General EYE ADVICE

Development of Astigmatism in Infants.

The following information is derived from the conclusions I have drawn from my study of available research and from personal clinical experience.

Most infants are born with a small amount of astigmatism of the eyes. Probably a little over half of the Caucasian (European type) infants are born with ATR astigmatism. The great majority of Chinese infants are usually born with astigmatism that is WTR. A greater proportion of pre-term infants compared to term infants is born with ATR astigmatism. In the embryonic stage, the cornea is normally formed with ATR astigmatism (horizontal meridian is steepest). The structure of the cornea is such that its growth involves a greater rate of flattening of the cornea in the horizontal meridian. The cornea thus progressively decreases in its degree of ATR astigmatism and then begins to increase in WTR astigmatism when the horizontal meridian becomes flatter than the vertical.

In some infants, such as the Chinese, the time at which the cornea begins to develop WTR astigmatism is most often prior to birth. However, in Caucasian infants, the time of this transition is usually around the time of birth or shortly afterwards. The rate at which the cornea develops WTR astigmatism is mainly genetically determined. A very small proportion of infants fails to ever completely lose the ATR astigmatism of the cornea, which then persists throughout life. By one year of age, most infants develop a small degree of WTR astigmatism of the cornea that is opposed by astigmatic focusing (see meridional accommodation (MA) below) involving the lens in the eye and to some extent, the cornea.

In some cases, the WTR astigmatism of the cornea cannot be prevented from increasing beyond the level of the opposing ATR astigmatism of the lens in the eye. This can occur if the drive towards WTR corneal astigmatism is abnormally high, or the neuro-muscular mechanism that limits the corneal WTR astigmatism (MA) is weak due to lack of iron etc. This results in the most common type of astigmatism of the eyes in children i.e. WTR.

MA involves a feedback mechanism to detect the presence of astigmatism before astigmatic focusing is stimulated. Thus adequate visual experience is also needed to provide feedback on the state of the vision. Some infants develop a high degree of WTR astigmatism before the retina matures to a level that enables clear vision and the operation of the visual feedback mechanism of MA (i.e. about six months of age). The high amount of MA that is then required to reduce the WTR astigmatism of the cornea may not be achievable, resulting in WTR astigmatism of the eye.

To detect any significant deviations from the normal at an early stage, all infants should be examined preferably around six months of age but no later than one year of age. Special emphasis should be paid to diet and visual experience (see Section A) if such deviations are noted. If there are no signs of improvement by eighteen months of age, corrective lenses may be necessary to prevent amblyopia (lazy eye) due to the astigmatism.

Even after two years' of age (up until about twenty years), the horizontal meridian of the cornea in most normal-sighted people continues to flatten slightly and at a decreasing rate. If it were not for MA, the eye would develop WTR astigmatism. The shape of the cornea remains fairly stable in young adulthood. From middle age and onwards, the horizontal meridian of the cornea usually becomes steeper for reasons explained below.

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