|Year : 1983 | Volume
| Issue : 6 | Page : 799-800
Beneficial effects of myopia
AMU Institute of Ophthalmology, Aligarh, India
AMU Institute of Ophthalmology, Aligarh
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Nath J. Beneficial effects of myopia. Indian J Ophthalmol 1983;31:799-800
Simple Myopic eye is superior to the emmetropic and the hypermetropic eye aesthetically, medically, socially and even technically. It is prominent with a large palpebral aperture, a large cornea and is simply beautiful.
Presbyopia, and early affliction in emmetropes and hypermetropes, appears late or not at all in simple myopes.
They suffer less from diabetic retinopathy. Jain observed that it is either less frequent, or if present, progresses more slowly in myopes. This observation was also made by Gonzales 2and Amalrie(3). The former did not find a single case of high myopia in 146 cases of vascular lesion while the latter found that the course of diabetic retinopathy is more benign in myopic eyes or does not occur at all. In a study4 carried outon Eaeles's disease at the Institute of Ophthalmology, Aligarh, it was found that simple myopes are comparatively rare sufferers from this malady, especially the higher ones.
Gonzales(2) also observed that hypertensive retinopathy is also less in myopes. In a similar study Kato(5) found that the A/V crossing change in hypertension were less marked in high myopes.
Glaucoma occurs less in these eyes due to the deep anterior chambers while cataract takes an unusually long time to mature. The glasses required after the operation are less bulky and hence is an added advantage to simple myopes.
Papilloedoema due to increased intra cronial pressure is also less common in simple myopes. This was shown by Marchisanio(6),Biette(7) and Morone(8). Consequently, the damage to the optic nerve fibres is less or delayed.
The reason of these advantages are not difficult to find if the evolution is considered.
The eye has passed through many phases of development. Diffuse sensitivity to light has been seen in Protozoa and Porifera. In invertebrates the most important fact, barring a few exceptions, is that the eye has developed from ectoderm while in Chordates the eye has developed from neurectoderm.
In Coelentrata, contact photoreceptors of the most elementarry types are present. The Echinodermata, have a photosensitive skin while the star fish have the optic cushions. The Platyhelmenthes have developed the rudimentary sense organs and in the Annelida the receptors are distributed in the epithelium and around the sub epithelial nerves. Molluscs have two cephalic eye which subserve the visual functions while the Arthropodes show a wide variation of eyes is simple, compound and even a median eye.
In the case of vertebrates, the eyes have arisen from the neurectoderm. The strangeness is the sudden appearance of the eye which later underwent a number of changes to suit different functions.
In fishes the eye is relatively much more hypermetropic as they surface in water. The shape of the eye is streamlined to facilitate swimming. The lens moves forwards and backwards in order to focus the prey.
Eyes, of Amphibia, are no longer streamlined but spherical. They also move the lens forward and backwards to accommodate. To survive, their objectives are limited. To moisten the eye the Marderian glands and the nasolacrimal ducts appear. Reptiles have a mobile iris and accommodate by deformation of the lens. Avians have good vision a well developed eye with nasal accentricity to help in a little binocular function but they must have a wide field to survive.
Accommodation or focussing of the eye at various distances is very important and has been achieved by various developmental acquisitions in different animals. There is the development of accessory retinae close to the diopteric apparatus (as in the developmental of tubular eye in deep sea fishes.) Variations in the positions of the visual cell in relation to the lens (bats or horse), the use of stenoepic pupil (the cat), deformation by the muscular apparatus of the eye (lamprey), pushing and pulling of the lens forward and backward (some fishes, amphibia and snakes), change in shape by squeezing it (reptiles and birds) or relaxing it (mammals).
Wild animals and primates should see better at a distance as this helps both in location, attack and defence. Wild animals are therefore usually hypermetropic or emmetropic while domisticated animals show a considerable amount of myopia.
In man, the functional requirement of the eye has tremendously changed. The eye of the hunter and caveman required a good distance vision. Later man settled down to be tiller of the land, started cooking his own food and did much more near work than his forefathers did. As the need for near work increased, the faculty of convergence, a comparatively recent acquirement on the evolutionary scale, has developed. This facilitated him to focus on near objects better and in all dimensions. The struggle for better near vision went on and this may have been a factor for evolving a simple myopic eye.
This earlier hypermetropic or emmetropic eye, suffered from various diseases and disadvantages like presbyopia, early cataract, diabetic and hypertensive retinopathy Eales's Disease, glaucoma, pappilloedoema etc. So nature in its experiments to improve this model has produced a beautiful myopic eye with increased axial length, larger cornea, larger chamber angle, and perhaps a larger and flatter lens which may he more secure in the struggle for man with the decreased incidence of above diseases peculiar to the other eyes.
If so, in times to come every man will he a myopic sample rather than otherwise. Mongolian race is a living example. This is against the Trpn'sa Hypothesis which is a statistical analysis of only Caus-asians.