BY DR. H.K. BASU, B.Sc., M.B. (CAL.), D.O.M.S. (LOND.), D.L.O. (LOND.), L.M. (DUB.)
Myopia (commonly known as short sightedness) is caused by various conditions. To understand them properly one should try to define the normal eye first. In normal eyes the parallel rays coming from infinity or very great distance come to a focus, when entering the eyes, on the retina, the sensitive nervous innermost layer of the eye, without any effort of accommodation on the part of the internal eye muscles called ciliary muscles which constitute accommodation. But in myopia these parallel rays entering the eyes, with accommodation at rest come to a focus in front of the retina and then again become divergent producing a circle of diffusion on the retina.
Thus myopic eye is unable to see things at distant. If now the objects looked at are brought nearer, the focus of the rays from these objects also recedes backwards and there will be a position of the object looked at when the focus will just reach the retina and the object is clearly seen. This position of the object is called far point with respect to that eye. So far no consideration of the power of the eye for accommodation is made.
Again if the object is brought nearer the eye the focus goes behind the retina causing the image blurred but now the power of accommodation of the eye comes in for help, and the converging rays inside the eyes are more converged by the change of shape of the lens by accommodation, thus helping the eye to see any object within a few inches from the eye. This point is called the Near point of the eyes. Thus myopic persons can only see all objects between these far point and the near point only.
The causes of myopia:
1. The eye may be longer in length in its anteroposterior diameter than normal eye.
2. The curved surfaces of the eye namely cornea & lens surfaces, may be more curved than the normal eye.
3. Certain changes in the densities of the medias of the eye may be so altered that they force the parallel rays to come to a focus in front of the retina.
Thus almost all protruding eyes are found myopic, where the anteroposterior axis of the eye – ball has increased and has become more than that of the normal eye – called axial myopia. This is generally due to some hereditary weakness of the coats of the eye – ball or acquired weakness from any devitalizing effects of the disease suffered in early life. The effect is seen as bulging backwards of the posterior pole of the eye – ball which is less supported in this region and thus cause the antero – posterior diameter to increase.
The curved surfaces of the eye may be altered as congenital deformity or acquired disease after birth, called curvature myopia.
The changes of the media are generally due to some disease of the media themselves or changes of the density of the media, in such conditions are early cataract, etc.; these types of case are called Index type myopia. It is a common experience that in early cataract due to increased density of the lens a previously normal person become myopic, a myopia more myopic and a hypermetropic less hypermetropic.
Clinical varieties met with in practice are:
1. Non – progressive type and
2. progressive type.
By non – progressive type one should not take it for granted that it does not increase but the increase is very low and start increasing from childhood and reaches as high as about 5 diopters, and this tendency to increase stop at the age of 28/29 when the maximum growth of the body also is completed. But the progressive type goes on increasing through out life and ultimately may reach as high a figure as 30 diopters.
For the point of the diagnosis of the type in early cases one should look into the fundus, when no pathological changes will be visible in non – progressive type but pathological changes in the shape of myopic crescent (effect of posterior staphyloma) and vitreous opacities are noticed in progressive group.
A false form of myopia may be caused by powerful accommodation in younger persons by changing the course of the rays entering the eyes to converge earlier by accommodation, thus giving a false impression that a degree of myopia exists. An error in estimation of refractive power will be introduced in all such cases if this point is not kept in the mind while correcting younger myopes.
Symptoms: Generally only symptom myopic persons complain is that they are unable to see distant object distinctly, but persons with high degree of errors in addition, is unable to do near work comfortably where there is unusual protrusion of the eyeballs as they cannot maintain convergence for a long period. Progressive types see spots and specks floating in front of their eyes and in many cases even after best attempts the vision cannot be brought down to normal.
Myopia once established cannot be eliminated but the progress can be kept in check to some extent by
1. Reducing the hours of near work.
2. Giving rest to the eyes for a few minutes in between the course of near work.
3. Reading or writing only in sufficient light 3 C.P. should be considered as minimum for the purpose.
4. Holding reading and writing materials good distance away from the eyes.
5. Correcting lenses once every year.