hatching to maturity under continuous illumination, the following findings were reported: myopia, astigmatism, increases in axial length and equatorial width, shallow anterior chamber, increase in corneal diameter, reduction of corneal curvature, increased intra-ocular pressure (IOP), low outflow facility, reduction in aqueous space, buphthalmos, macrophthalmos and glaucoma. The above mentioned changes were consistent in the majority of the studies. In a few experiments where a change in one of the above mentioned parameters was not found, no tendency for the opposite condition was reported When the illumination level of the visual environment was changed in the avian eye there was an increase in the total size of the eye as well as exophthalmos. Other parameters were not examined. It is quite possible that myopia and eye enlargement in avians are caused by entirely different processes than myopia and eye enlargement in mammals since they can be induced either by changes in the diurnal rhythm or by low intensity light. The involvement of the pineal gland in the control of eye growth in avians is therefore possible. The effect of continuous dark rearing was studied in the avian and in the mammalian eye. In developing chicks reared in continuous darkness some enlargement of the eye took place, but a condition of hyperopia was found as opposed to the expected myopia. This result is in agreement with the results of experiments performed on monkeys and cats reared from infancy to adulthood in complete darkness. The effect of near vision conditions during growth was studied in monkeys, cats and chicks confined to small chambers, cages or rooms. A slight myopia was usually obtained in all of the above species but the incidence of myopia increased consistently in the experimental animals compared to the normal controls. The effect of optically restricting the visual field during growth was studied in chicks. Using special occluders, the eyes of the chicks were exposed only to the frontal fields of vision. This manipulation induced a considerably high myopia and an increase in the axial length of the eye and in the depth of the anterior chamber. Removal of the occluders resulted in a reversal of the induced myopia. This myopia was also reduced following ciliary nerve section. The effects of reducing the level of visual contrast by corneal opacification or the wearing of translucent occluders or spheric or cylindrical lenses of high dioptric power were examined in developing monkeys, kittens and chicks. Myopia and axial enlargement usually resulted. The most potent and common experimental model used for the induction of myopia is surgical closure of the eyelids during development. This manipulation had been successfully used in monkeys, cats, tree shrews and chicks. Many studies show that this surgical manipulation may result in myopia together with axial enlargement, corneal flattening and/or changes in the lens and the ciliary apparatus. It is assumed that lid suture combines two parameters, a reduction in visual contrast and a diminution in the illumination level, both of which might facilitate the development of myopia. Finally, the effect of atropinization, which blocks accommodation, was studied in monkeys, cats and chicks during development. This manipulation yielded negative results in normal animals and positive results in animals in which myopia had already been induced. The results so far obtained demonstrate that the accommodative process(at least in mammals) is involved in the induction of myopia during development.