General EYE ADVICE

Vision Defects (Refractive Errors)

Jaundice or lack of iron can lead to refractive errors, especially hypermetropia (long-sightedness) and astigmatism. For an explanation of the optics involved in vision defects you may refer to OAA Refractive Errors.

In normal-sightedness the eye is set such that if one is looking at a very distant object, very little or no focussing is required to see it clearly. The closer an object is brought to the eyes, the greater will be the amount of focusing that is required in maintaining clear vision. This focusing is carried out by the neuro-muscular components of the visual system and requires a sub-conscious effort. It is dependent on the elasticity of certain tissues within the eye. Infants and children have the greatest focusing power (accommodation) due to the high tissue elasticity. At maximum effort, they can normally see clearly as close as 7 cm from their eyes (this amount of focusing power is given a value of 14 Dioptres). The elasticity in older people gradually reduces and at fifty years of age the accommodation drops to 2 Dioptres.

In long-sightedness, the eye is set such that one has to focus more than the slight amount that is necessary in normal-sightedness when looking at a distant object. The degree of long-sightedness determines the extra amount of focusing that is required compared to the normal. If one is only mildly long-sighted (1-2 Dioptres), distance vision is usually unaffected in younger people as this only involves focusing 1-2 Dioptres in total. However, prolonged close work can lead to more rapid fatigue as this extra amount of focusing must be done in addition to the normal amount of focusing. Higher degrees of long-sightedness may lead to fatigue with prolonged distance viewing (e.g. TV). The degree of long-sightedness, the intensity of viewing and any concurrent astigmatism or muscle imbalance usually determine the necessity for distance glasses. Glasses are usually required for prolonged reading when there is over 2 Dioptres of long-sightedness.

Most children are born slightly long-sighted. A healthy neuro-muscular system (dependant on a healthy diet as discussed above) and normal visual experience will most often prevent any progression of long-sightedness and in time, achieve normal-sightedness. In some individuals, long-sightedness can cause or aggravate a particular type of eye muscle imbalance called esophoria. In this condition, the eyes have a tendency to cross inwards. High degrees of long-sightedness may increase the inward tendency of the eyes to such an extent that the eyes cannot remain straight and one eye turns inwardly. Esotropia is the term used to denote an eye that is turned inwards and is discussed in some detail below.

In myopia (short-sightedness) the eye is set such that an object in the distance appears out of focus. Unlike its strong ability to focus in towards near objects, the human eye has negligible ability to actively focus away into the distance. In order to refocus into the distance, we mainly rely on the resting of our near-focusing system. This enables the elastic tissues within the eye to spring back into position for distance focusing. Thus when the eye is at "rest" (i.e. the reference point that we use to define its refractive state), it cannot succeed in focusing towards a more distant object. The closer the distance from the eye at which the eye is set or "rested", the more out of focus that distance objects become as focusing away from the rested state into the distance is not possible. Depending on the degree of myopia, near vision can be particularly good, as a lesser degree of focusing is necessary.

Only a small proportion of term babies is born myopic. The more premature that a baby is born the greater are its chances of being born myopic. With visual experience, many babies that are born myopic become normal-sighted. However, these babies are more prone to revert to myopia at school age. (See discussion below at end of this section on recommended measures to prevent myopia. These measures are also applicable to those children born normal-sighted.)

Please refer to Part II for a separate, detailed discussion of astigmatism, where information that is applicable to infants as well as adults can be found. (In particular, see development of astigmatism in infants.)

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